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Entry  Mon Sep 11 23:31:50 2017, johannes, Update, Cameras, post-vent camera capture comparison 13x
    Reply  Tue Sep 26 22:11:08 2017, johannes, Update, Cameras, post-vent camera capture comparison lens_distance.pdfImage__2017-09-26__18-54-29.jpgIMG_20170926_190157222_HDR.jpgIMG_20170926_185649325.jpg
       Reply  Mon Oct 2 12:44:45 2017, johannes, Update, Cameras, Basler 120gm calibration calib_20170930_152.pdfcalib_20170930_153.pdf
          Reply  Mon Oct 2 23:16:05 2017, gautam, HowTo, Cameras, CCD calibration 
             Reply  Tue Oct 3 01:58:32 2017, johannes, HowTo, Cameras, CCD calibration gige_calibration.pdf
                Reply  Thu Oct 12 01:03:49 2017, johannes, HowTo, Cameras, ETMX GigE side view IMG_20171011_164549698.jpgImage__2017-10-11__16-52-01.pngGigE_lens_position_helper.py.zip
                   Reply  Thu Oct 12 07:56:33 2017, Steve, HowTo, Cameras, ETMX GigE side view at 50 deg of IR scattering Image__2017-10-11__15-29-52_15k400g.pngImage__2017-10-12__15-50-18wipedRefocud2.pngcamCan16cm.jpg
                      Reply  Wed Oct 18 11:37:58 2017, johannes, HowTo, Cameras, ETMX GigE side view at 50 deg 
                Reply  Wed Oct 18 15:26:58 2017, johannes, HowTo, Cameras, Revision: CCD calibration calib_20170930_152.pdfcalib_20170930_153.pdf
Message ID: 13352     Entry time: Mon Oct 2 23:16:05 2017     In reply to: 13348     Reply to this: 13354
Author: gautam 
Type: HowTo 
Category: Cameras 
Subject: CCD calibration 

Going through some astronomy CCD calibration resources ([1]-[3]), I gather that there are in general 3 distinct types of correction that are applied:

  1. Dark frames --- this would be what we get with a "zero duration" capture, some documents further subdivide this into various categories like thermal noise in the CCD / readout electronics, poissonian offsets on individual pixels etc.
  2. Bias frames --- this effect is attributed to the charge applied to the CCD array prior to the readout.
  3. Flat-field calibration --- this effect accounts for the non-uniform responsivity of individual pixels on the CCDs. 

The flat-field calibration seems to be the most complicated - the idea is to use a source of known radiance, and capture an image of this known radiance with the CCD. Then assuming we know the source radiance well enough, we can use some math to back out what the actual response function of individual pixels are. Then, for an actual image, we would divide by this response-map to get the actual image. There are a number of assumptions that go into this, such as: 

  • We know the source radiance perfectly (I guess we are assuming that the white paper is a Lambertian scatterer so we know its BRDF, and hence the radiance, perfectly, although the work that Jigyas and Amani did this summer suggest that white paper isn't really a Lambertian scatterer). 
  • There is only one wavelength incident on the CCD.
  • We can neglect the effects of dust on the telescope/CCD array itself, which would obviously modify the responsivity of the CCD, and is presumably not stationary. Best we can do is try and keep the setup as clean as possible during installation.

I am not sure what error is incurred by ignoring 2 and 3 in the list at the beginning of this elog, perhaps this won't affect our ability to estimate the scattered power from the test-masses to within a factor of 2. But it may be worth it to do these additional calibration steps. 

I also wonder what the uncertainty in the 1.5V/A number for the photodiode is (i.e. how much do we trust the Ophir power meter at low power levels?). The datasheet for the PDA100A says the transimpedance gain at 60dB gain is 1.5 MV/A (into high impedance load), and the Si responsivity at 1064nm is ~0.25A/W, so naively I would expect 0.375 V/uW which is ~factor of 4 lower. Is there a reason to trust one method over the other?  

Also, are the calibration factor units correct? Jigyasa reported something like 0.5nW s / ct in her report.

Camera IP Calibration Factor CF 8.58 W*s 7.83 W*s

The incident power can be calculated as Pin =CF*Total(Counts-DarkCounts)/ExposureTime.


[1] http://www.astrophoto.net/calibration.php

[2] https://www.eso.org/~ohainaut/ccd/

[3] http://www.astro.ufl.edu/~lee/ast325/handouts/ccd.pdf

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