Re: [APML] more new pics

[Aplanatic@aol.com]

Hi Bobby:

> Also on the M-57 shot. If you look you will see evidence of my bit of
>  astigmatism with this scope. The corner stars in the lower right and upper
>  left of the image are a bit oval while corner stars in the upper right and
>  lower left remain round. The astigmatism is easily noticable in star tests.

I don't see the same thing that you see, but I didn't take a close look in 
Photoshop.  I see what appears to be a bit of coma in the UL, UR and LR 
corners.  My guess is that the teleconverter is slightly out of square or a 
bit off-axis.  It can also introduce its own set of aberrations (like coma) 
which complicates the picture.

>  I have not fiddeled a lot with the focuser to make sure I'm not
>  out-of-square but I have adjusted it some. I think that diagonal
>  mis-alignment can also introduce astigmatism from what I have read.

The diagonal, the focuser and the primary all act together to define the 
optical axis and the focal plane.  Diagonal mis-alignment in and of itself 
cannot produce astigmatism, but an optically bad diagonal sure can.  
And a mis-collimated system will show asymmetrical coma, which 
you might be incorrectly interpreting as astigmatism.  

Astigmatism is an overused word and means different things in different 
situations.  When seen on-axis it means that there is an axial asymmetry 
to the optics.  In a Newtonian this is most often due to the diagonal not 
being flat, but can also be due to a poorly made primary, pinched optics, 
and tube currents that never completely die down.  When seen off-axis, 
like in a R-C Cass, it produces round images that don't come to a well 
defined focus.  In the first case (on-axis) its not supposed to be there at 
all, whereas in the second case (off-axis) it may well be an inherent 
feature of the optical system.

Astigmatism (the off-axis kind) combined with a tilted focal plane will 
cause very interesting star images.  Where the film plane is inside the 
best focus, stars will be elongated in one direction and where the film 
plane is outside of the best focus the stars will be elongated in the other 
direction.  This can easily be misinterpreted as coma, and in some 
situations can be nearly indistinguishable from it.

Add a (potentially) misaligned teleconverter to this mess and things get 
quite out of hand.

So, your best bet is to go back to basics.  Make sure that you are very 
well collimated.  Use a laser collimator that has been carefully squared 
to the film plane and adjust the secondary and primary accordingly.  If 
you can, check the teleconverter alignment and centering.  Then, with 
an eyepiece look for on-axis astigmatism, rotate the mirror (if possible) 
45 degrees in its cell and test for astigmatism again.  If the astigmatism 
moves with the primary then that's where the problem is, otherwise it's 
in the secondary or the secondary holder.

>   If I do
>  find that it is in the main mirror, is that something that can be easily
>  taken care of with re-figuring?

Well, easy is in the eyes of the beholder, so to speak.  If the glass is 
stressed then it can sometimes be impossible to remove all the 
astigmatism from the mirror, but this is rare.  If the glass is OK then 
it's just a matter of work.  Not much different from figuring a new mirror.

A simple Foucault test in the hands of an experienced optician will reveal 
whether or not it's astigmatic.  So, make sure you have it tested by 
someone who knows what's up before re-figuring it.  Once again, it's 
much more likely that the secondary is bad or the optics are pinched.  

>  Of course exhaustive testing of all other
>  possibilities needs to be done before re-figuring is considered but as of
>  now I want to spend my time taking pictures instead of testing and 
adjusting
>  components. One of the disadvantages of a newt..........lots of parts that
>  can move around.

Yes, lots of parts and even more ways that things can get out of hand, eh?

Nice work, Bobby,

Dave Rowe

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