I live very close to where NEAF is held, about :20min away. So I saw the scope
in person. It looked great. The construction was very nice.

George
20" Obsession F5

Sent from my Verizon Wireless BlackBerry

-----Original Message-----
From: "abellhunter" <humphreys@...>
Sender: bigdob@yahoogroups.com
Date: Sat, 17 Jul 2010 20:08:47
To: <bigdob@yahoogroups.com>
Reply-To: bigdob@yahoogroups.com
Subject: [bigdob] Great Red Spot's 40in Dob

just opened Sky & Tele's Aug issue and found
the biggest dob since jimi's 48.

See:

http://www.greatredspot.com/Jupiter_Series_Telescopees.htm



------------------------------------

Yahoo! Groups Links

Hello,

I'm not sure if this is the place to post this question, but I cannot find a
Lumicon Yahoo Group. I just recently completed construction of my first Dob
(14.5 inch Kreige-style), and I have taken the old Lumicon Sky Vector I off my
old C-8 and installed it on the Dob. With a little creative tinkering to the
mounting hardware, everything is working fine. My problem is that where I live,
there are so many trees that I often cannot see two of the 28 alignment stars
more than 45 degrees apart (the manual recommends 60 degrees). I'd like to use
stars from the Star Catalog for alignment, but after all these years, I no
longer have the manual that lists what these stars are. The handheld unit only
displays cryptic numbers. After much searching, I have been unable to locate a
list online. If anybody happens to have a list of what stars the numbers in the
Star Catalog refer to, or could direct me to somewhere where I may find a list,
I would be most grateful.

Thanks in advance,

Arnie

Arnie,
 
Here is a list from the JMI site. The Sky Vector is a Tangent box, same as the
JMI.
 
http://www.jimsmobile.com/html_docs/ngcmax_db_stars.htm
 
Pat

--- On Wed, 8/4/10, alwittstein <astronomer9@...> wrote:


From: alwittstein <astronomer9@...>
Subject: [bigdob] Lumicon Sky Vector I on newly built Dob
To: bigdob@yahoogroups.com
Date: Wednesday, August 4, 2010, 2:21 AM


 



Hello,

I'm not sure if this is the place to post this question, but I cannot find a
Lumicon Yahoo Group. I just recently completed construction of my first Dob
(14.5 inch Kreige-style), and I have taken the old Lumicon Sky Vector I off my
old C-8 and installed it on the Dob. With a little creative tinkering to the
mounting hardware, everything is working fine. My problem is that where I live,
there are so many trees that I often cannot see two of the 28 alignment stars
more than 45 degrees apart (the manual recommends 60 degrees). I'd like to use
stars from the Star Catalog for alignment, but after all these years, I no
longer have the manual that lists what these stars are. The handheld unit only
displays cryptic numbers. After much searching, I have been unable to locate a
list online. If anybody happens to have a list of what stars the numbers in the
Star Catalog refer to, or could direct me to somewhere where I may find a list,
I would be most grateful.

Thanks in advance,

Arnie








[Non-text portions of this message have been removed]

Catalogs are not identical across all the Tangent based units,
but the ST catalog may be.  Once you have selected an object,
say ST001, hit the enter button to scroll the item information.
You can compare it to the handy listing that Patrick provided.

Pete Eschman

--- In bigdob@yahoogroups.com, Patrick Moore <prmo@...> wrote:
>
> Arnie,
>
> Here is a list from the JMI site. The Sky Vector is a Tangent box, same as the
JMI.
>
> http://www.jimsmobile.com/html_docs/ngcmax_db_stars.htm
>
> Pat
>
> --- On Wed, 8/4/10, alwittstein <astronomer9@...> wrote:
>
>
> From: alwittstein <astronomer9@...>
> Subject: [bigdob] Lumicon Sky Vector I on newly built Dob
> To: bigdob@yahoogroups.com
> Date: Wednesday, August 4, 2010, 2:21 AM
>
> Hello,
>
> I'm not sure if this is the place to post this question, but I cannot find a
Lumicon Yahoo Group. I just recently completed construction of my first Dob
(14.5 inch Kreige-style), and I have taken the old Lumicon Sky Vector I off my
old C-8 and installed it on the Dob. With a little creative tinkering to the
mounting hardware, everything is working fine. My problem is that where I live,
there are so many trees that I often cannot see two of the 28 alignment stars
more than 45 degrees apart (the manual recommends 60 degrees). I'd like to use
stars from the Star Catalog for alignment, but after all these years, I no
longer have the manual that lists what these stars are. The handheld unit only
displays cryptic numbers. After much searching, I have been unable to locate a
list online. If anybody happens to have a list of what stars the numbers in the
Star Catalog refer to, or could direct me to somewhere where I may find a list,
I would be most grateful.
>
> Thanks in advance,
>
> Arnie

Heading out for Stellafane tomorrow. I hope to see some of you there.
One of the few star events I get to go to. Weeee...




-Roger
18" Obsession SC/AN/FSA
www.astrocrumb.com

About 43 to 38 years ago Norman James of San Diego showed a sphere telescope at
RTMC.  It was a 12.5 inch reflector with a 28 inch fiberglas sphere merged with
a 14 inch tube, all one piece.  There was a wide window next to the tube for a
unity power finder made from a large lens and a shaving mirror.
The interesting thing was the mounting for the ball.  It was a pan of water on
short legs or large blocks.  This supported the ball firmly with no friction. 
An axle with suction cup extended from slightly below the pan at the 33 degree
latitude angle.  A hand air pump and valve stuck the suction cup onto the sphere
and a small DC motor drove the axle at sidereal rate.
Here in RTMC archive pages you can see a couple pix of the Norman James sphere
mount.
http://www.cloudynights.com/ubbthreads/attachments/3729721-JAMES%2C%20NORM%20-%2\
0spheremount%20-%201A.jpg
http://www.rtmcastronomyexpo.org/RDS/RTMC69.html
http://www.rtmcastronomyexpo.org/RDS/RTMC72.html
http://www.cloudynights.com/ubbthreads/attachments/3729712-RTMC%201970%20-%20Jam\
es%20%26%20spheremount%20-%20small.jpg
http://stellafane.org/convention/historic/photos2.html
Here is a book about his famous auto designs:
http://lottoworlddirectory.com/OfFirebirds&Moonmen.htm
Gregg

