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Old 10-07-2015, 07:16 AM
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Auto dark vs master dark

I always use MaxIM auto-darks for my images. Having said that, I do have a master dark library (differing temperatures etc).

MaxIM's auto-dark removes the single column defect perfectly. Of course, if I load the master dark and calibrate it against an already dark-subtrated image, I INTRODUCE the column defect back in

So, now comes the question - which is better - continue using the auto-dark function, or turn auto-dark off and use the dark master (said in a DEEP voice ). Are there any GENUINE advantages to using a combined master than a simple single auto-dark subtract for all images?

I am thinking to just go to MaxIM full calibration each session. Worth talking me out of it or not?
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Old 10-07-2015, 08:55 AM
garymck (Gary)
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Hi Lewis,

the "column defect" is not actually a column defect - it is a hot pixel that causes a bleed down the column - this was how SBig explained it to me (It doesn't run down the whole column) . I used to calibrate using master darks and master bias frames. The Bias frames got rid of the "column defect". I had no problems using a master dark library. Nor did I have any issue with scaling darks - as long as I used bias frames. Not sure ow Maxim does it, but I bought Astroart which I found much easier to use.

cheers
Gary
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Old 10-07-2015, 12:00 PM
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Are you scaling your darks?

I believe this artefact will cause issues if you scale your darks. I used to use 30min darks with my 8300 sensor and scale them, no major issues; however with the 16803, I can't get away with this because I can clearly see 1 or 2 darker columns, in addition to the fixed noise pattern generated from the RBI flash I apply. Now I've built 10, 15, 30min RBI flashed dark masters and will just match to those. If you use matched darks, don't subtract the bias, it's already within the dark anyway.

I guess just ensure you're not subtracting a bias from your subs and then subtracting a dark (non bias subtracted), else you're doubling up on the readnoise subtraction.
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Old 10-07-2015, 12:30 PM
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Quote:
Originally Posted by LewisM View Post
Are there any GENUINE advantages to using a combined master than a simple single auto-dark subtract for all images?
Yes... a single dark frame contains a bunch of noise as well as dark current (read noise and also noise associated with the dark current). When you calibrate with that individual dark frame you'll remove dark current from your sub but you'll also add all that noise. A master dark composed of many individual dark frames should have much lower noise and won't introduce much additional noise to your subs during the calibration process.

The key to understanding this is to realise that dark calibration doesn't remove noise. It removes unwanted signal (bias and dark current.)

Cheers,
Rick.
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Old 10-07-2015, 01:19 PM
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Quote:
Originally Posted by RickS View Post
Yes... a single dark frame contains a bunch of noise as well as dark current (read noise and also noise associated with the dark current). When you calibrate with that individual dark frame you'll remove dark current from your sub but you'll also add all that noise. A master dark composed of many individual dark frames should have much lower noise and won't introduce much additional noise to your subs during the calibration process.

The key to understanding this is to realise that dark calibration doesn't remove noise. It removes unwanted signal (bias and dark current.)

Cheers,
Rick.
What Lewis is talking about is using the auto calibration within Maxim. It allows you to either add, for instance, 50 individual dark frames OR a Master Dark that you've created from those 50 frames.

It does sound like a double bias subtraction that is reintroducing the "column defect".
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Old 10-07-2015, 01:57 PM
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Originally Posted by Atmos View Post
What Lewis is talking about is using the auto calibration within Maxim. It allows you to either add, for instance, 50 individual dark frames OR a Master Dark that you've created from those 50 frames.
I assumed he meant this:

Quote:
Simple Auto-dark tells the software to shoot a dark frame, matching the exposure in exposure duration and temperature. The auto-darks are saved in a rotating buffer, so that you can quickly switch between several different exposures without having to reshoot the auto-dark each time. If the camera has no shutter you will be prompted as required to manually cover the optics.
Perhaps you can clarify, Lewis?
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Old 10-07-2015, 02:28 PM
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Rick has it right. MaxIm auto-dark subtraction for an imaging run - the first frame of a set is a dark that matches the timing of the lights. This dark is then automatically calibrated to the downloaded light exposure. This obviates the need to do them later. The Lazy Man's approach
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Old 10-07-2015, 07:28 PM
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Rick's all over the math like a self saucing pudding!

