Choix d'un spectrographe echelle

[Norbert Jorun]

Je vais m'orienter vers un spectrographe multiordre de type echelle et j'ai repéré 4 modèles :

- Baches de chez Baader : https://www.baader-planetarium.com/en/s ... -2937.html
- Eshel de chez Shelyak : https://www.shelyak.com/produit/pf0011- ... ope-eshel/
- Lynx : pas trouvé de site web. Est ce un produit commercial ou un spectrographe "fait maison" ? Dans ce dernier cas, où est t'il possible d'avoir les plans ?
- Nou-T : idem, pas trouvé de site ni les caractéristiques de ce spectrographe.

Donc à défaut d'avoir des infos sur Lynx et Nou-t, je suis en train de comparer Baches et eShel et on constate des différences suivantes :
- Baches est fixé sur le télescope alors que l'Eshel est à fibre optique et déporté sur une table à proximié du télescope ou dans une enceinte régulée en température, donc on peut vraisemblablemnt dire que le Baches a un meilleur rendement mais une stabilité moindre, pas de régulation thermique et subit des flexions mécaniques selon son orientation et le pointage du télescope. La fibre apporte une perte plus importante qu'une fente il me semble ?
- Baches a une meilleur résolution que l'eShel : 18000 pour Baches, 11000 pour l'eShel
- Baches est indiqué avec un rendement de 30% alors que pour l'eShel je n'ai rien trouvé
- Baches va de 392 à 800nm alors que l'Eshel va de 390 à 760nm (pas vu sur le site de Shelyak, mais vu sur ce forum)
- Baches est équipé d'une fente, eShel d'une fibre.
- Masse de Baches : 1,3Kg, masse de l'eShel : pas indiqué sur leur site. Certe dans les 2 cas faut prendre en compte aussi la masse de la caméra voir celle d'autoguidage aussi.
- Précision et stabilité en température : aucun des sites n'indiquent quoi que ce soit à ce niveau.
- l'Eshel semble pouvoir s'adapter plus facilement à des rapport f/d différents à cause des dives modules d'autoguidage et adaptation de fibre ?

Donc difficile de ce faire une idée de ce qui est le mieux entre les 2.
Ce qui serait ludique pour moi c'est d'en construire un mais encore faut t'il avoir des plans (n'ayant pas de compétences/connaissances en optiques)

De plus, aucun des spectrographes mentionnés ici ne montrent ce que l'on voit à travers le capteur d'autoguidage (la fente ou la fibre).
C'est un critère important à prendre en compte : champ couvert par le système, efficacité de la surface réfléchissante, coma, vignetage, etc..

[James Foster]

To: Nobert,

Excuse me, my Franci is bad so here are my observations about your questions in English:

- Baches has a better resolution than eShel: 18000 for Baches, 11000 for the eShel
ANSW: True, but since this is "less stable" is this really desirable?

- Baches is indicated with an efficiency of 30% whereas for the eShel I did not find anything
ANSW: I read somewhere that the Fiber/eShel efficiency is similar depending on the purity of glass, the end of the fiber (1-direction is more efficient), and
calibration of the camera to the eShel.

- Baches goes from 392 to 800nm ​​while the Eshel goes from 390 to 760nm (not seen on the Shelyak site, but seen on this forum)
ANSW: Many advanced users of the eShelII have seen the CaII IR lines to 8662A (resolution suffers from order dispersion at this lower wavelength)
See for more details: http://www.astrosurf.com/buil/asi6200mm/

- Baches is equipped with a slot, eShel with a fiber.
ANSW: Yes

- Mass of Baches: 1.3Kg, mass of eShel: not indicated on their site. Certainly in both cases must also be taken into account the mass of the camera see that of autoguiding too.
ANSW: The eShel (without the camera and fiber) is about 10% more massive than the LhiresIII. The only mass on the scope is the rather small Injection unit (maybe 0.5Kg max), guider (i use the ZWO ASI120MM mini, less than 0.2Kg) and maybe 0.1-0.3Kg drag weight from the calibration and science fiber.

- Precision and temperature stability: none of the sites indicate anything at this level.
ANSW: You can find a detailed study from the Univ of Tel Aviv on using the eShel I with a 1m telescope: https://arxiv.org/abs/1703.09937
according to the end summery, they seem to achieve 200 m/s accuracy on binary orbital RV on stars as dim as 11th mag.

- the Eshel seems to be able to adapt more easily to different f / d ratios because of the different moduli of autoguiding and fiber adaptation?
ANSW: That is most likely correct. They're is more published work on using the eShel than any other similar spectroscope in this class; at least under $20K spectroscope!

James

[Olivier GARDE]

Pour la question concernant l'image du capteur d'autoguidage, voici une image réalisée avec le module d'injection de l'eShel



Ici la caméra est une ATIK 314L+ le trou de la fibre fait 50µm.
Le champ fait 16x10' (avec un RC400 ramené à f/6 avec un réducteur de focale, soit 2400mm de focale)

le champ est vignetté avec aussi de la coma dans les angles mais permet de bien se repérer dés lors que le télescope pointe bien.
La surface réfléchissante est en verre, ce qui donne une très bon flux vers la caméra de guidage.

