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AT6RC collimation under different loads

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#1 pfile

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Posted 09 November 2012 - 12:58 AM

well, i kind of posted this over in the CCD section, but maybe that was the wrong place.

the short question: is it normal to have collimation results that are wildly different when switching camera payloads on an AT RC telescope?

when using my DSLR+FR my collimation is spot-on and my field is flat.

when i switched to a heavier camera (STT-8300M), FHWM analysis across the field shows that the optical center is nowhere near the center of the CCD. the flattener has left the train due to plumbing issues. so i don't expect the field to be flat, but i expected it to be centered on the CCD.

is it normal to have to recollimate in this situation, or do i have some other problem?

#2 WadeH237

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Posted 09 November 2012 - 09:45 AM

It sounds like your focuser might be sagging a bit.

#3 pfile

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Posted 09 November 2012 - 10:37 AM

I thought of that - but the focuser is a brand-new moonlite. also I see the problem if the telescope is pointed straight up...

could it be that the moonlite is not up to the task? the weight adjustment is tightened down as far as I could with my fingers.

#4 Peter in Reno

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Posted 09 November 2012 - 11:27 AM

I believe the focuser tube (not external focuser) inside OTA is connected to the primary mirror which is not a good design and causes collimation changes from point to point if there is a somewhat heavy imaging train. So when the scope is pointing to Zenith, that probably creates maximum stress to primary mirror/focuser tube.

Peter

#5 pfile

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Posted 09 November 2012 - 12:19 PM

yeah, actually i should have linked my post in the CCD forum over here. one of my possible conclusions is that it's a 'cheap' scope and the way the primary is connected to the focuser tube is causing the problem.

the fwhm analysis shows what looks like sag, so i guessed that when the weight of the camera is pulling straight down on the mirror, that the tilt would be gone. but you are right that this situation represents the maximum force on the focuser tube and perhaps just the slight angle (i can't point exactly straight up due to the tripod) is enough to cause the tilt.

anyway, is it a dumb idea to recollimate the scope under this load?

#6 Peter in Reno

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Posted 09 November 2012 - 12:33 PM

If you collimate at Zenith, the mirror/focuser tube may still shift elsewhere in the sky.

Peter

#7 pfile

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Posted 09 November 2012 - 04:47 PM

i guess i'm boned then :) i hope the AT10RC is more tolerant of this heavier load. that's my final destination here...

#8 Peter in Reno

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Posted 09 November 2012 - 04:58 PM

Not necessarily. How far off is the collimation under heavy load? I have seen many fantastic images taken with AT6RC scopes. The focal length may be short enough not to notice collimation shifts at prime focus or with focal reducer.

You should double check with the manufacturer about their design in AT10RC. I believe AT8RC has same design as AT6RC.

Peter

#9 jjongmans

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Posted 09 November 2012 - 05:17 PM

All the GSO RC's (6", 8", 10" and 12") have the same design: the focuser is connected to the primary.

#10 pfile

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Posted 09 November 2012 - 05:33 PM

GAH :) maybe it's a tad stronger, we shall see. lots of people use these scopes successfully so in theory it should be possible.

#11 korborh

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Posted 09 November 2012 - 05:36 PM

pfile, are you planning to get the AT10RC scope? That is a much more expensive scope and it also has the focuser connected to primary mirror design flaw. So you may want to take this into consideration before buying.
These scopes are very hard to collimate correctly, so I think you should not try to tweak it for different parts of the sky. Apart from wearing out the mechanics of adjustments, it will wear you out also :p

#12 jjongmans

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Posted 09 November 2012 - 05:37 PM

Is the miscollimation only noticeable with software, or can you easily see it?

#13 pfile

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Posted 09 November 2012 - 05:37 PM

Not necessarily. How far off is the collimation under heavy load? I have seen many fantastic images taken with AT6RC scopes. The focal length may be short enough not to notice collimation shifts at prime focus or with focal reducer.

You should double check with the manufacturer about their design in AT10RC. I believe AT8RC has same design as AT6RC.

