CCD Specs

H.J. van Heerden wrote on Apr 24, 2007

Hi all.

I hope someone can help me on this. We are using a Apogee U55 16 bit, back illum CCD on our telescope. The datasheet for the camera spec. give it a LFW of 400 000 electrons and a dynamic range of 93 dB. It does not however give estimates of the gain and read-noise.

If someone can help me on these numbers. I need them for the photometry pipeline that i am building for our observatory. Rough calculations that I did from equations of the internet gives a gain = 7 epadu and a read-noise = 10 e. :?:

Thanx
Pat

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H.J. van Heerden wrote on Apr 24, 2007

Hi all

It is amazing the number of tasks and packages available in IRAF. Stumbled apon one while trying to find a procedure/experimental process on how to measure the gain and readnoise for the camara your using. :)

Task called "findgain" in the noao.obsutil package. For someone learning IRAF it is difficult to know about all these tasks. I expect that if there is time you can go through every package and discover what is available. :)

These is a Q in there. :wink: Is there a site that list all the tasks available and what they do?

Thanx
Pat

P.S. Solved my gain and readnoise problem. :D

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Mike Fitzpatrick wrote on Apr 24, 2007

Hi Pat,

Sorry your original message wasn't replied to directly. We don't have a web page with all available tasks, but you can do the search you want using the REFERENCES task, e.g.

cl> refer gain

would find all tasks with 'gain' in the one-line description of the task. If you really want a list of all available tasks, use

cl> references updquick+       # create a  quick-reference file
cl> references usequick+              # tell the task to use if from now on
cl> page uparm$quick.ref            # browse the file

This lists only what you have installed on your machine. For that's some 2700 help pages so browsing by hand has limited use 8-)

Cheers,
-Mike

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Philip Massey wrote on Apr 24, 2007

You might also gain (no pun intended) by looking through the various IRAF users guides in the documentation. Although written to help people reduce their KPNO and CTIO data, there is a fair amount of general information here as well.

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hanin wrote on Apr 24, 2007

I tried the task several times, using different combinations of images. The results are well different each other.

My questions are:
1) Should I use the same exposure time and filter for the flats?
2) Can I use sky flats instead of dome flats?
3) How do you define the "section" in which the flats are really "flat"? To me, I looked visually at DS9 (color code: b) to define the flattest (i.e. having the same color) region in my flat images.
4) What is the reasonable range for gain and readnoise? My typical findgain result for my CCD is g=0.65 r=1.04 whereas the instrument datasheet provides r=10~15 (I'm trying to determine the gain here).

Thanks..

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H.J. van Heerden wrote on Apr 24, 2007

Hi Hanin

For my experiemnts I used sky-flats, but it is reccomended that u use the flats that u use during observations, i.e. sky or dome flats to get a tru representation of the system performance.

It is best to use flat frames with an close enough mean data count (for 16 bit between 45000 and 55000, depending on saturation level and exposure times), just so long as the data count is high.

readnoise is usually in the region of 10 electrons/pixel, depending on the ccd
the gain is usually in the region of 1-3 electrons/ADU, depending on the ccd.

u might want to determine the dynamic range of the ccd, to get a tru reflection of the system capabilities.

Hope this helps :D

Pat

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Last post on Apr 24, 2007