Slant with Demetra

[David Cejudo]

Untitled-1.jpg (42.16 KiB) Viewed 11409 times

Hello.

I recently decided to change the slit from 25μ to 35μ. I found that the calibration lines are slanted, as well as the emision lines of background sky in long exposures.
If i am not wrong, this is due to the slit a bit turned.
When procesing the images with Demetra, in the step "geometry", correction is possible in tilt and smile, but i found difficult to correct for slant.
Trying the button "calculate", Demetra does a poor job as the slant is increased! I tried to chnge the numbers manually but it seems difficult.

Am i doing something wrong?


Thanks for advice.

David Cejudo.
Observatorio El gallinero.
El berrueco (Spain)

[Nicolas Durand]

Hello

Did you selected an area around a line before trying to detect the smile value ?

Nicolas

[etienne bertrand]

Just a question, with slit 35µm what is your resolution ?

[David Cejudo]

Hello, all.

Yes, i selected an area around a line while trying smile correction. But smile just fixes the curvature of the line, not its slant. I recently knew that Demetra does not correcft for slant. I suppose slant is already taken care of by fixing the slit correctly with the screws, mechanically, and following the procedure stated in the manual. I guess this is the best means, but whenever the slit is changed, the procedure must be done again as the square mirror (the slit) emplacement has some play, it does not sit snuggly. It does not fit exactly in its location. You may try either moving just very slightly the mirror with a small pick until no slant is observed or -better- dismount the spectroscope from the telescope and use those screws to fix it. The manual states that the guider should be used for this correction. I wonder -i did not try it yet- if could it be a good idea to use the calibrating lamp an its spectral lines -using the main camera- to perform this correction by checking the lines verticallity. Also with this procedure it is easy to check if at the acquisition camera's chip the spectrum is horizontal and is placed at its very center.
It seems to me that could be easier, but i am not sure if it is correct.

Anyway, i found a work around to handle the slant in the images:
1. Run Demetra step by step just until the step called "Geometry" with the "Save step files" ticked.
2. Open all the files with the suffix "G" with BASS software and correct all for slant.
3. Save those corrected files back overwriting the uncorrected ones.
4. Continue running Demetra step by step starting with "Extraction" until the last step, "Crop" (don't click the "RUN" button!).
That is it.

Regarding resolution, i can give you what the theory says:

23μ: R ≈ 1000; Spectral resolution ≈ 5.9 Å; Dispersion ≈ 1.76 Å/pixel
35μ: R ≈ 830; Spectral resolution ≈ 7.0 Å;
That is the theory, the real values are most probably lower. I still don't know yet as i just changed the slit recently.

[Robin Leadbeater]

But smile just fixes the curvature of the line, not its slant. I recently knew that Demetra does not correcft for slant.

That does not sound very useful. (Unless your spectrum is exactly on the radius there will always be a slant component) ISIS smile correction corrects both the curve and the slant

Robin

[David Cejudo]

Here i attach an image of the calibration lamp's spectrum lines at the the step "Geometry." First image shows the lines with a bit of slant, about 5º.
Second image shows the same after hitting the button "Preview". The lines are now obviously more slanted. Smile obviously does not work for slant in Demetra, unless i am doing something wrong.

I forgot to mention that my spectroscope is a LISA!