Re: Re : ATM Diag. size

[Bratislav Curcic <epabcc@epa.ericsson.se>]

Gary,

> I note that doing the same test (obstructing a good
> 7" APO) lead Terence Dickinson and Alan Dyer to conclude that the image
> degradation resulting from a central obstruction was for the most part a
> myth! 

I'm not surprised. This just confirms many of the studies done in
the past.

> To quote: "Even a 2-inch disc (29 percent)hardly marred the refractor's
> fine images.

This is also in a good agreement. It takes some REALLY experienced planetary
nuts to see defects at (and _maybe_ below) 20%.

>  This, however, does not explain Dickinson and
> Dyer's experiences - unless that 7" wasn't as superb as they thought.  For
> my own part, my experiments contradict this - one of the reasons I advocate
> as small a secondary as you can get away with.  There is no danger in this
> approach

... as long as you are absolutely confident that your diagonal is free
from edge defects, and flat to better than 1/20th wave. I wouldn't
trust ANY manufacturer's word for that. I'd insist in an interferogram,
and even then run my own test or two.

As far as quality of that 7", I'm pretty confident that 
1 - Roland wouldn't risk the reputation by selling the substandard
    scope to someone who has such a large audience to potentially ruin
    AstroPhysics' hard earned image; I'm pretty sure that _that_ scope
    was hand-picked
2 - both Dickinson and Dyer aren't excatly spring chickens when comes
    to telescope's quality

>  If one is not aware of the potential
> problems using a minimal secondary, then by all means go a size larger and
> be safe - you'll likely get a better image.

Spot on.

Bratislav