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

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

Are you sure that he was saying that R2 and R4 only have the effect of moving the focal point around, and that R1 and R3 were more difficult? I found that R1 and R3 were a piece of cake to fabricate, but that R2 and R4 were exceedingly difficult because they were so close to being a flat.
Our scope has an 8 inch diameter mirror, f/5 and corrector plates that are 7.6 inches in diameter. I used several different eyepieces (RKE, Plossl, Erfle, different lengths, no UWAs cause I can't afford that) and it didn't seem to make a difference, everything was awful. So awful, that I didn't even bother waiting for it to get clear and dark enough to do a star test. Imagine having a Trashco refractor that somebody has smeared LOTS of vaseline on the front of the primary, and that it's so hot that the vaseline moves around.
I am more and more beginning to think that the one thing that we didn't check ourselves, namely the actual indices of refraction and dispersion of the glass, has a good chance of being the problem. Unless all of the optical defects disappear when I make a device to enable me to make the corrector plates well and truly parallel to the primary.
I'm not complaining! It's been a very interesting and educational exercise!

Chris Dalla Piazza <dalchri@hotmail.com> wrote: Guy,
I think that he was talking about the two long focus surfaces on the
corrector. Sorry to hear about your troubles but I wouldn't doubt your optics just yet. My first experience was less than I expected too.  What sort of an eyepiece were you using?  What size scope is this?
Visually, I just don't see much of an advantage over my identical Newtonian using a 15mm Televue Possl.  The edge is a little better but not tack sharp edge to edge as I was expecting.  This is with a 6" f/4.5.  It seemed like the "good" part of the field on the LHN was wider though.  It's just that the edge deteriorates much more rapidly than the Newtonian when it does start to go bad whereas the Newtonian deteriorates linearly towards the field edge.

Again, I don't know how much of this is contributed by the eyepiece.  I sort of remember hearing that some eyepieces are designed for Newtonians, SCT's, or other systems with coma.  I'm not sure how much the match between eyepiece and telescope might also be at play.

It really shines on my CCD though.  The out of focus and near focus star
images are much rounder and symmetrical on the LHN.  And the nice thing about the off axis astigmatism that is present is that it does not make the star images asymmetric if you focus well enough.  It can actually aid in focusing as you watch one astigmatic light lobe disappear into the focused star image and the other emerge from the focused star image 90º around the star image.

What did your out of focus star images look like?  Were they round?  Did you notice any spherical aberration on either side of focus?  Something I want to try before I am satisfied is a star image knife edge test.  I am
interested to see if any defects show up in the BK7 glass.

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Guy Brandenburg
Sent: Monday, August 13, 2007 11:12 AM
To: Bob May; atm@atmlist.net
Subject: Re: [ATM] first light on a Lurie-Houghton

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  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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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

=============================
"The corroded and corrupt state of Saddam was replaced by the corroded, inefficient, incompetent and corrupt state of the new order," Ali A. Allawi concludes in "The Occupation of Iraq", newly published by Yale University Press...
=============================
_______________________________________________
ATM mailing list http://www.atmlist.net/