Talquence wrote:

> Rule of thumb for me. Diameter of tube would be diameter of mirror +
> size of focuser (.965", 1.25", 2" 3")+ diameter of mirror divided by f
> (in your case 4 or 1.5").

This makes me curious. Why the "diameter of mirror divided by f " term in
addition of focuser size?

If the tube/cage edge is one focal length away from the primary (a
reasonable assumption), adding the diameter of the maximum field of view
(less than the focuser) to the mirror diameter would in itself  ensure no
vignetting within the field (always supposing you collimate to center the
optical axis!). Using the full barrel dia. would mean some extra margin.

N O

--- Dale Eason <atmpob@...> wrote:

> Yes it is the Bath as described on the
> interferometer
> group.
>
> If you look through the group postings you will find
> were Dave Rowe quantified the error based on beam
> seperation.  Mine is less than 10 but greater than
> 5.
> The error term it generates is still not greater
> than
> about 1/10 wave on the surface if I remember.  I
> don't
> really remember the exact value.  I do remember
> verifying his estimate by doing the rotate and sum
> technique to remove the constant terms.

I think Rohr did a Zemax layout of the beams and it
came out to 1/8 wave for 5mm seperation.

People need to ask the kind of interferometer that was
used in any test reports - atm or pro,to get an honest
estimate of the wave rating.Waiting to see a
certificate of 1/15 wave on a sold mirror using a Bath
system - it will happen...

Ric
>
> On the Ultrathin, thru the technique of rotation,
> Steve and I have removed test stand and
> interferometer
> induced terms.
>
> What he had left truely rotated with the mirror and
> was not produced by the test system.
>
> Dale Eason
>
>
>
> --- Ric Rokosz <radicell2@...> wrote:
> >
> > I find your experiment witht the thin mirror
> > interesting,especially about the astig.There is
> one
> > point about your interferometer that has me
> > wondering,and that is the setup itself. You are
> > using
> > a variant of the Bath setup.From the discussion on
> > the
> > interferometer group if the two beams from the
> cube
> > exceed 5mm you induce astig. just by the
> limitations
> > of the Bath.Now your setup,from the photos I've
> seen
> > the seperation of the cube from the reflective
> > mirror
> > is more like 12-16mm.Aren't you inducing a heck of
> a
> > lot of additional astig into the equation just by
> > virtue of this seperation.So much so that it skews
> > all
> > your results?
> >
> > Ric


__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around
http://mail.yahoo.com

Ric,

> People need to ask the kind of interferometer that was
> used in any test reports - atm or pro,to get an honest
> estimate of the wave rating.Waiting to see a
> certificate of 1/15 wave on a sold mirror using a Bath
> system - it will happen...

You seem to imply by this that the Bath interferometer is incapable of
producing an accurate wavefront rating, because of its inherent
astigmatism.  I think it can, because the astigmatism is predictable
and can safely be removed from the results, once understood.

To my mind, the bigger challenges to accurate interferometry (Bath or
otherwise) are properly eliminating test stand deformations, which
often result in much more than 1/8 wave P-V of difficult to control
deformations.  Also, the Bath often gets used without nulling optics,
and properly removing null artificially requires accurate tracing of
narrow, curved fringes and very careful determination of the edge of
the mirror.

Finally, one important point in favor of the Bath is that it works
without needing a fine reference element, which could itself be a
source of errors in other setups.
--
Steve Koehler

Nils
I have interchangable focusers and to be quite honest the last factor
was used for my 50mm camera field set up with an oversized diagonal for
wide field photos.
talquence

Talquence wrote:

> Rule of thumb for me. Diameter of tube would be diameter of mirror +
> size of focuser (.965", 1.25", 2" 3")+ diameter of mirror divided by f
> (in your case 4 or 1.5").

This makes me curious. Why the "diameter of mirror divided by f " term
in
addition of focuser size?

If the tube/cage edge is one focal length away from the primary (a
reasonable assumption), adding the diameter of the maximum field of view
(less than the focuser) to the mirror diameter would in itself  ensure
no
vignetting within the field (always supposing you collimate to center
the
optical axis!). Using the full barrel dia. would mean some extra margin.

