Another Offset Question

Discussion of using SharpCap for Deep Sky Imaging
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cuivenion
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Another Offset Question

Post by cuivenion » Tue Oct 16, 2018 2:08 pm

My understanding of offset (or brightness/black level) is that it needs be be increased if you need to move the histogram to the right to avoid black clipping, and you don't want to increase your exposure time or gain during DSO imaging.

However I've come across this thread on cloudy nights where they seem to suggest that offset should increased with gain:

https://www.cloudynights.com/topic/5420 ... o-cameras/

It doesn't make sense to me as surely the more gain you use the more likely you are to be avoiding black clipping anyway and you'd need to use less offset with higher gains.

Of course its possible I'm misunderstanding this so any comments would be welcome.

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oopfan
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Re: Another Offset Question

Post by oopfan » Tue Oct 16, 2018 3:57 pm

Hi,

I subscribe to Dr. Craig Stark's "Set it and forget it" rule:

viewtopic.php?f=16&t=1163#p5786

You can determine the minimum offset for each setting you use in the comfort of your home. Simply cover the lens, like you are taking darks, and set your fastest exposure (mine is 0.125ms, yours may be different.) For each gain setting that you are likely to use, start with Offset 0, and take a snapshot. Launch FITS Liberator and look at the energy distribution in the histogram. If it is clipped on the left-side then you need to increase Offset. Repeat these steps until the histogram is no longer clipped and is symmetrical around the Mean. Jot down the results and protect it from loss.

Dave (turfpit) and I have an ongoing debate as to whether or not there are benefits to increasing Offset beyond the minimum. I am of the belief that it does not add any value.

Brian

cuivenion
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Re: Another Offset Question

Post by cuivenion » Tue Oct 16, 2018 4:07 pm

Thanks for the reply. Surely the exposure time influences this as well though? Does the offset have a use beyong making sure the image isn't black clipped? To be honest the only time I've used offset is when live stacking and subtracting a dark. I've never used it when doing more traditional DSO imaging.

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oopfan
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Re: Another Offset Question

Post by oopfan » Tue Oct 16, 2018 4:19 pm

Offset ensures that the energy is represented by positive real numbers after calibration. You use the same value for Offset, for a given gain, for any length of exposure. There is no need to modify Offset as you increase or decrease exposure for lights and darks. Offset is just a function of Gain.

Brian

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turfpit
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Re: Another Offset Question

Post by turfpit » Tue Oct 16, 2018 4:32 pm

in-progress-capture.JPG
in-progress-capture.JPG (69.43 KiB) Viewed 993 times
For this in progress capture of M31, mini histogram stretch applied, I reckon my histogram is textbook log display for a DSO. Tells me my capture settings are good. Final image here viewtopic.php?f=16&t=1204

However, YMMV.

Dave

cuivenion
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Re: Another Offset Question

Post by cuivenion » Tue Oct 16, 2018 6:07 pm

Thanks guys. At gain 0 my minimum value is 16. The histogram isn't very revealing though. It doesn't appear to be clipped.
Untitled.jpg
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oopfan
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Re: Another Offset Question

Post by oopfan » Tue Oct 16, 2018 6:12 pm

Dave tends to dial in more Offset than I do but one thing that we agree on: Never adjust Offset below a specific minimum value. If you do then all kinds of funky things will start happening, especially when you stretch the image to eke out faint nebulosity. Post-processing will become pure hell for you.

Here is an image of M1 that I accidentally took with Offset 0. Like I said it was pure hell to process. I had to try every technique under the Sun to get a decent final image.

Brian

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oopfan
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Re: Another Offset Question

Post by oopfan » Tue Oct 16, 2018 6:15 pm

We posted at the same time!

You need to increase Offset. The peak of the energy distribution is near the value of 16. The left-hand side of the distribution is being clipped.

Brian

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oopfan
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Re: Another Offset Question

Post by oopfan » Tue Oct 16, 2018 6:29 pm

Hi,

You said:
Thanks guys. At gain 0 my minimum value is 16. The histogram isn't very revealing though. It doesn't appear to be clipped.
Did you mean to say "At OFFSET 0 my minimum value is 16" ?

Brian

cuivenion
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Re: Another Offset Question

Post by cuivenion » Tue Oct 16, 2018 6:33 pm

No probs I'll try increasing the offset next time I image. One other question though, whenever I use the brain function it never increases the offset, I'm wondering why this is if increasing the offset (even at gain 0) is seen as good practice.
Last edited by cuivenion on Tue Oct 16, 2018 6:35 pm, edited 1 time in total.

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