5.3. Make a master bias image

Change the settings in the “DP” window to match those below.

Screenshot of settings for making master bias frame.

  1. Check only the following:
    • Bias Subtraction -> Build
    • Bias Subtraction -> med
  2. Find and enter file pattern of bias, e.g. “bias006” or “eyumabias” or “dark_bias”…can be weird sometimes, so watch out.
    1. You can use ? to replace one character or * to replace multiple characters, e.g. bias???.fit could replace a series which looks like bias242.fit, or *bias* could replace a series which looks like dark_bias_7.fit
  3. Make sure number in Totals next to Bias Subtraction top row is equal to number of bias you have
  4. Choose a name to save master as, e.g. mbias or master_bias
  5. Click start and watch for errors


  1. How many bias frames should be included by the this automated tool? Note: there is no fancy way to figure this out. Look at the file names.
  2. Does the master bias image that you produced look like a master bias should? By this point you should have some rough idea of what a master bias looks like and you should have notes on what the typical pixel value is in a bias frame. Check both of those things.
  3. Compare your master bias image to the appropriate Reference master images to make sure they are really the same. Do that by loading both your master bias and ours and using AstroImageJ to take the difference or the ratio (what would you expect for either?).