Welcome to the bigdob mailing list at egroups.

I started this group as a trial to see if denizens
of the bigdob-l@... would be happier here
because of egroups cool features and easy
admin.

However, everyone is welcome to join.

Btw, how big does a dob have to be to
be called 'big'?

clear skies.

-ad

Having spent a night at TSP2000 observing with the 82" cass
at McDonald observatory, I've moderated my notion of what 'big'.

I nolonger think its a number of inches.

I think what makes a particular scope big, at least for
newtonians, is weather you use a ladder to observe.

What has a ladder got to do with it? Well, I think if people
are willing to put up with a ladder, or some other major fuss
to observe, then I figure that scope is delivering an aperture
big enough to be worth the fuss.

Why this sophistry? Well, I used to think of my 25" as "big".
Then in reading David Kriege's book, it became clear that
he thought "big" started at 30 inches. Then I spent a night
with an 82" -- make my 25" look like a finder.

So it's probably best not to take the word "big" seriously.

I'd say we should welcome posts about dobs of all sizes here.

Clear skies.

-ad

This is a test to see if bigdob-l@... is getting
messages forwarded to from bigdob@egroups.com.

If this works, I will change the forwarding so that
daily digests (instead of individual emails) are
forwarded to bigdob-l@....

-ad

Hello fellow big dobbers!

Hope this gets the shot in the arm that we need.

Dan Harriman
Houston, Tx

At 10:52 AM 6/20/00 -0500, you wrote:
>     Given the positive replies to my question about weather we'd be
>happier with this list at egroup, i have created a trial
>egroup at:    www.egroups.com/group/bigdob    "" link to join the fun.   I


----------------------------------------------------------------------------
---
If at first you don't succeed, maybe you shouldn't try sky-diving!

Quite a few posts in the starmaster and obsession egroups may
be of interest of other dob owners. Here are some of the
threads:

tracking Platform
  http://www.egroups.com/message/obsessionusers/9?&start=1&threaded=1
tom O's platform:
   http://www.egroups.com/message/starmaster_scopes/94?&start=80&threaded=1
ramp for eq platoform:
   http://www.egroups.com/message/obsessionusers/177?&start=151&threaded=1
mirror coating:
   http://www.egroups.com/message/obsessionusers/87?&start=85&threaded=1
truss-mounted finderscopes:
   http://www.egroups.com/message/obsessionusers/87?&start=85&threaded=1
ngc-max az encoder:
   http://www.egroups.com/message/obsessionusers/117?&start=115&threaded=1
finderscopes for dobs:
   http://www.egroups.com/message/dobsonian/18?&start=1&threaded=1
finderscopes & balancing:
   http://www.egroups.com/message/obsessionusers/144?&start=115&threaded=1
nova optics:
   http://www.egroups.com/message/obsessionusers/149?&start=115&threaded=1
teflon sheet:
   http://www.egroups.com/message/obsessionusers/163?&start=151&threaded=1
keeping mirror box centered:
   http://www.egroups.com/message/obsessionusers/177?&start=151&threaded=1
alt bearing lubricant:
   http://www.egroups.com/message/obsessionusers/177?&start=151&threaded=1
covers:
   http://www.egroups.com/message/starmaster_scopes/40?&start=31
insurance:
   http://www.egroups.com/message/starmaster_scopes/147?&threaded=1
upper-cage baffles
   http://www.egroups.com/message/dobsonian/28?&start=27&threaded=1

Clear skies.

-ad

A ladder is what got me into trouble in the first place.

I never considered getting a big dob until I discovered
platform ladders by "caradon lite". I had been plain scared
of ladders. But caradon made ladders with big flat steps and
"jesus" handles at exactly the right place where I felt the urge to
grab something. After that, getting the big dob was inevitable.

But it's a 4 legged ladder. So it's sometimes hard to find a
level spot to plonk it down and still reach the eyepiece.
(I know some people use 3-legged or apple-picking ladders, but
I have grown attached to my particular caradon.)

