Re: ATM A question about angular size at a distance

[tkrajci@san.osd.mil]

>From: "PK" <pkreisle@earthlink.net>

>Scopes

>11in 5.8F  Newt
>6in 4.5F    Newt
>4.7in 8F    Refractor

How far do these scopes have to be so that ‘nearby’ test targets present 
about ¼ wave of overcorrection in spherical aberration?  (Compared to an 
ideal astronomical telescope that is designed to look at objects at 
infinity.)

According to Sinnot, Sky&Tel, May ’91 the formula is:

N = 28 (D/F)^2
Where N is in feet, D is inches, and F is the focal ratio.

So, for our ‘test instruments’:

11in 5.8F  Newt ...N = 101 FT
6in 4.5F    Newt ...N = 50 FT
4.7in 8F    Refractor  ...N = 10 FT

However, I recommend roughly doubling these values.  The 11 inch scope is 
the most sensitive to this.  Put all scopes at 200 FT and you should be OK 
in terms of distance.

There are other issues.  Will the Newt’s be vignetted because the 
secondary mirror no longer captures the converging light cone on these 
nearby objects?  Will there be mirror flop and loss of collimation because 
the Newt’s are pointed at the horizon?  Will the big Newt’s mirror be 
potato chip shaped when pointed at the horizon?  An so on....

I hope helps clear up at least the distance aspect of this test.

Tom Krajci
Tashkent, Uzbekistan