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Low resolution spectroscopy 2: Presenting your best profile




The weather continues to be very unfavorable for astronomy, so I continue to play in my 'shed of delights'. I have addressed some of the issues associated with using a 500 line diffraction grating as a simple filter cell.  I did not want to permanently adapt my QHY5-11 camera for spectroscopy and so designed a 3d printed block to allow it to be brought very close to the imaging chip at the optimum angle for a first order spectrum at 550nm. This appears to have minimised 'fish tailing' of the spectral image and aided obtaining focus upon the spectrum rather than the star. 
In between visits from grandchildren I tried my new spectrometer out on my 66mm Altair Astro refractor.  Sitting in our living room on 07 July I could just make out a few stars above our neighbour’s house roof ( approx South). Being a bit idle, I pointed my spectrometer-refractor combo through the patio doors at a brightish star at a convenient height and obtained my first spectral image with the new spectrometer.  I didn't obtain any darks and flats or any such 'technical marlarkey' as I was really just giving it a go in between the clouds. I was quite surprised that I captured a reasonable image of the 'mystery star’.
One of the not immediately appreciable downsides of using a 500 line diffraction grating is that, as the spectrum is more spread out than using  a 100 or 200 line grating, it produces a significantly dimmer image. Possibly this will be improved by using my 127mm refractor, by stacking more light frames and by taking darks and flats. 
I prepped the spectral image using the freeware IRIS and then had a go at producing an initial calibrated profile using John Paraskeva’s excellent BASS software. I am amazed at the generosity and skills of people who devise and contribute to the development of freeware that open up access to scientific hobbies for old blokes like me!
Having obtained a profile, I guessed the star in question was an A type star. I then used Stellarium to look for bright  A stars that were observable to the naked eye from our living room on 07-07-2021. The best candidate for my money is the A07 star Mu Serpentis. (This is probably wrong but ‘in for a penny in for a pound’!)
Next up I intend to:
Image Vega and obtain a camera response curve for the set up.
Learn more about using BASS software.
Complete a half constructed 3d printed 100 lines/mm set up for obtaining spectra from fainter stars.
Come up with a rudimentary grating design to address extended objects of interest.
Mess about with a fibre optic link between my telescope and a spectrometer.




Edited by Hawksmoor
poor format and missed a photo

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That is a nice clean setup you have made there .  How did you determine the angle for the grating ?


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Very neat and tidy!


The angle is approx 3.15 deg for a 100 l/mm grating.

The formula:

sin B = n*N*L

where B is the deviation angle, n, the number of lines (l/mm) N is the order of the spectra (usually one) and L the wavelength.

for a 100 l/mm grating, 1st order at 550nm (green)

sin B = 1*100*550*10^-6

B = 3.15 deg.



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