Peter


it's way off, check this out:

Posted Image

the stars in the corner look like seagulls.

#14 pfile

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Posted 09 November 2012 - 05:42 PM

here's a screenshot of the upper left corner and the lower right corner...

Attached Files



#15 pfile

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Posted 09 November 2012 - 05:42 PM

and the lower right

Attached Files



#16 Peter in Reno

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Posted 09 November 2012 - 05:46 PM

Not good.

Maybe if you collimate at Zenith, it might minimize collimation shift elsewhere in the sky.

Peter

#17 pfile

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Posted 09 November 2012 - 05:47 PM

pfile, are you planning to get the AT10RC scope? That is a much more expensive scope and it also has the focuser connected to primary mirror design flaw. So you may want to take this into consideration before buying.
These scopes are very hard to collimate correctly, so I think you should not try to tweak it for different parts of the sky. Apart from wearing out the mechanics of adjustments, it will wear you out also :p


well, i already have it, i have not deployed it for real yet. basically i have been working my way up to it, trying to get my mount tuned up and understanding how to use OAG with the AT6.

i freely admit that i did not think all of this out very well. the AT6 was a bit of an impulse buy since they were so steeply discounted. and i was pretty successful with it with a light load (DSLR + TRF-2008). so basically i never saw the problems that the mirror/focustube design can cause.

that experience made me think i could probably be successful with the AT10RC. i may be in for a surprise, but i will at least try, since i already have the scope. at least i bought it used :)

#18 jjongmans

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Posted 09 November 2012 - 05:48 PM

Did you adjust the primary already? If so, maybe you should try to tighten the collimation bolts.

#19 pfile

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Posted 09 November 2012 - 05:58 PM

Did you adjust the primary already? If so, maybe you should try to tighten the collimation bolts.


i have not adjusted this telescope at all, ever, mainly because of what korborh said above... the collimation looked really good with the DSLR/TRF so i never had to mess with it.

i am willing to try collimating it, because i've come to realize that if you own one of these telescopes, you must learn how to do it. having said that, i had the AT10 collimated professionally. of course, once there's a camera attached to it, it may be out of collimation again just as the AT6 seems to be.

#20 jjongmans

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Posted 09 November 2012 - 06:01 PM

How heavy is your ccd including filterwheel, oag, etc?

#21 pfile

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Posted 09 November 2012 - 06:21 PM

the camera is 1.8 pounds and the filter wheel is 2.5 pounds, for a total of 4.3 pounds (about 2kg). it probably weighs a tad bit more because there are 6 filters in the wheel.

#22 Jeff in Austin

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Posted 09 November 2012 - 08:28 PM

I'm using an AT12RC and had an AT8RC and didn't have any of the problems described with a 6+ lb load.

#23 pfile

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Posted 09 November 2012 - 08:29 PM

cool, well the weather is bad but maybe i will put the camera on the AT10 indoors and see what a flat looks like. did you upgrade the focuser on the AT8 or the AT12?

#24 korborh

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Posted 09 November 2012 - 09:51 PM

I had the AT8RC and also attempted to tweak collimation. What you need to know is there are many degrees of freedom for collimation adjustment ( or screw-up) on this scope (1) secondary mirror tilt (2) primary mirror tilt (3) Axial alignment of primary/secondary (4) distance between primary and secondary.

So one can appreciate how hard it will be to re-collimate if it gets messed up. In fact I tried for a week and then gave up, got it professionally collimated and sold it.

Warning - do not make large collimation adjustments to the primary as it can potentially mess up 3 of the 4 alignment requirements above.
So that is why I do not recommend tweaking collimation for the flexure issues or if you are getting good images. These scopes probably need a complex bench setup to bring them back to collimation if gone too far.

#25 pfile

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Posted 09 November 2012 - 10:53 PM

in fact i think astro-tech/gso initially recommended to people that they should not try to adjust the primary mirror, ever. but since the problem here seems to be shifting of the primary, it seems like that would be necessary.

i fear that my experience would mirror yours (ha ha) if i tried to collimate the scope, and unfortunately there's no way to find out without trying...






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