Transmission grating in a converging beam

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Transmission grating in a converging beam

Postby Andrew Smith » Mon Feb 15, 2021 1:11 pm

I have been looking at this configuration as a possible way to do very low resolution spectra of flare stars with less glass and unknown coating that my current spectrograph.

One issue that puzzled me was the correct focal surface to use as I wished to offset from the zero order focus to allow remote unattended operation.

Bowen and Vaughan ( ... ..85..174B ) note that the sagittal image is focused on a curve of radius s equal to the distance from the zero order to the grating and that the tangential image surface is on a curve of radius s/3. The continuum is in the direction of the sagittal image and the lines crossing it are in the direction of the tangential image.

Different authorities have used different choices for this when optimising the spacing or in calculating the resolution R.

I used OSLO Edu to experiment with a 400mm F6.8 parabola on axis and a grating with 100 or 200 l/mm with various spacings. To get the surface of best focus as a function of wavelength I performed a monochromatic optimisation of focus minimising the OPD (optical path difference) one wavelength at a time. The OSLO data is from 3000A to 7000A

The best focus lay between the two curve with radii s and S/3 and surprisingly fell on s/2 in all cases.

Grating in converging beam.png

Any comments or explanations would be most welcome. There is always the possibility I made a mistake but I did check my trigonometry.
Regards Andrew
Andrew Smith
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Joined: Fri Sep 30, 2011 12:23 pm

Re: Transmission grating in a converging beam

Postby Ken Harrison » Mon Feb 15, 2021 11:24 pm

I've replied on the astronomicalspectroscopy group. ... sage/16598
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before....
"Imaging Sunlight - Using a digital Spectroheliograph" - Springer
Ken Harrison
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Location: St Leonards, Australia

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