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Old 08-05-2013, 12:31 PM
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PRejto (Peter)
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First Partial Result With ONAG

http://www.pbase.com/prejto/image/150070295/original
http://www.pbase.com/prejto/image/150070295/large

I decided to post here rather than in the images section because these results are very preliminary and I need a lot more data. Still, this is my first attempt at guiding through the ONAG. Photo is the full 1x1 frame uncropped. Setup is as follows:

TEC140 - ONAG - Hutech IDAS Filter - AP BARADV 2x barlow - G2-8300
- ONAG - ST-i (at f7)

Plate solve indicates resolution of .59 arcsec = F13.5 (1890 mm)

60 min total (12 x 5 min subs) of M83 unbinned.

I used focus max and had FWHM ranging from 2.24 - 2.69. No processing on the image other than some mild deconvolution and DDP in CCDStack.

At first I found setting up the ONAG for guiding rather confusing and I had trouble focusing. While the focusing isn't very exact sliding the tube in and out of the locking mechanism I was able to get better focus by rotating the camera on the threads of the tube and then locking it down with the retaining ring. This works OK if you have enough free cable to twist and if you don't need/want the guide camera oriented a particular way. Unfortunatly the first night I tried to do this the seeing was quite poor and I couldn't seem to lock focus in. A better night made a huge difference not only in focus but with guiding as well. It was just my bad luck to try this out first on a bad night and to not recognize that seeing was the cause of my difficulties! I also had to work out a way to be at the scope without breaking down my whole imaging computer and to set up outside just to focus or move the stage of the ONAG to find a good star. I found a good way using an Ipad with remote desktop installed. I used a free app called LogMeIn and it worked perfectly. I was a little bit worried about latency but I couldn't detect any. Focused stars have a different shape after passing through the mirror of the ONAG. They look like a small cross. The star shape in TheSkyX guiding software also looks a bit different. The plotted star shape (see photo) in the graphs window looks fatter and often has two small peaks. I found I could guide well binning 2x2 with rather long exposures (6 sec +) and aggressiveness set to 4 or lower. The longer exposures seem to even out seeing. In any case I was guiding through the TEC at f7 while capturing at f13.5

I'm hopeful that with a lot more data this might turn into a nice image. I would appreciate any comments. I'm new enough at all of this that I might be missing something obvious.

Thanks,
Peter
Attached Thumbnails
Click for full-size image (Guiding May 6-03-M83.jpg)
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Click for full-size image (ONAG + Barlow & Filter.jpg)
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Last edited by PRejto; 08-05-2013 at 12:45 PM.
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  #2  
Old 08-05-2013, 07:35 PM
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Shiraz (Ray)
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very interesting post and the image shows real promise - looking forward to you getting more data.
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Old 08-05-2013, 09:33 PM
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h0ughy (David)
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excellent work and the guiding looks spot on
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Old 08-05-2013, 11:27 PM
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I don't have an ONAG but the process of focussing the guide camera should be similar to my spectrograph. Once it is par focussed with the camera you should not have to touch it again.
I just aimed at a really bright star with a bahtinov mask. Focus the main camera using a subframe and a very short exposure so that you have almost real time updating on the screen. Once you are happy with it then lock down the focusser. You then move the guide camera manually in its holder watching the screen in real time. With my STi I use a 0.1 sec exposure and this will still show the diffraction spikes from the mask. Once it is focussed, lock the position tightly. You should not need to touch it again as you just need to focus the main camera for each target.
Cheers
Terry
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Old 08-05-2013, 11:42 PM
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It looks like guiding was good but perhaps a small amount of coma from the Barlow?

Or some guiding errors still. How good is the polar alignment?

