Advanced calibration scheme

The standard calibration of CCD frames uses single dark frame and single flat frame. To obtain reasonable results, it is necessary to use dark frame of the same exposure duration as the scientific images. If you observe several objects during a night and those objects have different magnitude, different exposure durations are often used. But in this case you will need to carry out as many sets of dark frames as the number of exposures you have chosen. This task may be quite time consuming work. This disadvantage can be solved by advanced calibration, which involves scalable dark frames. The reduction process in this case is a bit more complicated than standard one, so it is recommended to experienced users only.

Calibration procedure

  1. Acquire a set of bias frames. They are carried out by the similar way as the dark frames, the exposure duration should be minimal (zero in ideal case). I used total number of 90 frames. The bias frames can be reused till some change to the camera hardware was made, so you need to acquire the set only once.
  2. Each night, acquire the set of dark frames using long exposure duration. It is recommended to carry out dark frames on the end of observation, when the camera is enough uniformly cooled. I took ten frames, 120 seconds each.
  3. Open the Muniwin program, open the Tools menu and click on the Preferences item. Select the Calibration tab and check the Use advanced calibration scheme option.
  4. Add all bias frames (see step 1) to the table of input files. Convert them to temporary folder, check and remove all severed frames. Run the Make master-bias frame tool to make single master bias frame.
  5. Clear the table of input files and add all dark frames (see step 2) to it. Convert files to the temporary folder, check and remove all severed frames. Open the Make master-dark frame tool, the dialog window opens. In the dialog, check the Make scalable dark frame option. Close the dialog by the OK button.
  6. In the Select a bias-frame dialog, select file, which contains the master bias frame (see step 4) from the list. Click on the OK button to continue.
  7. In the following dialog select a directory and enter the name of output file to be saved. Press the Save button.
  8. Clear the table of input files again, add the scientific images to it and convert them to the temporary folder. Open the Corrections menu and select the Bias frame item. A new dialog window opens, find and select the bias frame, which was made in step 4. Next, open the Dark frame dialog and select the dark frame made in step 7.
  9. Continue in reduction of source images by standard way.

Remarks

  • The scalable dark frame is automatically detected using the information stored in the file header. For all other frames which are not considered to be scalable ones, the standard calibration is used.

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