--- In bigdob@yahoogroups.com, "startestisbest" <mlinnolt@...> wrote:
>
> In the photo folder "Mike's 20" f/3.6 spheremount", I have placed a series of
images briefly showing the steps I went through over the past few years, of
making a very lightweight telescope of somewhat alternative design. This scope
was also written up in issue #58 of Amateur Astronomy magazine. Recently, more
sphere mounted scopes have been showing up at events such as Stellafane and
RTMC.
>
> Given this type of base is a significant variation of the traditional design,
can this still be considered a "big Dob"?
>
> I have been using this scope quite extensively the past few years, and had
many opportunities to compare it to a "standard" Dob =SD. Here are my
observations of the differences (advantages/disadvantages):
>
> ADVANTAGES
> 1. The fiberglass sphere design is much more lightweight than the equivalent
size SD. This entire scope including optics weighs 66lbs. Even though it is
designed with truss poles and quickly assembled, it can be lifted and moved and
transported in a van or SUV by a single person, in one piece.
> 2. Any position in the sky can be observed with equivalent ease, no "Dobsons
hole".
> 3. The eyepiece angle can be adjusted to the optimal viewing position
simultaneously with locating an object. The comfort factor is an order of
magnitude improvement over a SD.
>
> DISADVANTAGES
> 1. Construction of the sphere requires fiberglass work, which is somewhat more
messy and specialized than cutting a plywood base. But, due to the simplicity,
lack of bearings, etc. it doesn't appear to take that much more overall time to
construct.
> 2. The large smooth sphere needs to be more carefully protected from damage,
such as sand between the bearings and the surface, which can permanently scratch
it. (I have minimized the latter problem by using Teflon cloth over the
bearings, which absorbs most sand particles before they can scratch.)
>
> I would like to hear people's opinions on the relative merits of this sphere
design vs. a standard Dob.
>
> Thanks,
> Mike Linnolt
>

[ From bigdob@yahoogroups.com (peschman2001) on Wednesday, August 04, 2010 ]

> Catalogs are not identical across all the Tangent based units,
> but the ST catalog may be. Â Once you have selected an object,
> say ST001, hit the enter button to scroll the item information. Â
> You can compare it to the handy listing that Patrick provided.

> Pete Eschman

FWIW, the alignment stars in my old NGC Sky-Vector are listed by name:

Achenar
Albireo
Aldebaran
Alpha Centauri
Alphard
Alpheratz
Altair
Antares
Arcturus
Betelguese
Canopus
Capella
Castor
Deneb
Denebola
Fomalhaut
Mirfak
Navi
Polaris
Procyon
Rasalhague
Regulus
Rigel
Sirius
Spica
Suhail
Vega


--
Len Philpot - l e n @ p h i l p o t . o r g   (no spaces)
><>   http://philpot.org/ - http://lphilpot.zenfolio.com
       Read my story: http://www.mostimportantthing.org

[Non-text portions of this message have been removed]

In Texereau's 2nd Ed. Pages 403-406 are some of his detailed fiberglassing
methods and diagrams. Certainly Mr. James design was unique and revolutionary
for its time. Several commercial scopes were apparently offshoots - Edmunds
"Astroscan" and the Mag1 Portaball line.
The goals of my design were basically, simplicity of construction and
lightweight. This is quite a departure from James's design.
1. Rather than complex fiberglassing of a complete OTA, I just glass the sphere,
the rest is basically a truss tube with a unique top section.
2. The sphere is minimally 20% larger diameter than the primary.
3. To avoid extra counterweighting in the sphere, my primary blank was ground to
a truncated conical shape, so it fits as far as possible towards the back of the
sphere.
4. The mirror mount is a flat plate with rubber cushion, which if ground very
flat, works as well as an 18-pt flotation without the added weight and
complexity.
5. Edge support is foregone in favor of a central stud providing the lateral
force when tilted.
6. Collimation is done by adjustments of 2 of the truss poles nearest the
focuser. The mirror mount is fixed to approximately the correct position, with
fine adjustments done at the eyepiece end. This allows quick adjustments during
use and saves weight at the primary as well.

While my design shares a few basic elements of James's and other designs, it has
numerous new and unique features which set it apart. And, its probably the
largest aperture working sphere scope that I know of.

