Measuring CCD Linearity

I have the Steve Howell book - Handbook of CCD Astronomy - the method he gives is to measure a star field... I hate doing flats - though I do have a lightbox. Unfortunately, the light output of the lightbox seems to be too high.

Howell mentions that for a given star, if you double the exposure, you double the number of photons - and that all relates to electrons etc etc. Seems logical to me. Sure, I hear you on the flat-field pice - but if I took two images with the star in pretty much the same position on the CCD, surely the flat-field term disappears? Or is that too simplistic?

My head hurts. I'm going to lie down in a dark room.

Dave,

It is much easier than you probably realize.

ROI refers to the region of interest ... in our case and using CCDSoft, specify a square of 100 x 100 pixels around the center of the integration. The software will then simply download the 100x100 section and nothing more.

Then, using something like AIP4Win, get the flux for the complete "image" of 100x100 pixels and proceed as I describe above.

A comment was made that the light intensity from the lightbox may decline over time and to pursue two such series. I agree with this comment but downloading a 100x100 pixel "image" is super-super-super quick and you can knock off these exposures very fast, thus effectively eliminating the slow degradation of the lightbox.

Alternatively, take sky flats during a clear day using a piece of paper (or even a t-shirt) over the dewshield.

Anthony.

ayiomamitis wrote: Then, using something like AIP4Win, get the flux for the complete "image" of 100x100 pixels and proceed as I describe above.

Anthony - this is where I have the issue. Where do I get the "flux" reading? In MaximDL (I don't yet have AIP4Win but will have soon) you can do aperture photometry which gives a variety of reading - max pixel, average, intensity, etc etc but nothing that says "flux" or anythong like it... The help file in Maxim is also sparse on the detail.

Dave I'd take peak value of what's under the aperture and ignore the annulus.
This will probably be reported as ADUs or DNs or something (I don't have MaxIm).

You need to measure how a typical pixel value changes with integration time.
I suggest using the peak value (PV) because its the peak value that's going to stray from linearity before any other pixel in your image of a star.

Any form of averaging here is going to obscure the reading of where non linearity starts.

Make sure that the area of the CCD in which the star resides is reasonably flat - without flatfielding. You want to measure what the pixel does - not what it is supposed to do.

Doing the above will get you close enough. You'll probably find that its really quite subjective anyway working out where the straight line is deviating enough to say "non-linearity starts here".

I must re-visit my attempt - seem to recall it wasn't great but it was an exercise.

Best of luck.

Mark C.

dmcdona wrote:

ayiomamitis wrote: Then, using something like AIP4Win, get the flux for the complete "image" of 100x100 pixels and proceed as I describe above.

Anthony - this is where I have the issue. Where do I get the "flux" reading? In MaximDL (I don't yet have AIP4Win but will have soon) you can do aperture photometry which gives a variety of reading - max pixel, average, intensity, etc etc but nothing that says "flux" or anythong like it... The help file in Maxim is also sparse on the detail.

Dave,

One of the software packages I use is AIP4Win V2.2 and it is a trivial exercise to get the flux of the complete 100x100 "image". Just go into Measure -> Statistics and grab the value for "Mean" and also the value for "Number of Pixels" .... multiply them and you have the flux.

Alternatively, just use the mean for each of the subs since the number of pixels between all of the subs is precisely the same.

Anthony.

When Anthony refers to "flux" I believe he's just talking about pixel value in ADU's (please correct me if I'm wrong Anthony).

I think flux can have specialised meanings not intended here (like total light measured from a star - less background expressed in some unit or other).

Mark C.