ATM Longitudinal Aberration Measurement

[Bob Prabowo <BOB@bahana.co.id>]

Guys,

Last night, for the first time, I tried to measure the longitudinal
aberration. I have a mm dial gauge attached to my Foucault tester now,
thanks to Dave Bevel and Dan Fundo. This dial gauge really makes
measuring easier. IMHO, espescially because it's in metric :-)

The specs of my mirror is D= 330 mm, ROC 3633 mm.
Now, the formula I use is the one in Berry's book, i.e, delta R=r^2/2R,
because I use a moving ligt source. According to this, delta R is 3.747
mm.

To measure it, i follow Tex's method. I forgot in what page it is, but I
tried to match the shadow of my mirror with picture in fig. 37A to 37C
in Tex book and measure the difference between 37A and 37C. 
When the sahdow matched the pictures in 37 A, I moved the k.e backward
until the shadow appeared like fig. 37C, and measure the
difference/distance between the two. I took two readings, the results
are 4.7 mm and 4.95 mm. Please remember that I'm still trying to do it
right here. At COC it looked like fig 37 B in Tex book.

  Questions:
1. How do you tell when a mirror is hyperbola, ellips or parabola?  Do
you subtract the readings result with the theoretical result of delta R?

2. In Tex book, he says (if I remember it correctly) that for parabola
the b is -1. I'm confused with this. Can anybody enlighten me?

3. Ronchi test shows that the mirror is overcorrected. Should I continue
to try to figure it back to  "sphere" and then parabolize it from there?


Thanks,
Bob P. Sumitro
Jakarta, Indonesia

visit my still-under-construction astronomy and telescope making page:
http://www.geocities.com/CapeCanaveral/Hall/2670/