Littrow Spectrograph?

[DaveS]

Did a quick google on Hartridge Prisms, and the first entry was This. Damnit, I have one in my box of bits but have banged my head on the wall trying to get it to give clean spectra. Every so often I dig it out and think I've sussed it but just come up against the same problems of overlapping orders.

[Thalestris24]

2 minutes ago, DaveS said:

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.

Hi Dave

In case you still need one, I found that a 2" 28mm EP has a 32mm dia f= ~70mm, achromatic doublet. Also, the 6 x 35 finders have 31.2mm dia achromatic doublets f= ~120-125mm, as objectives

Louise

[DaveS]

Thanks Louise. As it happens I have a spare Skywatcher 6x30 finder kicking around that I can dissect for it's objective lens.

I also found This, Hartridge's original paper detailing his prism-grating and its characteristics,

Now to get myself an optical bench.

[vlaiv]

Quite cheap lens for collimation can be purchased here (not sure about quality, but it looks like lens for finders):

https://tavcso.hu/en/productgroup/kieg_lencsek

50mm FL 182.8mm

[andrew s]

If you plan a full range visible Littrow then I strongly suspect an achromat will not allow you to focus well across the spectrum.

Regards Andrew 

[DaveS]

I had a read of Hartridge's paper, and no realise what the slip of black velvet is for. I thought it was just to protect the grating, but it's to absorb unwanted reflected light. A UV/IR cut filter will also get rid of higher overlapping orders, without needing an order-sorting prism.

[DaveS]

2 minutes ago, andrew s said:

If you plan a full range visible Littrow then I strongly suspect an achromat will not allow you to focus well across the spectrum.

Regards Andrew 

Which is why I was looking at off-axis paraboloids.

Argh! My head is about to explode!

[andrew s]

3 minutes ago, DaveS said:

Which is why I was looking at off-axis paraboloids.

Argh! My head is about to explode!

Apochromatic objective, expensive but...

Another possibility is a well corrected camera lens often available second hand at reasonable cost.

Regards Andrew 

Edited November 14, 2019 by andrew s

[vlaiv]

How about this lens then - still very cheap:

https://www.surplusshed.com/pages/item/L3465.html

or maybe this:

https://www.surplusshed.com/pages/item/L3710.html

[vlaiv]

In fact, surplus shed has loads of different lenses rather cheap

[Thalestris24]

23 minutes ago, vlaiv said:

In fact, surplus shed has loads of different lenses rather cheap

I've ordered some from them - were shipped on the 7th Nov. Departed uk 'carrier facility' yesterday - I think that's probably Royal Mail. Should arrive in the next day or two! However, it's worth bearing in mind that they will be second hand lenses so no guarantee of quality. Hopefully good enough to prototype with and can be swapped later, if need be. The finder and ep doublets mentioned above are very good but a bit large in terms of diameter for the Lowspec design - would really need to make the design a bit bigger to accommodate them. As it is, it's not certain I'll be able to fit them in the optical path. I should be able to put everything together very soon so I'll know better then.

Louise

Ed. the mail follows a tortuous path in the States!:

Edited November 14, 2019 by Thalestris24

[DaveS]

I'll have a look in my collection of camera lenses, though my best lens is probably my 180mm f/3.4 Leitz Apo-Telyte-R. No, it's NOT going in a spectrograph lol.

The field lens from a 2" 28mm EP sounds like a good idea, as I think I have one kicking around. It came with the 180 Mak-Cass that I got from FLO in a sale a few years ago.

[Thalestris24]

21 minutes ago, DaveS said:

I'll have a look in my collection of camera lenses, though my best lens is probably my 180mm f/3.4 Leitz Apo-Telyte-R. No, it's NOT going in a spectrograph lol.

The field lens from a 2" 28mm EP sounds like a good idea, as I think I have one kicking around. It came with the 180 Mak-Cass that I got from FLO in a sale a few years ago.

I got the 2" EPs with each of the SW 130-pds and 150-pds scopes I think I got 1 6x30 finder with the 130pds - can't remember where the second one came from. I'll have to get the dust off and  try and resurrect the 130pds one day...

Edited November 14, 2019 by Thalestris24

[DaveS]

I had a look around and found this

 

It should be well corrected, but for finite conjugates. How far off it will be for infinite conjugates is another matter.

It's also a heavy lump of brass 'n' glass.

