Littrow Spectrograph?

[DaveS]

Just kicking ideas around ATM, and don't even know if I will proceed, but do any of the experts on here have any experience / thoughts on Littrow configurations?

ATM I'm thinking of an off-axis paraboloid for collimation and focusing, and a small flat mirror at 45 deg to divert the beam into the camera. Also considering a 45 deg pellicle beam splitter to divert a portion ( ~32%) of the input light to a guide camera.

Ideas are fuzzy as yet, and not even down on paper.

I've cobbled together a few spectroscopes in my time, but nothing as critical as this.

[andrew s]

I built one using an echelle grating and a Borg 55mm refractor. I introduced the light using an off-axis guider. A scaled down version of this http://www.astrosurf.com/buil/vhires/test.htm.

I have pondered using off-axis paraboloids but could not find suitable ones. I also gather they are hard to collimate so I would not try moving it to focus.

There is all the other stuff on Christian Buil's site including the LHIRES III

Good luck.

Regards Andrew

[Merlin66]

Interesting.

I've designed and built a few.....

What focal length collimator are you thinking about? 

What resolution?

A reflective slit plate is a better option than the beamsplitter (I've tried both!)

A microscope glass slide works well (4/92)

Checkout our website below for some ideas.

Edited November 12, 2019 by Merlin66

[DaveS]

Well, as it would go on (Most likely) the ODK12 at f/6.8 (Call it 7 for now), and to maintain throughput the f-ratio of the collimator should equal that of the telescope, then if I use a 25mm dia mirror then I get a fl of around 175 mm.

I'm not after super high resolution, but as a ballpark figure I would want to split the Na D lines, so 2000 or so. I hope to be able to put the visible-ish spectrum (Say 380-780 nm) on the whole of a sensor, which could be an ASI1600 or perhaps an ASI183. Maybe. Perhaps. I'm open to the possibility of having to move the grating to select narrower regions of the spectrum.

Note the vague design specs, this is because it is at a *very* early stage, of still kicking ideas around, I haven't even done any precise calculations yet.

[Merlin66]

OK.

You only need around R=1000 to split the Na lines.....

Have a look at the SimSpec spreadsheet, It may help to firm up your ideas..

http://www.astronomicalspectroscopy.com/instrument.html

[robin_astro]

Hi Dave,

Unless you are desperately needing to work across a very wide wavelength range, particularly into the UV  I would tend to steer clear of  reflective designs as they are  tricky to design and set to avoid inherent aberrations. There are a couple of practical examples around, the Starlight Xpress spectrograph which uses a curved grating and works but has significant astigmatism and Christian Buil's UVEX project design which is a Czerny-Turner configuration optimised to minimise aberrations and then uses a cylindrical lens to tame the remaining astigmatism and is producing impressive results.

http://www.astrosurf.com/buil/UVEX_project_us/

I agree about the reflective slit over a beam splitter for the guider. being WYSIWYG makes life so much easier to set up and use.

Cheers

Robin

[robin_astro]

Dave,

Here are some ideas for  Littrow designs

A compact Littrow design of Christian Buil's which can just split the Na D lines. 

http://www.astrosurf.com/buil/us/spa/test.htm

and the full design details for the now commercialised LHIRES III which can produce resolutions of 0.3A also designed by Christian Buil et al

http://astrosurf.com/thizy/lhires3/index-en.html

developed from this earlier version

http://www.astrosurf.com/buil/lhires2/plans.htm

Cheers

Robin

[DaveS]

Many thanks guys. I think I will apply the KISS principle to my design and avoid off-axis paraboloids, though it seemed a good idea to avoid a change in focus with wavelength.

Much to ponder in the linked articles. It's likely that I will use a "basic" ruled grating of 600 lines. Whether I will source the lens from Edmund or use a S/H photo lens remains to be seen.

Off to do some more research, but in the meantime I may well buy a SA to go in the filter wheel of one of my 'scopes.

[vlaiv]

23 hours ago, Merlin66 said:

A microscope glass slide works well (4/92)

Is this for reflective slit? I can't find any reference on the net for such slides (have no idea what 4/92 stands for).

Could you expand on it please?

[andrew s]

5 minutes ago, vlaiv said:

Is this for reflective slit? I can't find any reference on the net for such slides (have no idea what 4/92 stands for).

Could you expand on it please?

No it is just a beam splitter, 4% of the light goes to a guider 92% to a non reflective slit.

Used before reflective slits became readily available. 

Regards Andrew 

Edited November 13, 2019 by andrew s

[Merlin66]

Thanks Andrew....