Mike

--- In bigdob@yahoogroups.com, "starryskyn" <starryskyn@...> wrote:
>
> About 43 to 38 years ago Norman James of San Diego showed a sphere telescope
at RTMC.  It was a 12.5 inch reflector with a 28 inch fiberglas sphere merged
with a 14 inch tube, all one piece.  There was a wide window next to the tube
for a unity power finder made from a large lens and a shaving mirror.
> The interesting thing was the mounting for the ball.  It was a pan of water on
short legs or large blocks.  This supported the ball firmly with no friction. 
An axle with suction cup extended from slightly below the pan at the 33 degree
latitude angle.  A hand air pump and valve stuck the suction cup onto the sphere
and a small DC motor drove the axle at sidereal rate.
> Here in RTMC archive pages you can see a couple pix of the Norman James sphere
mount.
>
http://www.cloudynights.com/ubbthreads/attachments/3729721-JAMES%2C%20NORM%20-%2\
0spheremount%20-%201A.jpg
> http://www.rtmcastronomyexpo.org/RDS/RTMC69.html
> http://www.rtmcastronomyexpo.org/RDS/RTMC72.html
>
http://www.cloudynights.com/ubbthreads/attachments/3729712-RTMC%201970%20-%20Jam\
es%20%26%20spheremount%20-%20small.jpg
> http://stellafane.org/convention/historic/photos2.html
> Here is a book about his famous auto designs:
> http://lottoworlddirectory.com/OfFirebirds&Moonmen.htm
> Gregg
>
> --- In bigdob@yahoogroups.com, "startestisbest" <mlinnolt@> wrote:
> >
> > In the photo folder "Mike's 20" f/3.6 spheremount", I have placed a series
of images briefly showing the steps I went through over the past few years, of
making a very lightweight telescope of somewhat alternative design. This scope
was also written up in issue #58 of Amateur Astronomy magazine. Recently, more
sphere mounted scopes have been showing up at events such as Stellafane and
RTMC.
> >
> > Given this type of base is a significant variation of the traditional
design, can this still be considered a "big Dob"?
> >
> > I have been using this scope quite extensively the past few years, and had
many opportunities to compare it to a "standard" Dob =SD. Here are my
observations of the differences (advantages/disadvantages):
> >
> > ADVANTAGES
> > 1. The fiberglass sphere design is much more lightweight than the equivalent
size SD. This entire scope including optics weighs 66lbs. Even though it is
designed with truss poles and quickly assembled, it can be lifted and moved and
transported in a van or SUV by a single person, in one piece.
> > 2. Any position in the sky can be observed with equivalent ease, no "Dobsons
hole".
> > 3. The eyepiece angle can be adjusted to the optimal viewing position
simultaneously with locating an object. The comfort factor is an order of
magnitude improvement over a SD.
> >
> > DISADVANTAGES
> > 1. Construction of the sphere requires fiberglass work, which is somewhat
more messy and specialized than cutting a plywood base. But, due to the
simplicity, lack of bearings, etc. it doesn't appear to take that much more
overall time to construct.
> > 2. The large smooth sphere needs to be more carefully protected from damage,
such as sand between the bearings and the surface, which can permanently scratch
it. (I have minimized the latter problem by using Teflon cloth over the
bearings, which absorbs most sand particles before they can scratch.)
> >
> > I would like to hear people's opinions on the relative merits of this sphere
design vs. a standard Dob.
> >
> > Thanks,
> > Mike Linnolt
> >
>

Hi Folks,
I am new to the group. Joined as I am putting together a big dob from a nicce
17.5' F4 mirror.

Getting ready to build a base for a 17.5" F4  mirror and am facing two
questions, not having done this particular feat before, both regarding the
bearings.

Altitude bearing: I have seen mounts that use a simple V-groove bearing with
some felt or something for a bearing surface, and others with a more precision
surface with a circular bearing surface and perhaps felt/Teflon.  I am thinking
that the V-groove is more forgiving but probably more sensitive to imperfect
balance, lacking the friction surface of a longer circular surface of a cradle. 
On the other hand, the long circular bearing surfaces will probably require
greater precision in terms of gap clearance between the cradle bearing and the
shaft (so to speak). I am inclined toward the latter, but wondering if anyone
had any hands-on experience, as I can see having to do this more than once to
get it right.

Azimuth bearing:  Question here is "lazy susan" bearings  or something simpler
like two sandwiched LP records lubed with silicon spray.

Any suggestions, pointers or links would be greatly appreciated - otherwise I am
blazing my own trail!

Randy G

You need to get the book
"The Dobsonian Telescope" by Kriege and Berry.  It will answer all of those
questions and more.

http://www.willbell.com/tm/index.htm

Large circular bearings are about moving the altitude axis up higher on the tube
so that it will balance without adding a lot of weight to the rear end to make
it balance.  That is needed if you want to have the bearings mounted close to
the ground.  The shorter the rocker sides the easier it is too eliminate twist
and vibration of the side bearings.

Standard lazy susan bearing do not have enough friction to keep the scope
pointing where you put it.  The usual method for the azimuth is to use three
pads of Teflon supporting a bumpy laminate called Ebony Star or using a material
called FRP (Fiberglass Reinforced Plastic.)  It is getting a little hard to find
Ebony Star which once was easy to find at Home Depot but is usually not there
anymore.  FRP can be found in 8x10 foot sheets at building supply stores and are
not very expensive.

But you really should get the book as it explains a lot about what is important
and solutions to the usual problems.


Some good sources for supply of telescope part are

http://www.astrosystems.biz/
and
http://www.scopestuff.com/

Dale Eason

--- In bigdob@yahoogroups.com, "Randallo" <rogauntt@...> wrote:
>
> Hi Folks,
> I am new to the group. Joined as I am putting together a big dob from a nicce
17.5' F4 mirror.
>
> Getting ready to build a base for a 17.5" F4  mirror and am facing two
questions, not having done this particular feat before, both regarding the
bearings.
>
> Altitude bearing: I have seen mounts that use a simple V-groove bearing with
some felt or something for a bearing surface, and others with a more precision
surface with a circular bearing surface and perhaps felt/Teflon.  I am thinking
that the V-groove is more forgiving but probably more sensitive to imperfect
balance, lacking the friction surface of a longer circular surface of a cradle. 
On the other hand, the long circular bearing surfaces will probably require
greater precision in terms of gap clearance between the cradle bearing and the
shaft (so to speak). I am inclined toward the latter, but wondering if anyone
had any hands-on experience, as I can see having to do this more than once to
get it right.
>
> Azimuth bearing:  Question here is "lazy susan" bearings  or something simpler
like two sandwiched LP records lubed with silicon spray.
>
> Any suggestions, pointers or links would be greatly appreciated - otherwise I
am blazing my own trail!
>
> Randy G
>

Hi all,just put together a 15" obsession i bought on the cheap? ( 2700
about 3 years ago ). been a LITTLE BUSY, so just getting around to using
it.
had it collimated using lasermax 2" collimator, and had 22mm nagler
installed. trained on Jupiter last night and was kinda disappointed. not
a very sharp image, just saw a blurry reddish line across the middle,
not nearly sharp at all. moons were visible easily, but not perfect
Circles...all near the center of the EP.. I remember getting much
sharper images in a cheap 6" newtonian years ago.....
I can't collimate it any better than I did using the equipment i had, so
wondering if the mirror might be cause. maybe why i got it on the cheap
side [:O] ...mirror should have been at temp, was sitting outside all
night, and i was viewing around 1am...Here is writing on the mirror:
(sticker on the back)QSPOptical Technology IncSanta Ana, CA
(writing on back of mirror)im 6569 NOVA EAL # 398703
Also, I do have some torque on the secondary mirror adjustors. they are
very tight. had to do this to get the mirror adjusted...
So my big question is: Is there a method to determine if the primary is
warped while the thing is installed, without sending it back to the
factory??
Thanks for any advice.RgdsBob


[Non-text portions of this message have been removed]

Welcome to the group Randy!

