multiple object spectrographic telescope

Design, construction, tuning of spectroscopes
Information and discussion about softwares (telescope remote, autoguiding, acquisition, spectral processing ...)

multiple object spectrographic telescope

Postby Hamish Barker » Wed Dec 04, 2019 8:01 am

this concept looks interesting.

https://www.youtube.com/watch?v=ollpNYOrbcc


http://www.3dewitt.com/telescope/slide1

I have tried a little duct tape mockup using my L200 and an input lens after the objective grating, looking into the output port with an eyepiece, I can see the image of the scene the input lens is looking at, but with a spectrum overlaid. That is, each colour is mapped to a different spatial coordinate in the input image.

http://www.3dewitt.com/PDF/3DeWitt_Dittoscope-NIAC.pdf

My next plan is to build a neater setup with the biggest gratings I have (a 150mm x 75mm piece of 1000lpmm grating) as well as a couple of other 50x50mm reflection gratings I have.

Then I'll try to get a long piece of transmission grating film (150mm x 2000mm x 1000lines/mm.) and set it up as a shallow (grazing) EXIT angle objective grating in front of my 200mm f6 (stopped down to 150mm), with the L200 downstream (it has a 600lpmm grating.) I will post results and further investigations.

It's ingenious: the long, grazing angle objective grating maps a separate direction to each of the separate wavelengths for a single exit direction (which is selected by the telescope via input slit of the L200 littrow spectroscope). Multiple directions are therefore all mapped to the input slit of the spectroscope, but for each direction, only one colour arrives at the slit. The output of the spectroscope is then a spatial map with only one colour per direction.

To get full spectra for each star, drift scanning is required. this doesn't seem much of an avantage over a normal slit spectroscope. But the advantage is that due to the grazing exit angle, flatness tolerance on the objective grating is apparently relaxed (not sure that I agree, as what about the input angle of the objective grating also?), so large, thin, low specification plastic film gratings can be used as the primary collector. The proposal was to scale it up to a huge scope.

Has anyone else ever seen this sort of double dispersion (both before the telescope, and simultaneously in the usual location after the slit) before?

I think it's worth some further investigation.
Hamish Barker
 
Posts: 58
Joined: Thu Jun 20, 2019 12:11 am

Re: multiple object spectrographic telescope

Postby Robin Leadbeater » Wed Dec 04, 2019 1:27 pm

I dont understand what the advantage is compared with for example just scanning the field over the spectrograph slit without the first dispersing element, recording the spectrum of each object as it crosses the slit. (rather like in spectroheliography for example) Even assuming a 100% efficient objective grating, the number of photons collected is the same in both cases so there can be no overall efficiency advantage.

Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
Posts: 1343
Joined: Mon Sep 26, 2011 4:41 pm

Re: multiple object spectrographic telescope

Postby Robin Leadbeater » Wed Dec 04, 2019 1:36 pm

Also wouldn't the scanning the dispersed image convert noise from intensity variations due to seeing into the wavelength domain, increasing the noise in the spectrum ?
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
Posts: 1343
Joined: Mon Sep 26, 2011 4:41 pm

Re: multiple object spectrographic telescope

Postby Hamish Barker » Wed Dec 04, 2019 7:15 pm

Hi argument is that the primary objective grating can be very long, arranged so that exit angle towards the telescope mirror is grazing. more photons are collected. the grating efficiency does drop as exit angle increases, but not as fast as the increasing area of the long grating.

Also, the long primary objective grating means that in the dispersion direction, spatial resolution of the sources is proportional to the grating length. If the relaxed tolerance on the grating flatness is true, it may be easier to build a long thin primary grating (membrane) than an equivalent sized mirror.

If it's so good, the obvious thing would be to build a small scale demonstrator (more than the bench work they've done so far). I'll try it out and report back.
Hamish Barker
 
Posts: 58
Joined: Thu Jun 20, 2019 12:11 am

Re: multiple object spectrographic telescope

Postby Robin Leadbeater » Thu Dec 05, 2019 1:23 pm

Ah OK,

I'll have to delve more deeply into it :-)

Thanks
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
Posts: 1343
Joined: Mon Sep 26, 2011 4:41 pm

Re: multiple object spectrographic telescope

Postby Hamish Barker » Fri Dec 06, 2019 12:02 am

I wonder if the concept could also be used with a high quality grating upstream of objective lens or mirror to provide energy rejection function for spectroheliography by diverting most bandwidth away from the input slit.
Hamish Barker
 
Posts: 58
Joined: Thu Jun 20, 2019 12:11 am


Return to Spectroscopes and Softwares

Who is online

Users browsing this forum: No registered users and 0 guests

cron

x