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DIY low res slit spectrograph based on StarAnalyzer proposal / need a bit of help


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On 23/09/2017 at 15:09, Stub Mandrel said:

You trust a man who labels his graphs in Angstroms? :wink:

Given the preponderance of inches, lightyears, degrees, seconds and minutes on this site, at least Angstroms are metric. Maybe he has a Swedish grandmother.

Not to mention the Babylonian Right Ascension.

Even parsecs, which are universally accepted should be milliparrads or something.

I feel the whole unit thing is a complete mess.

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20 minutes ago, robin_astro said:

Lose the Barlow. It is only making the star image bigger and losing you resolution

I can't on the 114, not enough back-focus.  When I get the 200 off bricks (it's currently having it's focuser fixed) I can do without the Barlow. Meantime I understand now where all the boxes and mirrors come from. It's all to do with the curvature introduced by the diffraction angle. They are just so big and heavy, I wonder if there is a more lightweight solution.

On my photo it's why the blue seems more out of focus than the red.

Maybe I could take more than one photo and stitch them together.

 

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11 minutes ago, SteveBz said:

Given the preponderance of inches, lightyears, degrees, seconds and minutes on this site, at least Angstroms are metric. Maybe he has a Norwegian grandmother.

Not to mention the Babylonian Right Ascension.

Even parsecs, which are universally accepted should be milliparrads or something.

I feel the whole unit thing is a complete mess.

Fair point!

I think Angstroms are Metric but they aren't SI...

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5 minutes ago, Stub Mandrel said:

Fair point!

I think Angstroms are Metric but they aren't SI...

They aren't but my physics master taught Å in the 70s while my chemistry master taught nm.

I'd love a general agreement on units, but it seems as far away as ever.

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2 hours ago, Stub Mandrel said:

You trust a man who labels his graphs in Angstroms? :wink:

Ah, this is astronomy though.  No SI unit police there.   Plenty of Angstroms still around even in the latest literature, and the flux Y axis is often given in erg/cm2/sec/Angstrom. Mind you that's nothing  compared with some of the units we had in the paper industry  :icon_biggrin:

Robin

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3 minutes ago, SteveBz said:

Do prisms give better or worse spectra? Would it be possible to avoid the focus/curvature problem with a prism?

You cannot form a decent spectrum at all with a prism in the converging beam, the aberrations are just too severe. You need a parallel beam and the camera at an angle. You can also angle the camera with a grating to minimise the effect, or use a shallow angle wedge prism to deflect the beam back on axis but once you go beyond ~200l/mm various other aberrations rear their head and you really need collimator  to give a parallel beam and an objective to refocus on the detector to get a decent spectrum.

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12 minutes ago, robin_astro said:

You cannot form a decent spectrum at all with a prism in the converging beam, the aberrations are just too severe. You need a parallel beam and the camera at an angle. You can also angle the camera with a grating to minimise the effect, or use a shallow angle wedge prism to deflect the beam back on axis but once you go beyond ~200l/mm various other aberrations rear their head and you really need collimator  to give a parallel beam and an objective to refocus on the detector to get a decent spectrum.

So what do you think of this guy's spectrometer based on a inline prisms?

You'll need to scroll down to see the pics.

http://www.skyandtelescope.com/get-involved/pro-am-collaboration/the-revival-of-amateur-spectroscopy/

 

 

 

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14 minutes ago, SteveBz said:

So what do you think of this guy's spectrometer based on a inline prisms?

You'll need to scroll down to see the pics.

http://www.skyandtelescope.com/get-involved/pro-am-collaboration/the-revival-of-amateur-spectroscopy/

 

 

 

Yep that's  amateur spectroscopy pioneer Maurice Gavin. He's on here from time to time. He made a lot of innovative unusual designs and observations and is one of the people who helped me when I picked up spectroscopy.  

Yes  you can use a Barlow to collimate the beam from the telescope ahead of a prism (in this case an Amici prism which keeps everything in line) or grating and then use a camera lens to focus the spectrum. Similar to the junk box spectrograph but using a concave barlow lens as a collimator instead of an eyepiece.

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

The Barlow does not produce a secondary image ahead of the grating though so you cannot add a slit with this design.

 

Cheers

Robin

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I've been away for some time, but right before I went, I was entertaining idea of using a cheap plossl eyepiece as a source of collimation lenses, similar to @robin_astro junk box arrangement, but with only two simple doublets from disassembled plossl one acting as eyepiece part and other one acting as regular lens before camera part. This is of course based only on looking at plossl eyepiece design graphs - showing 2 doublets - have no idea of actual focal lengths of those lenses and if they are the same, or if not the same FL, whether they can still be used in this arrangement - which is basically: take cheap plossl - saw it in half, insert SA between first and second lens group and adjust distances of lens groups so that beam between first and second group is collimated.

There is always an option of buying actual lenses (and even really cheaply from places like surplus shed) but I'm wondering how difficult it would be to mount everything together, ideally there would be some sort of lens mounts with T2 threads on both ends to do that, but I don't think I found anything like that cheap online.

