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I am doing an image reduction with data from ESO but I can't find dark frames when I download the zip-folder even though I downloaded the associated callibration files. The image reduction I am doing is on NGC2264 with program ID 106.21VY and even when selecting all the data I still have no dark frames. Am I supposed to find them somewhere else or am I doing something wrong?

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    $\begingroup$ You might want to actually link your sources, so that we may actually see ourselves what you get and access, so that we can answer properly.. Edit your question accordingly $\endgroup$ Nov 12, 2023 at 2:22
  • $\begingroup$ You might not have got that answer on Physics SE! $\endgroup$
    – ProfRob
    Nov 12, 2023 at 18:18

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Calibration frames, including dark frames, for the ESO telescopes in general and the FORS2 instrument in this case, are taken by a general calibration program rather than the specific science program. (There can be exceptions to this if the program is using the instrument in an unusual way or in a mode that is not covered by the general calibration plan but you normally need to specify the need, and include the time budget, for the additional calibration in your proposal).

For FORS2 and other professional optical CCD imagers, the operating temperature is normally so low that dark current is normally negligible; for FORS2 at the normal 150 K/-120 C, the dark current is a few electrons per pixel per hour (see FORS User Manual Table 2.9). This means a long exposure time is needed to get any meaningful dark current and so these calibrations are only done a few times per month.

If you relax the program/target constraints on the archive query form and configure it to search for CATEGORY=CALIB, Type=DARK for FORS2 within plus or minus 10 days around the time of the NGC2264 (~2022-01-20) observations from the 106.21VY program as in this screenshot: Screenshot of the ESO archive showing it configured to search for Dark frames from FORS2 between 2022-01-10 and 2022-01-31

will produce results, a subset of which are shown in this next screenshot of the results:

Screenshot of the ESO archive showing the successful results of the query for dark frames from the previous screenshot This shows that 3x1800s dark exposures are taken by the 60.A-9203(D) proposal (which is the FORS Engineering Team) a few times per month, as indicated by clicking on the magnifying glass under 'More' which gives details such as the OB (Observing Block) Name which is f2ocal-dark-monthly which I'm going to reasonably guess stands for FORS2 Optical Calibration monthly darks....

Depending on what exactly you are doing with the science data, you likely won't need to use the darks unless you are looking for very faint/low signal features in very long exposures (it's often worth looking at some of the bias-subtracted dark frames to see if there is any structure in the dark that is not removed by the bias subtraction although this is uncommon with professional CCDs in the optical). Supporting this lack of need for the darks, the FORS Imaging Cookbook for data reduction with ESOReflex doesn't mention dark frames, only bias and twilight flat fields.

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