ATM 12.5" Double-Arched Mirror & Light Scatter

[Roger Ceragioli <rogerc@as.arizona.edu>]

Possibly this will interest a few people.

It has become clear to me that at least some of the TDE I've been seeing 
in my mirror is thermally induced.  A few nights ago, despite masking 
the outer 3mm of my mirror, there was terrible TDE in the image.  And 
you could easily see it by holding a Ronchi grating up near the focus of 
a bright star.  The night was hot and somewhat humid.  I was observing 
(of necessity) over gravel.

Then last night the weather started to improve and after midnight the 
air was much cooler and pleasant.  I took out the scope and it had 
almost no equilibration to do.  Presto!  Almost no TDE in the 
image--except in about one quadrant of the mirror (as judged by Ronchi). 
 The star test was much better.

This behavior amazes me.  I know that at least one other person on the 
list has seen such behavior in his mirrors.

Has anyone else had problems with TDE in the image which varied with 
temperature?  Of course, you expect such variance in the spherical 
correction as the night goes on.  But I've never seen the edge of a 
mirror so affected before.  Thanks for any info. you have.

Concerning light scatter from mirrors:  my 8" f/15 achromat often uses 3 
fold flats, and John Gregory's legendary 8" apo uses 4.  But the images 
from John's scope are exquisite.  I remember that at TSP '97, there was 
a night when Mars showed the most incredible detail through John's 
apo--Yes!--as though from a space probe nearing the planet.  My jaw 
dropped, because my nearby 8" f/6 reflector showed so little detail.  My 
achromat has never shown images like John's apo, of course, but I have 
seen "canals" on one occasion last year.  These were independantly 
confirmed by a fellow observer who didn't know that I had already seen 
them.  Not that they really were canals, but you have to get good images 
for this visual illusion to occur, I think.  

And the point is that John's scope folded the light path FOUR times with 
mirrors (mine three time), whereas my 8" reflector (not excellent, but 
certainly good) used only two mirrors.  So light scatter from the 
coatings can't have a huge effect, as long as the mirror surfaces are 
reasonably smooth and the coatings were properly applied.  Otherwise, 
John's scope should have shown nothing on Mars.

Moreover, I frequently silver my mirrors for star testing now.  And 
sometimes the surfaces come out visibly hazy.  Yet, so far I haven't 
noticed any significant visual differences in the images produced by 
these hazy silvered surfaces and well aluminized ones.  Of course, 
coronagraphs are a different story, but visual amateur astronomy doesn't 
need coronagraphic surfaces.

The real culprit is what Texereau and many others have been saying 
forever:  thermal problems frequently plague reflectors like nobody's 
business with devastating results.  It's amazing how many different 
thermal problems there are and variable they are.  They seem to be worse 
with imaging mirrors than fold flats, and are much worse when the 
mirrors are exposed directly to the sky.  Windows and full aperture 
correctors (e.g. Houghton correctors) can vastly reduce, but not 
eliminate these themal problems.  Fans can help too, but not cure. 
 Central obstructions of moderate size show almost no effect, as Suiter 
indicates.

Well, enough belaboring the obvious.  Now, I'll shut up.

Roger Ceragioli