Re: [ATM] Spherometer calibration, precision, etc.

["Thomas Janstrom" <thomas@moiler.com>]

Hi Dominic-Luc,

I use a spherometer and I routinely get the measured ROC to match the design
ROC as measured on the test stand to within 0.05mm not easy, and I only
really worry about it on short focus surfaces, but I can match mirrors to
better/less than 12mm (when polished out) difference in ROC on mirrors
ground and polished independently. 

To measure on the test stand I use calibrated metal rods and a pair of
callipers, my test area is at a constant 25'C....

My spherometer can be seen at http://www.tjanstrom.com/spherometer.html 

Cheers, Thomas.

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Dominic-Luc Webb
Sent: Sunday, 25 February 2007 3:31 AM
To: ATM Superheros
Subject: Re: [ATM] Spherometer calibration, precision, etc.


Mike, others....

OK, I did a little numerology this morning on this topic of measuring
the ROC of a Cass secondary with a spherometer. When I have wanted
tight precision of ROC, I have always actually measured the actual light
with a pinpoint light source and piece of paper beside. What raised a
red flag with me was not only that a spherometer was being used, but it
was being used for such a presumably small diameter surface with a low
F/ratio that will not be spherical (again, this is a Cassegrain
secondary). Most ATMs are after visual systems so I presume in this
project some field curvature (from a low F/ratio secondary) at the
image surface is acceptable. Consider this secondary surface:

Diameter: 150 mm
ROC_design: 1000 mm  (sagitta: 2.816466 mm)
ROC_actual: 1010 mm  (sagitta: 2.788503 mm)

I would say most of us would agree that 150 mm diameter for a Cass
secondary is large, and an ROC of 1000 mm (F/3.3333) presents a strong
sagitta for any measuring instrument. The sagitta for the two ROCs
above (design vs what is actually obtained) differ by only 0.027963
mm (0.00110"). I am pretty sure I could measure this with my pinpoint
light and paper test, but I am curious how well you are actually
measuring this with a spherometer, especially in light of the fact
you even move the spherometer around the surface a bit to detect
deviations. In this example of a rather large surface, the pins on a
typical spherometer (3 pins, spaced 120 degrees apart) could not be
more than 50 mm from the center tap. Smaller diameters would be even
more challenging. The exact point on the surface where the spherometer
is placed must affect the measurement of sagitta and people do not
usually have a precision technique for positioning a spherometer. Are
people here actually getting such good measurements, and if so, could
someone please point me to a picture of what was used?

The one time I actually compared a professionally built spherometer
against my pinpoint light measurement was with an 8" F/8 sphere. The
ROC from the spherometer reading differed from my light measurement
by roughly 2". At that time, I formed an opinion that there has to be
some allowance for deviations between actual and design ROC, if for
no other reason, because different measuring instruments and methods
give will not give exact same values. Staying within design
tolerances then becomes an issue of which ruler is used, so maybe it
is wise to use the same ruler for the entire optical system to hold
relationships between surfaces exact.

Come to think of it, at what point do you go for the hyperbolic
curve (fine grinding or polishing) for such low F/ratio. If it needs
to be introduced during fine grinding, wouldn't this make using a
spherometer difficult?

Part of the reason I ask is that I have a cored 12.5" F/5 that could
potentially be transformed from a Newt into a Cass. Predicting some
frequent testing ahead, I would not mind having a very fast test.

Actually, along similar lines, I never got round to purchasing one
of these high precision guages for a spherometer (best I have now is
1.968 turn/mm screw). Maybe someone can remind me of the source,
and I'll just go ahead and order it.

Dominic-Luc Webb

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