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Hi Phoenix,
my comments may be a bit misleading. They are a bit tongue in check in that The good Fuji films we have been using, both the Superia Xtra 400 (FSX 400) and the Superia 200 (FS 200) are readily available right now. The big question is the unkown quantity of, for how long. There have been good films available to us in the past that were around for several years until the film companies changed the chemistry. We are at the whims of the film companies on this, and beleive me they don't even consider the astrophotographers at all. My mate Noel thinks that in a few years time we not be able to get film at all because of the continuing developments in digital photography.
Widefield or telephoto focussing of the film camera is the easiest of all. Film camera lenses have focus point called, 'infinity,' which looks like a stretched side-ways figure 8. Filming through a telescope requires a different technique called, 'knife-edge focussing.'
First you need a knife edge device. This needs to have a bevelled edge like a knife edge. We lost our purpose built knife edge and so we used three Stanley Safety knife blades, which by nature are very sharp so you have to be careful. You slide the knife edge along the film rails in the back of the camera (see pic attached). by this stage you have attached the camera to the 'scope, opened the back of the camera, inserted the cable realease and started an exposure on the "B" setting (B for Bulb) which holds the aperture open for however long you want. You can directly view the star by moving your head around so that the star avoids the central obstruction caused by the secondary mirror.
You point the telescope at a fairly bright star. with the aperture open you look through the back of the camera and see the star as a bloated, out of focus disc. This is because you can't get your eye to the actual focus point (see diagram attached). The third diagram shows what happens to the appearance of the star disc as you go in and out of focus.
As you slide the knife edge across the star disc you may notice the knife-edge crossing from the direction that you are moving the device. Change the focus and see if you get a difference. When you hit the sweet spot of focus the star blinks out instantly. There is no gradual drop off in light intensity. Then you load the film into the camera and you are ready to start shooting. If you accidentally change focus at any stage then you will have to rewind the film and take it out of the camera and repeat the focussing again (avoid this at all times. It can be done but it aint no fun!)I
hope that this makes sense.
Cheers
Coops
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