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Old 09-04-2013, 11:39 PM
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Unusual dark frame artefact

I took some 10 min darks tonight as it was wet.... Atik 420 mono....
I was looking through them and noticed some unusual artefacts that donesn't look right - looks like dust/crud, but on a dark - I certainly don't see this on lights, flats, or other, shorter (3&5 min) darks I've done...
Also looks to be a gradient?
Scope had its dust cap on, only light in obs was PC and a power supply display... ?light leak?
I'll certainly redo them in scrupulous darkness in case, but seems odd....
I've done other darks at the same temp but shorter duration, and never seen this at all....

Any ideas??

The JPG is quite stretched, but I still wouldn't expect marks like this...
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Old 09-04-2013, 11:47 PM
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h0ughy (David)
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first thought was a left over medical slide

but could it be spores? how mouldy was the observatory when you cleaned it out. it was a very moist ay today it could have triggered a growth of them?
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Old 10-04-2013, 07:55 AM
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No mould.... I think it is a light leak after sleeping on it.... I didn't pay attention to shielding the PC/PSU, given I was shooting darks....

I took 6x10min frames.
Frame 1 started while I was there, had main light on - has average pixel intensity of 10000, and prominent black marks....
Frames 2-5 - average pixel intensity of 2500 or so, less prominent black marks.... main light off, but the PC display was on a 45 min power save before shutting off...
Frame 6 - looks fine, av pixel intensity of 1300 and no black marks - the PC display would have blacked during frame 5....

Will try these again soon....

I did notice similar on a scout frame the other night (attached), I think the main light was on at the time then also....
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Old 10-04-2013, 08:12 AM
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Looking at that dark frame, it seems to me that each of those patterns is circular - almost perfect (if incomplete) circles. My guess would be with a light intrusion up the focuser, and bouncing back from the inside surface of the objective. I've found that I have to use gaffer tape to mask off the focuser drawtube on some scopes and some focusers. I'd be looking for any LEDs in the vicinity of the scope/camera.
Peter
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Old 10-04-2013, 09:12 AM
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You're probably right - I had the scope tucked away in its sleeping position with the obs roof shut, given the improvement lining up with the PC display shut off, I think the answer is there somewhere.... when taking lights I'm careful to shield the monitor with some black cloth.... will experiment tonight with my 500W work light.....
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Old 10-04-2013, 09:54 AM
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Quote:
Originally Posted by Lee View Post
You're probably right - I had the scope tucked away in its sleeping position with the obs roof shut, given the improvement lining up with the PC display shut off, I think the answer is there somewhere.... when taking lights I'm careful to shield the monitor with some black cloth.... will experiment tonight with my 500W work light.....
Try wrapping the whole camer/focuser assembly in a black loth then take some darks. My guess is e problem will have disappeared.
Peter
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Old 10-04-2013, 09:55 AM
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That's the plan.....
It's looking like a nice clear few days for new moon though, might have to wait....
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Old 10-04-2013, 11:12 AM
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have seen something similar with my f4 newt - was light leaking in past (through?) the main mirror
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Old 10-04-2013, 12:32 PM
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It's looking like a nice clear few days for new moon though, might have to wait....
Half your luck!! The prognosis up here is for continuing cloud or worse.

Peter
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Old 11-04-2013, 03:01 PM
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Repeating these last night with strict attention to stray light - problem solved....
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