N O

Nils
Another thing. It incited others to direct the questioner to reliable
design programs which I out of curiosity downloaded for myself. As for
me, I just guestimated my designs leaning toward the over-size. You
shoud see the trailer-hitch I made for my truck.
talquence

First, decide how much vignetting is tolerable.   If your are going to do
any kind of photometery, the appropriate amount of vignetting is ZEROE over
the focal plane.   At the low powers where variable star observations are
done, diffraction is not a problem.   If you are working at high power on
planets and double stars, try for a minimum diagonal with zero vignetting,
but with a very small focal plane (u = .01).

Cut this program, and paste it into a memo for your palm.  Modify the first
seven non-comment lines to match your particular scope. Hotsync, and then
run it on your palm with HotPaw Basic.   (You can get HotPaw FREE and use it
for up to four applications.)  Be patient, and it will build a picture of
your actual aperture and will identify the weak links that need larger
aperture.

#vig05.bas
#vignettlng in cass or newt telescope
# Richard Schwartz
# richas@....
# new version Jun 2004 (after crash)
# set up
# distances
# setup for your problem:
r1=1.100 : d01=4.00 ' focuser rim
r2=1.200 : d02=10.00 ' diag. interior
r3=5.500 : d03=50.00 ' main mirror
r4=5.600 : d34=30.00 ' baffle
r5=r2+.1 : d35=40.00 ' diag. exterior
r6=4.800 : d36=55.00 ' tube end
u=0.700 ' distance off axis in focal
# (end user input. area)
draw -1
20 n=39 : nn=n+n
dim k1(nn,n),k2(nn,n),k3(nn,n)
dim k4(nn,n),k5(nn,n),k6(nn,n)
dim ks(nn,n)
d13=d03-d01 : d23=d03-d02
# projected radii and offsets
r1p=r1*d03/d01 : f1=u*d03/d01
r2p=r2*d03/d02 : f2=u*d03/d02
r3p=r3 : f3=0
r4p=r4 : f4=-u*d34/d03
r5p=r5 : f5=-u*d35/d03
r6p=r6 : f6=-u*d36/d03
# monte carlo integration- initialize
40 dr=r3p/n
for jj=0 to n-1
for ii=0 to 2*n-1
k1(ii,jj)=0 : k2(ii,jj)=0 : k3(ii,jj)=0
k4(ii,jj)=0 : k5(ii,jj)=0 : k6(ii,jj)=0
ks(ii,jj)=0 ' incremented by test
next ii
next jj
200 while 1
xr=dr*rnd(10000)/10000
yr=dr*rnd(10000)/10000
for jj=0 to n-1
y=jj*dr+yr
for ii=0 to 2*n-1
x=ii*dr-r3p+xr
#print"main ",x,y,sqr(x^2+y^2)
if x^2+y^2r5p^2 then
k5(ii,jj)=k5(ii,jj)+1
else
kk=0
endif
if (x+f6)^2+y^20 then draw color 250,160,250,1
if ks(ii,jj)=0 then draw color 0,0,0,1
moveto 80+2*jj,80-2*n+2*ii
lineto 80+2*jj,80-2*n+2*ii+1
lineto 80+2*jj+1,80-2*n+2*ii+1
lineto 80+2*jj+1,80-2*n+2*ii
draw color 0,255,0,1
next ii : next jj
return
end

-----Original Message-----
From: atm_free@yahoogroups.com [mailto:atm_free@yahoogroups.com] On Behalf
Of Arjan te Marvelde
Sent: Tuesday, February 28, 2006 11:32 AM
To: atm_free@yahoogroups.com
Cc: john.downie@...
Subject: [atm_free] Re: Focusers + Tube Clearance


John,

> range?  Also, is a 7" tube enough clearance (6.75" ID) for a 6"
> mirror?

It depends on what size focuser and what size secondary and what kind of
focal plane you want. A clearance of .75" seems pretty tight to me. It means
your unvignetted field will never be wider than approximately this same
size. Of course, that is provided your secondary is large enough.

Arjan te Marvelde





Yahoo! Groups Links

It would help to qualify the wave rating you are specifying.

PV or RMS?
Surface or wave front?

I have not always specified these either but I try. Mirror makers
like to talk surface error.  Interferometrits tend to talk wave
front error.  I am barely either.

I have measured my induced terms.  It is not hard to do.
My particular setup induces about 1/30 wave rms or 1/6.7 wave PV on
the wave front for astig.  My beam separation is 7.5 mm.  I just
measured that tonight again.  So I think that confirms both Rowe's
and Rohr's figures and my actual's are all close.  All are below the
1/20 RMS diffraction limit for wave front error.

