Slit width for low resolution spectroscope.

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Re: Slit width for low resolution spectroscope.

Postby Ken Harrison » Wed Jun 10, 2020 1:22 am

SimSpec V2 onwards spreadsheet has always used the CAOS slit efficiency calculation.
The original Buil spreadsheet was designed for a slitless configuration
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before....
"Imaging Sunlight - Using a digital Spectroheliograph" - Springer
http://www.astronomicalspectroscopy.com
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Re: Slit width for low resolution spectroscope.

Postby Robin Leadbeater » Wed Jun 10, 2020 11:21 am

Ken Harrison wrote:SimSpec V2 onwards spreadsheet has always used the CAOS slit efficiency calculation.
The original Buil spreadsheet was designed for a slitless configuration


Hi Ken,

Your "SimSpec" may have a different version numbering scheme to Christian's. On the link I gave all the spreadsheets there have an entry for slit width including the one labelled "Original Simspec V2.2" dated 2003. However the slit width there appears to be used only for the SNR calculation (sky/camera noise contribution) and the star FWHM is used for the resolution calculation. (ie it assumes a "wide slit" configuration.) Similarly for the transmission grating version SimTrans.

The version specifically for the LHIRES Littrow configuration (from 2006) however has an entry for slit width which is used for the resolution calculation and it also calculates the slit transmission using a lookup table. I dont know what the values in the table come from however.

Cheers
Robin
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Re: Slit width for low resolution spectroscope.

Postby Robin Leadbeater » Wed Jun 10, 2020 11:52 am

Hi Ken,

In your SimSpec V4 you mention using the CAOS group calculation for SNR. Do you have a link to that method on their website or a copy ?

Thanks
Robin
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Re: Slit width for low resolution spectroscope.

Postby Ken Harrison » Thu Jun 11, 2020 8:18 am

The SimSpec spreadsheets all use the CAOS slit efficiency calculations.
The original Buil spreadsheet was developed for slitless designs.
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before....
"Imaging Sunlight - Using a digital Spectroheliograph" - Springer
http://www.astronomicalspectroscopy.com
Ken Harrison
 
Posts: 166
Joined: Thu Oct 25, 2012 5:45 am
Location: St Leonards, Australia

Re: Slit width for low resolution spectroscope.

Postby Robin Leadbeater » Thu Jun 11, 2020 11:15 am

Ken Harrison wrote:The SimSpec spreadsheets all use the CAOS slit efficiency calculations.
The original Buil spreadsheet was developed for slitless designs.


Hi Ken,

I was not specifically referring to your SimSpec in my original post but the spreadsheets on Buil's page.
http://www.astrosurf.com/buil/us/compute/compute.htm

Although Buil includes V4 of your spreadsheet on his page, his original SimSpec v 2.2 there which predates your SimSpec
http://www.astrosurf.com/buil/us/compute/simspec.xls
does in fact allow for the inclusion of a slit but assumes a wide slit significantly wider than the star image where all the light passes through the slit (ie a "photometric" slit). The width of the slit however is still used to calculate the noise from the sky background to give the correct SNR.

Buil's calculator for the LHIRES on that page (called etcl.xls)
http://www.astrosurf.com/buil/etcl2/etcl_en.htm
is dated 2006 which I believe predates your SimSpec and does calculate the resolution and slit losses for any width of slit using a lookup table.

Your use of the CAOS SNR calculations in your version of SimSpec is interesting and relevant to Jarek's original post. I could not find the calculation on the CAOS website. Do you have a link or a reference to this so we can see how this differs from the calculation used by Buil in his spreadsheets ?

Thanks
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
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Re: Slit width for low resolution spectroscope.

Postby Robin Leadbeater » Thu Jun 11, 2020 1:10 pm

Robin Leadbeater wrote:
Your use of the CAOS SNR calculations in your version of SimSpec is interesting and relevant to Jarek's original post. I could not find the calculation on the CAOS website. Do you have a link or a reference to this so we can see how this differs from the calculation used by Buil in his spreadsheets ?


See your comment in the revisions for your SimSpecV4, highlighted here
simspec_KH_rev.png
simspec_KH_rev.png (12.15 KiB) Viewed 1250 times


Does this refer to a different way of calculating SNR based on a method from CAOS or just a modification of the spreadsheet calculation of SNR to take account of slit losses which was added in the same revison ?

Thanks
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
Posts: 1519
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