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2 hours ago, vlaiv said:

I've been away for some time, but right before I went, I was entertaining idea of using a cheap plossl eyepiece as a source of collimation lenses, similar to @robin_astro junk box arrangement, but with only two simple doublets from disassembled plossl one acting as eyepiece part and other one acting as regular lens before camera part. This is of course based only on looking at plossl eyepiece design graphs - showing 2 doublets - have no idea of actual focal lengths of those lenses and if they are the same, or if not the same FL, whether they can still be used in this arrangement - which is basically: take cheap plossl - saw it in half, insert SA between first and second lens group and adjust distances of lens groups so that beam between first and second group is collimated.

There is always an option of buying actual lenses (and even really cheaply from places like surplus shed) but I'm wondering how difficult it would be to mount everything together, ideally there would be some sort of lens mounts with T2 threads on both ends to do that, but I don't think I found anything like that cheap online.

I'm hoping you find the way and then let us all know.  It just too expensive a toy.

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15 hours ago, vlaiv said:

There is always an option of buying actual lenses (and even really cheaply from places like surplus shed) but I'm wondering how difficult it would be to mount everything together, ideally there would be some sort of lens mounts with T2 threads on both ends to do that, but I don't think I found anything like that cheap online.

Edmund Optics do T mounted components including lens mounts. I have used them in spectrographs but they are not cheep. I suspect 3D printing is the way to go in the medium term. I have seen prototype 3D printed spectrographs on other forums.

Regards Andrew

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3 hours ago, andrew s said:

Edmund Optics do T mounted components including lens mounts. I have used them in spectrographs but they are not cheep. I suspect 3D printing is the way to go in the medium term. I have seen prototype 3D printed spectrographs on other forums.

Regards Andrew

3D is obviously an excellent idea, sadly you could buy a full spectroscope for the price of a printer.

I did look at Edmund Optics, and that is probably the way to go.  You'd have to do Christian Buil's lens calculations.

Regarding the housing, I have a dead, black dishwasher, so I thought if I had a template, I could go at it with an angle grinder to create a kit which I could rivet or bolt together.

Mounting stuff inside then needs a generous junk box and fittings.

Again I saw a template somewhere, I'll have another look and print it out.

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46 minutes ago, SteveBz said:

My other thought was to take a cheap 4 or 6mm eyepiece and remove the lens, get a 2m fibre optic cable stick one end in the eyepiece/ota and feed the other end into any old spectrometer you can find on eBay.

I use home made fiber fed spectrographs but use a Shelyak fiber guide head. The key issue is getting the star onto the fiber. Your typically aiming at 50 micron fiber.

Regards Andrew

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For a simple slitless setup, I  always fancied trying something like the RS spectrograph, which is similar in concept to the junk box spectrograph.

http://www.astroshop.eu/rigel-systems-spektrograf-rs-spectroscope/p,51408

At this price it is silly but it could be done quite cheaply if you already have one of those big wide field eyepieces and a DSLR. It is a fully collimated design and the camera is angled so you can use a high dispersion grating (I think this one uses 500 l/mm) and get good resolution. The only possible drawback with some DSLR eg cannon is that the front ring rotates as you focus.

Robin

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14 hours ago, robin_astro said:

It is a fully collimated design

Very nice, it's just what I need, although not at that price.

It seems to be eyepiece projection, is that the same as fully collimated? I imagine it would work equally well without the lens, it's really the same as mine but with the angle I need.

I wonder if a 135 degree, 32mm pipe elbow (89p from Wickes) would do the trick with a bit of padding eg PTFE tape?

Steve

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Steve,

I think you need to re-think...

Spectrographs like telescopes are precision optical devices.

I speak from experience when I say there's only so far you can go with a B&Q solution.

I would honestly recommend starting (properly) with a classical design - say based on a couple of s/h bino objectives and a 600 l/mm reflective grating.

Buil has some basic designs...

http://astrosurf.com/buil/spectrographs.htm

Also check out my website:

http://www.astronomicalspectroscopy.com/

 

 

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1 hour ago, Merlin66 said:

Also check out my website:

http://www.astronomicalspectroscopy.com/

You're probably right.  I've downloaded Ken Harrison's article from your website, which makes good reading, and he also used 6mm ply, probably from B&Q :), although to be fair, it doesn't actually say.

The issue I have is that it's either a year-long project or at least a grand upwards.

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5 hours ago, SteveBz said:

Very nice, it's just what I need, although not at that price.

It seems to be eyepiece projection, is that the same as fully collimated? I imagine it would work equally well without the lens, it's really the same as mine but with the angle I need.

Yes it is optically the same as eypiece projection. It will not work using a 500 l/mm grating without the eyepiece and camera lenses though. The key is that the eyepiece makes the beam parallel through the grating which is essential if you are using a grating with more than ~200l/mm.  Here is a similar arrangement by Christian Buil using camera lenses. The LORIS spectrograph

http://www.astrosurf.com/buil/us/loris/loris.htm

Robin

 

 

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