SECRETS TO SUCCESSFUL CALIBRATION

There are some surprisingly analogue aspects to getting effective dark frames

Poor subtraction in dark frames can cause all sorts of problems. Issues like ‘digital rain’ in the background of images and poor subtraction of the starburst or ampglow can be a challenge to correct. To fix this, dark frames need to be captured for the same length time as the main images: if you’re capturing 60-second exposures, you need 60-second darks. If the camera is cooled, then capturing the dark frames around the same temperature as the light frames will help reduce background noise in the stacked image.

It’s common practice to capture dark frames at a different time to an imaging session. But if the darks are captured on different length cables to the light frames this can cause all sorts of problems in getting a good subtraction. In my setup the cables are 7m long, running to a warm room next to the observatory where the computers are based. They’re USB2, with the imaging camera on its own lead directly to the computer, not through a hub that has the guide camera connected. If the darks are captured on a short cable connected to a local computer the resulting subtraction can contain lots of coloured hot pixels and a digital rain across the image.

The Celestron RASA with a temporary cover to capture flat frames. Right: A dew heater controller mounted underneath the telescope