| article: prime focus astro photography |
Prime focus astro photography means that you use your telescope's optics directly with the camera to image.
It's like the telescope is used as a *huge* telephoto lens.
Typical focal lengths are between 600mm to 2000mm (23inches to 78inches).
The bigger the focal length, the higher the resolution of the image.
Mechanical issues
But at the same time all the tracking errors of your mount are magnified too and usually become visible in the picture.
That's why prime focus is much more challenging than the piggy-back method.
Typical mount tracking errors:
- Cheap mount bought together with a cheap telescope: 90 arc sec
- Better mount bought together with a better telescope (i.e. LX90 from Meade): 30 arc sec
- Very good mount bought separately (i.e. Losmandy or Fornax): 5 to 10 arc sec
- Best available amateur mounts (with *astronomical* cost): 3 arc sec
These errors are introduced by slightly unround worms in the gear and play in the bearings.
Even with a perfect round worm (cannot be manufactured!) there will be errors by dirt in the gear or variations in the motor speed among other causes.
Another big error is usually incorrect polar alignment.
Because of the refraction of the atmosphere there is even an error by tracking with the telescops through different altitudes.
This error can be lots of arc secs as shown in this article: The Influence of the Atmosphere in Astronomy.
But how much of error can be tolerated?
Let's have a look at different CCD resolutions given a telescope with 1000mm of focal length:
- Starlight Xpress MX-516 : 2.0 to 2.5 arc sec per pixel
- Starlight Xpress HX-916 : 1.4 arc sec per pixel
- Santa Barbara Instr. ST-7: 2.4 arc sec per pixel
According to the Nyquist theorem you need twice the resolution of your sampling instrument (in this case the camera's pixel size) to achieve a photo without loss o f information.
Giving a normal seeing of the atmosphere of 4 arc secs all of these ccd cameras are a good choice for the telescope with the HX-916 tending to hope for better seeing conditions.
Tracking errors will therefore be visible in the picture when they reach about 2 to 3 arc secs.
Even the best mounts cannot achieve this for the integration times of typically 30 minutes or so!
Hence you have to *guide* your telescope to keep track of a guide-star which is a reference for the whole picture.
You can do it either with a seperate guide scope or you can use the main telescope and an off-axis guiding system.
A third solution is Starlight XPress' Star 2000 selfguiding which uses the same ccd drame for guiding and for imagaing.
You can either guide with your eye and a star on the crosshairs or you can force another ccd camera to do the job for you.
Optical aberrations
Because of the bigger magnification compared to piggy-back photopgraphy there are some optical aberrations which are magnified too.
The good news is, that some of them (i.e. coma of a Newtonian Mirror, vignetting of a refractor) are the more prominent the more off-axis the picture is produced.
Because ccd frames are so small compared to traditional film the ccd is closer to the optical axis hence decreasing the errors.
This changes with the rapid growth of the ccd frames.
The latest (2004) CMOS based cameras like the Canon EOS 300D are already fairly close to film cameras.
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