Dealing with a Turned Down Edge
Michael Rapp
steady/clear nights. It's apparent that I have a turned down edge. That is,
my inside focus and outside focus images look almost exactly like these at:
http://www.aegis1.demon.co.uk/tutorials/edge.gif
It's just like the picture shows, outside of focus I have a very prominent and
bright outer ring while inside focus the rings are fuzzy and indistinct.
I'm going to TSP this year. I'm going to call John Hall at Pegasus Optics
tomorrow and see if he can refigure a mirror in a month. I'm not all that
optimistic as I know he's very busy and he also has to send the mirror to a
coater which could take up to four weeks in itself.
If I can't get my mirror refigured in time for TSP, what do you suggest for
optimizing my optical system in time for TSP? What is the best way, aside
from refiguring, to deal with a turned down edge? What effect does a turned
down edge have on my image quality? (Yep, I've got Suiter's book on order.)
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Chuck Gulker
(experiment) of the edge of the mirror. You might end up with a very good
mirror. Give it a try. It is free and you can always take off the mask.
Good luck, Chuck
Michael Rapp wrote in message <36fe5a19...@news.uh.edu>...
BillFerris
>What is the best way, aside from refiguring,
>to deal with a turned down edge? What
>effect does a turned down edge have on my >image quality? (Yep, I've got
Suiter's book
>on order.)
The principal effect of a turned edge is two-fold. It limits the aperture of
the telescope. Essentially, your mirror performs no better than a telescope
with an aperture equal to yours minus the turned edge. Also, turned edge
reduces contrast.
Suiter compares the loss of contrast caused by turned edge to the negative
effects of a dirty mirror. Both cause scattered light which in turn reduces
contrast. Turned edge is different in that the contrast loss is greatest at
spatial frequencies where most people do serious lunar and planetary observing.
Suiter suggests masking the outer portion of your primary as the best cure for
a turned edge. The outer few millimeters are of no use, anyway, and contrast
should improve due to the ellimination of the worst part of the mirror surface.
The masking approach suggested by Chuck Gulker seems reasonable enough. It
allows you to experiment with masks of various widths without permanently
damaging or otherwise changing the mirror.
Bill Ferris
Flagstaff, AZ
Bob May
working it???
Lots of questions but thats the way I see it.
Bob May
Herm
inevitable in mirrors and will automatically mask the edge. It would
be worth it to use the cardboard mask and see if it improves..if it
does mask the scope with a bit of black paint along the edge.
If its only the outer 1/4" and the scope seems to be good otherwise
then I would not bother having the scope refigured.
Herm
On Sun, 28 Mar 1999 11:49:25 -0500, "Chuck Gulker"
<cgu...@columbus.rr.com> wrote:
>You might want to take some black cardboard, and mask off the outer 1/4 inch
>(experiment) of the edge of the mirror. You might end up with a very good
>mirror. Give it a try. It is free and you can always take off the mask.
>
>Good luck, Chuck
>
>Michael Rapp wrote in message <36fe5a19...@news.uh.edu>...
>>I've been startesting my telescope (8" f/6) several times over the last few
>>steady/clear nights. It's apparent that I have a turned down edge. That
>is,
>>my inside focus and outside focus images look almost exactly like these at:
>>http://www.aegis1.demon.co.uk/tutorials/edge.gif
>>
>>It's just like the picture shows, outside of focus I have a very prominent
>and
>>bright outer ring while inside focus the rings are fuzzy and indistinct.
>>
>>I'm going to TSP this year. I'm going to call John Hall at Pegasus Optics
>>tomorrow and see if he can refigure a mirror in a month. I'm not all that
>>optimistic as I know he's very busy and he also has to send the mirror to a
>>coater which could take up to four weeks in itself.
>>
>>If I can't get my mirror refigured in time for TSP, what do you suggest for
>>optimizing my optical system in time for TSP? What is the best way, aside
>>from refiguring, to deal with a turned down edge? What effect does a
>turned
>>down edge have on my image quality? (Yep, I've got Suiter's book on
>order.)
>>
Michael Rapp
wrote:
>Michael Rapp <mrapp@*deletethispart*io.com> wrote:
>> What effect does a turned
>>down edge have on my image quality?
>
>Won't snap to focus and contrast will be "compromised", I think the
>modern term is.
What's interesting is that I've had this scope since 1991 but it hasn't been
until relatively recently that I've looked through comparable reflectors and
seen their images. My scope wasn't living up to those 8" reflectors and I
began to wonder why and I got interested in star testing. One thing I did
notice is that my scope doesn't have that snap to focus on stars . . sorta
blends in and blends out.
Thanks for the advice!
jerry warner
discover the extent of this TDE (1/4" etc?) so try several different mask (1/4 -
1inch).
You have one thing on your side - your system is f.6. - affects of tde varywith
f/number...
worse at f/4 vs f/8+. If the width of tde turns out to in excess of 1/4inch I
would have
this mirror refigured as the cost is minimal vs having to live with a lousy mirror
and lost
aperture, or take the same $$ and shop around for another better mirror (one
tested)?
I'll confess I never liked masking - if I had a badly tde'd mirror I would find
the extent
of that tde and just black it out (withpaint!) right on the mirror. I have
functioned with many
blacked mirrors amazingly well... andmy advice would be if you are going to
replace
or refigure this mirror, ince the extent of tde is determined just black it out
... it beats
masks blowing in the wind etc!
JWarner
Michael Rapp wrote:
> I've been startesting my telescope (8" f/6) several times over the last few
> steady/clear nights. It's apparent that I have a turned down edge. That is,
> my inside focus and outside focus images look almost exactly like these at:
> http://www.aegis1.demon.co.uk/tutorials/edge.gif
>
> It's just like the picture shows, outside of focus I have a very prominent and
> bright outer ring while inside focus the rings are fuzzy and indistinct.
>
> I'm going to TSP this year. I'm going to call John Hall at Pegasus Optics
> tomorrow and see if he can refigure a mirror in a month. I'm not all that
> optimistic as I know he's very busy and he also has to send the mirror to a
> coater which could take up to four weeks in itself.
>
> If I can't get my mirror refigured in time for TSP, what do you suggest for
> optimizing my optical system in time for TSP? What is the best way, aside
> from refiguring, to deal with a turned down edge? What effect does a turned
> down edge have on my image quality? (Yep, I've got Suiter's book on order.)
>
Stephen Tonkin
>What is the best way, aside
>from refiguring, to deal with a turned down edge?
Once you know the extent of it, masking or blacking the mirror edge will
do the trick. (You can use masks and star-testing to determine the
extent.)
> What effect does a turned
>down edge have on my image quality?
Won't snap to focus and contrast will be "compromised", I think the
modern term is.
Noctis Gaudia Carpe,
Stephen
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