What are those smarter imaging techniques I referred to earlier? The method I have evolved over the years involves taking short exposures - and lots of them! This has many advantages, which more than outweigh the disadvantages. The first is in combating light pollution. Long exposures would simply swamp the CCD at my location. Second, short exposures mean no guiding or even the hassle of having to set up an auto guider. Third, should anything go wrong, such as an airplane crossing the field of view or the telescope being bumped, then only one short exposure is lost. Fourth is bleeding, those unsightly vertical streaks from bright stars, which is much reduced by short exposures. The alternative, antiblooming gated (ABG) chips, come with a penalty of reduced quantum efficiency of up to 30%. This is too high a price to pay. The disadvantage is, of course, that every time an image is read out it suffers degradation (noise), and the more images we have the more readout noise we have.
What is a short exposure? That really means an exposure that is long enough to record a meaningful signal. In my case, with fast f/4.7 optics and big 20-micron square pixels it means 15 to 60 seconds for most objects. I have standardized on 30 seconds as a good compromise. The number I take varies depending on the type of object - 33 exposures are enough for star clusters, 66 for galaxies and 99 for anything really faint. These may seem like big numbers, and from a dark site we could get away with a lot fewer but, from a light-polluted site, the only way to get a decent signal-to-noise ratio is to increase the number of exposures.
Diffraction focusing using triple bars in front of the telescope. Image a bright star and adjust the focus until a single slender long spike is visible. Top right - a long way from focus; bottom left - getting close to focus; bottom right - at focus.
Before beginning an imaging session I prepare finder charts using a sky-charting program (SkyMap). Not only is this essential for confirming I am aiming at the right point, but it is very useful for framing. The program allows the positioning, on the finder chart, of the precise CCD field of view - in my case an 18-arcminute square. In that way I might be able to position the field of view to include other objects in the vicinity, such as faint background galaxies.
At the telescope, the first job after booting everything up is to align the telescope on a bright star to initialize the GOTO. This bright star is also used for focusing. I have always used diffraction focusing and found it simple and reliable. To do this I place three parallel bars across the front of the tube - the central bar I align exactly with one of the diagonal mirror supports. Then, with no binning (i.e., 1 x 1) and the visualization set to "MAX," I adjust the electric focuser until a single bright spike is visible (see Figure 9.5). It's as simple as that. The method is absolute. When you see a single slender long spike you know it's at focus. There's no need to try to see if it's better by pushing the focus button once more. The tricky bit is to remember to take the bars off!
The software I use for controlling the camera is MAXIM/CCD (see Figure 9.6). Its operation is very logical. It has a "sequence" mode whereby a generic name for the file is entered and the instruction is given for automatically taking a number of exposures one after the other. As mentioned, I set this to 33. I can then go indoors, and the software will take the exposures and save them, numbering each incrementally. This number of exposures is a practical limit for my setup - after this many the dome needs turning and, if there is any drift, I recenter the object before setting it off on another 33.
I take calibration frames the following evening, providing it is not raining! This does not waste imaging time, which is extremely valuable in Lancashire where clear nights are few and far between. I take 33 of each, i.e., bias, dark and flat. This is, of course, only possible because I can leave the camera on the telescope in the observatory untouched in the exact focus and orientation I used the night before. The bias (zero exposure) and dark (30-second exposure), both with the telescope capped, are straightforward. The flat I take of the evening sky - either clear or
cloudy - and time it to when it is just dark enough for a 1- to 2-second exposure to register about 70% maximum brightness. If clear, a few stars are usually recorded, but by taking a median these will be removed.
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