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Dear spectroscopers, Im just getting into Spectroscopy and have an Sa100 for now. I wanted to get to grips with ISIS and have the current version 6.1. I have downaded and follwed the excellent tutorial http://www.astrosurf.com/buil/isis/guide_sa/tuto_en.htm It has been translated by Robin. I get all the way through the tutorial until I superimpose the instrument response file and then the spectrum flatlines horribly as though I have added a negative image of the original spectrum.. I’ve shecked the instrument reponse profile you create and it looks as it should. Some screenshots attached in the PDF.
I have unladed, reinstalled and repeated the process and its always the same step that goes wrong.
Q: Is it me making some rookie error or is there something about 6.1 that is incopatible with the tutorial (which comes with all the test spectra and files you need, or is it something else?
Many thanks
Jonathan
Jonathan
Hi Jonathan,
Firstly congratulations on getting as far as you have with ISIS. Although it is very powerful, particularly for producing research grade spectra from more advanced spectrographs than the Star Analyser, it can be tough to master and most people use different software with the SA like Visual Spec, RSpec, BASS project which can be easier to use with the SA. (I tend to use mixture of VisualSpec and ISIS, mainly because that was what was around then). I think you actually are ok though with what you have done and I suspect the flat line is just because the resulting spectrum has some really high and low values at the edges outside the wavelength range where there is good data and ISIS has autoscaled to include them. If you rescale manually or crop the bad edges (using the crop tool in the profile display) hopefully you should be able to see the result.
There are also some other resources on the website here that you might find useful
https://britastro.org/section_information_/equipment-and-techniques-section-overview/spectroscopy
David Boyd’s “Scientific analysis of amateur spectra” at the top is an excellent overview of the whole subject.
Particularly for the Star Analyser, my slides for the tutorial “Low Resolution Slitless Spectroscopy” downloadable here
https://britastro.org/document_folder/baa-document-store/past-talks/spectroscopy-workshop-at-the-nlo-oct-2015
Also the document on my website here specifically tackles the subject of response correction (flux calibration)
http://www.threehillsobservatory.co.uk/astro/Relative_flux_calibration_20221222.pdfCheers
RobinI see cropping the spectrum to remove the bad edges is mentioned in the ISIS tutorial ~ 2/3 the way down. I think you have just missed that step
“Tip: The response profile changes shape very rapidly near the edges and it may not accurately represent the response in these regions as the signal there is almost zero. It is a good idea to crop the useful part before smoothing the spectrum. Use the “Cut” tool for this operation. The smoothing will then be easier and the results better. Here is the smoothing result under these conditions:”
Cheers
RobinThank you so much Robin for the encouragement – I was determined to master it. I didn’t spot anything on cropping but I see it now… I missed the tip as you suspected… seems more like an essential step!
I cannot tell you how excited I was when it worked – Thank you so much, I could not have done it without you – screenshot attached. All ready to do my own now once I have done my darks and flats and offsets – just need to work that out now and then wait for the clouds to roll away. 3C273, here we come! I wanted to learn ISIS because I suspect I will get an Alpy at the first opportunity.
I can do my flats with a tungsten lamp right? can I use an LED lamp instead, do you think?
I will have a look at the other resources – I did go through David’s US talk, but got a bit lost at flux and rectification – my career has been in life sciences not physics, so more work to do there – thank you, I’ll look at your website.
Many many thanks Jonathan
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