So I'm looking for ideas for quicky  leveling a 4-legged ladder.
Acutally, precise leveling isnt needed. Just something to
stop the rocking. (This is especially important at public
starparties. We can have the 5 year olds and the grannies
being flung about by a tottering ladder.)

Is there some kind of spring-loaded gizmo I could attach to
one of the legs to adjust its length to reach the ground? Or
should I just carry a big wedge around?

Clear skies

-ad

Hi,

It's the third time I subscribe to a dob oriented list this week ! what a
great week !
Glad to see that their is a new list for dobs.I hope that we will see some
intresting ATM ideas here.

About what is a big dob.... few years ago I thought a 12 inc was big, and a
16 inc and a 22... now, I don't know. Big dobs have a natural tendency to
reduce in size year after year :)


At the end of this email, their is a link for some dobs making stuff. Sorry,
it's a french web site (that's why I have so much trouble with english...).
Their is an article about mirror quality and astatic miror cell. We are
lucky enough to have some professionnal mirror maker and mirror cell
designer who help us sometime.


Frédéric Géa
GEAF@...
http://www.astrosurf.org/jupiter/altaz/index.htm#index

Anyone made their own equatorial platform for a large dob (18"-25")
based on the Warren Peters, or David Shouldice models?

I have a 20" dob, and am considering making one to save some money. I
live in Canada, and it is very expensive to import.

Also any idea of the latitude range for using it?

Brien Stratton

--- In bigdob@egroups.com, "Brien  Stratton" <bstratton@s...> wrote:
> Anyone made their own equatorial platform for a large dob (18"-25")
> based on the Warren Peters, or David Shouldice models?
>
> I have a 20" dob, and am considering making one to save some money. I
> live in Canada, and it is very expensive to import.
>
> Also any idea of the latitude range for using it?
>
> Brien Stratton

I havent made a platform. But I nose around for such.

You might want to talk to Jeff Collinson ( jcollins@... )
of the Kitchener-Waterloo RASC. Not only has Jeff built eq platform,
he was thinking of going into buisness making them in Canada. At
the time (~3 years ago) he thought he could build a better platform
than Tom Osypowski's.

Other readers might be curious to know about the platfoms
Brien mentioned. Try these links:

Peters:
http://home.hiwaay.net/~warrenca/Astronomy/Projects/Tracker/Manual.html

Shouldice:
http://www.users.uswest.net/~dalekeller/atm/atmother/shouldice/shouldice.htm

Just for fun, try these too:

The classic equatorial platform page is Chuck Shaw's:
http://www.atmsite.org/content/shaw/platform.html

My favorite eq platform design, for sheer mind-twisting creativity, is
Don Davies design. He manages to avoid using *any* curved surfaces:

http://members.loop.com/~raethrasher/eqmount.htm

-ad

Filter sliders are great. Using one with an OIII filter
to rapidly blink the filter was the only way that
Mike Wirths and I were able to observe Pease I,
a tiny planetary nebula inside M15. We tried blinking
the filter between our eyes and the eyepiece:
too hard. We wound up using a lumincon 1.25"
filter slider. However we needed a barlow to
get enough focus travel.

So I envy the slider for 2" filters that you
can get with Astrosystems TeleKit dobs.

I've ordered part of the filter slider,
just the metal plate with the threaded holes,
from Randy at Astrosystems. But I dont have
too many ideas yet as to how I am going to mount
it in a way that wont take up any focus travel.

So has anyone retrofitted a filter slider,
Astrosystem's or whatever, to thier dob?

