RE: ATM : Double star test results: flex mirror / cooling fan

["CSC" <optic@gwi.net>]

Your star test shows there is a correcting effect by flexing.  To try to get
it right by observing a focused star would be trial and error, and may be
alright.

The 1.5 sec double would only be split with everything in your favor:
	Still night.  The nights of arc second seeing are usually infrequent.
	No collimation errors.  A 6" f/8 should be forgiving.
	Cooled scope and optics. Setup scope in the right spot. No warm air
currents.
	Patience.


My method for setting the correction is to use the star test using the
comparison of inside and outside focus images.  Here it goes:

-Use a high power eyepiece on a very steady night. I use a 7.5mm Plossl on
my 6" f/10.5 or my 8" f/7.  215x and 190x respectively.  150 and up will be
good for any 6". I use Polaris because I do not use a drive on these scopes.

-Before you flex the glass, note how different the out of focus images are
when you rack the focus inside and outside the same distance, say 1/8 of a
turn.  The undercorrected sphere should show a brighter edge to the
diffraction image and a hollowed out center.  The shadow of the secondary
becomes visible almost immediately when going inside focus. The opposite
happens outside focus, the image having a strong center area and a dim edge,
the secondary shadow showing itself only after you back out a ways, much
further than when the shadow shows inside focus.  Your scope is about 1/2
wave undercorrected.

-Increase the flex tension until the in/out images are identical.  When you
are as close as possible, you're within maybe 1/4 wave; try the test on a
colored star like Gamma Andromeda, a nice colored double, or use a colored
filter. When you are out of focus, there will be fine rings seen inside the
diffraction image. These will look different inside/outside when there is
correction error.  If you can adjust these to look similar in/out, the
correction is done.

-You'll also see the effect of roughness, turbulence and collimation errors.

-The higher the magnification, the more precise the star test. I often go up
to 300-400x just to check the images closer to focus, but I'm a wacko, and
have a fetish for wavefront correction.

-My 6" f/10.5 is a tiny bit overcorrected, both by the Foucault test and the
star test. The outer zone is long.
-the 8" flex is right on.  Last Friday's images of Saturn at 250x blew
people away.

good luck.  It should blow your socks off on the right night.

CSC


One side project of mine is to mess around with a
little 6" f8 dobsonian that has a very simply flexed
spherical mirror (bolt glued onto the back - no foam,
no disc, just file the head of the bolt flat and glue
it on).  I recently had a nice steady night and
experimented with flexing and relaxing the mirror to
see changes in resolution.  I admit that I have little
in the way of comparison, and so would appreciate
feedback from more experienced users.

Separation:  2.4 seconds.  Unflexed - indication that
it is a double but no separation.  Flexed - clear
separation - wide split.  Cooling fan (16 cfm) - no
difference.

Separation:  1.5 seconds.  Unflexed - fat star.
Flexed - moments of "amoeba mitosis" - never any total
separation.  Cooling fan - no difference.

- Gil
"The mind is a funny thing"


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