ATM 36" optical test results

[<jrh@nofs.navy.mil>]

This is a continuation of Performance with Spherical Aberration:

Hello ATM's,

As several of your suggested, a Ronchi grating at the 
focus using Altair, which was high in the southern sky, 
was revealing.

I had to place the grating(s) outside of focus; the bands 
were not distinct inside of focus.  Even with the grating 
outside of focus, the bands were difficult to see.  Two 
different gratings of 80 and 100 lines per inch gave 
similar results.

With the Ronchi grating vertical (to my eye), the bands 
bowed outward (away from the mirror center) fairly strongly 
near the mirror center and less strongly near the mirror 
edge.  It appeared that the center two bands (of four 
total bands) became almost straight toward the top and 
bottom of the mirror.

The most troublesome aspect, however, is all the bands 
turned very sharply outward at about two inches from the 
edge.  The center two bands actually turned outward at 
right angles; the two bands closer to the side of the 
mirror turned outward at about 75 degrees (although 
they may have been at right angles to the curving 
bands - it was a very sharp turn). 

It appears this telescope system has zones with an 
especially strong one in the outer two or so inches.


I carefully examined the image of Altair around focus,
although focus is not an accurate term because this system 
does not really focus.  It appears there is also significant 
astigmatism when moving slightly to a moderate distance 
from "best" focus.  Far out of focus, the image looks 
reasonably round with just a trace of collimation error.
This is outside of focus.  The image inside of focus is 
ragged and difficult to analyze.

I also used Suiter's technique of estimating spherical 
aberration by noting when the dark center of the out-of- 
focus image appears.  I moved the focus knob about 1/4 
turn in and about 2-1/2 turns out to get approximately 
the same size shadow.  This suggests my original estimate 
of 1+ wave of spherical aberration is reasonable.

Closer inspection also showed the coating on the primary 
not to be in new condition, so we are loosing some light 
here too.  I believe the same person who made the optics  
also coated the mirrors.

Glare from Altair after it drifted out of the field of 
view showed that the baffling needs modification.

Another inspection of M-13 showed brighter images than I 
remembered from the last observation, but the stars just 
would not focus to the point I am used to.  The telescope 
owner agreed that M-13 looks brighter and better in my 
20", which has a reasonably good mirror.  Observing 
conditions were roughly similar for this comparison.

The telescope owner and I also noted a bright ring of 
light around the edge of the 55mm Plossl field of view.
This was not visible in a 26mm orthoscopic (twice the 
magnification).

The final target for the evening was Epsilon Lyra.  We 
could tell the pairs of stars were double, but there 
was no space between the 2 to 2-1/2 second images.  Of 
course, I have seen this multiple star the same way in 
my 20" due to seeing.  Epsilon Lyra was very high in 
the sky during this observation.  A nearby AstroPhysics 
6" showed Airy disks with some boiling due to seeing.


For an immediate course of action, I will recommend the 
owner mask off the outer 2 or so inches of the primary; 
I do not think that (large) area is contributing to 
good images or contrast.  He will not be happy to have 
a 32" telescope, but....

It appears to me that rework of both mirrors will be 
necessary to fix this optical system; a test of the 
primary alone will be necessary to verify this diagnosis.
The owner prefers a classical cassegrain design, and 
this will be an opportunity to provide that.  
Unfortunately, the owner has expended most of his 
financial resources on the original purchase and 
installation of this telescope. 

It also appears to me that the person/company that made 
this system is not competent to refigure the optics.


Thank all of you for pointing me in the right directions. 
It turns out that all of your suggestions addressed 
real problems in this telescope system.

Joe Hobart
Flagstaff, AZ