setting up the Alpy

[Mr Nicholas John Atkinson]

With astro-photography on the back burner with shorter nights I have been spending several days and nights concentraing firstly setting up the off-axis guiding module and achieving a level line as per the instuctions. I then took a sky spectrum but found that the spectrum was slightly tilted. Should this be a concern?

[Robin Leadbeater]

Hi Nick,

Are you saying the spectrum lines in the sky are slanted when the star spectrum is horizontal? It does not matter if it is not too far out as it can be corrected in the software (They will be slightly curved too)  Getting the star spectrum as close to horizontal as possible is more important.  The slanted lines mean the grooves in the grating are not exactly parallel to the slit. If it bothers you it can be adjusted in the ALPY core module. There are (two?) little Allen headed grub screws which if slackened off allow the grism to be rotated. Only slacken them off a tiny amount though otherwise the whole assembly can spring apart !. You can see the internal  construction on page 4 here

http://www.threehillsobservatory.co.uk/astro/ALPY200_VdS_BAAVSS_poster_2014.pdf

Cheers

Robin

 

[Mr Nicholas John Atkinson]

Hi Robin,

Yes I do mean that the slt was perfectly horizontal but the Spectrum was slightly tilted. From what you say is that the most important thing to do is to have the spectrun horizontal. I do not think I will try to adjust the slit to make it parallel.

One other point. Do you think Demetra is better than ISIS?

Cheers

Nick

[Robin Leadbeater]

Hi Nick,

My ALPY (one from the first production batch) somehow came with the grism significantly skewed so I fixed it with Francois Cochard’s guidance. (Also I have a habit of taking everything apart to “see how it works” ! ) If it is only slightly skewed then I would not worry about it. The main reason for correcting the slant/smile is so that any lines in the sky background subtract correctly. In ISIS you select a strong lamp or sky line and it automatically measures the slant and curvature and corrects for it.  I expect Demetra does something similar.

Spectroscopically I “grew up” with ISIS so I know most of its quirks well and knowing Christian Buil, I had confidence that it would give good reliable results. I was down as a beta tester of Demetra but to be honest there was no pressure for me to change over, particularly as (at least at the time. I have not kept up with its development) it was not as comprehensive and flexible as ISIS so I never really got into it. Similarly with BASS which I have never used. 

Robin

[Robin Leadbeater]

I dug out my before and after grism alignment images (spectrum of the sky). The spectrum is horizontal as seen by the lines generated by dust specks on the slit. 

[Hugh Allen]

Nick,

I think Demetra is ‘better’ than ISIS in terms of a more intuitive software. I was also a Demetra beta tester and the software was designed by Francois Cochard and Nicolas Durand to make the accurate processing of Alpy spectra as straightforward and reliable as possible and in that I think they brilliantly succeeded. BASS is a very intuitive software as well which I have used extensively for my data reduction. Both Demetra and BASS will help ensure that the complexity of data reduction is as low a barrier as possible to getting more people practicing amateur spectroscopy. That for me is their key attraction, whilst ISIS will remain the ‘gold standard’ in terms of, as Robin said, its flexibility and comprehensive performance

Cheers

Hugh

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