Re: [ATM] first light on a Lurie-Houghton

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

Well, I took the LHN out for its first light attempt at the Almost Heaven Star Party, and even attempting to view nearby terrestrial objects, it was obvious that the optics were simply very, very, very, very bad. Nothing was clear at all, and if you moved your eye around in the eyepiece, different parts would become clearish or more blurry. 
Disappointing and somewhat enlightening at the same time.
The primary is 8" f/5, approximately, and is very very nice and spherical and well aluminized.
Surface #3 is very nice and spherical, and surface #1 matches it very well, within a wave or less.
Surface #4 is nice and spherical, but surface #2 (convex) forms nice circles in the interference test, but appears to be about 1.4 inches different from #4 in ROC. It didn't seem to matter in OSLO.
I could tell that the corrector plates were not truly parallel to the primary, but didn't have the means to fix and adjust that at the time.
I have some hypotheses for what might be wrong:

(1) Possibly the glass of the corrector plates was not what we thought, which would make all of the curves incorrect.

(2) Possibly the tilt of the corrector plates is worse than I suspected and is more critical than I thought.

(3) Possibly the spacing between the corrector plates must be adjusted.

(4) Possibly the ROC's of #2 and #4 are too different.

(5) Possibly #2 isn't a good surface after all.

(6) Possibly the tolerances of a LHN are much tighter than I was led to suspect; the ROC of #2 is a bit different from what was  planned.

Since Bob May doesn't like to quote previous messages, I unfortunately don't know which surfaces he is referring to. Perhaps he could clarify? However, I would be extremely surprised if there were any astigmatism anywhere.

Bob May <rmay@nethere.com> wrote: From the calcualting that I've done for Dale Horn's 12.5" F4.6
design, I've found that those two surfaces do affect the focal
point but that almost seems all that they do.  The much more
difficult surfaces are the other two which Dale wants to have
within two waves of each other and those tend to move the color
correcction around a lot.
If you can, build a large focuser so that you can take a small
eyepiece that you put in a plate and let it move about on the end
of the focuser so that you can investigate the edges of the FOV
with the EP.  A Jap standardd four mm EP would probably be a good
choice fort the EP.  Sliding it about on the back of the focuser
will allow you to see if there is any focus shit over the field
and let you see the edges of the field for when astigmatism
starts to get in the way.  The focus shouldn't shift at all if
things are square and the astigmatism should be very tiny until
you get to the edge of the field where it will suddenly blow up.
Bob May



Guy Brandenburg, Washington, DC
My home page on astronomy, mathematics, education:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
or else 
http://tinyurl.com/r6fh2

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