Checking a newt focuser is square to the OTA

[JamesF]

I'm about to fit a new focuser to my dob. When I refitted my previous one I tried to make sure it was square to the tube with less success than I'd really desire. The adjustment isn't that hard. What gave me the trouble was marking a point on the inside of the tube opposite the focuser to check against. Are there any nice little tricks to make that easier?

James

[Moonshane]

to be honest I have never worried too much as long as it looks approximately square. although I don't image of course.

[JamesF]

I have no plans to use the dob for imaging, but it is f/4.7 so I'd really like to try to get it as close to spot-on as possible. And I've got all these collimation bolts to play with. It seems a shame not to use them

James

[mdstuart]

James

i also think its not too critical visually.

The ultimate test is the star test as I know only two well!

Mark

[JamesF]

i also think its not too critical visually.

It might not be. I am

The ultimate test is the star test as I know only two well!

I have my shiny new artificial star standing by Though actually I bought that for checking the collimation of the Mak and SCT as they're a bit of a pig to do under the stars unless the seeing is so good that you wouldn't want to be wasting time collimating

James

[laser_jock99]

I would have thought the focuser only needs to be 'roughly' square to the tube. The idea of collimation is to line up the optics with the optical axis of the focuser not vice versa. Obviously the focuser has to be fairly square to the tube because there's not that much adjustment on available on the mirrors?

The only way I can think of getting it critically square on is to remove the secondary and put a small, rectanglar flat mirror on the opposite side of the tube. Now provided this flat mirror sits squarely against the internal tube wall you could tweak the angle of the focuser so that a reflected laser beam from the focuser goes back along its own path?

A standard laser collimator could be used for this.

[JamesF]

I would have thought the focuser only needs to be 'roughly' square to the tube. The idea of collimation is to line up the optics with the optical axis of the focuser not vice versa.

Well, yes, but...

It strikes me that if the focuser tube does not move along the optical axis then it will hold the eyepieces at an angle to the focal plane, meaning one part of the image may be in focus but the rest isn't. In a slow scope that might not matter too much, but in a fast scope with 82 degree or 100 degree eyepieces, it might well. It also means the image moves across the field of view as the focuser moves in and out. Again that might not be too much of a pain if your eyepieces are all close to parfocal, but my 2" eyepieces and 1.25" eyepieces focus in quite different places.

I accept that it may genuinely make no difference in actual use, but when I want to get something right, I really do like to get it right

James

[Bizibilder]

Use a strip of paper:

Cut a long thin piece and wrap it round the inside of the tube at the focusser position. Make sure the ends of the paper overlap and the overlap covers the focusser hole. Mark the position of the focusser on both ends of the paper. Take the paper out, fold it in two so the two focusser positions overlap exactly. Mark the fold. Reinstall the paper and you can mark the tube exactly opposite the focusser hole.

Takes longer to type than to do!!

[JamesF]

I shall give that a go, Roger.

James