Fine Focus Obsessive

[sbooder]

When I had a bog standard R&P focuser, it was easy, I racked it out until I thought I was in focus and then back in a little and then back again…and that will do, I am in focus.

Now I have a dual speed Crayford, I have be come obsessive, I rack it out, and then back in again a little and then back out and stop. Now I ask myself, is that as good as just a millisecond ago, so I rack back in again…oh hang on that it not as good as where I stopped…hmm, are all four stars in the Trapezium dots and separate, not quite as good as I can get it, or is it. And on it goes.

Dose anyone else have this affliction?

[Adz]

Yep there is one way to fix it, a Bahnitov mask

[glowjet]

You can use a Bhatinov Mask if you want to but that will cost you, try a simple Y mask over the aperture of you scope, you can make the Y out of thin strips of black document folder material, you may have to experiment but for a large aperture 200mm to 300mm these will be in the region of 5mm to 10mm wide and for smaller diameters from 2mm to 4mm for 150mm and less, the diffraction pattern formed will act in the same way as the B mask, if you want to read up on it have a look at the PDF on Franhofer Diffraction a primer by Chris Lord

John.

[Si W]

As well as the Bhatinov Mask, there's the Hartmann mask as well. You could knock one up yourself, there's also templates on the net.