Re: [ATM] More Dall Kirkham Secondary questions

[Michael Peck <mpeck1@ix.netcom.com>]

At 04:29 27-11-04, Rod Brackenridge wrote:
>Hi,
>
>I think my secondary is finished.  I now have 4-8 lines arc lines across 
>the secondary when I place it on the reference.  There is no sign of any 
>zones other than a turned edge.  I am not worrying about the edge because 
>it will either be removed or covered.  The arc shaped lines bend by about 
>a half a fringe.

This may be overkill, but if you can take a picture of the fringes it might 
be worthwhile to analyze it with FringeXP, which you can download at 
<http://www.ceravolo.com/fringe/FringeXP/FringeXP.htm>. It can help you 
distinguish between curvature differences that you don't care about and 
correction errors or zonal defects that you should care about.


>My secondary is 100mm diameter for a 355mm primary.  Using Mike Peck's 
>spreadsheet I designed a system that shields the field of a 50mm diameter 
>eyepiece and fully illuminates the central 12mm.  The secondary baffle is 
>104mm diameter (about a 30% obstruction).  The useful

My spreadsheet calculation returns "optimal" baffle dimensions under 
assumptions that might or might not be relevant to your observing needs. I 
would definitely recommend using the spreadsheet as just a starting point 
for more detailed analysis.

If this were my project I think I would leave the secondary alone and maybe 
build a secondary baffle that masks off the edge. A short cylindrical 
baffle should be sufficient to prevent sky flooding without too much 
vignetting. YMMV, as always.

Mike Peck

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Michael Peck
email mpeck1@ix.netcom.com
Wildlife photography page http://home.netcom.com/~mpeck1/index.html
Amateur telescope making http://home.netcom.com/~mpeck1/astro/astro.html 

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