Calibrating Flat frames and bias frames?

[tomb18]

Hi,
I use CCDSoft with an STL11000.  When I acquire flat frames and bias frames, is it better to allow CCDSoft to automatically  subtract dark frames or should I acquire these separately?

Thanks

[Geoff]

I am not a user of CCDSoft, but I am reasonably sure that it aquires one dark at the beginning of the run and then subtracts the SAME dark from each image, so essentially you are using just one dark frame. You need to take many and average them to get a good measure of the dark current. Remember that each individual dark frame contains noise as well as the dark current. You need to knock down this noise by using multiple dark frames.

[Niall Saunders]

Hi Tom,

There's no straight answer here, as far as I can see.

A lot would have to depend on the exposure times you are using to acquire your flats. They longer they are, the stronger the argument would be for then calibrating the raw Flats with a MasterDark.

The only way you would really know is to take dark frames at the same exposure time as your Flats, and then analyse your Darks. If there is 'area-specific' or 'zone-specific' noise present at, rleatively-speaking = short exposures, then you may need to use a Master Dark to eliminate the noise that must therefore be present in your Flats as well.

And the same argument applies to Bias frames as well - except that you have nothing like a 'dark' frame to calibrate them with, and nor do you need to trouble yourself with flats. Neither of these matter, because Bias frames are, by their very nature, 'zero-length' exposure images.

However, grab a healthy collection of them and perform a simple 'stack' (to create a Master Bias) - and then have a good look at what yyou are getting.

How do they compare with your Master Dark (a stack of a series of un-calibrated Darks is what you want for this comparison). If there is little difference between the two (on my QHY10 OSC camera, I can't tell the difference - even at 1800-second exposure times for the Darks), then you really have to ask yourself the question, "Do I really need Bias Frames at all, in fact - do I even need Darks?". After all, if you can't tell the difference, then you might as well just use Bias Frames in place of Darks - they tale a lot less time to acquire!

Would you let CCDSoft do partial frame calibration automatically? Or would you prefer to do this in PI? Well, that depends on whether you are prepared to fully get to grips with image calibration, and the extremely powerful tools that are available to you, under your full control, in PI.

If you are not happy with 'getting your hands dirty' doing all of the calibration stages yourself, then you might as well lett CCDSoft do things for you - and its not just CCDSoft in the frame here - even the likes of Nebulosity will doo all of this for you, leaving you with just the post-processing stage to complete in PI.

Will these other packages do things as well, worse, or better than PI? You have to understand the steps involved, and how your specific equipment performs, before that question can be answered. Just remember that PI has had some not insignificant effort put into its development, to ensure that what it does do, it does extremely well.

I know which package I rely on for image calibration (but I also run my images through Nebulosity - as I acquirethem - to give me a first glimpse as to what the final data might yield under all the statstical analysis of the PI engine.

[sreilly]

This is something I revisit from time to time. Personally I don't use CCDSoft or TheSky Camera Add-on but I've taken flats and calibrated them with a master bias frame only and can visually see no difference when also using a scaled master dark. Now keep in mind that my flats are generally very short exposures, say 1-2 seconds for anything but narrowband filters where they may go for as much as 10-15 seconds. I have been told that for the STL-11002M cameras it's important to take the bias frames at the same temperature as the lights and I've always followed that advice as I usually have seasonally darks/bias library anyway. I group my exposures in varying temps with steps of 5 degrees. Spring and Summer is almost always -10, -15 with fall and winter -20,-25, -30. I've just redone a series of flats and will process them using dark and bias masters and compare with bias master only. As for DSLRs, I have no experience at all.

Steve

[tomb18]

Thanks for all the comments.
Looks like I will be taking separate darks for all me flats.
Tom