Since you are starting from scratch, maybe consider a sphere-mount scope
http://groups.yahoo.com/group/bigdob/photos/album/1174616503/pic/list

For a 17.5" mirror a 22-24" diameter sphere should work well.

Mike

--- In bigdob@yahoogroups.com, "Randallo" <rogauntt@...> wrote:
>
> Hi Folks,
> I am new to the group. Joined as I am putting together a big dob from a nicce
17.5' F4 mirror.
>
> Getting ready to build a base for a 17.5" F4  mirror and am facing two
questions, not having done this particular feat before, both regarding the
bearings.
>
> Altitude bearing: I have seen mounts that use a simple V-groove bearing with
some felt or something for a bearing surface, and others with a more precision
surface with a circular bearing surface and perhaps felt/Teflon.  I am thinking
that the V-groove is more forgiving but probably more sensitive to imperfect
balance, lacking the friction surface of a longer circular surface of a cradle. 
On the other hand, the long circular bearing surfaces will probably require
greater precision in terms of gap clearance between the cradle bearing and the
shaft (so to speak). I am inclined toward the latter, but wondering if anyone
had any hands-on experience, as I can see having to do this more than once to
get it right.
>
> Azimuth bearing:  Question here is "lazy susan" bearings  or something simpler
like two sandwiched LP records lubed with silicon spray.
>
> Any suggestions, pointers or links would be greatly appreciated - otherwise I
am blazing my own trail!
>
> Randy G
>

Update:
Checked Lasermax TLC-202 laser colllimator in focuser. rotated it 180, then back
again. found the spot on the center of the primary rotates by about 1/8" to
3/16" ( about 1/2 to 2/3 of cutout on sticker in center of primary. hole in
sticker is .25".. )

Will adjust laser tonite, but wondering if this amount of offset is big enough
to cause the "blurry bands of jupiter"??

thx,
Bob


--- In bigdob@yahoogroups.com, "xrpb11a" <rsh@...> wrote:
>
> Hi all,just put together a 15" obsession i bought on the cheap? ( 2700
> about 3 years ago ). been a LITTLE BUSY, so just getting around to using
> it.
> had it collimated using lasermax 2" collimator, and had 22mm nagler
> installed. trained on Jupiter last night and was kinda disappointed. not
> a very sharp image, just saw a blurry reddish line across the middle,
> not nearly sharp at all. moons were visible easily, but not perfect
> Circles...all near the center of the EP.. I remember getting much
> sharper images in a cheap 6" newtonian years ago.....
> I can't collimate it any better than I did using the equipment i had, so
> wondering if the mirror might be cause. maybe why i got it on the cheap
> side [:O] ...mirror should have been at temp, was sitting outside all
> night, and i was viewing around 1am...Here is writing on the mirror:
> (sticker on the back)QSPOptical Technology IncSanta Ana, CA
> (writing on back of mirror)im 6569 NOVA EAL # 398703
> Also, I do have some torque on the secondary mirror adjustors. they are
> very tight. had to do this to get the mirror adjusted...
> So my big question is: Is there a method to determine if the primary is
> warped while the thing is installed, without sending it back to the
> factory??
> Thanks for any advice.RgdsBob
>
>
> [Non-text portions of this message have been removed]
>

The simplest way to check for collimation is look at a star at high power. If it
is perfectly circular and symmetric in the center of the field, then you are
collimated without vignetting. Any sign of a "comet" shaped star in the center
implies misalignment along the direction of the "tail" of the comet.

There could be many reasons for poor planetary performance. Poor seeing is the
first thing you should rule out. Thermal issues with the mirror and/or air
currents in the optical path come next. Then you can start examining the figure
of the primary mirror. Doing a classic star test would resolve that beyond any
question. Lastly, there is always a chance of a poor secondary or eyepiece
problem, but those are infrequent.

Mike

--- In bigdob@yahoogroups.com, "xrpb11a" <rsh@...> wrote:
>
>
>
>
> Update:
> Checked Lasermax TLC-202 laser colllimator in focuser. rotated it 180, then
back again. found the spot on the center of the primary rotates by about 1/8" to
3/16" ( about 1/2 to 2/3 of cutout on sticker in center of primary. hole in
sticker is .25".. )
>
> Will adjust laser tonite, but wondering if this amount of offset is big enough
to cause the "blurry bands of jupiter"??
>
> thx,
> Bob
>
>
> --- In bigdob@yahoogroups.com, "xrpb11a" <rsh@> wrote:
> >
> > Hi all,just put together a 15" obsession i bought on the cheap? ( 2700
> > about 3 years ago ). been a LITTLE BUSY, so just getting around to using
> > it.
> > had it collimated using lasermax 2" collimator, and had 22mm nagler
> > installed. trained on Jupiter last night and was kinda disappointed. not
> > a very sharp image, just saw a blurry reddish line across the middle,
> > not nearly sharp at all. moons were visible easily, but not perfect
> > Circles...all near the center of the EP.. I remember getting much
> > sharper images in a cheap 6" newtonian years ago.....
> > I can't collimate it any better than I did using the equipment i had, so
> > wondering if the mirror might be cause. maybe why i got it on the cheap
> > side [:O] ...mirror should have been at temp, was sitting outside all
> > night, and i was viewing around 1am...Here is writing on the mirror:
> > (sticker on the back)QSPOptical Technology IncSanta Ana, CA
> > (writing on back of mirror)im 6569 NOVA EAL # 398703
> > Also, I do have some torque on the secondary mirror adjustors. they are
> > very tight. had to do this to get the mirror adjusted...
> > So my big question is: Is there a method to determine if the primary is
> > warped while the thing is installed, without sending it back to the
> > factory??
> > Thanks for any advice.RgdsBob
> >
> >
> > [Non-text portions of this message have been removed]
> >
>

Even more sensitive is to defocus the high power (300x) view of the star both
inside and outside of focus.  Look to see if the shadow of the diagonal is in
the center of the large brighter area.  This must be done when the star is at
the center of the image however.  Best to use Polaris since it will stay put. 
Also check to see if the bright area stays circular.  If not then start looking
for the cause.