Clear skies.

ad

Enter your vote today!  Check out the new poll for the bigdob
group:


Just for mutual curiosity, what size
(in inches) dobsonians
do people on this list actually have?
Names are not recoded in this poll, but
anonymity is not a reason to
exaggerate. :)

   o 4
   o 6
   o 8
   o 10
   o 12
   o 14
   o 16
   o 17
   o 18
   o 20
   o 22
   o 24
   o 25
   o 27
   o 30
   o 32
   o 36
   o 40
   o 41
   o Way, way bigger than that


To vote, please visit the following web page:

http://www.egroups.com/polls/bigdob

Note: Please do not reply to this message. Poll votes are
not collected via email. To vote, you must go to the eGroups
web site listed above.

Thanks!

When you follow the link listed below click on
the text in order to vote.  This may be obvious
to most, but I thought I'd pass this along in
case there are some others out there (like
myself) who are a bit slow when it comes to using
the internet.
Mark

bigdob@egroups.com wrote:
>
> Enter your vote today!  Check out the new poll for the bigdob
> group:
>
> Just for mutual curiosity, what size
> (in inches) dobsonians
> do people on this list actually have?
> Names are not recoded in this poll, but
> anonymity is not a reason to
> exaggerate. :)
>
>   o 4
>   o 6
>   o 8
>   o 10
>   o 12
>   o 14
>   o 16
>   o 17
>   o 18
>   o 20
>   o 22
>   o 24
>   o 25
>   o 27
>   o 30
>   o 32
>   o 36
>   o 40
>   o 41
>   o Way, way bigger than that
>
> To vote, please visit the following web page:
>
> http://www.egroups.com/polls/bigdob
>
> Note: Please do not reply to this message. Poll votes are
> not collected via email. To vote, you must go to the eGroups
> web site listed above.
>
> Thanks!
>
>
>
> ------------------------------------------------------------------------
> Accurate impartial advice on everything from laptops to table saws.
> http://click.egroups.com/1/4634/9/_/_/_/961696808/
> ------------------------------------------------------------------------

--
ÿWPC�

I have replace my astrosystem filter system not long ago because of a its
weight. My dobs has a low center of gravity and the astrosystem device is
heavy. I have made a prototype to test the idea. It will be replace soon
cause the proto seems to work quite well. It's a plywood disk (more comact)
with 5
possible position for filters (One empty slot and 4 filters). It's a 6,7 inc
disk in diameter. Currently filters are hold by small pieces of plastic. The
axe of the filter wheel is attached to the focuser board. Their is 5 small
holes on the disk and a plastic screw on the focuser board, so when I turn
the wheel, it stops on each position. The whole wheel is inside a plastic
(kind of kydex) that protect the filters. This plastic "square box" is open
on one side (thin side) so I can turn the whell by puting the hand inside
the diag cage. Their is a round hole of course on both side of the filter
"box" to see acrossot. It's probably possible to have a rotative big button
just bellow the focuser to turn the wheel more easily. It's lightweight and
that give me a good focuser bafle. It's really funny to use caus one someone
ask me to see the image with a filter, "click" that's it ! To know the
position of the filter wheel, I have put the longest barlow I have and full
rackin the focuser, the filter are closed but don't touch anything. The
final wheel will probably be made of alu with threated holes. An other
useful thing is a drawing board on the focuser board. I have remove the
system but soon I will put it again. It's a system that allow drawing thanks
to a red light just bellow the focuser. No need to go back again and again
to draw.

A question about this list, is it possible to send pic or drawing of , let's
say, 20 ko at max ? Could be useful.

Frédéric Géa
GEAF@...
http://www.astrosurf.org/jupiter/altaz/index.htm#index


-----Message d'origine-----
De : Attilla Danko <attilla.danko@...>
À : bigdob@egroups.com <bigdob@egroups.com>
Date : jeudi 22 juin 2000 17:10
Objet : [bigdob] Filter sliders?


