Two ways for
focusing an astrophoto
In astrophotography everything must be accurate and one of the most difficult steps is to reach a correct focus on the film.
The camera viewer is not sufficient to get good results. I use two alternative methods: in both cases first I focus on a bright star, then move the telescope to my subject. Anyway I have found useful a graduated scale stuck on the focus knob.

 

1. The "Hartmann's mask"

You can buy this accessory, but you can also very very easyly built it with a plastic sheet: it does not need to be very strong cause it just has to stay few seconds on your telescope. For a 20cm (8") SC I made two 5cm (2") holes.

It is very easy to use: point a bright star (mag. 0-1) and directly look in the camera. Out of focus you will find the star doubled. Move the focus knob until you will see the two images perfectly overlapping.
Merits: cheap and easy to use
Faults: not very accurate with short focal lengths, typical of prime focus photography.

 

2. the Focal Point

This is an Italian product by Laborfoto and I do not know if you can find it in other Countries, but I suppose it is quite similar to Pointsource by Spectra Astro Systems.


It is a Ronchi grating that must be temporarily attached to the OAG (or to the photographic adapter) for focusing, then sobstituted by the camera with a T-ring. The grating and the film plane will exactly be in the same position: this way focusing on a quite bright star by the grating (as in a Foucault test) the film will be perfectly focused too.
Merits: very very accurate!
Faults: not very easy to use.

 

 

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