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Old 14-06-2013, 04:36 PM
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pvelez (Pete)
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What's the diagnosis?

I've been wrestling with collimation on my RC8. I had used CCD Inspector and determined I had an error of about 10 arcseconds - huge.

So I did what any of us would do - I started fiddling. Of course I stuffed it up

I bought a Tak collimation scope - still no improvement.

I decided that the primary was out and - you guessed it - I fiddled with that too.

The result - not pretty.

Funnily enough, the Tak scope says its quite close - though perhaps the centre spot on my RC8 is not centred.

Anyway, here is the patient - I'm interested in the diagnosis from the assembled doctors. I took an image of Centaurus A - 300 seconds in L binned x 2 guided. The guiding chart looked quite good.

I ran the image through CCDIP - the data is in the attached. Collimation out by 10.6" with a tilt of 9% and curvature of 19%. That all said, the viewer is still a satisfying indigo.

Then I've given you the image so you can see it in all its ugly glory.

Next I have a close up of stars in the four corners of the image - all are eggy but not to the same extent. I've presented them in order moving clockwise from the top left. The bottom right corner seems most round

So - my question. Is my issue solely miscollimation? Or is my camera/focuser/rotator not orthogonal? Or is there something else weird going on.

I have my Medicare card handy - just waiting for the doctor's report...

Pete
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  #2  
Old 14-06-2013, 04:47 PM
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Pete try this method , I find it puts me pretty much dead on and also makes for an easy quick check .
http://www.davidcortner.com/slowblog/20110815c.php
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Old 14-06-2013, 06:52 PM
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Quote:
Originally Posted by Marke View Post
Pete try this method , I find it puts me pretty much dead on and also makes for an easy quick check .
http://www.davidcortner.com/slowblog/20110815c.php
Thanks for this

I feel I've trawled through dozens of websites lately. I read this one but skipped over it as I assumed it was dealing with the technique to use an artificial star. I've reread the "Hall of mirrors" section and that sounds like something to try in daylight. Is that what you were referring to?

Pete
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Old 14-06-2013, 07:34 PM
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Yes Pete try it in day light it will show you if you are out or not and it's very accurate and easy to adjust. I have a tak scope but I find this method easy , the tak scope does help getting the primary right tho.
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Old 15-06-2013, 09:58 AM
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Quote:
Originally Posted by Marke View Post
Yes Pete try it in day light it will show you if you are out or not and it's very accurate and easy to adjust. I have a tak scope but I find this method easy , the tak scope does help getting the primary right tho.
Thanks Mark

I had another go today. Maybe its me, but the collimation looks quite good using both this method and the Tak scope. I still have rubbish stars.

I used CCDIP to look at some old images. It looks as if my collimation has been pretty poor for quite a while - though I have images from 2011 that have roughly the same reported miscollimation. The stars in that image are heaps better than the images I posted yesterday.

So I'm still scratching my head - is it only collimation or is there something more sinister afoot?

Pete
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Old 15-06-2013, 10:01 AM
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One last point to help with remote diagnosis.

When star testing, I found that the stars were elongated in 1 axis but both directions. So if I have a defocussed star in the centre of the FOV, the donut may be thicker at both 12 and 6 o'clock. How do I adjust to address that ? This is what led me to believe that my primary was out - looking at it today, I don't believe that it is.

I'm v confused.

Pete
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Old 15-06-2013, 04:15 PM
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Is your camera straight if you are using the stock focuser there may also be a bit of sag in it . First thing I did was swap over to a Feather touch .
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Old 17-06-2013, 08:21 PM
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Using the tak scope is good but not the entire solution to the collimation resolution.

From your descriptions you could have warped the secondary holder and mirror just a little. You cannot tighten these things are really tight, just snug is the go.

I used the tak scope until I had it as good as I could then I did a star test that night with the camera in place.

Your focusor might have some issues too. Which focusor are you using?

Rotator - if Optec just check to make sure the back adapter is snug all the way around. It can be just a bit out and that will also create this sort of thing.
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Old 18-06-2013, 08:35 AM
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Quote:
Originally Posted by Paul Haese View Post
From your descriptions you could have warped the secondary holder and mirror just a little. You cannot tighten these things are really tight, just snug is the go.

I used the tak scope until I had it as good as I could then I did a star test that night with the camera in place.

Your focusor might have some issues too. Which focusor are you using?

Rotator - if Optec just check to make sure the back adapter is snug all the way around. It can be just a bit out and that will also create this sort of thing.
Thanks Paul

you might be right re the warping. I have a suspicion that its not simply a case of having the secondary misaligned. I had read somewhere that the RC design is very sensitive to the distance from primary to secondary so if I have gone too far - either in or out - I won't be able to get it right. Is this your experience?

Focuser is an Optec TCF-S 2 inch focuser. Its a heavy beast and there is precious little movement in it.

More interesting perhaps is the rotator - a Pyxis 2 inch also. I have a few adapters in the imaging train to get everything to focus. The miscollimation has become more apparent since including it in the mix. That said, once I started adjusting the collimation I have been unable to get reasonable results with or without the rotator.

Pete
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Old 18-06-2013, 09:03 AM
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I am not sure you can move the secondary that far to have an influence on the required distance.

Focusor sounds alright but it could be the adapters. I am a little amazed that the rotator has fit together adapters and not screw threaded. I recently checked one of my images on CCD inspector and it reported I had miscollimation in my refractor images. I thought that was puzzling and so when down at the observatory I was checking over everything and noticed that there was about .5mm of warp on the camera side adapter from one side to the other. So it is worth a check.
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Old 18-06-2013, 09:32 AM
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Quote:
Originally Posted by Paul Haese View Post
Focusor sounds alright but it could be the adapters. I am a little amazed that the rotator has fit together adapters and not screw threaded. I recently checked one of my images on CCD inspector and it reported I had miscollimation in my refractor images. I thought that was puzzling and so when down at the observatory I was checking over everything and noticed that there was about .5mm of warp on the camera side adapter from one side to the other. So it is worth a check.
Thanks Paul - all very helpful

I have found the nosepiece arrangement with the QSI/TCF-S a bit clunky - just 3 screws rather than something more substantive. So I was pleased when I picked up the Pyxis as it came with a number of adapters that seemed more robust.

Now I am wondering. I can use the Tak scope with the focuser but not the rotator. Perhaps there is some sag somewhere which means that the collimation looks good (or good enough) with the Tak scope but lousy with the CCD.

Will need to investigate further - may need to ditch the rotator (bugger)

Pete
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Old 18-06-2013, 02:20 PM
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Is there any chance you can use bolt up or screw fit spacers and adapters? Because as you say, the collimation may look good, but using nosepiece adapters and thumbscrews is more than likely going to make your stars look ugly, especially if your using a full frame chip.

Josh
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