monture alt-azimutale en spectro

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monture alt-azimutale en spectro

Postby etienne bertrand » Sat Sep 01, 2018 9:27 am

Bonjour,

Suite au message de Christian sur le spectro UVEX et les aberrations chromatiques, il est dit que l'on peut utiliser les montures alt-azimutales en spectro.
Qu'en est il vraiment ? Une monture alt-azimutales est aussi utilisable qu'une équatoriale ?
Hormis d'être intéressante pour améliorer les défauts des aberrations chromatiques est ce qu'une monture alt-azimutales peut s'utiliser tout le temps par exemple avec un C14 par exemple à la fois pour travailler bas sur l'horizon mais au zénith et partout dans le ciel ; quels seront les désavantages d'une telle monture ? Au niveau suivis ça tient ou on perd bcp de flux par rapport à une monture équatoriale (suivis en escalier..) ?
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Re: monture alt-azimutale en spectro

Postby Robin Leadbeater » Sat Sep 01, 2018 12:01 pm

Hello Etienne,

I have never tried it but even if the guiding is good enough I imaging some problems with field rotation in long exposures of faint objects. (The star drifting off the slit when guiding on field stars, field stars in the sky background moving in and out of the slit? )

Cheers
Robin
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Re: monture alt-azimutale en spectro

Postby Christian Buil » Sun Sep 02, 2018 1:49 am

My opinion is that the field rotation is not a real problem for stellar spectroscopy (the field is punctual). Tracking on a stellar point (if the mount is well designed) is not a source of difficulty (to be confirmed, of course, for a given setup).

Christian
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Re: monture alt-azimutale en spectro

Postby Vincent Bouttard » Sun Sep 02, 2018 2:59 pm

You have to autoguide on the star which is on the slit to avoid field rotation. How could you guide during few minutes or more on a star on a field which is turning due to the field rotation ? Will the field turn around the guiding star or not ?

Alt-az mounts are really interesting for several reasons. Even more if you could put all your spectroscopic setup on a coudé focus, like professionnal telescopes.
The dream is a RC telescope on alt-az mount equipped with a coudé focus. :)

Vincent
VBO @ ARAS database : Newtonian 250 mm F/4 / Alpy600 / Atik 460EX
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Re: monture alt-azimutale en spectro

Postby Christian Buil » Sun Sep 02, 2018 9:51 pm

If the guide star is also the spectroscopy target, no problem (a very general situation). If the guide star is not the target, yes a field rotation is present, and it's a difficulty. But, a correction guiding consigne correction is not very difficult to write for fix the target star on the slit for compensate rotation of the guiding objet (a spherical trigronometric problem for mathematician + effort an effort guiding software writer)...

Here, a determinant test made last night :

Image

For simulate an AltAz mount, all the UVEX spectrograph is rotated and the long axis of the slit is now (approximatively) vertical. Compared to the equatorial mount, the spectrophometric error is dramatically reduced (the processing of the two narrow slit (23 microns) spectra is the same).

Definitively, AltAz mount is more efficient (yes, Vincent, a RC on an a AltAz + coudé, is a dream !).

(an alternative to an AltAz is an optical image rotator system in conjunction with an equatorial mount, not very difficult to construct...).

Christian
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Re: monture alt-azimutale en spectro

Postby Vincent Bouttard » Mon Sep 03, 2018 7:57 pm

Christian,
The best way could be the famous "ball mount", made in Calern. I've just read a thesis by Olivier Lardière about this mount and how it works. All can rotate, even the tube to put the focuser always vertical. An incredible mount... :D
Optec is one of the only who made de-rotators for telescopes. It's expensive and I don't know if it really works fine.

Vincent
VBO @ ARAS database : Newtonian 250 mm F/4 / Alpy600 / Atik 460EX
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