Taking bias frames?

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ScottD
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Taking bias frames?

#1

Post by ScottD »

Hello,

The other night I attempted to take bias frames. In SharpCap, the only options for capture are flats and darks. So, I set the exposure all the way down and snapped off a bunch of darks, thinking I could use these as bias. When importing into DSS as bias, they messed up my image. I'm attaching what happens when these subs are used as bias in DSS. When importing as dark flats, they seemed to work, image also attached. Can someone tell me how to do this correctly?

Thank you,

Scott
Attachments
Horsehead Nebula Resized.jpg
Horsehead Nebula Resized.jpg (373.32 KiB) Viewed 1971 times
Horsehead stacked bias.jpg
Horsehead stacked bias.jpg (473.07 KiB) Viewed 1972 times
PetePeterson
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Re: Taking bias frames?

#2

Post by PetePeterson »

Hi Scott,

I'm still working SharpCap at the entry level. Haven't found any bias capability that would split bias out to a separate file that could be used in post processing. But there is a bias frame add on in the flats menu, and I'm assuming that factors bias into the flat so that it can be ignored elsewhere.

Have no idea as to what's going on with the normal procedure of subtracting darks from flats. And am only guessing as to the bias thing.
ScottD
Posts: 43
Joined: Mon May 04, 2020 2:45 pm

Re: Taking bias frames?

#3

Post by ScottD »

Thank you for your reply. It appears to be working when I import these files as dark flats, so maybe that's how I need to work it in the future. Again, I appreciate your feedback.

Scott
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oopfan
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Re: Taking bias frames?

#4

Post by oopfan »

Scott,

Are you Live Stacking or capturing in a traditional manner? Perhaps if I could view one of your bias frames, I could help. Did you capture in FITS file format? If so, then download and install FITS Liberator, open your bias file, and take a screenshot. The histogram will tell me a lot.

Thanks,
Brian
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oopfan
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Re: Taking bias frames?

#5

Post by oopfan »

Scott,

If your bias files look normal, is there a possibility that you added them to your DSS project as something other than "offset/bias files"? Perhaps you accidentally added them as light files?
DSS Bias.jpg
DSS Bias.jpg (63.8 KiB) Viewed 1944 times
Brian
ScottD
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Re: Taking bias frames?

#6

Post by ScottD »

Hi Brian,

Thank you for your willingness to help! Attached is the stacked bias frame from Sharpcap, brought into PixInsight and exported as a jpeg. I was live stacking. When I take darks, flats and bias, I only get a master, no single frames. I also attached a screen capture of the same bias master with histogram from Fitswork, as well as a close up of the histogram. I am positive I'm adding it in DSS correctly. I did it at least five times.

I look forward to your input.

Scott
Attachments
Capture histo2.JPG
Capture histo2.JPG (27.62 KiB) Viewed 1942 times
Capture histo.JPG
Capture histo.JPG (220.59 KiB) Viewed 1942 times
Bias Horsehead snapshot.jpg
Bias Horsehead snapshot.jpg (50.81 KiB) Viewed 1942 times
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oopfan
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Re: Taking bias frames?

#7

Post by oopfan »

Hi Scott,

Sorry to say that this is out of my range of experience. I'm old school: I use SharpCap to capture lights, darks, flats, and bias as individual files, and then I calibrate and process in Astro Pixel Processor (APP) but I've also used DSS.

I replied because you mentioned DSS in your original post. Let me see if I understand: you are taking the master bias, flat, and dark files from SharpCap and importing them into DSS? I've read it somewhere on the forum that SharpCap's masters are not compatible with DSS. The masters are to be used/reused within SharpCap only. I think that might be the answer.

I am curious. Your Horsehead Nebula is excellent! Is there something about it you want to improve?

Brian
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admin
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Re: Taking bias frames?

#8

Post by admin »

Hi,

normally there's no need to use Bias frames if the exposures for your dark frames match those for your light frames. Bias frames are a way to 'bodge' things so that a 60s dark can be used with a 120s light (for example) - at least that's always been my understanding of how it works.

That being said, there's nothing special about them - if you capture them at exactly the same settings as your darks, but with minimum exposure then you should be fine. What you should avoid doing is using the averaged dark frame created by SharpCap's 'Capture Dark' along with manually created bias frames. This is because the averaged dark created by SharpCap is already part processed (by averaging) potentially increasing the bit depth among other changes. A good rule is to do all your processing in a single application, rather than splitting it across 2 or more.

cheers,

Robin
ScottD
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Re: Taking bias frames?

#9

Post by ScottD »

Brian, I'm pretty happy with the image. I was just perplexed by the strange image created when using the bias frame in DSS. Again, thank you so much for your willingness to help!

Robin, my understanding from your response is that as long as I'm capturing flats, darks and dark flats, there is no need for bias. Am I correct with that assumption? Also, I'm not manually capturing any of these calibration frames. I'm simply hitting "Capture Dark" or "Capture Flat" and letting SharpCap do the work and save the file. The way I captured this bias was to leave the settings exactly as they were for the lights and darks, but put the exposure time all the way down to the minimum/fastest exposure. I believe that's how you're supposed to capture a bias frame. I'm very new to all this imaging stuff, so please forgive my many questions.

Regards,

Scott
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oopfan
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Re: Taking bias frames?

#10

Post by oopfan »

Scott,

Like Darks, Bias frames require for you to cover the objective. If you don't then a bright star like Alnitak will still cause a print at 1ms exposure.

Brian
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