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Re: [ATM] Re: Star testing with a ronchi test to quantify a TDE
Thanks Mike for your interesting reply
I use a 6 mm radian and a 2" 2X powermate for star testing.
I have to reread carefully D.Suiter's Star testing astronomical telescope to
check about 5th order spherical aberration.
Effectively the 5th order spherical aberation in aberrator may look like a
TDE except the outer part of the image.
I have to recheck a star test with an annular mask (to mask the outer part
of the mirror) to see if I still see what I call a TDE but that could in
fact be a 5th spherical aberration.
Maybe I miss something but it seems to me that I can't simulate a TDE with
my V3 beta test version of aberrator
Raphael
----- Original Message -----
From: "Mike Spooner" <spoon@PageAmerica.net>
To: "Raphaël GUINAMARD" <rguinamard@infonie.fr>; <atm@atmlist.net>
Sent: Friday, December 24, 2004 9:39 PM
Subject: Re: [ATM] Re: Star testing with a ronchi test to quantify a TDE
> Your mirror is very fast and it would not be hard to have some higher
order
> spherical effects that will look similar to TDE. Check it out in Aberrator
> as positive 5th order spherical of .1 or .2 waves. This amount hardly
> affects the MTF (at least compared to 3rd order) and combine it with 2%
TDE
> to see what happens. Since the Ronchi shows you have some TDE, then a mask
> would help but determining the width of it is often difficult and probably
> needs trial and error efforts. People obsess over TDE when it may not even
> be affecting the infocus images compared to figure errors. It takes very
> little to upset the out of focus symmetry. At f/3.3 the only eyepieces I
> know of that will give a correct star test for spherical are the Radians.
> Older orthos and Plossls will be a wave or two undercorrected and Barlows
> and Powermates also have problems with these wide light cones so they
aren't
> reliable help. If the mirror is showing good spherical correction with the
> older designs than it is overcorrected and adjusting for the eyepiece
> inadequacies (which is fine if those are the eyepieces that are going to
be
> used but astigmatism isn't as well corrected off axis compared to the new
> widefields).
>
> Are the infocus images being affected badly by the TDE (or whatever else
may
> be there)? That is the ultimate discriminator.
>
> Best,
> --Mike Spooner
>
>
> ----- Original Message -----
> From: "Raphaël GUINAMARD" <rguinamard@infonie.fr>
> To: <atm@atmlist.net>
> Sent: Thursday, December 23, 2004 11:20 PM
> Subject: [ATM] Re: Star testing with a ronchi test to quantify a TDE
>
>
> It seems this message didn't reach the list.
> I send it again.
> ----- Original Message -----
> From: Raphaël GUINAMARD
> To: atm@atmlist.net
> Sent: Thursday, December 23, 2004 9:59 PM
> Subject: Star testing with a ronchi test to quantify a TDE
>
>
> The star testing of my mirror shows a Turned Down Edge.
> It doesn't seem very big but I think I 'm going to put an annular mask
to
> avoid the image degradation.
> I've also seen this TDE using a ronchi grating, and if I take a picture
of
> the image, I think I could mesure where the TDE is starting -at least on
the
> picture itself-.
> My question is :
> I know the ronchi test is not very sensitive with fast scope (mine is a
> 24" f/3.3) so is the measure of the TDE on the picture would be a good
> approximation of the TDE on the real mirror?
> I mean, if I measure for example on the image that the 2% outer part of
> the image has a TDE, does this mean I could confidently make a mask for
the
> 2% outer part of the mirror?
> Maybe the low sensitivity of the ronchi test could show a 2% outer TDE
> with a 4 % outer TDE mirror? no?
> I know I could try different annular width and star test with each, but
my
> question is to go as quickly as I could to the solution
> Thanks in advance
> Raphaël
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>
>
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