RE: [APML] Help! Advice on collimating a C14

["Andrew Clegg" <andy@w4je.com>]

I do have the FastStar-compatible OTA. I haven't checked carefully that the
corrector plate and secondary assembly are snug--I will do that (although
when I installed Bob's Knobs, there was no indication of any movement of
either the secondary or the corrector plate). That might explain the loss of
collimation between sessions. But I'm still at a loss at getting the thing
collimated very well at all. I've had the thing for six months, and
regardless of my attempts at collimation, I have yet to get any decent lunar
or planetary views, even after hours of cool-down time. I have yet to see
any semblance of a decent diffraction pattern through the 'scope. At high
power (heck, even at medium power), most star images look like blobs
(regardless of seeing conditions). Out-of-focus, the shadow of the secondary
is as centered as I can get it, but I still get blobs at best focus (hey, at
least it makes guiding with an illuminated reticle easy!!). When collimating
on a star, I have worked my way up through higher and higher power
eyepieces, and closer and closer to focus, centering the secondary shadow
all the while. Still no luck at getting good images.

When observing the moon, the effect is that the best focus still makes the
moon a bit soft, as if I have a soft-focus filter in the path.

BTW, all optical surfaces are essentially pristine (corrector plate,
primary, secondary, diagonal, and eyepieces), so that's not the problem.

I'm beginning to think I have a bad apple. How much can a five foot long,
80-pound package, insured for $3500 and shipped across the country cost,
anyway??

-----Original Message-----
From: astro-photo-bounces@seds.org
[mailto:astro-photo-bounces@seds.org]On Behalf Of Duncan Munro
Sent: Monday, November 29, 2004 12:59 AM
To: Discussion of Film Astrophotography
Subject: Re: [APML] Help! Advice on collimating a C14


Andrew,  I have collimated lots of SCTs and in my experience they do not
lose their collimation easily if collimated properly, and they are easy
to collimate unless something is wrong. You should start by tightening
down (firm but not too tight...)all the collimation screws and then
starting the collimation process again. This time you will back the
screws off as you collimate so that when you are finished the screws
will still be reasonably tight. I'm not really a fan of bobs knobs
because I rarely have to recollimate and I find the allan keys allow for
finer rotation of the screws.

Does your C-14 have the removeable secondary? It's possible that
something is loose which is allowing the secondary assembly to move
around. Also check the corrector plate screws and see if they are all
snug. I used a Meade 16" SCT extensively and the corrector plate had a
tendency to move around. IIRC we put some extra spacers around it to
hold it in place.

When you  observe a star at high power are you seeing concentric
diffraction rings?

t will probably take a C-14 a couple of hours to cool down. Planetary
viewing will suffer greatly until the tube currents start to go away.
Tube currents can mimick rough optics and/or poor collimation.

cheers

Duncan

Andrew Clegg wrote:

>Gang,
>
>This may not be the best place to ask this question, since everyone seems
to
>use RCs or refractors, but I thought I'd ask anyway...
>
>I have a C14 CGE, and I am having one &$%#^ of a time getting the thing
>collimated. Deep sky views are spectacular, but lunar and planetary views
>are basically horrible (my Orion ED80 gives better views). This is
>frustrating because in the reviews of the C14 CGE that I've read, they
often
>mention that the 'scope provides some of the best lunar and planetary
images
>the reviewers have ever seen.
>
>As I eventually want to move past piggyback astrophotos to shooting through
>the C14 itself, I need to find a way to get good collimation. I've tried
>collimating on stars (with the help of a friend), but the process is very
>slow, tedious, and frustrating (even with Bob's Knobs), and never ends up
>with greatly improved images. And when the 'scope is packed up for the
>night, collimation is lost, and the next observing session is once again
>spent mostly on collimation, in a vein attempt to improve the images.
Arghh.
>
>I've read almost everything I can on the tricks of star collimation, but it
>just ain't working for me. So the next option is one of the specialized
>laser collimators for SCTs, which shine through the optical path of the SCT
>in reverse and project a scattered laser spot onto a distant target. I
>wouldn't mind trying that, but I really would like to know if anyone has
any
>experience collimating SCTs (either with a laser or something else) before
I
>shell out $200 for the laser collimator. Another choice would be
collimating
>on an artificial star, but again, it would be good to hear some opinions
>before sitting down at the workbench to make one of these.
>
>Has anyone had the same trouble as me getting a large SCT collimated?
Should
>I send the OTA back to Celestron to make sure there isn't something wrong
>with the optics? That would cost me a couple hundred bucks, but it would be
>worth it if that would cure my problems.
>
>Help!
>
>Clear skies,
>Andy
>www.w4je.com
>
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>
>
>

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