Manual Calibration Bias and Darks - with certain hardware (CMOS cameras)

Scott Iver

Hi Adam,

I wonder if you might comment a bit on an issue I ran into previously with using Bias frames and a cooled CMOS camera.  Early on when I first got my camera I was treating it as if it were a CCD camera (used the superbias process etc), but I was running into odd artifacts when processing.  On Cloudy Nights several folks have had threads concerning the cooled CMOS cameras, and it's to the point where the consensus seems to be to *not use bias frames at all for these cameras.  That said I'm currently a bit in "limbo" as to what I should be doing.   Some have recommended "dark flats" and offered workflows for that.  Others have recommended taking something like 300 bias frames to integrate those instead of using the superbias process.

I'm very interested to hear your thoughts on this subject.

Thanks in advance!

BlockHead

Hi Scott,

I don't have much experience with cooled CMOS cameras- only processed a handful of data sets coming from these kinds of cameras. A couple of comments though... First, with a sufficiently large enough set of biases there shouldn't be a real-world difference between a "superbias" and the combination of many individual frames. Next, the idea of "dark flats" sounds odd to me (always has). Flats are generally such short exposures that with a cooled camera- I can't imagine there being a significant difference between a "dark flat" of 5 or 10 seconds and a bias frame. (It is a good test to make some difference images...)  CMOS chips in particular are supposed to be *less* noisy- so it doesn't make sense to me that 300 biases will be "better" than just 50 or so. 

So... I think the answer lies in the "bit of an issue" that you started with.  There can be changing pattern noise or other things that could be a culprit- and to be honest I would say let me look- but I am not certain I would be able to identify the issue. 

Not certain if the above is helpful... but it is all I've got.

-the Blockhead

Scott Iver

Thanks Adam, it struck me as I was driving home today that I be used this camera to try planetary imaging. I took video of Mars and the exposures were around 20-30 milliseconds. Supposedly this camera is “unstable” under 0.2 second exposure.

That all said my friend and I have the same camera and lately neither of us have been using Bias frames.

I have a lot of testing to do

Scott Iver

Interesting enough I just found that by only only using flats with my camera the masters are coming out far better than ever. One of my friends speculates I have an issue with my dark library, maybe some light leaked in when I was taking the darks or something...