[Norbert Jorun]

Thanks James and Olivier for your answers.
For the precision part, I therefore have information on Baches and eShel.
On another topic on this forum, I found this publication which confirms the one indicated by James for the eShel :
https://iopscience.iop.org/article/10.1 ... 115001/pdf

accurency for echelle spectrograph :
Baches : 1,3km/s
eShel : 0,25 or 0,2 km/s
Lynx : ?
Nou-T : ?

is there no one who can give me technical information on the Lynx or the Nou'T echelle spectrograph ? Nothing found on my side for performance and characteristics.
It's a DIY spectrograph or a commercial unit ?

[Joan Guarro Flo]

Hi Norbert,

The LINX ( year 2017 ),and MUSSOL ( year 2016 ), there are the firsts spectrographs that I made, you can see the LINX informations in,
http://www.spectro-aras.com/forum/viewt ... LINX#p8200, and the MUSSOL, in, http://www.spectro-aras.com/forum/viewt ... lit=MUSSOL, one time I made them ready, always I used them successfully in numerous observations, until I changed these ones for the NOU_T.

I put them both for sale, you can see, http://www.spectro-aras.com/forum/viewt ... =44&t=2633. They have inside a transmission grating
instead of an equilateral prism and have an optical fiber system. I think that its price is very reasonable, but anyone is required to. On the other hand, all echelle with optical fiber need a mechanical agitator, because the optical fiber produces a modal noise that eats a lot of fine details and produces ripples on the graphic results. I remember Ernst Pollmann rejected a lot of my observations of HeI 6678, until I used the optical fiber agitator.

Why do I put LINX and MUSSOL for sale ? Because Tim Lester showed his new echelle I realized it was another way, another thing, compact, without fiber and with new possibilities. You can see a lot of information here, http://www.spectro-aras.com/forum/viewt ... f=8&t=1767, and, http://www.spectro-aras.com/forum/viewt ... f=6&t=2367.

Is the NOU_T echelle perfect ? No. Perfection doesn't exist in this world. Its main problem is its atmospheric correction, if you want a good one you need to observe a standard A or B star every night, the same way that you need to make on all the other spectrographs that you put directly at the telescope. In the practice I use other soft metodes.

By the way, I do have the possibility to transform the NOU_T yourself to a NOU_T with an optical fibre system within just 25 relaxing minutes, it permits other more large objective lenses, cameras, etc. Don't forget your fiber agitator. And of course, you are able to come back the first way when you want.

In my free time I make some NOU_T mechanical parts, they are available for observers that don't afford commercial echelles, tell me something if you are interested in it.

You can see Norbert, you have some options and all of them work very well, it all depends on what you want.., and your telescope possibilities.

Cheers, Joan.

[Norbert Jorun]

Thank you Joan for your answer but I didn't find this following caracteristics for Nou-T :
- Weigth
- Efficiency
- Accurency/precision

And do you have an image of the autoguiding field of the Nou-T spectrograph ?

[Norbert Jorun]

Do people use the Baches model on this forum ?
In this case what's your feedback on this spectrograph ?

[Joan Guarro Flo]

Well Norbert,

I left here the photo that you want. The NOU_T characteristics are, Range from 3769.5 to 9495 Angstroms, R_= 8500, weight around 2600 without cameras.

But could you tell us what your equipment - telescope, etc. - is ? And what are your observational projects ? It would be very interesting to know them.

Cheers, Joan.

[Norbert Jorun]

Joan, Thank you for your answer.
I will try to do spectroscopy on binary stars to find the compagnion period or more periodic event on multiple stars. I will use a Planewave 17' on a GM3000 HPS 10 micronI have many camera like ATIK 460 or ASI 6200.

Could you please give more information about efficiency and accurency of Nou-T ? I need this information to compare with others echelle spectrograph. (see up in this topics)

What about the field size of your autoguiding image and your focale length for this image ? Is it normal to have a circle ?

Nou-t is a commercial spectrograph or a DIY spectrograph ? if it's a DIY, where can I find construction details ?
I hope you have a website with all information like the SOl'Ex website : http://www.astrosurf.com/solex/

[Joan Guarro Flo]

Bonjour Norbert,

I'm very impressed with your equipment, I think that you would make very useful work with it and also you have serious and concrete aims in the future.

The circular field shape is due to the fact that the NOU_T uses an air 50 microns pinhole from National Aperture Inc. (USA), it represents a util field around 10.6' diameter at 2600mm. focal, so that are 9' over the 2939 mm. in your Planewave 17'

NOU_T isn't a commercial echelle nor DIY one, Nou_t is an artisanal echelle because it is composed of pieces done in mechanical precision shops with a very robust design. At the moment I remite you again that youcan see its details in http://www.spectro-aras.com/forum/viewt ... f=8&t=1767, and in, http://www.spectro-aras.com/forum/viewt ... f=6&t=2367. With a bit of patience you can find a lot of information there, also some NOU_T optical fiber version pictures.

But nowadays an observer, very experienced and everyone knows, has started a web page about a free study over his NOU_T performances and results, it is possible we will see in few weeks. So I don't answer you the efficiency and accuracy levels in an honest way now.

And on the other hand I'm not a commercial seller, I don't want to find customers and clients and I don't want to be anyone's business competence. So first, over all, I'm -- I think -- a good and experienced observer who makes Tim Lester's design spectrographs and tries to make friends with it. In my opinion, it's essential too, that an observer knows how his echelle works, an echelle design is easy, its build is laborious, not difficult, but there isn't any mysterious or something of the other planet in it. So everybody wouldn't be scared to access inside it for something.

Cheers, Joan.