Roland tests a mushy ETX
Duane Sand
I'm reposting an article that Roland just posted into the
the "Re: S&T and Burning Bridges" thread, to give it a
more descriptive thread title and to help everyone notice it...
chri...@aol.com (Roland Christen) wrote:
>I just had the pleasure of testing one of the hot new 90mm Maksutov
>telescopes (Questar wannabes on the cheap). It shows almost perfectly
>symmetrical star patterns inside and outside of focus, but is surprisingly
>soft on the Moon and planets. It was also very sensitive to atmospheric
>disturbances (see the S&T review of the 7" Meade Mak). My own 8" Mak set
>up right next to it showed almost no atmospheric disturbances.
>
>Since I have an interferometer, I was able to test it. It came out 1/3
>wave P-V with a Strehl ratio of 81%. This means that the 1st diffraction
>ring has twice as much light in it as theoretical. The reason it "tests"
>perfect on the inside-outside test is that the 1/3 wave in this telescope
>was fifth order spherical, not 3rd order, or the classical aberration in a
>sphere. 5th order is more complex, and is difficult to get rid of. It is
>inherent in the Gregory design, of which this 90mm is an example. This has
>been an eye opener for me, especially since many an amateur has reported
>those perfect inside-outside diffraction patterns with their inexpensive
>commercial scopes. It apparently does not necessarily mean a perfectly
>corrected optical system.
>
>Roland Christen
>ASTRO-PHYSICS
-- Duane Sand, d.s...@ix.netcom.com
Don Tabbutt
Turn this around. Say a Meade optician tests a sample of 1 (one)
Astrophysics telescope and comes away with the same conclusions. This list
would be on fire.
I get soooo tired of this crap...
Don
Chris1011
To Mr. Tabbutt who wrote>I get soooo tired of this crap...<
I'm sorry. I should not have mentioned that this scope was a 90mm Mak. It
should have been aimed at any commercial Mak now in volume production.
These scopes have inherent 5th order aberration which cannot be detected
in the star test. This aberration, however, severely impacts the ability
to see low contrast planetary detail (one of the reasons to own a long
focus Mak?) Does this mean every commercial Mak has this defect? No. It
depends whether the manufacturer has made an effort to hand aspherize the
optic.
My main point was to say that the much touted star test does not
necessarily work. This means that the amateur does not have a good way to
judge the state of correction of this type of instrument.
Roland Christen
ASTRO-PHYSICS
Jay Reynolds Freeman
Roland Christen writes:
> My main point was to say that the much touted star test does not
> necessarily work. This means that the amateur does not have a good
> way to judge the state of correction of this type of instrument.
Roland,
Suiter discusses higher-order spherical aberration and asserts that
it is detectable in the star test, though with more difficultly than
the lowest-order defect. Discussion and figures are in section 10.7
of his book. Do you have any comment or elaboration?
I'm not trying to start a controversy here -- but we don't all have
access to interferometers, so it would be extremely useful to have a
clear understanding of how far we can push the star test, and of what
its limits are.
(For those of you just joining the discussion, the cited reference
is _Star_Testing_and_Astronomical_Telescopes_, by Harold Richard
Suiter, Willmann-Bell, 1994 -- and the previous poster, Roland
Christen, is one of today's foremost producers of world-class
amateur-sized astronomical optics.)
(Incidentally, there is a confusing difference of convention as to
how the different orders of spherical aberration are labeled -- some
authors refer to the lowest two orders of the defect as "third-order"
and "fifth-order", some as "fourth-order" and "sixth-order". Roland's
previous posting used the former convention, and Suiter used the
latter. This is basically just semantics, don't worry about it, the
subject is confusing enough already...)
--
Jay Reynolds Freeman -- freeman at netcom dot com -- I speak only for myself.
Peter Bealo
> to see low contrast planetary detail (one of the reasons to own a long
> focus Mak?) Does this mean every commercial Mak has this defect? No. It
> depends whether the manufacturer has made an effort to hand aspherize the
> optic.
>
corrector?? I have recently been corresponding with guys from the old
OTI (Quantum 4, 6 and 8 Gregory Mak 'scopes) and they have said that
their correctors were aspherized.
Peter Bealo
PS...You guys Coming up to Stellafane???
Chris1011
>Suiter discusses higher-order spherical aberration and asserts that it is
detectable in the star test, though with more difficultly than
the lowest-order defect. Discussion and figures are in section 10.7
of his book. Do you have any comment or elaboration?<
This particular scope's 5th order was readily apparent to me at low power,
but virtually disaapeared at high powers. It manifested itself as an
extremely large looking central obstruction as the high power eyepiece was
slowly defocused. In other words, the "donut shape" had a huge hole in it
on either side of focus, something like 60-70% instead of the 33% that the
central obstruction actually measured.
Roland Christen
Fred & Julie Burger
Hello Peter,
Some Questar's have aspherized correctors and some don't. It
depends partly on when the unit was made.
Fred and Julie Burger
replace 'nospam' with 'sos' to send us e-mail!
e-mail: f...@nospam.net
URL: http://www.sos.net/~fjb