Message posted by lone wolf on January 30, 2005 at 15:58:12 PST:

As I stated in my other post,I think you are limited to about 1ft of resolution at the base from Tikaboo. You can figure out how large this is on the film plane by using similar triangles, assuming you remember your trig. Basically:

(image size at target)/(distance to target) = (image size on focal plane)/(equivalent focal length of telescope)

Most CCDs are on a 6um pitch, but they use a color mask, so you would probably want to have 1ft at the base lead to 18um on the focal plane Ii.e. cover 3 pixels). Film is a bit different in that you are dealing with the modulation transfer function (MTF), which basically related the intensity fall off with the lines per mm on the film. For Astia 100f, I'd target about 30 lines per mm, which is about a 16um pitch since you need to produce both a dark and light region to realize a line. This comes out to about 2500mm. This all assumes you are focused well, which isn't all that easy.

As you magnify the image more, it becomes easier to focus. However, the light intensity falls off as the square of the increase in magnification. That is, if you doubled the focal length, you get 1/4 th light. It's been my experience that about 1/30th is about as slow as you should go.

You can use the sunny 16 rule to get a handle on the upper limit of magnification is you want to keep the shutter speed around 1/30th of a second. Say you were shooting with iso 400 film. Using the sunny 16 rule, you would set your focal ratio to 16. At F32, the shutter speed would be 1/100th. At F64, the shutter speed would be 1/25th.

Take some mythical refractor that is f8 and 640mm focal length, which would make it 80mm aperture. If you use it prime (i.e at 640mm), the shutter speed would be 1/1600th. [Sounds kind of fast to me). Through in a 2x barlow, and now you are at 1/400th with 1280mm equivalent focal length. Use a 4x barlow and you are at 1/100th and 2560mm equivalent focal legth. I wouldn't go for another factor of two since you are close to the theoretical resolution limit. I'd be more inclined to use a slower film to get more contrast, i.e go to iso 200 and shoot around 1/50th of a second.

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