No tools collimation - what do you think??

[5ash (Philip)]

This months "Australian sky and telescope " has an article "no tools collimation" which seems to show a simple way of final star collimation . The author describes the out of focus image as having a dark hole near its centre which he alludes to asi the shadow of the secondary. Is it really that ? If so what effect does secondary offset in a fast scope have on its central position? The technique looks so simple and obvious that final collimation of fast newtonians can be easily achieved using it??
Philip

[Jason D (Jason)]

I have not seen the article but star collimation is a well-known method. If it is done correctly, secondary offset should not be an issue.

The proper way to star collimate is to use high magnification and defocus by a small amount. If you defocus to the point where you can see the diffraction pattern of the spider vanes then you have dofocused too much. Secondary offset will be visible and will impact the accuracy of your collimation when the star is defocused too much.

Bear in mind that star collimation can only fix the primary mirror collimation. It does not fix secondary mirror. Besides, it can only be executed reliably on nights with acceptable seeing.

Personally, I prefer to use quality collimation tools. It is an easier and a more reliable method.

Jason

[barx1963 (Malcolm)]

There are a couple of limitations, which Gary touches on in the article. The main one is that for a large dob or if you are trying it alone, adjusting can be difficult unless it is motorised. This is exacerbated by the high poer required, if I following the recommendation of 25x per inch I would need 500x on my 20" so a star wouldn't stay centred very long.
Also seeing needs to be pretty good as Jason said.

That said, doing a star test at the end as a confirmation that everything is OK is still a good idea.

Malcolm

[Don Pensack]

I might add that if your collimation tools are accurate, and the scope is meticulously collimated, I have never seen a star test that could improve on the collimation, even at very high powers.
and star collimation doesn't help with secondary alignment.
I have seen poorly-collimated scopes that could be improved with star collimation, however.
But, the primary downside to star collimation is that seeing needs to be very very good to see the near-focus, high-power image with enough resolution to check collimation.
I suppose that's one reason to have a tool for primary collimation.
No tool collimation works fairly well on f/10 and longer newtonians, but not on today's faster scopes.