Re: Secondary Size

[rgb@postman.ncube.com (Bob Bond)]

Have you tried newt20?  It's a very nice PC program for modeling the
sizes of secondaries, focuser interaction, etc.  It's available in
best.com's archive.

I'd go for the 1.5.  With a couple of assumptions about your focuser
height and tube thickness, I get a fully illuminated angle of almost
1/2 degree.  So you can see the full moon with no vignetting.

The falloff for that combination is pretty dramatic though, so if
you're looking for wide field things, you might go bigger.  Also, if
you expect to use many 2" eyepieces, you will want to go bigger.  A 32mm
Panoptic gives you a 1.8 degree field so your illumination would drop
off to almost nothing at the edges.  The exit pupil for that
combination is a modest 5.3mm so it is something you would want to
consider.

                Bob Bond

>From DAVEL@cc.usu.edu Thu Oct 26 14:40 PDT 1995
>Date: Thu, 26 Oct 1995 13:33:22 -0600 (MDT)
>From: DAVEL@cc.usu.edu
>Subject: Secondary Size
>To: atm@efn.org
>X-Vms-To: IN%"atm@efn.org"
>Mime-Version: 1.0
>Content-Transfer-Encoding: 7BIT
>
>Hi,
>       I have been agonizing over the proper size secondary for the 8"
>f6 newt that I am building.  This is to be a general purpose scope - planets
>and deep sky.  I have tried to optimize everything as much as possible (low
>profile focuser, minimum excess focus in travel).  What is the group 
>concensus on the size of the 100% illumination zone at the eyepiece versus
>obstruction of the primary by the secondary?
>
>       If I go with a 1.5" secondary, I will only have a .25" (6.4mm) zone
>of 100% illumination at the eyepiece.  The obstruction of the primary is 19%.
>(I am assumming that the secondary holder's lips will subtract approx. 1/8"
>from the secondary minor axis.)
>
>       If I go with a 1.83" secondary, the zone of 100% illumiation will
>be .6" (15mm), while the obstruction of the primary will be 23%
>
>       Contrast versus illumination?  I would appreciate any suggestions.
>
>                                       David
>