I believe Dave Rowe's calculations and they confirm Mr Rohr's if
those are wave front PV. There were derived by at least one other
person.  Then I verified that they were close to the values I was
seeing with experiments with my interferometer.  For my experiment I
measured the difference caused by two different beam settings. The
difference in measured astig from one to the other was as predicted
by Dave Rowe's calculations.


The issue with the Bath as Steve Koehler has pointed out is more
with the artificial nulling in software.  It is very critical to
correctly measure the actual ROC and the diameter of the mirror in
the interferograms.  If not done accurately that will cause an error
in the measured correction.  If the Bath is used with nulling optics
then that limitation falls to the quality of the nulling optics like
most other interferometers.

For most of the mirrors I have tested their measured errors are
several times larger than those induced by the test system.  The
ultra thin had the largest astig I have ever seen.  I have not seen
many. I also could change the value by several waves by applying a
few once force on the edge of the mirror.  None of that was caused
by the interferometer.

I have only seen 3 out of the 10 or so I have tested that have
strehls above the .9xx where all that starts to matter.  I have a
Foucault based system that measures correction pretty well and use
it to confirm the Bath measurements.

With any certification the tester should specify the conditions and
error uncertainty of the measurements.


--- In atm_free@yahoogroups.com, Ric Rokosz <radicell2@...> wrote:
>
>
>
> I think Rohr did a Zemax layout of the beams and it
> came out to 1/8 wave for 5mm seperation.
>
> People need to ask the kind of interferometer that was
> used in any test reports - atm or pro,to get an honest
> estimate of the wave rating.Waiting to see a
> certificate of 1/15 wave on a sold mirror using a Bath
> system - it will happen...
>

--- In atm_free@yahoogroups.com, "Dale Eason" <atmpob@...> wrote:
>
> It would help to qualify the wave rating you are specifying.
>
> PV or RMS?
> Surface or wave front?

The 1/8 wave rating I got for a 5mm seperation was from a posting
Rohr made on the interferometer group .At 1/8 it better be p.v....:)
>
> I have measured my induced terms.  It is not hard to do.
> My particular setup induces about 1/30 wave rms or 1/6.7 wave PV on
> the wave front for astig.  My beam separation is 7.5 mm.  I just
> measured that tonight again.  So I think that confirms both Rowe's
> and Rohr's figures and my actual's are all close.  All are below
the
> 1/20 RMS diffraction limit for wave front error.

The Raylieght limit for diffraction limit on a reflecting concave
mirror is 1/8 wave,not sure if this is p.v.,but again I hope so.

I'm just pointing out that the Bath is working at the limits if the
beam seperation isn't watched very closely.

Ric

Hi all,
   I've got a strange (to me) problem with a dob I'm trying to put
together.  It's a fast (f4) 8" two pole collapsing dob.
Mechanically it seems fine but when I take it out I get very soft
images.  When I focused on Saturn I noticed the "strange" part.
When I focused rather automatically for the smallest image it was
very soft.  But refocusing I was able to get a rather sharp image of
Saturn (complete with cassini division and banding on the surface of
the planet), but it was surrounded by a bright halo that was not
circular but roughly elliptical.  The edges were sharply in contrast
with the dark sky.  Turning the focuser back in the other direction
simply blurred everything.   The halo was not the type one gets with
dirty or dewed up optics.  I used a couple of tried and true orthos
and a UO konig.  None of the EP's have ever been a problem before.

   I've checked for mirror pinch, rechecked collimation with
cheshire, laser and barlowed laser.  I even tried a different
secondary.  No change.  I assume the primary is just bad, but when I
bought another (cheap) mirror it was the same.

   Has anyone seen an image like this?  Any ideas of what might be
wrong if not the mirror?

Thanks for any ideas.
Dave

Dave,

>   I've got a strange (to me) problem with a dob I'm trying to put
> together.  It's a fast (f4) 8" two pole collapsing dob.
> Mechanically it seems fine but when I take it out I get very soft
> images.

As you guessed, most likely it is an optical problem with the primary.
  Your best tool for diagnosing this in the scope is the star test.
Are you familiar with the basics of the star test?  If so, report what
you find and we can help diagnose the problem.