>
>Filter sliders are great. Using one with an OIII filter
>to rapidly blink the filter was the only way that
>Mike Wirths and I were able to observe Pease I,
>a tiny planetary nebula inside M15. We tried blinking
>the filter between our eyes and the eyepiece:
>too hard. We wound up using a lumincon 1.25"
>filter slider. However we needed a barlow to
>get enough focus travel.
>
>So I envy the slider for 2" filters that you
>can get with Astrosystems TeleKit dobs.
>
>I've ordered part of the filter slider,
>just the metal plate with the threaded holes,
>from Randy at Astrosystems. But I dont have
>too many ideas yet as to how I am going to mount
>it in a way that wont take up any focus travel.
>
>So has anyone retrofitted a filter slider,
>Astrosystem's or whatever, to thier dob?
>
>Clear skies.
>
>ad
>
>
>
>------------------------------------------------------------------------
>SALESFORCE.COM MAKES SOFTWARE OBSOLETE
>Secure, online sales force automation with 5 users FREE for 1 year!
>http://click.egroups.com/1/2658/9/_/_/_/961686640/
>------------------------------------------------------------------------
>
>
>
>
>

Also about filter slide, their is an article by Ed Stewart in Amateur
Astronomy #16, page 46.


Frédéric Géa
http://www.astrosurf.org/jupiter/altaz/index.htm#index


-----Message d'origine-----
De : Attilla Danko <attilla.danko@...>
À : bigdob@egroups.com <bigdob@egroups.com>
Date : jeudi 22 juin 2000 17:10
Objet : [bigdob] Filter sliders?


>
>Filter sliders are great. Using one with an OIII filter
>to rapidly blink the filter was the only way that
>Mike Wirths and I were able to observe Pease I,
>a tiny planetary nebula inside M15. We tried blinking
>the filter between our eyes and the eyepiece:
>too hard. We wound up using a lumincon 1.25"
>filter slider. However we needed a barlow to
>get enough focus travel.
>
>So I envy the slider for 2" filters that you
>can get with Astrosystems TeleKit dobs.
>
>I've ordered part of the filter slider,
>just the metal plate with the threaded holes,
>from Randy at Astrosystems. But I dont have
>too many ideas yet as to how I am going to mount
>it in a way that wont take up any focus travel.
>
>So has anyone retrofitted a filter slider,
>Astrosystem's or whatever, to thier dob?
>
>Clear skies.
>
>ad
>
>
>
>------------------------------------------------------------------------
>SALESFORCE.COM MAKES SOFTWARE OBSOLETE
>Secure, online sales force automation with 5 users FREE for 1 year!
>http://click.egroups.com/1/2658/9/_/_/_/961686640/
>------------------------------------------------------------------------
>
>
>
>
>

>A question about this list, is it possible to send pic or drawing of , let's
>say, 20 ko at max ? Could be useful.

>Frédéric Géa
>GEAF@...

Yes, you can send small pictures as attachments.

Also, feel free to upload larger pictures to the files area.
Goto:

http://www.egroup.com/files/bigdob/

and click on "add file" or "add folder".

Clear skies.

-ad

-----Message d'origine-----
De : Attilla Danko <danko@...>
À : bigdob@egroups.com <bigdob@egroups.com>
Date : vendredi 23 juin 2000 18:12
Objet : RE: [bigdob] Filter sliders?

>A question about this list, is it possible to send pic or drawing of , let's
>say, 20 ko at max ? Could be useful.

>Frédéric Géa
>GEAF@...

Yes, you can send small pictures as attachments.

Also, feel free to upload larger pictures to the files area.
Goto:

http://www.egroup.com/files/bigdob/

and click on "add file" or "add folder".

Clear skies.

-ad

Enter your vote today!  Check out the new poll for the bigdob
group:


What size aperture do you
consider 'big' for routine observing?

   o >= 8 inches
   o >= 10 inches
   o >= 12 inches
   o >= 14 inches
   o >= 16 inches
   o >= 18 inches
   o >= 20 inches


To vote, please visit the following web page:

http://www.egroups.com/polls/bigdob

Note: Please do not reply to this message. Poll votes are
not collected via email. To vote, you must go to the eGroups
web site listed above.

Thanks!