You can check primary figure problems by the same defocus test.  The diffraction
rings should look the same on either side of focus.  That is one side should not
have brighter more contrasty rings than the other for a perfect mirror.

A quick quality check is to focus and then defocus on one side until you see the
shadow of the diagonal just break out into the image.  Remember how far you had
to move the focus knob.  Then do the same to the other side of focus.  If you
had to move the know less than 2x time the other side of focus then the mirror
is diffraction limited at least for Primary correction.

Also notice how the white area wiggles and stuff moves through it at high power.
This is an indication of seeing in the upper atmosphere and heat in the scope
itself.

If you want to learn more about testing telescopes using these methods see.

Star Testing Astronimical Telescopes by Richard Suiter. Available from
WillmanBell.

http://www.willbell.com/

Dale Eason

--- In bigdob@yahoogroups.com, "startestisbest" <mlinnolt@...> wrote:
>
> The simplest way to check for collimation is look at a star at high power. If
it is perfectly circular and symmetric in the center of the field, then you are
collimated without vignetting. Any sign of a "comet" shaped star in the center
implies misalignment along the direction of the "tail" of the comet.
>
> There could be many reasons for poor planetary performance. Poor seeing is the
first thing you should rule out. Thermal issues with the mirror and/or air
currents in the optical path come next. Then you can start examining the figure
of the primary mirror. Doing a classic star test would resolve that beyond any
question. Lastly, there is always a chance of a poor secondary or eyepiece
problem, but those are infrequent.
>
> Mike
>
> --- In bigdob@yahoogroups.com, "xrpb11a" <rsh@> wrote:
> >
> >
> >
> >
> > Update:
> > Checked Lasermax TLC-202 laser colllimator in focuser. rotated it 180, then
back again. found the spot on the center of the primary rotates by about 1/8" to
3/16" ( about 1/2 to 2/3 of cutout on sticker in center of primary. hole in
sticker is .25".. )
> >
> > Will adjust laser tonite, but wondering if this amount of offset is big
enough to cause the "blurry bands of jupiter"??
> >
> > thx,
> > Bob
> >
> >
> > --- In bigdob@yahoogroups.com, "xrpb11a" <rsh@> wrote:
> > >
> > > Hi all,just put together a 15" obsession i bought on the cheap? ( 2700
> > > about 3 years ago ). been a LITTLE BUSY, so just getting around to using
> > > it.
> > > had it collimated using lasermax 2" collimator, and had 22mm nagler
> > > installed. trained on Jupiter last night and was kinda disappointed. not
> > > a very sharp image, just saw a blurry reddish line across the middle,
> > > not nearly sharp at all. moons were visible easily, but not perfect
> > > Circles...all near the center of the EP.. I remember getting much
> > > sharper images in a cheap 6" newtonian years ago.....
> > > I can't collimate it any better than I did using the equipment i had, so
> > > wondering if the mirror might be cause. maybe why i got it on the cheap
> > > side [:O] ...mirror should have been at temp, was sitting outside all
> > > night, and i was viewing around 1am...Here is writing on the mirror:
> > > (sticker on the back)QSPOptical Technology IncSanta Ana, CA
> > > (writing on back of mirror)im 6569 NOVA EAL # 398703
> > > Also, I do have some torque on the secondary mirror adjustors. they are
> > > very tight. had to do this to get the mirror adjusted...
> > > So my big question is: Is there a method to determine if the primary is
> > > warped while the thing is installed, without sending it back to the
> > > factory??
> > > Thanks for any advice.RgdsBob
> > >
> > >
> > > [Non-text portions of this message have been removed]
> > >
> >
>

ok, used the barlowed laser method with a hotech laser....
cut the paper circles, all that...
centered er up....big noticable difference in stars tonite. very round and
sharp..
put a 7mm on there, 248X, ran focus in and out, both sides, everything looks
centered....

biggest question now is can i drag myself outta bed at 4am to recheck
jupiter.....got the alarm set!