Greg.
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Old 09-05-2013, 03:13 PM
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PRejto (Peter)
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Quote:
Originally Posted by Terry B View Post
I don't have an ONAG but the process of focussing the guide camera should be similar to my spectrograph. Once it is par focussed with the camera you should not have to touch it again.
I just aimed at a really bright star with a bahtinov mask. Focus the main camera using a subframe and a very short exposure so that you have almost real time updating on the screen. Once you are happy with it then lock down the focusser. You then move the guide camera manually in its holder watching the screen in real time. With my STi I use a 0.1 sec exposure and this will still show the diffraction spikes from the mask. Once it is focussed, lock the position tightly. You should not need to touch it again as you just need to focus the main camera for each target.
Cheers
Terry
Hi Terry, You are correct. Once the guide scope is focused you don't need to change it. I'm just not very fond of the focusing options built into the ONAG. It consists of a short tube held in a clamp that can be loosened or tightened down...there is no other way to focus other than sliding the tube in or out. The only way I could fine focus was to rotate the camera on the camera threads. Not ideal, but it works. Next time I do it I will try a mask as you suggest, but due to the distorted star shape coming through the mirror I'm not sure how the mask will look. Many thanks!
Peter
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Old 09-05-2013, 03:49 PM
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PRejto (Peter)
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Quote:
Originally Posted by gregbradley View Post
It looks like guiding was good but perhaps a small amount of coma from the Barlow?

Or some guiding errors still. How good is the polar alignment?

Greg.
Hi Greg,

Many thanks for your comments! I think both factors are at play. The field is highly curved using the barlow (ca 33%), and whilst my guiding looks good on the screen I think I am getting elongation in RA. But, it's confusing because some stars look round and others don't. I cannot seem to make rhyme or reason of what I'm seeing.

My polar alignment is as good as I can get it. I have MA of 7.3 arcsec, and ME = 2 arcsec. I also have a pointing model of 270 points (I can only see 50% of the sky) with RMS = 8. Protract is engaged and I have done PEC.

I have done some experiments trying to guide with DEC disabled. It does slowly drift away in declination but only after a rather long time (longer than 5 min which these subs were taken). But that leads to a question. If guiding is primarily taking place in RA then how can guiding not lead to distorted stars if there are no corrections to DEC. I assume the star diameter will be a function of seeing, but the star shape a function of correction in RA. Is this not a paradox of sorts? One is told not to guide too quickly (short edxposures) because you will be guiding on seeing, yet if you guide longer than "seeing" then the star would be distorted in RA. Anyway, this is my paradox because obviously those of you out there are guiding very successfully! I really could use some help here. I feel I've guided very successfully with a guide scope (80mm, fl=500), but now guiding at 980 mm is much harder than I anticipated. Advice from Innovations Foresight (ONAG) has been to guide 4-6 sec to average out the seeing. When I do that I get a very tight graphed result (see photo in first post), but when I guide shorter intervals I get quite erratic results - guiding might seem ok for a little while followed by large unexpected moves. Whatever, I've yet to find the right combination of aggressiveness and exposure. One thing I will do is look again at PEC. I did replace the belts in my MX so perhaps something is different now.

Another factor I'm confused about what looks to be a pronounced difference between the blue and red filters concerning star aspect. For example I took a series of 10 short exposure in blue and red and compared the results in CCDInspector. The blue filter averaged FWHM = 2.20, Aspect = 10, but the red filter FWHM = 2.28, Aspect = 16.5. When I expose much longer subs these difference become quite pronounced. I don't know why this should be but imagine it might be atmospheric effect (though I though perhaps blue would be more trouble than red), or something in the ONAG or barlow. For example, last night I shot 11 luminance frames of M83 where the aspect varied from 16 to 32, but red varied from 25-37. And yes, I did refocus.

Any insight into this would be greatly appreciated.
Many thanks,
Peter
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Old 10-05-2013, 07:26 AM
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PRejto (Peter)
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Well, I checked my PEC last night and it wasn't correcting properly. I ran new curves but so far cannot get correction. My uncorrected error is peak to peak 2.2 arcsec, but when I apply a new correction curve I get an increase in PE regardless of whether the "west" is checked or unchecked. I've posted the issue at Software Bisque and hope for a quick resolution. I'm very hopeful this is the cause of my erratic guiding results.
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