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Old 13-08-2008, 04:52 PM
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strongmanmike (Michael)
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Bias frames..?

I have a question regarding Bias frames

Other than to remove any sytematic interference present in a frame what else are they used for exactly and how much difference do they really make to a final image?
Noise??? did I say noise? Aaaaarg ...oh no, it's ok, I said interference, phew.....

MIke
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Old 13-08-2008, 05:20 PM
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Ive often wondered how important they are, but who cares !. They are so easy to take and use, might as well, why not?. I use the same darks and bias for over a year. Take em once and forget it.
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Old 13-08-2008, 05:24 PM
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Bias frames eliminate negative values for pixels. This is done by raising the floor so to speak. It is really a computational problem.

bert
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Old 13-08-2008, 05:32 PM
jase (Jason)
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Yes, as the others have said...

Actually, a more important feature/function is that they allow you to scale your darks...which is pretty cool.
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Old 13-08-2008, 06:06 PM
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Quote:
Originally Posted by strongmanmike View Post
I have a question regarding Bias frames

Other than to remove any sytematic interference present in a frame what else are they used for exactly and how much difference do they really make to a final image?
Noise??? did I say noise? Aaaaarg ...oh no, it's ok, I said interference, phew.....

MIke
Not sure about FLI cam's Mike, but the pedestal set by...oh say..SBIG... takes the bias pattern into account, hence it is subtracted along with the dark signal during dark frame calibration.

I have used bias calibration with Apogee cameras and found absolutely no differences in bias-dark-flat vs dark-flat calibrated images.
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Old 13-08-2008, 06:18 PM
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Have a read here Mike, tells ya what you need to know.

http://www.starlightccd.com/faq/platt/platt5.htm

Theo.
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Old 13-08-2008, 06:19 PM
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I'm in with Jase. Unless you are scaling your darks, my understanding is you don't need them as they are built into the darks anyway. If you subtracted bias from the lights it assume you would then have to subtract them from the darks and then from the flats as well to keep everything on an even keel so to speak.

The only other time I could think of would be when you were testing the transfer, linearity and read noise of your camera.
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Old 13-08-2008, 07:56 PM
jase (Jason)
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Peter,
I'm not sure where you are going with the pedestal. Can you please elaborate? The pedestal is removed during either a dark and/or bias subtraction, though it can also be removed manually using pixel math. Most processing software use a pedestal value of 100 counts as many cameras write this value to the FITS header. This is to ensure the lowest ADU is still positive (zero is still considered positive). I'm not sure how relevant this is to bias subtraction, thus hoping you can shed light on your statement. Thanks.
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Old 15-08-2008, 08:30 AM
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Quote:
Originally Posted by avandonk View Post
Bias frames eliminate negative values for pixels. This is done by raising the floor so to speak. It is really a computational problem.

bert
I would have put this slightly differently. A bias is added to a frame by the camera to prevent -ve pixels. A bias frame subtraction removes this. It is needed to scale darks but little else.
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Old 15-08-2008, 10:21 AM
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Peter,
I'm not sure where you are going with the pedestal. Can you please elaborate?
Jase....this is my understanding of Bias frames...which may well be wrong...but it works for me.

Short explanation:

Separate Bias exposures: you simply don’t need them

Long explanation:

A bias frame is the same as a dark frame of zero seconds exposure. But taking a zero second exposure can result in negative pixel values. This is why a small and constant positive value, ie the pedestal is added to all exposures by the camera’s readout software.
It ensures the bias values are all positive, and while it could be a very large number, only needs to be large enough to make the negative values zero or positive. Hence my statement the bias is “contained within the pedestal”.

In post processing, a non zero second dark contains the zero second exposure (ie bias) plus the normal dark (ie long time) exposure. Thus the one step “dark frame subtraction” takes care of both.

Cryogenically cooled cameras (ie professional CCD’s) have effectively no dark signal, but the underlying bias structure will still be present, so typically only that is subtracted out in these instruments (ie no need to wast time with a non zero dark frame)
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Old 15-08-2008, 03:25 PM
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I have noticed a difference sometimes using a pedestal level set to 100 on some images.

It seems to take out a bit more noise.

My understanding is that bias frames are subtracted from the flats so you don't need to use them again and that is what I have been doing.

But I agree, I don't think bias frames are super important.

Greg.
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Old 16-08-2008, 11:24 PM
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I note I get completely black backgrounds, some of the histogram is cut off if I dont use bias frames, so I always use them now, easy enough to do with a DSLR, just take a dark frame with a 4000th sec shutter speed
With astro CCds theres generally a function to take a bias frame in the control software.
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