>   From: "Brien  Stratton" <bstratton@...>

>Anyone made their own equatorial platform for a large dob (18"-25")
>based on the Warren Peters, or David Shouldice models?

Yes, I used Warren's site for most of the ideas, and then made mine with
wood shop tools (sectors were hardwood), with hard plastic casters (non-ball
bearing type).  Worked quite well observing planets at 400x.

>I have a 20" dob, and am considering making one to save some money. I
>live in Canada, and it is very expensive to import.
>
>Also any idea of the latitude range for using it?


Within about 5 degrees of your desired latitude it's not hard to shim the
north/south feet to get close enough alignment for tracking.

One idea I've seen...one guy's home made eq. platform had a peep hole in the
top platform that looked at a mirror on the bottom piece of plywood, with a
painted reticle pattern on it.  Once it was calibrated well enough this was
his "Polaris sight" so he could align elevation and azimuth of the platform
fast and easy.

However, now that I've motorized the dob in both axes, with computer
control....the eq. platform gathers dust!  ;-)  Seriously, a well working
alt-az drive system is almost unbeatable.  Right now I get about 4
arcminutes RMS all sky pointing...and I hope to cut that in half with
another improvement in software.

Depending on your observing goals, you may want to look into motorizing your
scope in alt and az instead of making an eq platform.

Tom Krajci

----- Original Message -----
From: Tom & Lou Krajci <krajcit@...>
To: <bstratton@...>
Sent: Friday, June 23, 2000 4:54 PM
Subject: Re: [bigdob] Homemade Equatorial Platforms


> Brien,
>
> I tried posting this to the new egroups bigdob list, but it say's I'm not
> subscribed...funny, I thought I was.
>
> Anyway, here are my comments.  If you find them useful, then please post
> them to the list for me.
>
> Thanks,
> Tom Krajci
>
> =================================================
>
> >   From: "Brien  Stratton" <bstratton@...>
>
> >Anyone made their own equatorial platform for a large dob (18"-25")
> >based on the Warren Peters, or David Shouldice models?
>
>
> Yes, I used Warren's site for most of the ideas, and then made mine with
> wood shop tools (sectors were hardwood), with hard plastic casters
(non-ball
> bearing type).  Worked quite well.
>
> >I have a 20" dob, and am considering making one to save some money. I
> >live in Canada, and it is very expensive to import.
> >
> >Also any idea of the latitude range for using it?
>
>
> Within about 5 degrees of your desired latitude it's not hard to shim the
> north/south feet to get close enough alignment for tracking.
>
> One idea I've seen...one guy's home made eq. platform had a peep hole in
the
> top platform that looked at a mirror on the bottom piece of plywood, with
a
> painted reticle pattern on it.  Once it was calibrated well enough this
was
> his "Polaris sight" so he could align elevation and azimuth of the
platform
> fast and easy.
>
> However, now that I've motorized the dob in both axes, with computer
> control....the eq. platform gathers dust!  ;-)  Seriously, a well working
> alt-az drive system is almost unbeatable.  Right now I get about 4
> arcminutes RMS all sky pointing...and I hope to cut that in half with
> another improvement in software.
>
> Depending on your observing goals, you may want to look into motorizing
your
> scope in alt and az instead of making an eq platform.
>
> Tom Krajci
>
>

>you may want to look into motorizing your
> scope in alt and az instead of making an eq platform.
>
> Tom Krajci

ok... details please! I have a 30 and am working on the concepts - I
am collecting the guts. Whose "control"" did you choose to use? Mel's
or some other?

-----Message d'origine-----
De : Attilla Danko <danko@...>
À : bigdob@egroups.com <bigdob@egroups.com>
Date : vendredi 23 juin 2000 18:12
Objet : RE: [bigdob] Filter sliders?

>A question about this list, is it possible to send pic or drawing of , let's
>say, 20 ko at max ? Could be useful.

>Frédéric Géa
>GEAF@...

Yes, you can send small pictures as attachments.