Judging from your description of the problem, a strong halo close to
the planet would indicate a fairly large-scale defect such as
zoniness, severe primary ripple, or TDE.  I think the halo due to
small things (micro-ripple or dust) would be wider and probably more
diffuse.  I suspect the problem will turn out to be TDE.  The star
test should be sufficient to distinguish these causes.
--
Steve Koehler

Several ways to test.  The Star Test is one way as Steve
mentioned.
Also there is the Ronchi grating test where you are looking for
straight lines.  A fine toothed comb can be used as a crude
grating if necessary.
There is also the true Foucault test where you put a straight
edge across the focal plane and look for a "flat" shading just
like a spherical mirror on the test bench.
Bob May
bobmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
Replace the obvious words with the proper character.

--- In atm_free@yahoogroups.com, "Bob May" <bobmay@...> wrote:
>
> Several ways to test.  The Star Test is one way as Steve
> mentioned.
> Also there is the Ronchi grating test where you are looking for
> straight lines.  A fine toothed comb can be used as a crude
> grating if necessary.
> There is also the true Foucault test where you put a straight
> edge across the focal plane and look for a "flat" shading just
> like a spherical mirror on the test bench.
> Bob May
> bobmay at nethere.com
> http: slash /nav.to slash bobmay
> http: slash /bobmay dot astronomy.net
> Replace the obvious words with the proper character.
>

Do you mean straight lines iwth one of those ronchi eyepices because
other wise for an  8" F/4 the lines should be very curved in a
regualr ronchi test correct?

Mitch

Mitch,

> Do you mean straight lines iwth one of those ronchi eyepices because
> other wise for an  8" F/4 the lines should be very curved in a
> regualr ronchi test correct?

When the Ronchi test is done at the focal plane of a completed
telescope using a star as the light source, the Ronchi lines are
ideally perfectly straight for a perfect mirror.  This is so,
regardless of the focal ratio.  That's the beauty of a null test that
eliminates all the nasty spherical aberration seen when testing a
paraboloid at COC.

In other words, testing a paraboloid at focus with a star source is
ALMOST the same size sphere at COC.  I say almost, because the light
cone is much shallower when testing at COC (twice the focal length).
As a result, the Ronchi grating appears to have twice the line count
when testing at focus, versus at COC.

Bob mentioned doing the knife test at focus.  This (of course) is also
a null test.  A perfect paraboloid shoud "gray" all at once like a
good sphere at COC will.

--
Steve Koehler

Your suggestion reminded me I had an Easy Tester in the closet
(duh). I used it and found that the mirror is horribly
undercorrected, so badly in fact that I'm pretty sure it is just a
spherical mirror.  Probably came from Ebay or from a scope with a
built in corrector.  Knowing that I was able to make sense of my
star testing and again undercorrected (sharp clear lines inside
focus, a total blur outside focus).

Well that will teach me.  Thanks a lot for your suggestions everyone.

Clear skies,
Dave

--- In atm_free@yahoogroups.com, "Stephen Koehler" <s.c.koehler@...>
wrote:
>
> Mitch,
>
> > Do you mean straight lines iwth one of those ronchi eyepices
because
> > other wise for an  8" F/4 the lines should be very curved in a
> > regualr ronchi test correct?
>
> When the Ronchi test is done at the focal plane of a completed
> telescope using a star as the light source, the Ronchi lines are
> ideally perfectly straight for a perfect mirror.  This is so,
> regardless of the focal ratio.  That's the beauty of a null test
that
> eliminates all the nasty spherical aberration seen when testing a
> paraboloid at COC.
>
> In other words, testing a paraboloid at focus with a star source is
> ALMOST the same size sphere at COC.  I say almost, because the
light
> cone is much shallower when testing at COC (twice the focal
length).
> As a result, the Ronchi grating appears to have twice the line
count
> when testing at focus, versus at COC.
>
> Bob mentioned doing the knife test at focus.  This (of course) is
also
> a null test.  A perfect paraboloid shoud "gray" all at once like a
> good sphere at COC will.
>
> --
> Steve Koehler
>

Has anyone seen, come up with or made "telescoping" struts for
the "tube" assembly?
The idea being that one could just extend the top sipder/diagonal up
from the mirror-cell box. This would ease setup, enable transport in
one peice; and elimnate the need to screw together the struts, etc.
Just wndering if this was ever tried?
Thanks
Bill

