[ATM] Ross Null Test-would this lens work?

[DrewClan@aol.com]

Yes, your 1/4 wave pcx lens will certainly give you a null for the  parabola. 
I do think the focal length is a bit short, but more on that  later. As far 
as the quarter-wave accuracy, one of the things to keep in mind  with the Ross 
Null is that it's a double-pass test (the light goes through the  lens twice, 
going out to the mirror and reflected back from the mirror). So any  error in 
the figure or the smoothness of your test lens will be doubled.  The centers 
of these lenses tend to have the best figure, with most of the  errors out 
towards the edge. This is an advantage since you will typically  only be using the 
center 2/3 diameter of the Ross lens. This is why Peter  Ceravolo says 
"bigger is better." Hmm, aperture fever in testing lenses  too?
 
Other sources of error in the Ross Null Test include spacing errors. The  
excellent rossnullxp program will actually figure this error for you in the  
little function called "how good is your ruler?" How good does your ruler have  to 
be? I work within 1/4 inch with good results, but more finicky fiddlers will  
have better nulls with careful measuring. I work with a simple tape measure 
and  try to keep most of the sag out of it when I measure. Keep a close eye on 
your  spacings since you'll find that as you work with the tester things will 
creep  around a little.
 
Collimation of the optical path between the tester, test lens, and mirror  is 
critical as well. Good quality optical glass is worth the extra money,  
especially if you're shopping surplus. Your lens has a relatively short focal  
length for the Ross Null test, and I think it will be difficult to set up and  
use. It will also have a bit of aberration built in because of this.  You might 
find that a longer focus pcx lens works better and is easier to set  up. For 
80mm lenses, I recommend 300-400mm focal lengths as a better  choice.
 
How do you calculate the radius of the convex surface of your pcx lens? The  
best way is with a spherometer, but failing that a simple calculation will  be 
close enough, assuming a thin lens made of BK7 glass. Call the flat surface  
of the plano-convex lens R1 and the convex side R2. The radius of R2 is equal 
to  half the focal length (R2 = F/2). If you're sure you have BK7, you can get 
a  better calculation using the "refractive index" of the glass: R2 = F * 
(1.51509  - 1). 
 
I do think that this is an under used test for amateurs and hope that you  
have good results with it. Once you've done a couple of mirrors with the Ross  
Null test, I'll bet you have a hankering to purchase a certified lens from 
Peter  Ceravolo. Incidentally, the letter to the editor about cheaper Ross Null 
lenses  that I mentioned is in TM #43, not 49. Sorry for the misinformation on 
that  one.
 
Jay

 
In a message dated 9/29/2006 12:56:02 AM Central Standard Time,  
gilboastro at yahoo.com writes:

I  believe these lenses (it is by Melles Griot) have at
least a 1/4th surface  accuracy, so not perfect.  

How does one compute the R of  C?