First spectrums. Alnitak

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David Cejudo
Posts: 45
Joined: Mon Apr 23, 2018 9:01 pm

First spectrums. Alnitak

Post by David Cejudo »

Hello.

At last a night of good weather let me have a few shots with my new LISA.

Here is attached an spectrum of Alnitak, not calibrated with flats or darks. I am having a question here.
Are the spectrum endings at the picture normal? I mean, it looks like the spectrum spreads out, it is wider, at the left end. And at the right end, it is divided in two lines.
Thanks for comments.
Attachments
Alnitak 15sec_15s_20210105_235210-4.jpg
David Cejudo.
Observatorio El gallinero.
El berrueco, Madrid.
Rainer Ehlert
Posts: 135
Joined: Thu Sep 29, 2011 9:36 pm

Re: First spectrums. Alnitak

Post by Rainer Ehlert »

Hola David,

Lo primero que veo es de que está sumamente sobreexpuesta tu imagen. Así como lo veo es un problema de enfoque.

Se puede analizar mejor si le bajas la exposición para ver como se ve bien expuesta y así tener más información y poder darte algunos consejos.

Saludos
regards Rainer____Observatorio Real de 14_____MÉXICO_____N 22° W 101°
Olivier GARDE
Posts: 1243
Joined: Thu Sep 29, 2011 6:35 am
Location: Rhône Alpes FRANCE
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Re: First spectrums. Alnitak

Post by Olivier GARDE »

First of all, the spectrum is not well focused, we can see it because the spectrum is doubled. It's also over exposed and not horizontal.

For the blue part which goes in "fish tail" it is normal because there must be in your optical chain of your setup a focal reducer or other lens which is not achromatic
LHIRES III #5, LISA, e-Shel, C14, RC400 Astrosib, AP1600
http://o.garde.free.fr/astro/Spectro1/Bienvenue.html
David Cejudo
Posts: 45
Joined: Mon Apr 23, 2018 9:01 pm

Re: First spectrums. Alnitak

Post by David Cejudo »

Thanks for all your comments.

I will work on the focusing. I will try Demetra for that, i intalled and tried it already. I used to acquire and reduce a few spectrums but i did not try its focusing tool yet. I did not find anything about it in the Demetra manual, by the way. Or i missed it!

Regarding the fish tail in the blue part, you guessed it. I have a .5x Optec reducer ahead of the LISA. So i suppose it explains it.

On saturation, i am not sure. You say that the spectrum is saturated, but the maximum point value in the raw FITS image is around 24,000 ADU. Saturation in my Atik 460 EX starts around 50,000 ADUs, if i am not wrong. Why do you think it is saturated?
David Cejudo.
Observatorio El gallinero.
El berrueco, Madrid.
Rainer Ehlert
Posts: 135
Joined: Thu Sep 29, 2011 9:36 pm

Re: First spectrums. Alnitak

Post by Rainer Ehlert »

David Cejudo wrote:Thanks for all your comments.

I will work on the focusing. I will try Demetra for that, i intalled and tried it already. I used to acquire and reduce a few spectrums but i did not try its focusing tool yet. I did not find anything about it in the Demetra manual, by the way. Or i missed it!

Regarding the fish tail in the blue part, you guessed it. I have a .5x Optec reducer ahead of the LISA. So i suppose it explains it.

On saturation, i am not sure. You say that the spectrum is saturated, but the maximum point value in the raw FITS image is around 24,000 ADU. Saturation in my Atik 460 EX starts around 50,000 ADUs, if i am not wrong. Why do you think it is saturated?
Hi,

Possibly you did stretch wrong the image when converting to JPG.

I also have a Optec reducer 0.6X in front of my LISA and my spectra look like this on the blue end.

Alnitak is a very bright star and the highest Luminance is on the blue side and perhaps that makes it also worse. Below a spectrum from me on ALnitak and my exposure time is only 0.5s but I am binning 3X in the vertical and 1X in the horizontal.

I always check my focus point on my guiding image and adjust it there. In order that this fits and I guess you did it is well explained in the LISA manual, page 14 ff. What is your native focal length ? You did not say what telescope you are using and if you guide or not using the guide port of the LISA.

This is how Alnitak looks in my LISA setup. Will need to make a test with Binning 1X and 1Y and compare results.

Alnitak_O9.5Ib_5s_LISA_600mm-0010_C_CC_HT0.15_DnB_16bit.jpg
regards Rainer____Observatorio Real de 14_____MÉXICO_____N 22° W 101°
David Cejudo
Posts: 45
Joined: Mon Apr 23, 2018 9:01 pm

Re: First spectrums. Alnitak

Post by David Cejudo »

Hello.

I have a 12 inches LX200 GPS Meade telescope, f/10. With an Optec NextGen 0.5x reducer. Using the reccomended spacing for the reducer i am measuring very close to f/5, that is a focal length about 1500 mm. In theory is the ideal for LISA, althougth i know a focal reducer has its problems for spectroscopy.
I am using a Titan camera to guide through the LISA guiding port. To focus the image in the slit i use FocusMax. The image is sometimes not very clear in the Titan but i manged to obtain some V curves and succeded focusing it. From then on, the focuser keeps the image focusing automatically with temperature changes.
But if i am not wrong that is not too important for the final spectrum focus. Only to make the star as small as possible at the slit, correct? And so reducing exposure times.
To properly focus the spectrum i did it very roughly, using my eye and my fingers. Next time i will try Demetra focusing tool -and FWHM instead of my eye- and my fingers again! I guess i can do it anytime using the calibration lamp.
David Cejudo.
Observatorio El gallinero.
El berrueco, Madrid.
Olivier GARDE
Posts: 1243
Joined: Thu Sep 29, 2011 6:35 am
Location: Rhône Alpes FRANCE
Contact:

Re: First spectrums. Alnitak

Post by Olivier GARDE »

There are two things to consider for the focus of LISA :

1) Focusing the spectral camera with the LISA :
You must have the correct back focus between the sensor and the LISA box : 54,85mm
See this diagram page 3 : https://www.shelyak.com/wp-content/uplo ... nsions.pdf
Put the Ne/ar lamp on and adjust the internal doublet to have the best FWHM on a Ne/ar lamp close to the middle of the spectrum.

2) Focusing a target into the LISA :
- First of all, you need to have the right back focus on your auto guiding port between the ATIK Titan and the auto guiding port.
See also the diagram page 3.
- Take a star close to the zenith or high enough in the sky (for less turbulence)
- Focus the star on the slit using your telescope focusing knob (on the auto guiding camera)
- Adjust the height of the spectrum (with the spectrum camera) to have a spectrum as thin as possible by using the focus knob of your telescope
- Then adjust the position of your guiding camera to have the best focus on the star (do not use the focusing knob of your telescope)
LHIRES III #5, LISA, e-Shel, C14, RC400 Astrosib, AP1600
http://o.garde.free.fr/astro/Spectro1/Bienvenue.html
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