Re: [ATM] Improving my focault tester / testing skills

["Bob May" <rmay@nethere.com>]

Got a bunch of good advice already.
As far tas the bulb is concerned, if you loook at it and you
don't see the filament then the ight source is good enough.
Problems here will reflect not in horizontal errors with the
zones blinking out but rather that the zones may blink from top
to bottom or the other way around.
I don't use the Couder mask as I find that it is hard to select
each zone for anylisys so I make two masks with the odd zones on
one mask and the even zones on the other mask.  You can even go
to the ultimate and do a mask for each zone if desired.

I do my testing in an almost dark room so that the mask is fairly
dark to the eye of the camera that I use and also compare the
gray scale against the gray of the mask as I am going along.  I
do find that it is a lot easier to see the shadow zip across the
zones than to try to see a steady reading of that zone as the eye
is a lot more sensitive off-axis to movement than it is to seeing
a level of gray.  When the two zones are blinking out at the same
time, you won't be able to see a differencde in which one goes
first.
Approach the reading from each direction and keep at a zone until
you keep getting the same reading on the measuring tool.  This,
if done correctly, will improve your skills at reading the
measurement.
The slitless tester is just a slit tester that doesn't have a
second edge to limit the light.  When you look at how the test
really is run, you'll see that you need to have a very small slit
in order to be able to see the effects of that slit on the image
of the light from the mirror and you'll be getting huge
diffraction effects from that second edge as a result.
With skill, you should be able to get the readings down to two or
three thouisandths of an inch in the repeatability dept.
There are cheap video cameras that are now being sold for
surveilance work and I recently bought a color camera at a drug
store for thirty five dollars and it came complete with power
supply and an extra lens.  That is quite cheap especially when it
also had sound and the vables to hook up to your TV set
(assuuming that you ahve A/V inputs to your TV set).  The camera
makes things nice as it is a lot steadier than your eye behind
the KE but it does want to have a longer focal length lens
attached to it rather than the very short FL lensees that they
come with.  You'll probably want a cheap lens of about an inch of
focal length for this job and it doesn't matter whether the lens
is a single lens or a doublet as you really don't need a lot of
focusing ability to see the grayness of the surface.  Just put
the camera in a box with the lens on a hole in the box and adjust
so that it can see the wall across the room and you're focused
well enough for the job.  The lens wants to be right behind the
KE, touching it if possible for best results.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net


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