--- In atm_free@yahoogroups.com, "ronin06" <bgiannelli@...> wrote:
>
> Has anyone seen, come up with or made "telescoping" struts for
> the "tube" assembly?
> The idea being that one could just extend the top sipder/diagonal up
> from the mirror-cell box. This would ease setup, enable transport in
> one peice; and elimnate the need to screw together the struts, etc.
> Just wndering if this was ever tried?
> Thanks
> Bill
>

Yes, it has been tried before. If your "truss tubes" are parallel,
there is little incintive to resist twisting due to parallelism of the
system. This means that the trusses are just as happy as a
parallelogram as a square.

If the trusses are set up as trusses proper in a pyramidal shape, the
results are a much more stable system but to collapse it, you need to
make the top and bottom attachment points pivot or hinge as the
secondary cage approaches the prinary mirror.

This also creates looseness and poor operation of the system.
Not as easy to do as you might first imagine.

Ken Hunter

Remember that when you test on the bench, you're testing at the
ROC of the mirror.  Thus a parabola has curved lines as the ROC
is constanly changing and the sphere has straight lines.  When
testing with a star, the test is done at the focal length so the
parabola will produce straight lines - like the sphere on the
bench, you get a good point focus with a parabola with a star.
I'll note that the KE at the focus point of a parabola and a star
as the light source is really the proper way to do the Foucault
test as Leon actually did .  He looked at elliptical surfaces on
his way to a parabola by setting up the light source at a
different distance from the mirror than the KE.  His final
testing was done against a distant object and did the final
figuring with that.
Bob May
bobmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
Replace the obvious words with the proper character.
----- Original Message -----
From: Mitchell <Funnybone101@...>
To: <atm_free@yahoogroups.com>
Sent: Friday, March 03, 2006 7:20 PM
Subject: [atm_free] Re: Strange mirror problem


> --- In atm_free@yahoogroups.com, "Bob May" <bobmay@...> wrote:
> >
> > Several ways to test.  The Star Test is one way as Steve
> > mentioned.
> > Also there is the Ronchi grating test where you are looking
for
> > straight lines.  A fine toothed comb can be used as a crude
> > grating if necessary.
> > There is also the true Foucault test where you put a straight
> > edge across the focal plane and look for a "flat" shading
just
> > like a spherical mirror on the test bench.
> > Bob May
> > bobmay at nethere.com
> > http: slash /nav.to slash bobmay
> > http: slash /bobmay dot astronomy.net
> > Replace the obvious words with the proper character.
> >
>
> Do you mean straight lines iwth one of those ronchi eyepices
because
> other wise for an  8" F/4 the lines should be very curved in a
> regualr ronchi test correct?
>
> Mitch
>
>
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
>

I'd probably more want to do the bungee cord in the middle of the
rods ala the collapsing/folding rods for the lightweight tents
these days.  This method only requires precision machined rod
ends rather than something expensive in the way of tubing that
fits well together.  In addition, the locking mechanism on the
extended rod needs to be precisely done.  Lastly, the tubing
getting dented will make it all difficult to extend the rod to
full length.
Bob May
bobmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
Replace the obvious words with the proper character.

Thank you
Yes, I can see your point.
Bill
--- In atm_free@yahoogroups.com, "Ken Hunter" <kb7h@...> wrote:
>
> --- In atm_free@yahoogroups.com, "ronin06" <bgiannelli@> wrote:
> >
> > Has anyone seen, come up with or made "telescoping" struts for
> > the "tube" assembly?
> > The idea being that one could just extend the top
sipder/diagonal up
> > from the mirror-cell box. This would ease setup, enable
transport in
> > one peice; and elimnate the need to screw together the struts,
etc.
> > Just wndering if this was ever tried?
> > Thanks
> > Bill
> >
>
> Yes, it has been tried before. If your "truss tubes" are parallel,
> there is little incintive to resist twisting due to parallelism of
the
> system. This means that the trusses are just as happy as a
> parallelogram as a square.
>
> If the trusses are set up as trusses proper in a pyramidal shape,
the
> results are a much more stable system but to collapse it, you need
to
> make the top and bottom attachment points pivot or hinge as the
> secondary cage approaches the prinary mirror.
>
> This also creates looseness and poor operation of the system.
> Not as easy to do as you might first imagine.
>
> Ken Hunter
>

