Focuser issues

[johnkirkpatrick]

I have just collimated my scope but notice that although collimation is almost perfect. its only just out. but if I put a little force on the whole focuser and hold it over a bit its bang on and when i let go its slightly off. How do I configure the focuser and move it a bit. hope this makes sense to someone cause I have no idea how to explain what I mean lol

[AndyH]

Is the focuser unit flexing the tube, when you do this?

Andy.

ps: congrats to you and your better half, on the 2 new arrivals !!

[johnkirkpatrick]

Yeh it does mate.

Thanks mate.

[AndyH]

Mmm. Two possibilities.

I'd collimate it so it's bang on without you forcing the focuser. Or you could use some washers between the focuser screws and tube, so the focuser is tilted slightly. Personally I'd do the former.

Andy.

[brantuk]

Check out the hex screws under the focuser tube - the middle one is usually a tension adjuster to firm up the focuser. But the tension needs to be firm enough to reduce flexure but loose enough to allow easy focusing. It's a fine balance depending on what equipment is attached - particularly heavy 2" ep's and cameras etc

[johnkirkpatrick]

Its only a mm out so might just leave it as i'll just make it worse. I have tried to get the secondary mirror as central as poss. and have it almost there. The spider vains all measure at 115mm each to the centre screw on the mirror holder. I have a circular shape through the focuser and can see 3 mirror clips. and all looks good through the cheshire ( once i had the hang of it ), I'll try get a pic and put it one here in a few mins.

[brianb]

The "sweet spot" (diffraction limited) on an f/4 scope is 3mm in diameter, and much bigger than that with a slower mirror. If the flexure in the focuser is less than that, I wouldn't worry overmuch.

If you start wanting to hang heavy cameras etc. from the focuser, then it's a different matter ... time to invest in a Feathertouch ...