RE: [ATM] wire spider, and regular spider, reason not to blacken

["Jerry" <wa4guu@verizon.net>]

-----Original Message-----
From: Jay Kirkland
Subject: [ATM] wire spider, and regular spider, reason not to blacken

In the recent thread about wire spiders, I believe it was Mel Bartels who
said not to blacken the spider. On various web sites, I've seen many scopes
with their spiders and secondary holders left bare, and I wondered why.
Aside from paint just not adhering to stainless. Well, I just read the
reason in a book I just got. From Lecleires' excellent A Manual for Amateur
Telescope Makers, pg 272-3: 
	"Don't paint the spider and the secondary support black... Black
paint
will absorb infrared light from the sky, 

******************


This is counter to the comments by others on primary mirrors radiating their
heat to the cold sky causing a temperature gradient from the front of the
mirror to the back. 

Whichever is the case, if I can see it looking through the focuser with no
eyepiece in daylight I would blacken it. I think the stray light bouncing
off of un-blackened parts into the eyepiece might be more harmful than
whatever heat might be absorbed from the night sky.

Jerry  


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