Also, feel free to upload larger pictures to the files area.
Goto:

http://www.egroup.com/files/bigdob/

and click on "add file" or "add folder".

Clear skies.

-ad

Hi from a new dob owner.  After 5 years of using a Meade 10" LX200,
aperature fever got serious and I bought a used and well-worn Tektron
15" F5 dob.  Is this considered big enough to join?

The scope is fantastic!  My wife and I were blown away last night by
M13. My shortest EP is a Nagler 9mm, and it is not even close to
maximum power.  The Meade may be gathering dust for a while. I now
understand fully why so many serious amateurs are totally hooked on
these scopes.

I am curious about diagonals.  The secondary measures about 2.5" on
the minor axis.  Even with a low-profile focuser, I would have
figured
that the axis dimension should have been about 3".  Is this correct?
When I look into the eyepiece tube, I cannot see the edges of the
main
mirror.  Does this mean that I am not using the entire mirror
surface?
  Is it possible to just change the mirror holder without changing the
spider?  Would it make a difference?

I am so incredibly hooked by this image that 15" may not be big
enough! I am starting to think about home building a 20" f/4.

I amusing a Telrad, which seems adequate.  Does anyone use a DSC
system?  Sorry for so many questions, but I am new at this.  Hope to
be an active participant in this group.    Lou

>I am so incredibly hooked by this image that 15" may not be big
>enough! I am starting to think about home building a 20" f/4.

I have jump from 8 inc to 16 and from 16 to 22, worth the effort !!


Frédéric

bigdob@egroups.com wrote:
>
>
> Enter your vote today!  Check out the new poll for the bigdob
> group:

I noticed 13" is conspicuously absent.

--
Mike Lindner

----- Original Message -----

From: Michael Lindner

To: bigdob@egroups.com

Sent: Saturday, June 24, 2000 3:56 PM

Subject: [bigdob] Re: New poll for bigdob

bigdob@egroups.com wrote:
>
>
> Enter your vote today!  Check out the new poll for the bigdob
> group:

I noticed 13" is conspicuously absent.

--
Mike Lindner

>   From: GMYERS2@...
>
>>you may want to look into motorizing your
>> scope in alt and az instead of making an eq platform.
>>
>> Tom Krajci
>
>ok... details please! I have a 30 and am working on the concepts - I
>am collecting the guts. Whose "control"" did you choose to use? Mel's
>or some other?


I use Mel Bartels' system (software, schematics, and a laptop), driving
steppers at 24VDC, using home made, large worm gears made of JB Weld, driven
by nylon threaded rod.

An excellent example of this type of system is shown at:
http://www.america.net/~erg/intro.html

My scope slews at about 3 1/2 deg/second, and all sky pointing accuracy is
about 4 arcminutes RMS, and tonight I'm testing out another software mod.
that should cut that error in half.

Mel's system can be a mouthful to learn at first, but it's very capable and
powerful.  I know of no other amateur telescope drive system (especially the
software!) that has similar capabilities.

Tom Krajci

> I noticed 13" is conspicuously absent.

Fat fingers on the return key. There should be a smiley after that ;^)

--- a Coulter customer.

> My 15" f4.4 has a 3.1" diagonal which was recommend by the mirror maker.  For
an
> F5.0 I would think a 2.9" would be plenty for those widefield views.  Try
> punching the numbers into the Newt program to check for vignetting.
>
> I am curious about diagonals.  The secondary measures about 2.5" on
> the minor axis.  Even with a low-profile focuser, I would have
> figured
> that the axis dimension should have been about 3".  Is this correct?
> When I look into the eyepiece tube, I cannot see the edges of the
> main
> mirror.  Does this mean that I am not using the entire mirror
> surface?
>  Is it possible to just change the mirror holder without changing the
> spider?  Would it make a difference?
>
> I am so incredibly hooked by this image that 15" may not be big
> enough! I am starting to think about home building a 20" f/4.
>
> I amusing a Telrad, which seems adequate.  Does anyone use a DSC
> system?  Sorry for so many questions, but I am new at this.  Hope to
> be an active participant in this group.    Lou
>
> _

