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Old 09-08-2013, 12:10 PM
LewisM
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Elongation

Attached is a sample of stars near the centre of a single unprocessed 20 minute guided sub I took recently (out of focus, as were the ENTIRE 2.5 hours worseluck, but using this to draw some conclusions). Magnified 200%

Does this slight elongation appear to be polar alignment issues, flexure, camera tilt?

20 mins single exposure, using a Lodestar through an f/9 guidescope (secured to scope with solid rings, not guidescope rings). Imaging scope Vixen FL102S, reduced. SXVR-M25C camera. All screwed components in the imaging train - NO compression rings etc. Mount is a Vixen GPD2 using a Skywatcher EQ5 GOTO upgrade kit (that I tuned). Seeing was 9/10, nil wind.
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Old 09-08-2013, 01:01 PM
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PRejto (Peter)
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20 min is pretty long, no? Maybe it's PE that couldn't be guided out. Do you know how your camera is aligned vis a vis RA and DEC?

Peter
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Old 09-08-2013, 03:58 PM
LewisM
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I had been running 10 min subs, and no noticeable elongation at all, so I am ASSUMING the 20 mins are possibly PE as you mention. This Vixen is so precise that PHD makes miniscule corrections (so much so the mount is almost silent, unlike the NEQ6 I had that was forever being adjusted with PHD). In fact, the PE logging is also tedious, as the star barely moves (hey, not complaining! 37 minutes with non-noticeable movement is pretty darned good in my opinion)

This elongation is apparent across the field, so to me that's either slight PE or some camera tilt (though would expect it worse on the left than right if camera tilt)
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Old 09-08-2013, 04:06 PM
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multiweb (Marc)
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TBH at 200% I can't see any noticeable elongation. PE is well under 10min so if it's not in a 10min sub it won't be in a 20min sub. What you're looking at is likely to be flexure between your guider and imaging scope.
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Old 09-08-2013, 04:25 PM
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Bassnut (Fred)
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That's pretty good for a 20min sub and ext guide, most would be happy with that. A small tweak in PS would fix it. Your up for OAG guide or a lot more other work to get it better.
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Old 09-08-2013, 04:39 PM
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rmuhlack (Richard)
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Quote:
Originally Posted by multiweb View Post
TBH at 200% I can't see any noticeable elongation. PE is well under 10min so if it's not in a 10min sub it won't be in a 20min sub. What you're looking at is likely to be flexure between your guider and imaging scope.
I agree with Marc - i can't really see any elongation at 200% either. If you need to zoom in that far to see it, then is it really an issue? Personally i'd be happy with guiding like that

Nevertheless, is the elongation you are seeing parallel with the Dec axis by any chance? If polar alignment is slightly out, the mount will be making more frequent Dec corrections. When this occurs with my NEQ6 and VC200L (a reasonably heavy setup), i tend to see a bit of elongation in the Dec direction as i find guiding corrections in Dec are not as smooth as RA if I have PHD set to "auto" dec adjustment. For me, i find that if I set to either "north" or "south" dec adjustment instead (depending on the direction of the polar alignment error, which is different for every image session for me as i don't have a permanent setup) then guiding performance and subsequent elongation is much better - just make sure to flip the North-South correction when you do a meridian flip. Dec backlash is possibly the cause here in my case which might not apply to you, but it might be worth experimenting with those settings in PHD anyway if you're worried about it with your imaging runs. Just an idea ...

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Old 09-08-2013, 05:10 PM
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tlgerdes (Trevor)
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+ 2 in agreement with Marc.
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Old 09-08-2013, 05:36 PM
LewisM
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I am pretty happy with the guiding,ecstatic with the mount, but I did notice the elongation, probably because the stars were false-bloated thanks to the focus error. I have compared the stars to a circular tool in PS, and they are certainly not perfectly round. I'd like to somehow "fix" that, being a pedant.

I could call the data I got "The Squished Bug Nebula"... the focus error makes it look like a blurry red smudge
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Old 09-08-2013, 06:03 PM
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I'd probably be more concerned about why the focus was so far out?

I find manually focusing by watching FWHM numbers to be rather tricky, since it fluctuates so much even in good seeing. I like using a Bahtinov mask in combination with the Bahtinov Grabber program. It provides real-time feedback for how far out-of-focus you are, and whether you're in the critical focus zone for your optical configuration.
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Old 09-08-2013, 06:18 PM
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Quote:
Originally Posted by naskies View Post
I'd probably be more concerned about why the focus was so far out?

I find manually focusing by watching FWHM numbers to be rather tricky, since it fluctuates so much even in good seeing. I like using a Bahtinov mask in combination with the Bahtinov Grabber program. It provides real-time feedback for how far out-of-focus you are, and whether you're in the critical focus zone for your optical configuration.
Good point. I also now use Bahtinov Grabber to quantify exactly how far out-of-focus I am, and if I am within the CFZ. Much easier than just eye-balling it, and far more repeatable.
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Old 10-08-2013, 02:58 PM
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If its like that over the whole image, its unlikely to be tilt. Could be collimation, but very hard to diagnose with out of focus stars. I would suggest try again, this time focused and only expose for say 20 - 30 sec. This will / should eliminate guiding issues if there are any and polar misalignment.

seeing you said the elongation was even across the field, this would rule out field rotation from polar misalignment i would have thought.

you can test for tilt with a good repeatable digital focuser and an auto focus routine.

Josh
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