Re: [APML] Novak cells, clip diffraction patterns (was Alnitak)

["Joseph O'Neil" <joneil@multiboard.com>]

>    The clips are flat black.   The R200SS has a large secondary
>(short focal length).  The entire mirror (with clips) is visible from the
>focuser.  Is that the case in each of your scopes, i.e. is the
>entire light cone (including the clips) visible?.  If so, how wide
>are the clips.  The Vixen clips are fairly wide.

         That always bugs me in commercail mirror cells (warning - since I 
sell oour own brand of mirro cells, I am heavily biased in this area).  But 
I have been following this thread,a nd a poor quality mirror cell, or more 
commonly, a poorly mounted mirror can cause all sorts of 
problems.  Sometimes the eye will forgive these faults, but film never 
will.  Not sure if this applies to your images, but cehck out a couple things:

         - glued in mirrors.  Two problems.  first, on some scopes liek 
Meade, they use huge pads of glue that prevent the mirror form poperly 
expanding and contracting during any heating/cooling.  There should only be 
little, small dots, no larger than say 6 to 8mm in diameter.
         Secondly glue should be soft, flexible, and pliable - such as a 
good quality silicon caulking is, even after a full cure.  Some glues I see 
used look like hard expoy style glue,a nd this is never good.

         - too tight mirror clips.   Properly glues in, you should never 
need mirror clips to beging with.  Even not glued in, mirror clips over the 
front surface should be small, and NOT tight.  A good place to start is try 
to leave the spacing between the surface of the front of the mirror and the 
mirror clip equal to the thickness of a peice of paper.

         Now, as you can image, with this "looseness", during long time 
expsoures, you will definatgely get some mirror shift.  Whenever we modify 
a mirror cell for imaging use, we always, always, tell teh customer to 
remove the ornt mirror clips, and glue the mirror into place.  Up here in 
Canada, we reccomend RTV gasket seal, avaialble in most automotive supply 
sotres, comes in black, is highly pliable in extreme heat and cold, 
inexpensive, easily removeable with fishing line, and Peter Ceravol uses it 
on his mak-newt mirrors.
         When you do this, you do not have the front surface of the mirror 
being pinched by any mirror clips.  This can cause distortion on images and 
views, both visually and on film/CCD.

         Well, i could go on a almost write a book about this, but these 
two areas seem to be the amin fualts i see with a lot of "commercial" 
mirror cells - that is, mass produced stuff.  Novak cells are just fine, 
IMO (of course, I think ours are better :), but with a Novak cell, the only 
real fault is improper installation or re-installation of the 
mirror.  Anybody else, if the mirror is glued in properly, you shoudl never 
need fornt clips.  You should be able to hand your scope completely upside 
down during a major earthquake and never worry about it letting go if the 
job is done right.  20 years ago maybe this was not true, but  some of the 
new adhesives being developed, I would say right up to 10", you can get 
away with no clips.  Anything larger, just maybe, almost just maybe...
:)


anytime
joe



http://www.oneilphoto.on.ca
http://www.multiboard.com/~joneil


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