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

James P Crombie
Summerside, Prince Edward Island
Canada
Machinist - Amateur Astronomer - CadCam Guy

http://www3.islandtelecom.com/~jpcrombie        My Astronomy website
http://www3.islandtelecom.com/~jpcrombie/rhino/ My RhinoCad web page

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

--- In bigdob@egroups.com Lou (mostly) wrote:
> > My 15" f4.4 has a 3.1" diagonal which was recommend by the mirror
> maker.

If your focal plane was 9.5 inches from the center of you
primary (a guess for a low profile focuser), a 3.1" secondary
would have a 0.94 inch fully illiminated field. Thats about 0.82
degrees, which is not bad.


For an
> > F5.0 I would think a 2.9" would be plenty for those widefield
> views.

Hmm (sound of calculator keys clicking) I figure that combo would
have a 1.0 inch fully-illuminated field, which sounds better.
But it's 0.76 degrees, which is still quite good.

> >Try
> > punching the numbers into the Newt program to check for
vignetting.

Yes.
http://home.earthlink.net/~flyj/scopcalc.html
is an example of such.
Scale drawings work well too and may be easier to use.
Draw the marginal light
rays. In either case, the key distance to know is the distance
between focal plane position and center of secondary.


> >
> > I am curious about diagonals.  The secondary measures about 2.5"
on
> > the minor axis.  Even with a low-profile focuser, I would have
> > figured
> > that the axis dimension should have been about 3".  Is this
correct?

Depends how big a fully illuminated field you want and how far
the secondary is from the focal plane.



> > When I look into the eyepiece tube, I cannot see the edges of the
> > main
> > mirror.  Does this mean that I am not using the entire mirror
> > surface?

Make sure you put your eye where the focal plane of the scope is.
On most scopes this is about 1/2 to 1 inch inside the draw tube.

If you can rack your focuser in far enough so you can place your
eye at the focal plane, *and* if you cant see the whole primary,
then yes, that means something is wrong. Usually too small a
secondary. Watch out for other causes too, like the inside of
the focuser being too small and vigneeting. (usually not problem
on low-profile focusers.)

> >  Is it possible to just change the mirror holder without changing
the
> > spider?

Depends on the spider, but probably yes.  AFIK: FPI-Protostar, Novak,
Astrosystems and Gary's Astro-fabricating spiders would allow you
to replace secondary holders. If you change secondary mirror sizes,
you will need to re-adjust the scondary holder. For example,
a larger secondary will probably appear off-center in your
drawtube. In extreem cases that means re-mounting the spider at
different spot.

>Would it make a difference?

Depends how big a change you make. Too big of a secondary (>20% of
primary diameter) reduces contrast, too small reduces brightness
(by vignetting the primary). There is lots of room in between
where the only effect is change in the fully-illiminated field,
which, for visual use, can range quite a bit before people notice
a difference. Film is not so forgiving. For photography, you want
to carefully arrange your fully-illuminate field at the focal
place to to cover your film or ccd chip.


> >
> > I am so incredibly hooked by this image that 15" may not be big
> > enough! I am starting to think about home building a 20" f/4.
> >

Get the biggest scope that you will use a lot. Thats a different
size for different people. That reminds me, anyone selling an
82" dob yet?


> > I amusing a Telrad, which seems adequate.  Does anyone use a DSC
> > system?

Lots of people do. Check out the DSC/Sky Commander/JMI threads on both

http://www.egroups.com/groupobsessionusers
and
http://www.egroups.com/group/starmaster_scopes



>Sorry for so many questions, but I am new at this.

Isnt there an aphorism (Zen perhaps?) that says:  "everybody is a
beginner"?



Hope to
> > be an active participant in this group.    Lou
> >
> >

Clear skies.

-ad