Mesure FWHM dans Demetra

[Frederic Metz]

Bonjour / Hello,

FR :
Pour la focalisation de mon Alpy, j'ai utilisé le module d’acquisition de Demetra.

Pour cela, j'ai utilisé le bouton "Focalisation" (Focus ), je choisis "FWHM X" puis j'ai encadré une portion de raie venant d'une lampe fluo-compacte.
A chaque nouvelle image, un graphe de la FWHM est mis à jour, ce qui permet d'affiner la mise au point.

Cependant, si sur la même portion de raie, je fais un clic-droit-> FWHM, j'obtiens une valeur différente
Je peux également obtenir cette même valeur en utilisant le bouton FWHM.

Je ne comprend pas pourquoi la valeur est différente par le bouton Focalisation et par le bouton FWHM

Voilà une copie écran qui sera peut-être plus parlante

EN :
Hello,

To focus my Alpy, I used the Demetra acquisition module.

I used the "Focus" button, I choose "FWHM X" and then I framed a portion of line coming from a fluorescent lamp.
With each new image, a FWHM graph is updated, which makes it possible to refine the focus

However, if on the same line portion, I right-click-> FWHM, I get a different value
I can also get this same value using the FWHM button.

I do not understand why the value is different by the Focus button and by the FWHM button

Here is a screenshot that may be more meaningful

Mesure FWHM Demetra

Quelqu'un peut-il m'expliquer la différence ? Ou bien peut-être que je n'utilise pas l'outil de la bonne façon.
Merci pour votre aide.

Fred

[Robin Leadbeater]

The FWHM measured by binning a wide vertical region of the line would be an over estimate because of the curve and slant of the line. Perhaps one of the methods corrects for slant and curve ?

Robin

[Robin Leadbeater]

Also 1.68 pixels is an impossibly low value for FWHM unless your pixels are very large >~14 micron. In which case you would be under sampled. The FWHM of an in focus line should be approximately 23/pixel size for the ALPY with a 23 micron slit

Robin

[Francois Cochard]

Hello all,

Intéressant... cet outil de focalisation avait été imaginé au tout début de Demetra, mais il ne marche pas vraiment dans des conditions "normales"... avec un Alpy. On l'a laissé dans Demetra pour le moment, mais c'est à améliorer. Mais je suppose qu'avec un LISA, ça peut très bien marcher pour la mise au point.

Sur le plan technique, le calcul de FWHM est le même dans les deux cas, mais le chiffre indiqué en haut à droite est une moyenne sur trois mesures.

Salutations,

François
----------------------------------
English : interesting question... this tool was developed at the very beginning of Demetra, but we still have to improve it (as it is it cannot work properly for the Alpy focus). But... it could work fine for the LISA, I think. We kept it in Demetra, because it can be useful, but this is really to be improved.

Technically speaking the FWHM calculation is the same for the two values, but the top right one is an average of the left one, which explains the difference.

Best regards,

François

[Frederic Metz]

Also 1.68 pixels is an impossibly low value for FWHM unless your pixels are very large >~14 micron. In which case you would be under sampled. The FWHM of an in focus line should be approximately 23/pixel size for the ALPY with a 23 micron slit

Robin

Yes you should be right !
With Atik 314L (6,45µm pixel size), I can expect a FWHM of 3,45 at a minimum.

Fred

[Frederic Metz]

Hello all,

Intéressant... cet outil de focalisation avait été imaginé au tout début de Demetra, mais il ne marche pas vraiment dans des conditions "normales"... avec un Alpy. On l'a laissé dans Demetra pour le moment, mais c'est à améliorer. Mais je suppose qu'avec un LISA, ça peut très bien marcher pour la mise au point.
François

Focusing function will definitively help guys like me to start with spectro
Even if FWHM value seems to be false for my configuration, it remains relevant to find the focus point of my Alpy.

Fred

[David Cejudo]

Also 1.68 pixels is an impossibly low value for FWHM unless your pixels are very large >~14 micron. In which case you would be under sampled. The FWHM of an in focus line should be approximately 23/pixel size for the ALPY with a 23 micron slit

Robin

Hello.

I am trying as well to focus my LISA using Demetra. I wonder how does the focus tool calculate FWHM X. Does it use the whole spectrum or just the framed in blue color one with the mouse as the FWHM tool does? I suppose it uses the whole spectrum unless an area is framed?

I am getting also different values in the focus tool and the FWHM tool.

Also, how do you calculate the theoretical FWHM X of an in focus line? Robin said 23/pixel size for Alpy. What about LISA?
I am also using a 23um slit, but with a LISA, and i am obtaining my best values around 2.8 for the lines of the NeAr lamp. I guess that value mesures pixels, correct?

Also, what does FWHM Y means in a perfectly horizontal spectrum? I guess it is not the spectrum heigh, or it is?

I am doing my best to learn, but weather around Madrid is not helping much. I am a sea of doubts!
Thank you.

[David Cejudo]

I forgot to mention my set up.
Atik 460 EX camera with 4,54 microns pixels.
12"LX200 GPS Meade telescope with an Optec focal reducer making it a f/5 telescope.

[Nicolas Durand]

Hello

Does it use the whole spectrum or just the framed in blue color one with the mouse as the FWHM tool does? I suppose it uses the whole spectrum unless an area is framed?

The focus tool does not use the blue spectrum area. You should set a working area with the mouse around a line if you want to have a meaningfull value. When no mouse working area is set, the entire image is used...

Also, what does FWHM Y means in a perfectly horizontal spectrum?

It means nothing: The focus tool can be used for various cases so I let the possibility to use X or Y FWHM value. For horizontal spectrum, the X FWHM should be used.

Nicolas