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Old 09-05-2013, 02:49 PM
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PRejto (Peter)
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PRejto is offline
Join Date: Jan 2011
Location: Sydney, Australia
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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?

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,
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