Bias Changing over Last 9 months...

[PRejto]

What do you think? I took new bias frames last night and they look rather different compared to 8-9 months ago. This is my Moravian G2-8300.

Thanks!

Peter

[RickS]

Peter,

Screen stretches can be very deceiving with linear data. I'd suggest that you compare them statistically and ideally compare an old master bias with a new one.

I get quite a bit of bias drift on my cameras, even over a short period of time, so I use overscan calibration which accounts for the variation. My KAF-16803 also has a habit of developing new dodgy columns over time. Updating the Defect List when a new one pops up sorts that out.

Cheers,
Rick.

[PRejto]

Thanks Rick,

Can you explain what you mean by "statistical comparison?"

Thanks,

Peter

[Peter.M]

A two sample t test should sort you out.

[RickS]

I'd start with some simple stuff.

Compare mean and standard deviation for the whole frame and a representative subframe or two (don't include any obvious regions with amp glow, bright/dark columns or lots of hot pixels). Any significant differences between the old and new bias frames would flag a change in behaviour.

If you have at least two "before" and two "after" bias frames you can compare read noise. Subtract one before bias from another before bias and you get a frame that contains only the read noise from the two bias frames. Take a representative subframe and calculate the standard deviation. Divide this by SQRT(2) and the result is the read noise in ADU. Multiply by the camera gain to get the read noise in e-. Repeat for the new bias frames. If all was well you'd expect to get similar results.

Cheers,
Rick.

[PRejto]

Thanks Guys!! I see what you are after now.

Peter

[Somnium]

how do you read out the data in numbers rather than represented in light? can you use a tool to extract it to an excel spreadsheet or is there something that i am missing?

[Atmos]

You can do it easily in either PixInsight or Maxim DL (I personally prefer Maxim). In PI it is under Process > Image > Statistics. This brings up a window that will give you the average, STD and other stuff over the entire frame. The raw numbers.

If you subtract one bias frame from another the resulting image will have just the read noise (it is more accurate if you use a Master Bias). You do this with multiple bias frames just to calculate the average residual.

Craig Stark (wrote Nebulosity and PHD) has a lot of information on various aspects of astronomy.
http://www.stark-labs.com/craig/resources/Articles-&-Reviews/CCD_SNR3.pdf

This goes quite deeply into doing the calculations but he does explain it pretty good.