DT
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Old 10-07-2015, 07:47 PM
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I guess I was thinking about the wrong auto calibration!
In that case, the auto dark will remove some defects but increase the overall amount of noise in the image. In theory anyway.
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Old 10-07-2015, 08:31 PM
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Rick's all over the math like a self saucing pudding!
It's cool that the stuff I hated at Uni is actually useful for my hobby even if it's irrelevant in my professional life
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Old 13-07-2015, 07:03 AM
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If you are imaging in a very bright urban sky an autodark is probably OK. The shot noise from the sky will overwhelm the random noise introduced by subtracting the dark frame.

Somewhat counter intuitively, darker skies need more darks than bright skies.
Geoff
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Old 13-07-2015, 07:11 AM
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Quote:
Originally Posted by SpaceNoob View Post
Are you scaling your darks?

I believe this artefact will cause issues if you scale your darks. I used to use 30min darks with my 8300 sensor and scale them, no major issues; however with the 16803, I can't get away with this because I can clearly see 1 or 2 darker columns, in addition to the fixed noise pattern generated from the RBI flash I apply. Now I've built 10, 15, 30min RBI flashed dark masters and will just match to those. If you use matched darks, don't subtract the bias, it's already within the dark anyway.

I guess just ensure you're not subtracting a bias from your subs and then subtracting a dark (non bias subtracted), else you're doubling up on the readnoise subtraction.

Did you notice much of a difference using RBI flashing? Are your images cleaner?

Greg.
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Old 13-07-2015, 07:26 AM
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Did you notice much of a difference using RBI flashing? Are your images cleaner?

Greg.
Greg, we're using RBI flush with a PL16803 at SRO now. The RBI flush actually adds additional noise. The current that leaks out of the traps after the flush, like any signal, has shot noise associated with it. What the flush does is prevent ghost images from previous exposures. It's not intended or expected to reduce overall noise.

Cheers,
Rick.
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Old 13-07-2015, 09:24 AM
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Originally Posted by RickS View Post
Greg, we're using RBI flush with a PL16803 at SRO now. The RBI flush actually adds additional noise. The current that leaks out of the traps after the flush, like any signal, has shot noise associated with it. What the flush does is prevent ghost images from previous exposures. It's not intended or expected to reduce overall noise.

Cheers,
Rick.
Rick is correct, but the added noise is pretty low from my very limited observation as I've only had the camera a week or so now, it was around 50ADU added noise. I am running it at -35 and RBI stands out, a lot. If I was running it warmer and the camera had a higher read noise I probably wouldn't notice the ghosting. I'll take the predictable and subtractable noise pattern over a variable ghosting impact which will no doubt degrade the stacked end result... We'll see what happens for SII and OIII subs though.

If you use RBI flashing, you can't scale your darks, and in your darks/subs you will notice fixed pattern noise as a result of the physical silicone chip that the 16803 is cut from. It's like a ripple but it disappears once you calibrate.
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Old 13-07-2015, 09:31 AM
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Last nights images were all done with NO auto-darks - all dark/bias masters subtrated in CCDStack.

The difference is VERY apparent.

No more lazy-bum autodarks here
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Old 13-07-2015, 10:09 AM
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Rick is correct, but the added noise is pretty low from my very limited observation as I've only had the camera a week or so now, it was around 50ADU added noise. I am running it at -35 and RBI stands out, a lot. If I was running it warmer and the camera had a higher read noise I probably wouldn't notice the ghosting. I'll take the predictable and subtractable noise pattern over a variable ghosting impact which will no doubt degrade the stacked end result... We'll see what happens for SII and OIII subs though.
Chris: if you're getting 50 ADU from the flush then that's adding about 8.37e- of shot noise [sqrt(signal*gain)]. That's comparable to the read noise and not insignificant. Unlikely to be an issue for LRGB imaging but it probably will result in a SNR hit with narrowband imaging.

