Petzval refractor collimation...

[neotesla]

I'm have a Cheshire EP to collimate the aforementioned scope (a 127mm F5) and I was looking at the secondary set of lenses in the optical path, and my question is whether this corrector will be a problem.  The Cheshire works off the reflection of light on the primary objective, but wouldn't this second fixed lens in the path reduce the effectiveness of this method?  Anyone here have experience with this type of refractor?  The one I have is definately off, and I have been tweaking it as best as I can, but the Cheshire method is the most often mentioned way to collimate refractors.

Thanks!

[drclay]

The Cheshire eyepiece can be quite effective, but NOT as effective as the old and more difficult way (with a refractor the difficulty becomes exponential) as a bright star overhead with a Barlow lens inserted into the train and the refractor as close to focus as possible yet not quite focused....this will reveal the outer diffraction Fresnel rings and the bring core (Airy disk) together, whereas too far out of focus shows only the Fresnel pattern overall and no central definition that you can use to center.

Nonetheless, the optics of the Cheshire eyepiece (I am assuming these are the lenses to which you refer) should NOT be a negative factor PROVIDED that they are properly aligned in the Cheshire themselves.  Aligning with a tool that is out of alignment obviously is going to have disastrous results.

Doc

[neotesla]

Thanks...

I have used a few collimation methods, and the tools tend to be the rough fix until the star tests can be done.  As you pointed out there appears to be atendency for the tools to be part of the problem.  The biggest I find is centering the tool in the lense holder, focuser etc.  That little bit of wriggle room becomes painfully obvious, when making the initial adjustments.

My worry with the Petzval design was the second lens mucking things up, but I guess with that lense fixed in position, it would only defocus the final reflection... I just have to quit looking at it problem and just get this done.

[drclay]

I ran a quick ray trace schematic last night of the placement of the correcting lens set (actually an achromatic assembly) and you may indeed experience some reflected glare from the correction lens set; nonetheless, this will be annoying but should NOT impeded your ability to use the eyepiece at least for rough adjustments.  I believe that you are going to be far better off setting up the scope in a makeshift optical bench and aiming at an artificial star for the collimation, without the Cheshire.

Doc