Dummy Camera

[Aster]

Looking at my new Meade DSI Pro in my hot little hand I started thinking about Focusing.

Prime Focus.

In the the old days, twenty years ago or more, it was easy to measure the distance from the film carrier rails, where the film was pressed against by the back pressure plate, to where the lens butted against.

One just got hold of a micrometer made the appropriate measurement and made a dummy camera which then gave you a dead sure way of giving you 100% focus at the prime focus plane, only varies with atmospheric turbulence.

By Dummy Camera I mean a metal tube with a flat shoulder on one end and a razor blade glued across half the diameter of the other end, which sits in the focuser, the same as an eyepiece or any other equipment.

The distance between the razor blade and the shoulder being equal to the distance of the film carrier rails and the end of your camera adator which locates against the top end of your focuser.

One finds a bright star with an eyepiece, center it, exchange with dummy camera. Looking down the dummy camera one should see a brightly illuminated field showing the central obstruction and spider vanes, if you have a newtonian. If one now taps the focuser gently one should see a the razor blade intruding into the field. By moving your focuser either in or out one will notice when tapping the focuser on its side that the appearance of the shadow across the field will change.

If one continues to do that one will come to a point were the slightest touch on the focuser wil blacken the whole field instantaneously. This is the absolute focus, Prime Focus, of your telescope. Your DSI taken at that position could not be any better in focus. Disregarding what your computer screen or software tells you.


Now, after all this rambling comes the BIG QUESTION. Anyone know the distance from the shoulder inside the Meade DSI Pro, the one where your supplied nosepiece butts up against, and where your image is formed on the CCD chip ???

Would make focusing so much more accurate.

Alexander

[[1ponders] (Paul)]

I havent' heard of anyone building a knife edge focuser for a DSI but I'm sure someone somewhere would have done so. 'fraid I can't help you with the measurements either.

Good luck with the hunt.

[Aster]

Originally Posted by [1ponders]

I havent' heard of anyone building a knife edge focuser for a DSI but I'm sure someone somewhere would have done so. 'fraid I can't help you with the measurements either.

Good luck with the hunt.

Thanks, am surprised that someone actually knows what I am talking about.

[Lee]

It doesn't take long to focus a DSI with a Hartmann mask.....