[ATM] WG: Focault (sic!) Help

[Berthold Hamburger]

It is also interesting to observe the Strehl ratio for the isolated
astigmatism analysis.
Even the 1/10 wave astig remaining in the mirror currently will reduce
Strehl ratio to 0.8.

Berthold

-- 
Berthold Hamburger - Cellist/Spain
Email: behambu at artinso.com
http://www.artinso.com
http://www.astro.artinso.com


-----Original Message-----
From: atm-bounces at atmlist.net [mailto:atm-bounces at atmlist.net] On Behalf Of
Berthold Hamburger
Sent: miércoles, 26 de mayo de 2010 1:16
To: 'ATM List'
Subject: Re: [ATM] WG: Focault (sic!) Help

Hi again,

I uploaded a new file with test results taken after another 30' figuring
session:
http://www.atmlist.net/contrib/b-hamburger-at-artinso-dot-com/AstroArtinso/A
stig_analysis_26052010.pdf
These files are meant mainly as simple showcase examples for newbies to
demonstrate what to look for when testing for astigmatism. 

Astigmatism is much reduced as one can see. One thing to mention that I
forgot in my previous mail is that Foucault and Ronchi images include both
mirror AND test setup related astigmatism, while for interferometric
analysis I average 2 rotational results (0deg and 90ccw) in order to isolate
mirror astigmatism. To achieve the same in Foucault and Ronchi, one has to
measure on 2 or more rotations as well. 
To visualize the difference between isolated mirror aberration and
mirror+test astig., I included in the present document also an analysis for
the 0deg rotation that corresponds to the Foucault and Ronchi images. I
marked with arrows what to look for in the shadowgrams. Notice also the
slight angular difference in inclination in the Ronchi images.
Following a suggestion from Dale, I present only Zernike terms for
astigmatism in the interferometric analysis to help visualize the magnitude
of the aberration. The last image however displays the complete figure of
the mirror.

Regards

Berthold 

-- 
Berthold Hamburger - Cellist/Spain
Email: behambu at artinso.com
http://www.artinso.com
http://www.astro.artinso.com


-----Original Message-----
From: atm-bounces at atmlist.net [mailto:atm-bounces at atmlist.net] On Behalf Of
Berthold Hamburger
Sent: martes, 25 de mayo de 2010 2:13
To: 'Jeremy Royston'
Cc: 'ATM List'
Subject: Re: [ATM] WG: Focault (sic!) Help

Hi Jerry,

I did not want to argue your argument without having proof on hand. Much to
my dismay, I have now, as I overlooked a machine glitch and successfully
"figured" 1/5 wave astigmatism in my mirror ;-)
It can be seen on the document I uploaded to:
http://www.atmlist.net/contrib/b-hamburger-at-artinso-dot-com/AstroArtinso/A
stig_analysis_25052010.pdf

The Foucault and Ronchi are very sensible to astigmatism once you know what
to look for and once you know, you never forget :-)
The Foucaultgrams speek for themselves and the Ronchi shows opposite lean of
the line inside and outside focus. For completion I added interferometric
analysis that gives a quantitative measure of the amount.
Don't bother looking at the rest of the figure, as this is a very early
stage of figuring.

Enjoy! (I don't....)

Regards

Berthold


-- 
Berthold Hamburger - Cellist/Spain
Email: behambu at artinso.com
http://www.artinso.com
http://www.astro.artinso.com


-----Original Message-----
From: atm-bounces at atmlist.net [mailto:atm-bounces at atmlist.net] On Behalf Of
Jeremy Royston
Sent: sábado, 22 de mayo de 2010 12:44
To: atm at atmlist.net
Subject: [ATM] WG: Focault (sic!) Help


The Foucault and especially the Ronchi test are not very sensitive to
astigmatism, and you need a certain amount of experience to see it.
The easiest way to see astigmatism is to set up your mirror as for the
Foucault test, use the reflection of an LED (high power, narrow beam)off a
clean small steel ball (4-6mm) as a point source. Look at the returned
immage with a short focus eyepiece (about 5mm). If there is no astigmatism
the image is round inside and outside focus.
The test is described in Texereau
If you want a quantitative test, use the 2D Hartmann test, my hobbyhorse.
See http://home.earthlink.net/~burrjaw/atm/odyframe.htm The measurement
needs no more time than with Foucault, needs no experience, and gives you a
picture of the whole mirror 

Jerry

Berthold wrote:

... snip
The only aberration worthwhile looking at right now would be astigmatism
which could be detected by Foucault and Ronchi at this point, but demands a
little bit of experience in order to be correctly identified. If you work by
hand, rotate your glass diligently and support it well, it is unlikely that
astigmatism is an issue.

Regards

Berthold


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