Astrophotography 101

Astrophotography through a telescope can be done with two types of camera, either a general purpose DSLR
(such as Canon or Nikon models) or with a dedicated CCD camera.

There are two principle differences between these:

  1. the DSLR uses a sensor which captures colours in one shot, while most dedicated CCDs are monochrome, so that separate filters need to be placed into the light path, one at a time.

  2. the dedicated CCD is actively cooled to a constant low temperature of -20°C or lower to reduce noise

 

Apart from that the overall scheme is essentially the same.

Astronomical objects (with the exception of the Sun, Moon and planets) are faint and require very long exposures, commonly running into many hours. 

Naturally the telescope needs to track the object being imaged, as all celestial objects move across the sky during the night, and this requires a sophisticated control system in the telescope’s mount.

Before the sub-exposures can be combined they need to be calibrated, which compensates for the fixed patterns
of so-called hot pixels in the image as well as for dust on the filters and elsewhere and other artefacts of the telescope optics. This is done using specially prepared calibration images: dark frames of the same duration as the sub-exposures are subtracted to remove the hot pixels, and evenly-illuminated flat frames are used to compensate for dust etc.

A final step before images can be combined is to align them all, which compensates for errors in tracking or
other issues.

At this point all of the calibrated and aligned images for each filter are combined, using statistical techniques to eliminate outlying pixels. Finally a set of individual master images corresponding to each filter is produced.

These monochrome masters may then be combined into a colour image. 

Final processing includes non-linear transformation as well as noise reduction and other enhancements.

For many practical reasons the long exposure is achieved by dividing the time into a set of sub-exposures which can be accumulated into the equivalent of a single long exposure. Each sub-exposure can be anything from 5 to 30 minutes long. By dividing the time in this way the chance of losing the whole image due to an aircraft flying through the frame or some other issue is reduced. In the case of the cooled CCD camera separate sets of sub-exposures need to be taken for each colour filter.

The colour filters are usually either a natural colour set (red, green blue) or narrow-band filters tuned to the light emitted by certain gases (hydrogen, oxygen and sulphur) which are commonly found in astronomical objects. Usually a “luminance” filter is used as well to provide more detail to the final image.

Ideally some 10 or more sub-exposures with each filter will be taken. Acceptable results can be produced from less, but the more data are used the less noise will be visible in the final image. So about 40 or more sub-exposures will be taken, taking many hours, usually spread over different nights.

QSI filter wheel with 3 narrow-band filters
QSI 582wsg camera on the Officina Stellare UCRC 360 scope
ST2000XM camera system, includes 110mm Hasselblad lens, Falco guidescope and SBIG RGH