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Old 24-08-2016, 09:23 AM
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Crushellon (Tim)
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Bias/dark frames with CCD

Hi guys,

So I've got all of my new gear and am preparing to image with it for the first time. I have no experience with CCD cameras so I am just wondering if bias and dark frames are required? I will be imaging with an rc8, stf8300m and Lrgb filters using apt for capture.
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Old 24-08-2016, 09:34 AM
glend (Glen)
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In a word, yes, they are required for most cameras. I don't know much about your particular camera, but you probably have some form of cooling and temperature control. Whatever temperature you are shooting your light subs at should also be used to shoot bias and dark frames. I build a library of bias and dark frames for the typical narrowband and LRGB sub lengths that I shoot with my various scopes (and if you have gain/offset type controls on the camera you should shoot bias and darks using the same settings as the lights). Once you have built up a library of bias and darks you can build Master Bias and Darks using a stacker like DSS or other processing tools, this means you do not have to restack individual bias and dark frames all the time if your settings remain the same.
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Old 24-08-2016, 11:18 AM
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Yes you do. There is no gain or offset on the SBIG STF8300.

Chose a temperature you should be able to achieve on a regular basis. I chose -20C for my ST8300.

Cool the camera to -20. Then set the software to bias ( I use CCDsoft and sometimes SkyX and you set what type of image you want it to take).

I take about 18 for a bias. Some take more but I am not sure it really makes any difference to do more. I used to use 6.

Take say 16 at 1x1 binning and 16 at 2x2 binning.

Settle on a likely exposure time you will be taking. That would depend on how good your tracking mount is (worse mounts need shorter exposures) and if you plan to do narrowband or LRGB only. Narrowband generally need longer exposures to reduce the noise.

I use 10 minutes for both but my scope is large and fast. I also use 5 minutes.

So take say at least 6 darks 1x1 at both 5 and 10 minutes -20C.

Now you need to make these into a master bias 1x1 -20C, bias 2x2 -20C, dark 10mins 1x1 and 2x2 -20C and 5 minutes 1x1 and 2x2 -20C.

Save the masters with the file names as above for easy finding later when you need them.

I label mine Bias or Darks and then what type for example
Bias 1x1 -20C
Dark 10mins 1x1 -20C.

So later when you are processing all your darks will be together and biases together so they are easy to find.

You will also need to take flats. That's another discussion.

Having done the above when you now image you use either 5 or 10minute subexposures and set at -20C so everything matches up.

Darks need to be redone every now and then, say 6 months or when they start to not work as well. CCDs deteriorate over time. The 8300 may develop new vertical lines you want to see removed when processing, new hot pixels show up.

Greg.
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Old 24-08-2016, 06:46 PM
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If your dark frames are the same duration as your light frames, my understanding is you do not need to take bias frames, as when you subtract the dark from your light frame, of the same duration, the bias is subtracted with the dark (its in the dark frame).

However, if you chose to have your dark frames longer than your light frames, you will need to do adaptive dark subtraction, in which case you will need to subtract the bias first, then your dark frames are scaled.

Josh

Last edited by Joshua Bunn; 24-08-2016 at 08:22 PM.
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Old 24-08-2016, 07:56 PM
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Quote:
Originally Posted by Joshua Bunn View Post
If your rark frames are the same duration as your light frames, my understanding is you do not need to take bias frames, as when you subtract the dark from your light frame, of the same duration, the bias is subtracted with the dark (its in the dark frame).

However, if you chose to have your dark frames longer than your light frames, you will need to do adaptive dark subtraction, in which case you will need to subtract the bias first, then your dark frames are scaled.

Josh
I do find though you get more reliable flat fielding if you do either a bias subtract on the master flat at the time of calibration or a flat dark subtract. I do both. On some systems or in light polluted imaging conditions flat fielding is a bit of an art and it can make a huge difference.

Greg.
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Old 24-08-2016, 08:21 PM
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Quote:
Originally Posted by gregbradley View Post
I do find though you get more reliable flat fielding if you do either a bias subtract on the master flat at the time of calibration or a flat dark subtract. I do both. On some systems or in light polluted imaging conditions flat fielding is a bit of an art and it can make a huge difference.

Greg.
Yes, I agree Greg. I was only referring to light frames, I forgot to mention Flat frames
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Old 25-08-2016, 06:23 AM
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Great replies guys, thanks.
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Old 25-08-2016, 06:47 AM
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I usually check the box for bias with darks as you say for those times when the light exposure length is different to the dark length. CCDStack will scale your darks to match the exposure. It seems to work well but I prefer an exact match.

So I prefer to try to standardise exposure lengths to avoid that situation where possible.

Greg.
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Old 25-08-2016, 11:33 AM
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A bias is a zero second dark, so by definition, it is already contained in every dark frame. I simply don't bother with bias frames.

There is also some non-linearity in SBIG dark frames, so best practice it to take a library of darks for a series of exposure times and temperatures, that will match your light frames.

Darks also change over time, so it is also good practice to grab a new set of dark frames every few months.

As for flats...I still find median combined sky flats to be the most accurate, but have recently installed a A1 sized EL sheet in my dome which I'm still coming to grips with...and will likely make a separate post on later.
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