--- In bigdob@yahoogroups.com, "atmpob" <atmpob@...> wrote:
>
> Even more sensitive is to defocus the high power (300x) view of the star both
inside and outside of focus.  Look to see if the shadow of the diagonal is in
the center of the large brighter area.  This must be done when the star is at
the center of the image however.  Best to use Polaris since it will stay put. 
Also check to see if the bright area stays circular.  If not then start looking
for the cause.
>
> You can check primary figure problems by the same defocus test.  The
diffraction rings should look the same on either side of focus.  That is one
side should not have brighter more contrasty rings than the other for a perfect
mirror.
>
> A quick quality check is to focus and then defocus on one side until you see
the shadow of the diagonal just break out into the image.  Remember how far you
had to move the focus knob.  Then do the same to the other side of focus.  If
you had to move the know less than 2x time the other side of focus then the
mirror is diffraction limited at least for Primary correction.
>
> Also notice how the white area wiggles and stuff moves through it at high
power.  This is an indication of seeing in the upper atmosphere and heat in the
scope itself.
>
> If you want to learn more about testing telescopes using these methods see.
>
> Star Testing Astronimical Telescopes by Richard Suiter. Available from
WillmanBell.
>
> http://www.willbell.com/
>
> Dale Eason
>
> --- In bigdob@yahoogroups.com, "startestisbest" <mlinnolt@> wrote:
> >
> > The simplest way to check for collimation is look at a star at high power.
If it is perfectly circular and symmetric in the center of the field, then you
are collimated without vignetting. Any sign of a "comet" shaped star in the
center implies misalignment along the direction of the "tail" of the comet.
> >
> > There could be many reasons for poor planetary performance. Poor seeing is
the first thing you should rule out. Thermal issues with the mirror and/or air
currents in the optical path come next. Then you can start examining the figure
of the primary mirror. Doing a classic star test would resolve that beyond any
question. Lastly, there is always a chance of a poor secondary or eyepiece
problem, but those are infrequent.
> >
> > Mike
> >
> > --- In bigdob@yahoogroups.com, "xrpb11a" <rsh@> wrote:
> > >
> > >
> > >
> > >
> > > Update:
> > > Checked Lasermax TLC-202 laser colllimator in focuser. rotated it 180,
then back again. found the spot on the center of the primary rotates by about
1/8" to 3/16" ( about 1/2 to 2/3 of cutout on sticker in center of primary. hole
in sticker is .25".. )
> > >
> > > Will adjust laser tonite, but wondering if this amount of offset is big
enough to cause the "blurry bands of jupiter"??
> > >
> > > thx,
> > > Bob
> > >
> > >
> > > --- In bigdob@yahoogroups.com, "xrpb11a" <rsh@> wrote:
> > > >
> > > > Hi all,just put together a 15" obsession i bought on the cheap? ( 2700
> > > > about 3 years ago ). been a LITTLE BUSY, so just getting around to using
> > > > it.
> > > > had it collimated using lasermax 2" collimator, and had 22mm nagler
> > > > installed. trained on Jupiter last night and was kinda disappointed. not
> > > > a very sharp image, just saw a blurry reddish line across the middle,
> > > > not nearly sharp at all. moons were visible easily, but not perfect
> > > > Circles...all near the center of the EP.. I remember getting much
> > > > sharper images in a cheap 6" newtonian years ago.....
> > > > I can't collimate it any better than I did using the equipment i had, so
> > > > wondering if the mirror might be cause. maybe why i got it on the cheap
> > > > side [:O] ...mirror should have been at temp, was sitting outside all
> > > > night, and i was viewing around 1am...Here is writing on the mirror:
> > > > (sticker on the back)QSPOptical Technology IncSanta Ana, CA
> > > > (writing on back of mirror)im 6569 NOVA EAL # 398703
> > > > Also, I do have some torque on the secondary mirror adjustors. they are
> > > > very tight. had to do this to get the mirror adjusted...
> > > > So my big question is: Is there a method to determine if the primary is
> > > > warped while the thing is installed, without sending it back to the
> > > > factory??
> > > > Thanks for any advice.RgdsBob
> > > >
> > > >
> > > > [Non-text portions of this message have been removed]
> > > >
> > >
> >
>

The circular surface doesn't have to be precise since the tube actually rides on
the teflon pads, but it gives you the option of adjusting the position of the
teflon pads.  A v shape wouldn't allow that.

I've never heard of using felt, except maybe on very small scopes.  Teflon is
standard.

-John




----- Original Message ----
> From: Randallo <rogauntt@...>
> To: bigdob@yahoogroups.com
> Sent: Sun, August 8, 2010 12:37:12 PM
> Subject: [bigdob] Big Dob Mount  Questions
>
> Hi Folks,
> I am new to the group. Joined as I am putting together a big dob  from a nicce
>17.5' F4 mirror.
>
> Getting ready to build a base for a 17.5"  F4  mirror and am facing two
>questions, not having done this particular  feat before, both regarding the
>bearings.
>
> Altitude bearing: I have seen  mounts that use a simple V-groove bearing with
>some felt or something for a  bearing surface, and others with a more precision
>surface with a circular  bearing surface and perhaps felt/Teflon.  I am
thinking
>that the V-groove  is more forgiving but probably more sensitive to imperfect
>balance, lacking the  friction surface of a longer circular surface of a
>cradle.  On the other  hand, the long circular bearing surfaces will probably
>require greater precision  in terms of gap clearance between the cradle bearing
>and the shaft (so to  speak). I am inclined toward the latter, but wondering if
>anyone had any  hands-on experience, as I can see having to do this more than
>once to get it  right.
>
> Azimuth bearing:  Question here is "lazy susan"  bearings  or something
simpler
>like two sandwiched LP records lubed with  silicon spray.
>
> Any suggestions, pointers or links would be greatly  appreciated - otherwise I
>am blazing my own trail!
>
> Randy  G
>
>
>
> ------------------------------------
>
> Yahoo! Groups  Links
>
>
>
>

This could be just bad seeing, or an error in the laser.

-John




----- Original Message ----
> From: xrpb11a <rsh@...>
> To: bigdob@yahoogroups.com
> Sent: Sun, August 8, 2010 3:18:58 PM
> Subject: [bigdob] Poor jupiter view in 15" obsession..
>
> Hi all,just put together a 15" obsession i bought on the cheap? ( 2700
> about  3 years ago ). been a LITTLE BUSY, so just getting around to using
> it.
> had  it collimated using lasermax 2" collimator, and had 22mm nagler
> installed.  trained on Jupiter last night and was kinda disappointed. not
> a very sharp  image, just saw a blurry reddish line across the middle,
> not nearly sharp at  all. moons were visible easily, but not perfect
> Circles...all near the center  of the EP.. I remember getting much
> sharper images in a cheap 6" newtonian  years ago.....
> I can't collimate it any better than I did using the equipment  i had, so
> wondering if the mirror might be cause. maybe why i got it on the  cheap
> side [:O] ...mirror should have been at temp, was sitting outside  all
> night, and i was viewing around 1am...Here is writing on the  mirror:
> (sticker on the back)QSPOptical Technology IncSanta Ana,  CA
> (writing on back of mirror)im 6569 NOVA EAL # 398703
> Also, I do have  some torque on the secondary mirror adjustors. they are
> very tight. had to do  this to get the mirror adjusted...
> So my big question is: Is there a method  to determine if the primary is
> warped while the thing is installed, without  sending it back to the
> factory??
> Thanks for any  advice.RgdsBob
>
>
> [Non-text portions of this message have been  removed]
>
>
>
> ------------------------------------
>
> Yahoo!  Groups Links
>
>
>
>

----- Original Message ----

> From: startestisbest <mlinnolt@...>

> 6. Collimation is done  by adjustments of 2 of the truss poles nearest the
>focuser. The mirror mount is  fixed to approximately the correct position, with
>fine adjustments done at the  eyepiece end. This allows quick adjustments
during
>use and saves weight at the  primary as well.
>

That's a very interesting concept.  It certainly saves on complexity.  By "the
mirror mount is fixed" do you mean the primary?  Is _all_ collimation (both
primary and sec) done with these two truss adjustments?