I prepared a write-up describing the conversion of a Vixen flip mirror housing to a beamsplitter using the microscope slide, "Astronomical Spectroscopy for Amateurs", p220.

Vixen mod.pdf

[DaveS]

Thanks for the replies and apologies for not responding earlier, I'm still giving this thought. Question: Where to get a reflective slit? I've seen a rotating multi-slit plate in @Thalestris24's build thread, Thorlabs?

ATM I've been looking on the Edmund Optics site, they have air slits, but I didn't see the type I would want. As an aside, the slits I've made before were cut from brass and beveled tonear knife-edge.

[Merlin66]

Dave,

The reflective multi-slit plate is available from Jeulin.

They and some other useful suppliers are listed here:

http://www.astronomicalspectroscopy.com/supplier.html

Ken

 

[andrew s]

Shelyak sell slits intended for their spectrographs but you can buy them separately. 

Regards Andrew 

[DaveS]

Thanks, the OVIO slit plate looks a lot less frightening that the EO offerings.

[Thalestris24]

https://en.jeulin.fr/simple-radial-slits-212076.html

I've only purchased mirrors and a reflective grating (so far) from Thorlabs. They are very good. They also send a box of 'lab snacks' with every order!

Louise

NB Jeulin charge more + shipping to export my total cost was £59.10 for the radial slit

Edited November 13, 2019 by Thalestris24

[DaveS]

That's still a lot less than the £95+VAT for an air slit from EO.

[Merlin66]

The OVIO slit plate also gives you the option of 12 different slit widths...

Allows you vary the slit gap depending on the telescope, object magnitude and required resolution - SimSpec will show the probable results.

 

[Thalestris24]

6 minutes ago, DaveS said:

That's still a lot less than the £95+VAT for an air slit from EO.

Edmunds are an expensive supplier - more for professional users, I think. Their slits do look more rugged, though. The Jeulin/Ovio one needs careful handling - as do all the spectrometer optics, of course!

Louise

[DaveS]

If I was content with a transmission type I could knock up another Hartridge prism, though the ones I'd made before were only for school level use, a 600 l/mm grating on the hypotenuse of a 90 degree acrylic (Yeah, rough) prism gave DV near the centre of the visible spectrum. I think the PH DV spectroscopes use a similar arrangement.

Thinking, Jeulin do a 600 l/mm grating blazed in the 1st order for not too much money, cementing that on a right angle prism might be an idea.

Mind you, I have a small Amici prism in my "box of bits" though the non-linear spectrum makes it no-go for imaging. OK for purely visual perhaps.

[Merlin66]

Dave,

I'm pretty sure the PH spectroscope will use a glass prism.

Fulvio "converted" a PH DV spectroscope to his Spec 600

http://www.lightfrominfinity.org/Spec 600/Spec 600.htm

An Amici prism can be used successfully (the late Maurice Gavin's set-up is illustrated in "Astronomical Spectroscopy for Amateurs", p 170) using a barlow lens as a collimator.

The available processing software (BASS Project etc.) can easily handle the prism non-linear spectral image.

 

Edited November 13, 2019 by Merlin66

[robin_astro]

46 minutes ago, Merlin66 said:

Dave,

I'm pretty sure the PH spectroscope will use a glass prism.

 

Nope, they are 600 l/mm grisms.  Two different diameters in the two versions. I think the one in Fulivo's "ALPY/TRAGOS copy" is the larger size

Robin

Edited November 14, 2019 by robin_astro

[robin_astro]

1 hour ago, DaveS said:

 

Thinking, Jeulin do a 600 l/mm grating blazed in the 1st order for not too much money, cementing that on a right angle prism might be an idea.

PH make the gratings that Jeulin sell

You could make a TRAGOS using the Star Analyser 200.

http://www.burwitz-astro.de/spectrographs/tragos/index.html

or an ALPY copy using the 600l/mm grism from the PH DV spectrograph. (Not as good though as the ALPY uses special optics with reduced chromatism)

Robin

[Merlin66]

Robin,

My bad.. I meant to say that the PH grism prism was glass and not acrylic.

[DaveS]

Thanks for the links. I'm going to have another read of Sidgwick in the section about spectroscopes. He gives some equations for designing Hartridge Prisms. I think using a std 600 l/mm grating and a BK7 right-angle prism should give near DV, close enough anyway to put the spectrum on a sensor. I do have a few achromatic doublet lenses in my "box of bits" which I'll dig out.

*sigh* Why didn't I start this while I was working? At school I had a whole load of optical bench stuff I could have used for breadboarding, and a prep room where I could set things up.