Collimation Evaluation - Advice please!

[coxy322]

I have just had my first attempt at collimating my 200p today. Its approx 6 months old, brought from new and used approx 10-15 times.

Scope is outside now, waiting for darkness and myself hoping that I get at least enough break in the cloud so I can do the star test.

I had trouble though, maybe somebody can comment;

I adjusted the spider vains - one needed adjusting slightly by 3mm to achieve balance. Felt that was satisfactory so moved on...

Next I checked the secondary had the right circle when viewed against paper/card inserts. No adjustment necessary...

Primary to Secondary alignment - PROBLEM... Could not see all three clips of Primary inside the secondary. Attempted adjustment using three allen screws and even centre bolt in Spider - but it felt way too clumsy and the spider vains bent slightly on turning the keys/screwdriver. So left alone, went back to check secondary circle test and then proceeded...

Finally, using Cheshire I aligned the Primary as necessary - it was wayyy out! Although not a lot of offset for my f/5 as explained/expected in Astrobaby guide ... Altogether it felt very scary to go back and check on the three clips, is this where it might be wrong?!

Any advice folks?!

[umadog]

The key things are that you need to have the primary mark aligned in the cheshire reflection (this corrects for so-called primary axial alignment error). You also need to have the mark under the cross-hairs of the sight-tube. This corrects for the so-called focuser axial alignment error. You iterate between the two, going back and forth until both errors are minimised. You now have axial alignment. This is the most critical thing.

The mirror-clip thing is a slightly separate issue. You also want the secondary to appear round as viewed from the focuser and to be in the centre of the focuser. The sight-tube can be used to guide this step. If the secondary is centred and round then completing the axial alignment should be sufficient to centre the primary in the secondary. That is what the mirror cip thing is for. It's just a guide to help you know that the primary is in the centre of the secondary and that you can see all the primary. It's not always a good guide, mind you, since some secondaries are under-sized and you will never be able to see all the clips in this situation. Keep that mind so you don't go attempting the impossible. It is easier to adjust the secondary if you replace those horrible set-screws with "Bob's Knobs" (Google it). Don't worry if the spider flexes a little but if it does then maybe loosen the setscrews a touch .

Here are some links to help. Check out the secondary alignment link. Rob Campbell's Home Page