Focus problems

[Alastair Rae]

I think my reflector is correctly collimated (looks fine in the Cheshire) but I still can't get stars to focus as points. The brighter ones form something like a triangle of overlapping discs. I compared my view with the examples at the bottom of Astrobaby's collimation page but none seem to match what I'm seeing.

I can just about see the bands on Jupiter and can get no detail at all if I use a Barlow.

My scope is a 114 mm parabolic reflector and I'm using X-Cel EPs.

Anyone got any suggestions how to get a sharper view?

[8kids]

It could just be the viewing conditions, I've been outside this evening, my eyes are showing me it's clear but my telescope is showing me otherwise. Stars are just not sharp points of light. I'm leaving the scope outside and I'll have another go later, it might improve or it might not.

[Peter Drew]

Triangular star images usually indicate a too tightly held primary mirror. It would be worth checking for this and if necessary slacken off the mirror clips untill you can pass a piece of paper between them and the mirror.

[Alastair Rae]

Triangular star images usually indicate a too tightly held primary mirror. It would be worth checking for this and if necessary slacken off the mirror clips untill you can pass a piece of paper between them and the mirror.

Thanks, that makes a lot of sense as the clips are the only part of the assembly that forms a triangle. Taking the rear assembly off was a bit nerve-wracking but it went ok. The clips were locked down very tight indeed.

I'll report back on the results when I next get a clear night.

[brantuk]

The clips only hold the primary mirror in place and you should be able to slide a piece of thin paper between each clip and the mirror surface. When using the locking screws after collimating - these should only be snicked up "finger tight" enough to support the mirror underneath and hold it at the correct angle of tilt to ensure the reflected light path is well aligned with secondary and focuser tube.

If any of the screws are too tight it has the effect of bending the mirror surface (which is a very shallow parabolic or conical curve) and distorts the reflected light path. It manifests itself in the eyepiece as you descibe with mutliple images and diffraction spike shapes. It's known as "pinched optics" - the same effect can happen with glass lenses that are screwed in too tight as well. Hth

[Moonshane]

I agree with Peter and Kim. it's likely to be the clips and personally, I'd say lightly finger tight (i.e. not as tight as you can get them with your fingers) for the locking bolts. I tend to take out the locking bolts as they really seem to serve no useful purpose.