Interesting idea! Thanks
Bill
--- In atm_free@yahoogroups.com, "Bob May" <bobmay@...> wrote:
>
> I'd probably more want to do the bungee cord in the middle of the
> rods ala the collapsing/folding rods for the lightweight tents
> these days.  This method only requires precision machined rod
> ends rather than something expensive in the way of tubing that
> fits well together.  In addition, the locking mechanism on the
> extended rod needs to be precisely done.  Lastly, the tubing
> getting dented will make it all difficult to extend the rod to
> full length.
> Bob May
> bobmay at nethere.com
> http: slash /nav.to slash bobmay
> http: slash /bobmay dot astronomy.net
> Replace the obvious words with the proper character.
>

Ricardo Dunna
I've never needed to do it but woudn't the opticle element from an
eyepiece (minus the case/tube) mounted so you can focuse it on the
knife-edge do the job your asking. Of course you would have to use an
eyepiece whose focal plane was in far enough in front of the leading
element/lense.
talquence

Ricardo Dunna
I've never needed to do it but woudn't the opticle element from an
eyepiece (minus the case/tube) mounted so you can focuse it on the
knife-edge do the job your asking. Of course you would have to use an
eyepiece whose focal plane was in far enough in front of the leading
element/lense.
talquence

-----Original Message-----
From: atm_free@yahoogroups.com [mailto:atm_free@yahoogroups.com] On Behalf Of Ricardo Dunna
Sent: Sunday, March 05, 2006 6:26 AM
To: atm_free@yahoogroups.com
Subject: [atm_free] Focault auxiliary telescope

Hi,

 

My vision isn´t the same anymore, I´m having problems to compare the grays on the focault tester specialy with long FL mirrors.

 

Anyone with a project for a small auxiliary telescope to my focault ?

 

Regards

Ricardo Dunna

--- In atm_free@yahoogroups.com, "Ricardo Dunna" <rdunna@...> wrote:
>
> Hi,
>
> My vision isn´t the same anymore, I´m having problems to compare the
grays on the focault tester specialy with long FL mirrors.
>
> Anyone with a project for a small auxiliary telescope to my focault ?
>
> Regards
> Ricardo Dunna
>

Or as richard said you could rig up some type of camera. I have tried
using my monochrome electronic eyepice focused through an
eyepice/magnifying glass. The power this is backwards as i recall,
meaning a low power eyepice gives alot of zoom and a high power
eyepice is closer to the naked eye. A zoom eyepice such as my cheap
orion one is ideal.

Mitch

Know i just got to figure out a way to mount the thing..........

FWIW, the monitor that I like to use for viewing images has a 4X
amplifier on the input so that the contrast is really high.  This
works very nicely on monitors/TVs that don't do autogaining with
the input signal's sync level.
Bob May
bobmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
Replace the obvious words with the proper character.

Fellow Astronomers,

I recently replaced my homemade finder scope with a neat 60mm one.
Better and clearer over the cheap poorly constructed ones often found
on our scopes. I have created a detailed narrative on the project
here:

http://tinyurl.com/f44b6

But I have included just a bit here:

I have never been satisfied with the finder scopes that the telescope
companies provide with our instruments. Most are cheap junk, some
even with plastic lenses and not even worthy for spying on the
neighbors.

The better constructed ones do a decent job for what they are
supposed to do...locate the object in low power, but I have always
been disappointed in the "Straight-thru" mini refractor types,
regardless of superiority. What is needed is a right-angled diagonal
for comfort and ease of use. Straight thru finders not cool.

For some years now, I have been using a homemade finder scope, made
from 10X50 binocular parts and other doo-dads, and have been quite
satisfied with it's performance. It utilizes a focuser, right angled
diagonal and interchangeable eyepieces...and sat on a modified
microphone stand. But I always wanted to use one of my 60mm
refractors as a low powered finder scope for some time now. What kept
me from accomplishing this was proper mounting rings. Now, I have
experimented in constructing my own rings, but it never worked the
way I wanted, so I abandoned the idea...until recently. And besides,
the longer focal length of a regular 60mm tube would put the eyepiece
waay on the other end of the main OTA...close to the primary mirror.
It would work, but very inconvenient...

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Chicago Astronomer Joe
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