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Old 21-05-2008, 08:21 PM
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[1ponders] (Paul)
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Wot tha h....appened to my dark frame

I'm out imaging NGC3201 tonight and snagged a dozen or so light frames first and then started to take some darks. Here are the results. First shot is the unprocessed glob and the second shot is the dark. And yes I did put the cover on the scope first. The first 4 darks are like this but getting progressively fainter until the 5th dark looks clear.

Details: SBIG ST7XME, -15deg, exposures 2 min.
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  #2  
Old 21-05-2008, 08:39 PM
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Oh dear, Paul that dosen't look good, not that I can help you, I have trouble getting it right myself..

Leon
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Old 21-05-2008, 08:55 PM
jase (Jason)
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There is simply charge left it the chip Paul. Nothing to be worried about. Flush the chip by performing a few dark or bias exposures (as you've done) and you're good to go. This happens from time to time. I get the same issue with the STL11k.
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Old 21-05-2008, 09:00 PM
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[1ponders] (Paul)
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Sazin' frazin' Dang thang. Here I was hoping that I could get away with rattlin' off 3 or 5 dark frames so I could get onto my next object.

Patience Grasshopper.
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Old 21-05-2008, 09:01 PM
Ian Robinson
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Speculative answer : the pixcels had not lost the charge in them and were acting something like a capacitor , it was taking a while for the holes or electrons or both produced by the photons to diffuse back to the n and p layers.

Maybe you need to give the sensor more time between light and darks frames ??

Jase ... ya beat me to it .... a little knowledge of microelectronics or solid state physics goes a long way.

Question : does this happen with DSLR chips too ? (they aren't as sensitive as cooled SBIG ST7XME chips).

Last edited by Ian Robinson; 22-05-2008 at 07:55 PM.
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Old 21-05-2008, 09:16 PM
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[1ponders] (Paul)
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I've never noticed it with either of my DSLRs, modded or unmodded
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Old 23-05-2008, 02:48 PM
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I've never had this happen with my CCD that uses the came chip as the ST7 or with our clubs ST7. When mine starts a new exposure the CCD is first cleared by the software controlling the CCD. Maybe try a different program to take the exposures.
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Old 23-05-2008, 02:54 PM
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I've never had it happen to me before either Terry. That's what took me by surprise. I thought the chip gets "flushed" after each image. I'll see how it goes next time.
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Old 23-05-2008, 04:41 PM
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Quote:
Originally Posted by [1ponders] View Post
I've never had it happen to me before either Terry. That's what took me by surprise. I thought the chip gets "flushed" after each image. I'll see how it goes next time.
The other problem it could be is a shutter malfunction. Did you take a dark frame with the shutter closed or actually cover the scope?
I have had the shutter not fully close and let some light in.
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Old 24-05-2008, 08:05 AM
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I actually covered the scope Terry, which is why I was scratching my head.
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