Collimation Cap Hole size

[Bizibilder]

I wish I'd not started this!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!1

[brantuk]

Seems a lot of trouble to go to for a simple cap with a hole in it?

[Roel]

1.5 to 2mm would be ideal.... just to keep your eye on the optical axis

What about the center spot (ring). I can imagine that when that ring is small, a 2mm hole could be to big. I mean when you look though the cap, the view of the hole may be bigger than the inside of the center-ring.

Shouldn't it be "the smaller the better", also because a smaller cap hole will improve the overall sharpness of for example an optional crosswire in a chesire tube (because of the diaphragm effect)?

Isn't there some sort of calculation that uses the focal length and the center ring diameter to calculate the best hole size?

[TopHouse]

That's for a sight tube, not a collicap, and it's F X ID (F of scope x inside diameter) eg for a skywatcher 200 which is F5, length of tube would be 5 x inside diameter of tube.

[Merlin66]

The collicap only positions the eye on the optical axis. The FOV through the cap is pretty wide ie you can see the inside of the focuse tube etc. There's no relationship I know of that links the size of the hole to any accuracy in the results. The object of the exercise is to get all the optical components to appear concentric.

You can check this out by using a larger hole....the relative positions of the optical "circles" doesn't vary that much.

[Roel]

But the final step in the collimation process is to align the center ring on the primary with the hole in the collimation cap. Using either a cheshire, sight tube or a cap only: that step is always the last one.

Off topic:

"The object of the exercise is to get all the optical components to appear concentric."

That is not correct. The reflection of the secondary mirror should not be concentric, but slightly off center (away from the focuser). This is what perfect collimation looks like:

http://www.astrowiki.nl/images/7/76/Collimationyellow.png

When you carefully follow all the right steps of collimation, that will be the image you'll see through a collimation cap.

The peephole This is a small cap that fits in the focuser, with a small center hole (1-2 mm or so) to look through. It is easy to make one. You can adjust the collimation to make the following appear concentric (from outside in):

• The inner end of the focuser drawtube
  
• The secondary mirror (not so easy to see, since the outer edge reflects the black interior of the tube)
  
• The primary (main) mirror reflected in the secondary
  
• The reflection of the secondary mirror - but note that this image (and only this) is not concentric.
  
• The reflection of the drawtube and peephole cap (if transparent or semi-transparent - if not, it will be too dark to see).
  
• The mirror spot
  

Source: http://web.telia.com/~u41105032/kolli/kolli.html

[Merlin66]

I stand corrected on the concentricity issue... I was over simplifying.

I accept that the reflection of the rear of the hole in the collicap should be aligned to the centre ring on the main mirror. This is even more obvious if you use a cheshire or a null collimator.

[great_bear]

due to "them" attempting to force their tool through MY annular

[Roel]

:D:D

[Shure]

Heres a thing. I'd been looking for one then found on my AE ali dust plugs that they are part drilled on the reverse in the dead center. Got the drill out and finished the hole off all the way through (I of course removed the AE badge)

Got myself a nice collicap now!

Thought this thread was worth resurrecting as I have ordered one of the AE dust plugs to do this mod only mine has come already drilled the entire way. Well worth the £3.50!

[JamesF]

I shall try to remember that next time I need to make up the cost of an astroboot order to avoid paying postage

James