Deep space astrophotography is not 'point and click'!
When you are photographing distant galaxies and nebulae, you are
trying to capture images of objects that are not only incredibly distant, but
also, in general, very faint.
Compared to a normal daylight photograph, even after many hours of capturing this distant light, the image will be very underexposed.
Also, the earth is rotating, giving the impression that the sun, moon, stars and planets are moving across the sky.
In a normal photograph, the exposure will only be for a small
fraction of a second. Because of the faintness of most astronomical targets,
the exposures can range from 2 or 3 minutes, up to sometimes half an hour or
more (2 minutes is considered a short exposure)
This means that the telescope will have to track the object with incredible accuracy as it moves across the sky, and is why some astrophotographers spend a lot of money on a very good mount for their telescope and camera.
In the days before computers, astronomers would take images using photographic plates, and expose for sometimes as much as 5 or 6 hours on a single exposure, and sit in the freezing cold manually correcting any errors in the telescope tracking .the superb results they obtained are a testament to the sheer determination and dedication of these unsung heroes of the night sky.
So much can go wrong in a very long exposure that it was common to throw away many of the images .even a small cloud passing in front of the telescope for a couple of seconds can ruin an exposure.
Nowadays things are a lot easier than that, but still tricky!.
The telescope tracking is computer controlled using a second telescope to lock onto a star, and we don't generally go for really long exposures anymore.
Instead, the final image is built up from many sub exposures (subs) which are then combined in the computer to get the equivalent of a single very long exposure.
The length of the exposure is limited by the accuracy of the mount, sky conditions etc etc .I like to go for 10 minute exposures if possible, but if, for example, it's a very windy night, I will shoot shorter ones as there is less chance of wind vibration ruining all of them than if I shot longer ones.
The more sub exposures that you get, the stronger the image becomes, and the more any background noise caused by camera electronics, cosmic rays, and many other sources is overcome.
The balance between the unwanted background noise and the actual object you are imaging is called the signal to noise ratio (S/N ratio) and a good S/N ratio is what you are after lots of signal (the object you want to image) and very little noise.
The cameras I use are not like conventional cameras, with a shutter,
a viewfinder and a lens, instead, they contain a sensitive chip and have no
lens, the telescope itself being the lens. They also have internal cooling devices,
as the colder the chip is, the less thermal noise you get, so mine are cooled
to 20 degrees celcius below the surrounding temperature.
The camera is also monochrome, so only takes black and white images. This is because a colour camera is less sensitive, so I have to build up a colour image by taking seperate black and white images through coloured filters, generally red, green and blue, plus a monoochrome image that has the most time spent on it in order to bring out really faint detail, known as a 'luminance' image, which is combined with the red, green and blue using Adobe Photoshop to produce a detailed colour image.
On this site, the imaging details are given with the images, and these will be something like
L. 25 x 10 minutes
R. 10 x 5 minutes
G. 15 x 5 minutes
B. 15 x 6 minutes.
This means that I have taken 25 x 10 minute exposures that are
just black and white (L= Luminance), 10 x 5 minute exposures through a red filter
The image is constructed in the same way as a TV picture, where most of the detail is in the black and white image, and the images taken through the filters are used for colour, but not fine detail.
Certain deep space objects where certain elements are being ionised
glow at frequencies of light which are specific to these elements, and to capture
this light filters called 'narrowband' filters are used. These only allow a
very narrow bandwidth of light through, cutting out everything else, and are
used to capture certain types of object, and have the added advantage that they
are not generally affected by light pollution or moonlight, which normally swamps
conventional (broadband) images.
The most common narrowband filters are, Hydrogen Alpha (Ha), Oxygen 3 (OIII), Sulphur 2 (SII) and Hydrogen Beta (Hb).These relate to the certain gases being ionised, and are a very specific colour. SII is a very deep red, Ha is slightly less deep, OIII is a bluish green, and Hb is a greenish blue.
Many of the Hubble telescope images use SII, Ha & OIII for red, green and blue, and are actually false colour. The reason for this is that for scientific purposes, these images show the distribution of these particular elements in the object being imaged (and they look nice too!)
Where these filters are used in images on this site, this will
be seen in the imaging details.