Re: [APML] Re: Star Measuring WAS: Horsehead and Pleiades testshots with 6x9 camera/OAGand BRC

[Aplanatic@aol.com]

Regarding star image sizes and central obstructions:

1) I have done the diffraction analysis of a telescope system with various fractional central obstructions and combined this with the contrast transfer function and characteristic curves of several films including TP and PPF.  In the absence of seeing effects, the minimum full diameter at half density using TP should be approximately 9 microns with a 25% central obstruction, increasing to about 13 microns with a 50% central obstruction assuming an f/ratio of 5.  For PPF, one should achieve about 15 microns and 18 microns for 25% and 50% central obstructions respectively at the same f/ratio.  For PPF, the CTF of the film is the dominate resolution-reducing mechanism, regardless of the central obstruction.  For TP, the central obstruction has a significant effect at this f/ratio.  

2) I have seen TP images from an 8" f/1.5 Schmidt camera with a 2.7" diameter film holder.  This is a 34% central obstruction.  The smallest measured star images were no larger than 10 microns, about the theoretical limit of the film itself.  In other words, for very fast systems, the central obstruction will have a negligable effect because the diffraction limited spot size is so small to begin with.

3) For instruments with focal lengths of about 2 meters or larger, star image sizes on TP are usually dominated by seeing, not by film structure nor by diffraction caused by the central obstruction.  This depends, of course, on how bad the seeing is.

Dave Rowe

--  APML Archives at <http://astro.umsystem.edu/apml/>  ---
             Unsubscribe at <majordomo@seds.org>