[ATM] Re: Illumination Field-Another consideration

[Jeff Morgan <jeffmorgan@mindspring.com>]

John,

Your assumptions on the focuser are right on the money, I'm using a 
FeatherTouch with 1.5" travel.

I had a rather interesting day yesterday pondering over illuminated 
field. I downloaded sec.exe and the accompanying article from Sky & 
Tel. Essentially, the article said for visual scopes a large amount of 
vignetting would not be apparent (40% illumination at the edge!) - 
calculating it and seeing it two different things. It reminded me of my 
old 10" f/9 scope with the 1.25" secondary. I used that scope with a 
38mm Erfle, and the views were great.

I also called Bryan Greer at ProtoStar about ordering a larger (1.83") 
secondary. The conversation was a real eye-opener and I got schooled. 
He described a couple of simple tests I could perform that illustrated 
his points. To paraphrase what he told me, the rules of the game change 
a bit when using a barlow (which most binoviewers need for Newtonian 
application). That is, you need to be concerned with the diameter of 
the fully illuminated spot at the barlow lens, not the focal plane. If 
you are good at the barlow field lens, you are good at the focal plane. 
He went on to say that if the scope was dedicated to binoviewing, you 
could safely go one size *smaller* on the diagonal due to the effect of 
the barlow. In hindsight, I have read about people using relay lenses 
to reduce secondary size, a very similar situation.

Back to my current problem, I think I can move the mirror up the tube 
by 3/4", solve the binoviewer intrusion at the medium and high power 
modes, and still be ok with a 2" wide field lens.

Jeff


On Jan 26, 2005, at 6:56 AM, John Lynch wrote:

> Jeff,
>
> The other component of optimization is that, for
> everything that you're going to put into your focuser
> (I assume it's a 2" Crayford with 1.6" minimum
> height),
> you've got to be aware of your total travel, drawtube
> obstruction into the light cone, and margin.  All of
> your eyepieces, or Binoviewer, are going to expect the
> focal plane to be at a certain distance from the
> shoulder, which is the physical stop when the eyepiece
> or Binoviewer is slid into the focuser.  You can
> measure this distance with eyepieces by pointing the
> field lens at a distance light source and measuring
> where it focuses the light inside with a piece of
> graph paper with a little bend at the end.   I don't
> know how you would do this with the Binoviewer.  It
> may well be internal like the Nagler eyepieces, so you
> might be able to get manufacturer's specifications
> about the field stop (essentially the same point in
> the case of the Naglers).  These measurements will
> help you to decide where to place the focal plane to
> avoid obstruction and give you margin at the top end
> of the focuser travel for everything you're going to
> use.
>
> One other point:  If you've got to move the mirror
> forward by 1", that's equivalent to moving the focuser
> and secondary mirror 1" toward the mirror.  In doing
> so, you increase the distance from the secondary
> mirror and the focal plane.  SO, you're diverting a
> fatter part of the light cone outside the telescope
> tube and that's why the larger secondary might be
> necessary to give the larger illumination fields.
>
> Hope this is helpful.
>
> Good luck.
>
> John Lynch
>
>
> 		
> __________________________________
> Do you Yahoo!?
> Yahoo! Mail - now with 250MB free storage. Learn more.
> http://info.mail.yahoo.com/mail_250
>

_______________________________________________
ATM mailing list http://www.atmlist.net/