[robin_astro]

The old Olympus SLR lenses are pretty good and can be picked up at a reasonable price eg as used here with an eyepiece as collimator

http://www.threehillsobservatory.co.uk/astro/spectroscopy_19.htm

or here in an objective grating setup

http://www.threehillsobservatory.co.uk/astro/spectroscopy_17.htm

[Helen]

There are some Thorlab bits on ABS at the moment - not sure if they are any use to you Dave, Louise??

Helen

[Merlin66]

I agree with Robin!

The Oly Zuiko lenses are pretty good.

I still have some lenses from the ol' film days and use a 50mm f1.4 Zuiko on the MG80 spectrograph.....

 

Edited November 15, 2019 by Merlin66

[Thalestris24]

15 minutes ago, Helen said:

There are some Thorlab bits on ABS at the moment - not sure if they are any use to you Dave, Louise??

Helen

Nothing of interest to me but thanks anyway!

Louise

[Thalestris24]

I have an old Olympus C-770UZ compact with optical zoom (UZ = ultra zoom!). Only 4MP but an actual ccd! I think it has a lens of about 17mm diameter but it would be a shame to break it apart. I've not used the camera in many years. I put one of the old batteries in it, switched on, and it powered up! No card though - I think it uses an obsolete type. Oh, found a card in the case - a 512MB XD card. Anyway, I put it back into its soft case and it can sit and wait for when someone comes along to do an inventory of my belongings ha ha. No commercial value.

Louise

[Thalestris24]

On 14/11/2019 at 16:18, DaveS said:

I had a look around and found this

 

It should be well corrected, but for finite conjugates. How far off it will be for infinite conjugates is another matter.

It's also a heavy lump of brass 'n' glass.

That's quite a long fl - 190.5mm, and 42mm dia? 😮 Is it a doublet?

Btw surplus shed are offering random discounts over this weekend! Their doublets are usually only $7.50, but pot luck, of course. My ones might come tomorrow but could be held up in customs for all I know...  

Louise

[DaveS]

FL is a bit long, but should be doable. No, it's not a doublet, I would guess between 4 and 6 elements. It's a large-format enlarging lens, the last time I was using it was for enlarging 5x4 negs. it will certainly have a big enough corrected image circle to allow a modicum of offset for the right-angle mirror.

OK a search shows it to be a Tessar type construction, so a modification of the old Cook Triplet with a cemented doublet field lens.

[Thalestris24]

2 minutes ago, DaveS said:

FL is a bit long, but should be doable. No, it's not a doublet, I would guess between 4 and 6 elements. It's a large-format enlarging lens, the last time I was using it was for enlarging 5x4 negs. it will certainly have a big enough corrected image circle to allow a modicum of offset for the right-angle mirror.

OK a search shows it to be a Tessar type construction, so a modification of the old Cook Triplet with a cemented doublet field lens.

The Lowspec design uses 25.4/1" (collimator), 30mm (camera)  and 12.75/0.5" for the guide lens. They seem quite small diameters, in some respects.

Louise

[andrew s]

2 hours ago, Thalestris24 said:

The Lowspec design uses 25.4/1" (collimator), 30mm (camera)  and 12.75/0.5" for the guide lens. They seem quite small diameters, in some respects.

Louise

It's the focal ratio that counts and the collimator .ens should match the focal ratio of the feed telescope. The camera lens needs to be faster to catch the divergent Ray's of the spectrum. The SIMSPEC design software gives the details.

Regards Andrew 

 

[Thalestris24]

1 minute ago, andrew s said:

It's the focal ratio that counts and the collimator .ens should match the focal ratio of the feed telescope. The camera lens needs to be faster to catch the divergent Ray's of the spectrum. The SIMSPEC design software gives the details.

Regards Andrew 

 

Yes, I know

[DaveS]

Well, if I go for a littrow design then the collimator and camera lens are the same. Therefore it needs to be faster than the telescope, since the aperture of the input beam will be automatically limited by the geometry of the diverging cone, while the lens will be big enough to catch the diverging rays from the grating.

BTW my choice of the Ektanon was determined by Andrew's point about it needing to be well corrected for achromatism, plus I had it hanging around, plus it's fitted to a flange with four screw holes already drilled.

But it is a bit of a beast.

OK, first bit of arithmetic. (7.5 x 25.4) / 6.8 = 28.0 mm which is the diameter of the beam feeding the grating. Question: Do I go for a grating big enough to take the whole beam, or one covering the diagonal, ie 28mm or 19.8mm? Or more likely go for a 25mm square grating. Will need some dead black around the grating to kill the stray light.

 

Edit: Edmund do 30mm square gratings with 27mm clear aperture. Might be worth the punt.

Edited November 15, 2019 by DaveS