[Aavso-photometry] CCD Linearity testing

[Tom Krajci]

I'm doing more detailed/denser evaluations of CCD linearity.  It's 
easier these days with scripts and good software.

And, working with the folks that make Photometrica software...we may use 
this info to linearize CCD output and improve photometry data quality.  
Maybe this can be applied to ABG CCD's?...at least up to a point?

Below is an email exchange I'm having with Richard Berry.

If anyone wants the spreadsheet (about 250Kb size) of my first test-run, 
just ask.  (Same for the Orchestrate script I used to take the light 
frames.)

I'm open to suggestions on how to do this analysis better.

Thanks in advance,
Tom Krajci
Cloudcroft, NM. 9,440FT

===========================
Tom Krajci wrote:
> Attached is my first attempt at dense testing of CCD linearity.  
> You'll see the sequence of exposure times in the spreadsheet...from 
> 0.5 to 80 seconds...covering about 55 different exposure times in a 
> geometric progression...stepping up/down the exposure times in an 
> alternating pattern similar to that described in the AIP4WIN 
> Handbook.  I took almost 1000 light frames through the 
> night...repeating the exposure sequence until about 530AM.
>
> AIP4WIN's Series Analysis Tool examined a large ROI on the frames 
> (about 80% of the total pixel count).
>
> This CCD is a new ST-8, bin 1x1, -10C.  Using AIP4WIN's Characterize 
> Camera routine (and measuring multiple sets of bias/flat/dark frames), 
> I get read noise of 18.9e-, and gain of 2.72e-/ADU.
>
> The spreadsheet plot of exposure time versus ADU rate looks pretty 
> consistent   The light source seems to remain stable to a little less 
> than the 1% level, and repeated measurements may help improve 
> confidence below the 1/2% level.
>
> If you want a simple number, it looks like this CCD is linear up to 
> about 44,000ADU...typical for this camera/bin mode.  But if you want 
> to try your hand at fancy curve fitting...I think this CCD can be used 
> reliably up to about 58,000ADU.  (I've glanced at a few of the longest 
> exposure time light frames that had average values near 
> 61,000ADU...you can see that some pixels are saturating, the frame 
> looks streaked/mottled...I would not trust the statistics for these 
> images, they are probably very deviant.)
>
> Note that I *did* use auto dark matching in this test run.  However, 
> my 120-second dark frames have the extreme hottest pixel at about 
> 26,000ADU...this is well within the linear limit of this CCD.  I hope 
> this means that dark scale matching worked well for all but the 
> longest exposure light frames.
>
> I have not tested/evaluated residual bulk image effects.  If this ST-8 
> is like the ST-7 and ST-10 I've been using recently...it has this 
> problem...but then, what CCD does not have this problem?  (How much 
> does residual bulk image effect spoil my measurements?  Is it a factor 
> only at the lowest light levels?)
>
> How can I improve upon what I've done?  It's monsoon...there are many 
> nights of CCD testing remaining.
>
> Tom Krajci
> Cloudcroft, NM. 9,440FT
> ==============================
> Richard Berry wrote:
>> Hi Tom--
>>
>> What you're suggesting seems reasonable. Several comments:
>>
>> 1.) Test the whole procedure on a small number of images at each 
>> exposure, perhaps 10, and check that it all works as planned.
>> 2.) Don't do automatic dark matching, use matching dark exposure 
>> times. Dark matching makes assumptions about the linearity of hot 
>> pixels with integration time, and that's not necessarily true when 
>> the ratio of exposure times becomes large, like 120-second darks with 
>> 0.5-second flats. Use 0.5-seconds with 0.5-seconds, and so on for all 
>> exposure times.
>> 3). Instead of a shooting 100 each of time series like 0.5, 1, 2, 4, 
>> 7, 10, 14, 20, 28, etc., space the exposures closer together--steps 
>> of 1.2 times--and shoot maybe 32 at each time. Same total number of 
>> exposures, but it will fill the linearity curve with more points.
>> 4). Check for residual bulk image effects. With my KAF3200, when I 
>> shoot flats and flat-darks, the darks have higher mean levels when I 
>> interleave the flat-darks with flats than when I shoot them 
>> separately, and the flat-darks sometimes display patterns especially 
>> following the I-filter flat exposure.
>> 5.) Test the constant light source by including exposures of the same 
>> duration in every data set. LED output depends on temperature, and 
>> LEDs lose output over long run times. It's very hard to make a light 
>> source constant. If you can afford to, the standard method now is to 
>> use a pulsed LED, and use the number of pulses as the intensity of 
>> the light falling on the CCD, so 50 pulses is 50 times more light 
>> than 1 pulse. You can buy standard sources like this for $$$.
>> 6.) Use a large ROI in the middle of the frame. Exposure will be more 
>> uniform, and the edges of CCDs are usually not as free form defects, 
>> etc. as the middle.
>>
>> --Richard
>>
>>
>>
>> Tom Krajci wrote:
>>> Hi Richard,
>>>
>>> It's monsoon - I am going to take my CCD linearity testing to the 
>>> next level.
>>>
>>> Here's what I'd like to do:
>>> - set the CCD to one temperature
>>> - set a constant light source to give about 8,000ADU in a ten-second 
>>> exposure
>>> - take a bunch of flat-light frames of varying exposure time...from 
>>> 1/2 to 80 - 100 seconds...for most of the night...hundreds of them, 
>>> running a camera script
>>> - take a bunch of bias frames to make a master bias
>>> - take a bunch of 120-second darks to make a master dark
>>> - make a 'dummy' master flat frame...same x/y size as the other 
>>> frames...just make all the pixels the same value, such as 1.000000
>>>
>>> Set up advanced calibration in AIP4WIN...master bias, dark, and 
>>> dummy flat.  (My hope is to bias subtract and dark-match-subtract 
>>> all the various flat exposure times...but not apply any flat frame 
>>> correction, hence the dummy flat.)
>>>
>>> Multi-Image | Auto-Process (calibrate only) all the flat light 
>>> frames...save the calibrated versions of them in a new directory
>>>
>>> Measure | Series Analysis Tool...for all these calibrated images 
>>> (entire image, or a large ROI in the middle of the frame?)
>>>
>>> Plot the results in a spreadsheet...determining ADU rate from 
>>> average pixel value and exposure time.
>>>
>>> Am I making any mistakes in my protocol?  Are there any ways I can 
>>> streamline this process further?
>>>
>>> Thanks in advance,
>>> Tom
>>>