ATM Re: Secondary Holders

["Bryan Greer" <bgreer@fpi-protostar.com>]

Jeff Medkeff wrote:
> I used my ProtoStar unit to replace a home-made (and
> only adequate) secondary holder I had put together
> with metal vanes. I find the plastic material that
> the vanes of the ProtoStar are made of are for some
> reason much less prone to making big diffraction spikes.
> I have heard that metal vanes can set up an area of
> colder air around the vanes, causing some refraction
> inside the tube. If so, perhaps the ProtoStar vanes are less
> prone to this.
>

I believe the reduced intensity of the diffraction spikes is primarily due to the 
somewhat thinner vane material used (which is STAINLESS STEEL...wouldn't want a 
"plastic vane" rumor to start! <g>).  Of course, if it's a 3-vane unit, the 
diffraction spikes will be significantly dimmer if you're used to 4-vane spiders, 
since opposite vanes don't reinforce each other (though there will be 6, instead of 
the usual 4 spikes).

> Perhaps the larger units are already made this way. Thoughts, Mr. Greer?
>
(Who's Mr. Greer? <g>)  Yes, for tube sizes over 10", or so, the vanes each have 
two lugs.


Ken Lowther wrote:
> I have a larger 4 vane, and it appears to be metal vanes.  I plan on installing
> it tonight.
> 
<my Ed McMahon voice> You are correct, sir!

I appreciate the pos/neg feedback.  Off to Astrofest!! (I'm late, again) 

Sincerely,
Bryan Greer
ProtoStar Astronomy Products