Re: [ATM] Re: Aplanatic Gregorian

[Mauritz Andersson <mauritz@physi.uni-heidelberg.de>]

Yes one can think of such ("compensating") designs if one gives up the
coma-free condition. One can lower the conic constant of the primary to
get to a reasonable null focal distance (see the formulas in the post by
vladimir).

Example design

Diam 200 mm
R1 900 mm  k1=-0.75
R2 249.06 mm k2=-0.859
Mirror separation 600 mm
Sec obstruction 34%
Final focal ratio f/11

Here the primary null distance would be 6718 mm
and the secondary null distance 3403 mm,
both quite reasonable.

BUT, I would advise against this design since the coma is pretty
severe...

Another possibility for null test is to take instead the aplanatic
system, k1=-0.979, k2=-0.472 (rest is the same). Then first polish a
sphere in the secondary and use that to test the primary. An example 
null test configuration would be a point source at 7000 mm distance and
an increased primary-secondary distance of 621.2 mm. (small enough
change that the two mirrors perhaps could even be mounted in the final
telescope tube)
Then finish up by polishing the secondary to the k2=-0.472 which then
has a small enough k so that it could be null tested at a reasonable
distance of 795.6 mm.

Regards,
Mauritz

On Sun, 2004-11-28 at 05:38, lou krajci wrote:

> I did not state my desire clearly enough.  I'm looking to see if there is a
> Gregorian variant possible that has both mirrors capable of being null
> tested at conjugate foci...i.e. they are both ellipsoids.  This means your
> optical testing is only null testing with the simplest and minimum of
> equipment (and no additional optics)...you only need a light source and a
> knife edge.  You don't need to worry about measuring longitudinal aberration
> as with the Foucalt test.
> 
> Does such a Gregorian variant exist?
> 
> >Remember that you can consider
> >the primary as a less than fully corrected paraboloid and the secondary
> >can be null tested.  Null testing the primary is also possible but
> >probably not too practical.  Look at the numbers that Valdimir has
> >supplied and you will see what I mean.
> 
> I must have not received that particular digest issue of this forum.  Can
> someone send what Vladimir posted so I can review it?
> 
> Thanks in advance.
> 
> Tom Krajci
> Albuquerque, New Mexico
> ---
> Outgoing mail is certified Virus Free.
> Checked by AVG anti-virus system (http://www.grisoft.com).
> Version: 6.0.801 / Virus Database: 544 - Release Date: 11/24/2004
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/


_______________________________________________
ATM mailing list http://www.atmlist.net/