-John

Yes, The primary mirror mount is fixed into position. That approximate position
is determined by measuring that the distance from the edge of the mount(plate)
are about the same distances from the truss ring all around, so the mirror
basically looks straight out (normal) to the opening in the sphere.

The two of the 6 truss poles nearest to the eyepiece are adjustable length, and
those two motions alone are enough to fine adjust the collimation in the same
way as if you had a 3-pt mirror cell adjustment.

But, the secondary is adjusted separately by 3 screws that bear against the back
of it, which is pretty standard for secondary adjustment. The two adjustable
truss poles is not enough to collimated BOTH primary and secondary at once.

Mike

--- In bigdob@yahoogroups.com, John Mahony <jmmahony@...> wrote:
>
> ----- Original Message ----
>
> > From: startestisbest <mlinnolt@...>
>
> > 6. Collimation is done  by adjustments of 2 of the truss poles nearest the
> >focuser. The mirror mount is  fixed to approximately the correct position,
with
> >fine adjustments done at the  eyepiece end. This allows quick adjustments
during
> >use and saves weight at the  primary as well.
> >
>
> That's a very interesting concept.  It certainly saves on complexity.  By "the
> mirror mount is fixed" do you mean the primary?  Is _all_ collimation (both
> primary and sec) done with these two truss adjustments?
>
> -John
>

The comet-flare is a good technique for collimating the primary without a laser,
and certainly a way to double check the laser's accuracy.

I wrote an article about it here for SCTs but have used the same technique on my
10" Newt and the Club's 18" Newt with good effect. I'd like to say it's
foolproof but I'm sure there's a way to mess it up, haven't encountered it yet.

http://www.astromart.com/articles/article.asp?article_id=718

If you don't have tracking, use Polaris, and ignore the companion star, which
may start to get conspicuous in a Big Dob.

Another thought is that your scope isn't *holding* collimation.  To check that
you need to check the accuracy of the collimation at different angles, and you
may then enter the difficult world of sling adjustments, etc., but for my 2c,
the Howie Glatter support system is excellent.

MEANTIME, if you want to observe without messing with the mirror support
hardware, there is a workaround:  When you are on Jupiter, put one of the moons
*dead center*, rack it out of focus, and center the doughnut hole using your
collimation screws in the back.

You will then have fine tuned your scope for that part of the sky where
fine-tuning matters most.  Seeing and thermal issues also count for a great
deal.

regards
Greg N



--- In bigdob@yahoogroups.com, "startestisbest" <mlinnolt@...> wrote:
>
> The simplest way to check for collimation is look at a star at high power. If
it is perfectly circular and symmetric in the center of the field, then you are
collimated without vignetting. Any sign of a "comet" shaped star in the center
implies misalignment along the direction of the "tail" of the comet.
>
> There could be many reasons for poor planetary performance. Poor seeing is the
first thing you should rule out. Thermal issues with the mirror and/or air
currents in the optical path come next. Then you can start examining the figure
of the primary mirror. Doing a classic star test would resolve that beyond any
question. Lastly, there is always a chance of a poor secondary or eyepiece
problem, but those are infrequent.
>
> Mike
>

The important star test for quality of the primary is to carefully look at the
slightly defocussed star with a very high power under good seeing, and compare
the outer edge appearance on both sides of focus. It should not look fuzzier or
spiky on one side of focus compared to the other. This is the acid test that
determines the crucial edge correction.

The diagonal shadow size/breakout is not a reliable test for overall quality
because it concentrates the measure in the center of the mirror, which is
furthest removed from the crucial edge. Many mirrors will have non-uniform
variations in correction between the center zones and the edge, which
essentially invalidates the test done this way.

For example, you could have a high-Strehl ratio mirror essentially perfect from
the 30% zone out to the edge, but with a half wave hole in the center, which
will show a large difference in diagonal shadow breakout, but otherwise will
perform essentially like a perfect optic in focus.

Mike

--- In bigdob@yahoogroups.com, "atmpob" <atmpob@...> wrote:
>
> You can check primary figure problems by the same defocus test.  The
diffraction rings should look the same on either side of focus.  That is one
side should not have brighter more contrasty rings than the other for a perfect
mirror.
>
> A quick quality check is to focus and then defocus on one side until you see
the shadow of the diagonal just break out into the image.  Remember how far you
had to move the focus knob.  Then do the same to the other side of focus.  If
you had to move the know less than 2x time the other side of focus then the
mirror is diffraction limited at least for Primary correction.
>

The morning of 8/19 I got up around 2 am to some amazing views of Jupiter
through my 20".  I had a box fan on the mirror all night until observing, and
images at 400x had hardly any negative effects from seeing.  Amazing detail.

I decided to wake up again at 2 am the morning of the 20th (this morning) as I
knew GRS would transit.  Views at 155x were good sometimes, and then at other
times the only feature visible was the visible equatorial belt.  I was able to
observe the transit, but internal structure to the GRS really needs more than
155x so It'll have to wait.

I have seen features on Ganymede through this telescope before, but getting
those exact observing conditions is often like chasing a ghost.  Don't get me
wrong, it's worth waking up to at 2 am to try, but I sure am tired!

Get out and observe Jupiter.  It is really well placed.

Shane

http://cantinygamboa.es/en.php

Hello group,

Heading out for Black Forest Star Party and hope to see some of my fellow Big
Dob
cohorts. One of the two star parties I get to do a year. Hope the weather holds
out!

