Collimation of a Newt at night with a Cheshire EP?

[jetstream]

Hi,

Regarding Astrobaby's treatise on collimation, it is very good.  In her last photo she says "near perfect collimation" for the fast Newtonian.  And it is.  But she has a rotation of her secondary mirror, which is evident in the photo as you can see the secondary mirror looks elliptical, and not in line with the optical axis. (Though it is perfect in the sketch).

I did too, but fixed it.  Of course while observing you are not likely to see this effect, which might only be to cause some very slight vignetting in the off-axis portion of the field of view, that is the outer edges, since the primary would not be "filling" the secondary uniformly all the way around.

I have images of Astrobaby's photo where I superimposed a circle to show the effect the rotation causes.  I also have actual photographs where I had the same exact situation but corrected it by rotating the secondary mirror until its elliptical axes were lined up correctly with the telescope's optical axis.  But I can't figure out how to post or attach images on this forum  :/

After I center the secondary under the focuser with the sight tube and center the primary dot in the crosshairs of the sight tube and then center the primary dot in the Catseye cheshire,does this just happen as a result of my type of collimation?I realize I am not using the full offset method of collimation,but I notice no vignetting.

[bellavia]

Unless it is severe, I don't think the human eye can detect vignetting.  And the offset is not really absolutely necessary either.  That is mainly for capturing as much off-axis light as possible.  I am just a perfectionist.

These are the images I tried to post before but couldn't.  One is from www.astro-baby.com, with a circle drawn to show the slight ellipticity of the reflection of the secondary mirror, followed by own photo, where I had the same exact issue.

A correction on the rotation of the secondary support fixes that problem, and then it looks like the Jason Khadder sketch for a well-collimated, fast Newtonian (also on Astro-baby's excellent site, also attached).

The last photo is my scope collimated with the rotation error removed.  Nearly perfect.

Of course, when this is all done, you should do a star collimation - if your conditions allow it.  I rarely get skies good enough to do that, and my Dob does not track, so at high powers, you are chasing ,more than collimating,  in which case an artificial star is the best way to go.

But I am no expert.  Just learning and sharing.  

[Moonshane]

great post above. you are quite right about the rotation. thankfully though as you suggest, the secondary is......secondary......when it comes to effect upon observing visually at least. the impact is much less and I doubt many people would see a difference between Astrobaby's scope (or one with even more rotation) and your scope. a badly adjusted primary though makes a big difference which I think most people would see. obviously I'd encourage anyone to get it right all round but spend time/accuracy if needed on the primary on a regular basis for sure.

[Jason D]

Hello Vacuum,

Your observation is correct. The photo in astro-baby's site does include what is referred to as rotation/tilt secondary mirror error. That is why the secondary mirror silhouette appears elliptical. However, this error is the least critical collimation error. Below is an animation that shows this error (click on the photo to see the animation)

Keep in mind that there is nothing magical about getting the secondary silhouette to look circular. Imagine you have a square shaped secondary mirror which means the secondary mirror silhouette will appear square shaped. Does that mean your telescope is miscollimated? Will that introduce coma? The answer is "no" to both questions.

Below are photos from my well-collimated scope.

Jason