Quote:
Originally Posted by rumples riot
Don, I have the 2" triplepack, that includes the autocollimator. There is still room for some tweeking with a barlowed laser even after using the entire collimation kit.
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An autocollimator provides an order of collimation accuracy well beyond that of a barlowed laser. Once collimation has been achieved with an autocollimator no further collimation adjustment can be achieved with a barlowed laser unless the collimation has drifted off. A barlowed laser is, essentially, the same as a cheshire (the Catseye BlackCat), and the autocollimator can achieve a better collimation than either a sight tube or a cheshire.
So I'll go over a way of using the autocollimator that you might not be familiar with. Some day an inventor may come up with a more accurate collimator, but today it's the top of the heap. If an autocollimator shows the scope to be perfectly collimated, distrust your other tools.
After using the sight tube and cheshire and getting it as accurate as you can, the autocollimator will show the 4 reflected images of the centermark to be not completely perfectly "stacked" into one image. Which mirror do you adjust? How do you know what to do to make the 4 images perfectly stacked?
Reach down and miscollimate the primary by turning one of the collimation screws (I recommend the top screw) backwards or forwards about 1/4 turn.
In the autocollimator, the 2 reflected centermarks that are attributable to the primary will move away from the center of the field to the edge of the field, leaving the two images from the secondary still nearly stacked.
This is known as the "Carefully Decollimated Primary" protocol.
Using the screws on the secondary, carefully, and perfectly, stack the two images that are still nearly stacked in the center. You will find that doing so also brings the two decollimated images in from the edge slightly.
Then, using the primary collimation screw you moved, reverse the decollimation to bring those two images back to the center and stack them on the already stacked image from the secondary. You may find, as I always do, that to perfectly stack the two images from the primary requires adjusting more than just the collimation screw you decollimated.
After a little practice at this, all 4 images will be perfectly stacked and appear as one image with a slightly fuzzy edge.
Why the fuzzy edge? Because the differences in focal length the 4 images are seen cannot be simultaneously focused by the human eye.
This collimates the scope to a point 7 focal lengths out on the central optical axis, a multi-pass collimation that is not achievable by any other tool.
If any other tool shows the scope to be out of collimation at this point, there is something wrong.
What could be wrong if the other tools still show miscollimation?
Some possibilities:
1) misregistration. If the collimation tools are not tightened in place (despite what Jim Fly advises, all collimation tools need to be tightened in place to avoid misregistration caused by slop between the tool and focuser), then each individual tool can sit slightly askew in the focuser and this would make acieving a perfect stack with the autocollimator impossible. The AC is so sensitive that slight differences in sag in telescope components will decollimate the stacked 4 images. Some focusers aren't cut squarely on top of the drawtube. Others have drawtubes that aren't concentrically drilled (up and down motion not coincident with the center line of the hole). There might be slop between the fit of the barlow and the focuser.
2) flexure/sag in some component. This can be secondary movement because the spider isn't tight enough, or actual movement of the focuser drawtube in the focuser, or movement of truss tubes at either end, or sag in the truss tubes, or movement of the primary on its springs.
3) inaccurate tools. The tools can be non-round, so that putting the tool in ten times will get ten different results. Even your autocollimator may be misregistered and the perpendicularity of its internal mirror may be suspect. This is unlikely with a Catseye AC, but it can't be ruled out if rotating the AC in the focuser causes the stacking to change. With the exception of Glatter, many lasers aren't particularly round, I've found.
The gist of all this is that the AC should be able to do a final tweak (note spelling--it's not spelled with a double e) of collimation that goes beyond that accomplishable with any other tool, and which agrees with all the other tools.
And if you use a coma corrector (such as the TeleVue Paracorr), the tolerances for collimation tighten to 1/6th what they are without one, so achieving excellent collimation becomes even more important.
I'm not sure why you find the scope incompletely collimated with the barlowed laser AFTER using an AC, but I hope the above protocol will help in using the AC. I'll have to think about what other issues might be at play.
Best to you,
Don
