Re: [ATM] Eyepiece testing.

[Mark Cowan <toolontop@yahoo.com>]

Hi Jim:

I'm designing a setup for testing EPs currently,
stemming from a discussion (see "figuring with the
star test") over on Cloudy Nights ATM forum.  The
question basically was "what effect do EPs have on
star tests" and the conclusion was - surprise - not
much is to be found in any quantitative form.

The device will start with a 5-7/8" f/2 (or f/2.25)
reference sphere, figured with null test and likely
confirmed via interferometry.  An Airy-disc size
precision aperture (around 3 micron) fed (likely) by a
laser diode at low power will provide the source, goes
through a beamsplitter or pellicle to provide a
coincident light path at ROC.  The EP will be used to
image this effective f/4 or f/4.5 system and the
aberrations evaluated visually in the simulated star
test, and/or captured for reduction via Roddier (sp?)
software that's freely available.

The goal would be to identify the best EPs for star
testing (Mel says the simpler the better, with
symmetrical designs preferred) but you can also then
identify the best on-axis performers and even evaluate
manufacturing consistency if enough examples are
sampled.  And theoretically provide compensation for
preferred EPs in mirror figuring.

Have no idea what my time frame on this is, I have the
 blank for the sphere, at least. :)

Best,
Mark Cowan
Salem, OR
BTW DID I MENTION THE OSP WAS AMAZING? 

--- Jim Burrows <burrjaw@earthlink.net> wrote:

> The recent discussion of eyepiece correction
> prompted an idea for eyepiece 
> testing.  If I ever get my RC cass secondary
> refigured, a possible use of 
> the Hindle sphere is to test eyepieces.  With a mask
> on the sphere and a 
> point source slightly displaced laterally from the
> sphere's ROC, position 
> the eyepiece and a webcam with its lens removed in
> the return cone of light 
> to get an image.  The image could be processed with
> good ol' FFTs to find 
> the eyepiece aperture's transmission function.  Then
> a neat problem to be 
> solved:  given the functions for a collection of
> eyepieces, what is the 
> optimum profile of an objective mirror to be used
> with that eyepiece 
> collection?  Somewhat the same mess that I'm in -
> figure a secondary to 
> make an RC match with a given primary.
> 
>          -- Jim Burrows
>          -- mailto://burrjaw@earthlink.net
>          -- http://home.earthlink.net/~burrjaw
>          -- Seattle N47.4723 W122.3662 (WGS84) 
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 


--- Jim Burrows <burrjaw@earthlink.net> wrote:

> The recent discussion of eyepiece correction
> prompted an idea for eyepiece 
> testing.  If I ever get my RC cass secondary
> refigured, a possible use of 
> the Hindle sphere is to test eyepieces.  With a mask
> on the sphere and a 
> point source slightly displaced laterally from the
> sphere's ROC, position 
> the eyepiece and a webcam with its lens removed in
> the return cone of light 
> to get an image.  The image could be processed with
> good ol' FFTs to find 
> the eyepiece aperture's transmission function.  Then
> a neat problem to be 
> solved:  given the functions for a collection of
> eyepieces, what is the 
> optimum profile of an objective mirror to be used
> with that eyepiece 
> collection?  Somewhat the same mess that I'm in -
> figure a secondary to 
> make an RC match with a given primary.
> 
>          -- Jim Burrows
>          -- mailto://burrjaw@earthlink.net
>          -- http://home.earthlink.net/~burrjaw
>          -- Seattle N47.4723 W122.3662 (WGS84) 
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 


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