Quote:
Originally Posted by LewisM View Post
Last nights images were all done with NO auto-darks - all dark/bias masters subtrated in CCDStack.

The difference is VERY apparent.

No more lazy-bum autodarks here
Good to hear you're back on the straight and narrow, Lewis
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Old 13-07-2015, 10:36 AM
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Greg, we're using RBI flush with a PL16803 at SRO now. The RBI flush actually adds additional noise. The current that leaks out of the traps after the flush, like any signal, has shot noise associated with it. What the flush does is prevent ghost images from previous exposures. It's not intended or expected to reduce overall noise.

Cheers,
Rick.

Yes trapped charge leaks out during the exposure hence the need for high cooling to reduce the amount that leaks out. But the question really is do you notice any difference (improvement?). I am familiar with the RBI argument but if your tracking is very accurate and subs sit in the same spot image after image which they do when everything is schmick or if you dither and take enough subs then RBI would simply be either nonvisible in the first instance or an outlier in the 2nd instance.

If I flick through a long series of subs its hard to notice any artifact apart from cosmic ray hits. Where I do see it is if a plane flies through a sub or you take darks right after some lights which had some bright stars in it.

The worst I saw was when a plane flew through the image and the trailing lights were visible in the next 3 or 4 subs as it faded eventually.

Greg.
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Old 13-07-2015, 10:55 AM
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Yes trapped charge leaks out during the exposure hence the need for high cooling to reduce the amount that leaks out. But the question really is do you notice any difference (improvement?). I am familiar with the RBI argument but if your tracking is very accurate and subs sit in the same spot image after image which they do when everything is schmick or if you dither and take enough subs then RBI would simply be either nonvisible in the first instance or an outlier in the 2nd instance.

If I flick through a long series of subs its hard to notice any artifact apart from cosmic ray hits. Where I do see it is if a plane flies through a sub or you take darks right after some lights which had some bright stars in it.

The worst I saw was when a plane flew through the image and the trailing lights were visible in the next 3 or 4 subs as it faded eventually.

Greg.
Greg,

I'm happy to use my U16M at -30C without RBI flush. The only ghosting I ever notice is an afterimage of the focus star in the sub taken after FocusMax runs. Sigma rejection takes care of that.

The camera at SRO is shared and someone in the group wanted to use RBI flush. If it had been entirely up to me I would have run without it and evaluated the results to see if it was necessary. If you can get away without using it then you benefit from not incurring the extra noise.

Cheers,
Rick.
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Old 13-07-2015, 10:59 AM
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Chris: if you're getting 50 ADU from the flush then that's adding about 8.37e- of shot noise [sqrt(signal*gain)]. That's comparable to the read noise and not insignificant. Unlikely to be an issue for LRGB imaging but it probably will result in a SNR hit with narrowband imaging.



Good to hear you're back on the straight and narrow, Lewis
Ok, it didn't seem that large a difference... I did a few tests before capturing a dark library, 50adu wasn't the exact value but on the bias' there was around 2-5% increase in base level ADU.

Like I said, we'll see what happens with SII and OIII subs, but if its around a 2-5% difference and I don't end up with ghosting, I'll be keeping it turned on
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Old 13-07-2015, 11:10 AM
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Ok, it didn't seem that large a difference... I did a few tests before capturing a dark library, 50adu wasn't the exact value but on the bias' there was around 2-5% increase in base level ADU.

Like I said, we'll see what happens with SII and OIII subs, but if its around a 2-5% difference and I don't end up with ghosting, I'll be keeping it turned on
I'd be interested to hear how it goes, Chris. At -35C the residual charge will leak away more slowly over a longer period so you may notice more RBI than I do at -30C. I suspect that there may be some variation between sensors too.

Cheers,
Rick.
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