-Roger
18" Obsession SC/AN/FSA
www.astrocrumb.com

http://www.pfo0.dsdrugs.com

I all,pat here, i seem to have inhereted a coulter red tube dob,17.5 inch in
what appears to be in good shape for being about 25-30 years old.Still has the
silver coulter sticker at the mirror end of the tube. The mirrors look good. It
has been modified with a 12 point cell with 4 bolts holding the cell in. Has
curved spiders. I currently own a 5 inch refractor and do mostley planetary
work, so im wondering how much this thing might be worth.[Could i get a 6 inch
refractor for it] or are they considered junk?
I looked thru jupiter at it and was surprised how big the planet was so i dont
know i might enjoy keeping this monster. I tried to alighn it but i dont think i
did a very good job having never done it before, but i could see plenty of
planetary detail none the less. I was also thinking,if i keep it i would buy a
harbor freght small trailer and build a rig for it. I live in a relatively dark
city but not dark enough for this scope, except for the occassional comet i
guess ha ha ha .ANY INFO OR HELP WOULD BE APPRECIATED. Are they worth more
repainted,knicks bondoed, or do people like them original
Oh,it came with a 50mm 2 inch occular and a 1.5 reducer which i used to look at
jupiter with a parkfield 20mm. Thanks all, pj so cal

It depends on how good the mirror is.
Word is that the quality control on these left a little to be desired.
I have one of the good ones, and enjoy the heck out of it.

If you have a ronchi grating that you can hold to the focuser, while you look at
a star, or have someone who has a ronchi eyepiece; that will be the quickest way
of finding out what you have. When star testing withe a ronchi, the lines in the
image need to be straight. Any deviations from straight, indicate problems. You
should focus until you get about 4 bands across the face of the mirror. (More
bands will be a less senitive test.)

[Yes, you can get Suiter's book on star testing and test in and out of focus,
and yes that is very sensitive. But you will spend a lot of time learning how to
do it, when all you really need to know is a rough idea of how good it is.]

We my scope, an a night with steady air, I see about 18 bands on Jupiter, and
festoons.  With something like this, you have to have a little more patience
than with a smaller aperature scope. The moments of time when 17.5" of air is
simeltaniously steady are rarer than when 6" of air is steady at the same time.
When those moments come, they are spectacular. A lot of guys that do planetary
work with big scopes, like to look shortly before dawn, because the air often
will be steadier for larger chunks of air.

For "sidewalk" star-parties, where I am showing people that haven't looked at
planets before, I put a 7" off-axis mask on the scope. They are more likely to
see a clear moment right away. They see less than with full aperature, but they
don't have the patience to wait.  Being off-axis, I don't have any diffraction,
so the image is still pretty good. They get to see lots of bands, but typically
no festoons.

When you ronchi test, you may find that you have a turned edge. This apparently
was common for them. (Mine has one.) A black paint pencil, or black drafting
line tape, or a cardboard mask, will make the image better if you want.  You
will still have LOTS of aperature to play with.

When you want to take a break from planetary work, you will be surprised at how
good deep sky objects look. You may discover a new passion, now that you have an
instrument that can show it to you.

You will want a hand truck,and a set of nylon straps (quick release, or velcro).
Transporting it around your yard will kill you without one. [My daughter and I
used to say that Astromomy wasn't a hobby, it was a sport!]

Leaving it like it is, means you will have a very quick set-up.
At one point in my life, I no longer had a big vehicle, and had to start
transporting in a hatch-back. I then carved it up, and turned it into a
truss-tube.  I takes longer to set up, but transports easier.

You will find that collimation is a big deal.  It is easy to do, particularly
with a laser.  The quality of image, quickly goes down-hill if you get out of
collimation. I collimate every evening that I use the scope.  Once you see the
difference, you will to.

I would play with it for a while, before you decide to part with it. I have had
a great time with mine.  Being in the City, I would seriously consider getting a
light pollution filter. It will make the deep sky objects even more enjoyable.

Scott


--- In bigdob@yahoogroups.com, "clustersurviver" <clustersurviver@...> wrote:
>
> I all,pat here, i seem to have inhereted a coulter red tube dob,17.5 inch in
what appears to be in good shape for being about 25-30 years old.Still has the
silver coulter sticker at the mirror end of the tube. The mirrors look good. It
has been modified with a 12 point cell with 4 bolts holding the cell in. Has
curved spiders. I currently own a 5 inch refractor and do mostley planetary
work, so im wondering how much this thing might be worth.[Could i get a 6 inch
refractor for it] or are they considered junk?
> I looked thru jupiter at it and was surprised how big the planet was so i dont
know i might enjoy keeping this monster. I tried to alighn it but i dont think i
did a very good job having never done it before, but i could see plenty of
planetary detail none the less. I was also thinking,if i keep it i would buy a
harbor freght small trailer and build a rig for it. I live in a relatively dark
city but not dark enough for this scope, except for the occassional comet i
guess ha ha ha .ANY INFO OR HELP WOULD BE APPRECIATED. Are they worth more
repainted,knicks bondoed, or do people like them original
> Oh,it came with a 50mm 2 inch occular and a 1.5 reducer which i used to look
at jupiter with a parkfield 20mm. Thanks all, pj so cal
>

With my 20" mirror finished, now comes the coating issue. I had some
thoughts of trying silvering myself, but having no experience with it and issues
regarding getting a good result, I am tending to have it aluminized as usual.

My previous mirror was coated by Nova Optical (Steve Dodds) who also coats
Zambuto's mirrors. The newly coated mirror (std. Al with SiO2 overcoat) was
excellently done and performed perfectly, at the beginning. However, within just
a year the coating showed significant deterioration - multiple tiny pinholes and
general loss of reflectivity. My mirror has been exposed to a corrosive
atmosphere (mainly sulfur dioxide) which is a likely explanation. However, my
secondary from Antares Optical has shown no apparent deterioration under the
same conditions. The secondary has been enhanced coated (EAL) with ion assist
and a chromium underlayer.

So, my feeling is that enhanced coatings with SiO2 and TiO2 and ion assist may
be more durable under corrosive conditions?

Any opinions regarding this issue?

Thanks!
Mike

Currently I have installed on my 16 "f5 telescope dobsonian classic Kriege
design three 120 mm fans, one in the back of the mirror and the other two
sweeping the primary boundary layer. Regarding the latter two, is it better to
introduce or extract air into the mirror box?
Clear skies
Víctor