From diffraction grating to spectroscope

[Stub Mandrel]

OK, I have a proper photographically produced diffraction grating 4" by 1" and 1000 l/mm.

My assumption is that I can use this with a webcam and a slit to make a 'proper' spectroscope, but I'd rather try and do something more sophisticated than the simple CD spectroscopes on line.

I also have a few surface silvered mirrors (from old laser printers) that could come in handy and a well-equipped workshop.

But what I don't have is a straightforward 'plan' for making a working spectroscope or any tips. For example, I'm sure we used two opposed razor blades to make a slit when I was in school - is that the best way, and how narrow a slit should I aim for? What's the best configuration of slit, grating and detector?

[Oddsocks]

Hi Neil,

I am a bit further on with my project spectrometer having begun last summer but have hit a bit of a problem with the guiding module.

The original design uses a 250 lpmm grating and a slit made from two razor blades, the detector is a pair of photodiodes, one kept dark and the other sensing, the difference between the two used to provide temperature compensation, a wafer optical flat made from an old Cokin camera filter that I sent to be silvered is moved by a stepper motor from an old CD-ROM disk drive read/write head and scans the output of the grating across the slit and the whole thing is run by an Arduino including digitising the output of the photodiode (after amplification). Each step of the disk-drive head motor moves the flat so that the spectrum from the grating advances by approx 10nm across the slit. A small lens from a scrapped web cam focuses the output of the slit onto the photodiode.

Originally I wanted to be able to adjust the width of the slit by stepper motor as well but decided this was an unnecessary complication so just machined the razor blade holder with one blade mounted in a sliding frame that is moved by an old micrometer drive and the handle part just reaches outside the casing.

I also made a mistake with my first version of the moving mirror mechanism and tried to use two stepper drives, one pulling the mirror in 10nm steps the other pushing back against the slop in the swivel axle holding the mirror frame to the first stepper, since there was around the equivalent of 5nm of slop I thought I could achieve a 5nm step size by alternately pulling on one drive and pushing or pulling with the other but it just proved to complicated in the space available, I might return to this idea later if the 10nm steps prove too course.

The problem I have is with the self guiding module, originaly I had intended using a 30/70 semi-silvered optical mirror from an old medical camera I had kept, set at 45 degrees to the incoming beam and use that to send 30% of the incoming light to a loadstar but I found that the semi-silvered mirror had been multicoated to attenuate light near the IR band and this reached quite far into the red, it originally came from an image intensifier used on an X-ray machine so was designed to pass light in the green and blue, in practice it provided a non linear response so I have had to think again.

I looked at buying or fabricating a second optical flat, set at 45 degrees to the beam and drilling a hole in the centre to allow the wanted star light through to the grating and guiding off the reflected light from the rest of the mirror.

In my first build I had milled the casing from a single slab of Ali and changing the semi-silvered mirror to a drilled mirror has caused problems with the focal path lengths, the hole drilled in the mirror has to be large enough to let the full wanted starlight pass to the grating and at the dimensions used the hole that the loadstar sees is more that half the loadstar sensor size, tilting the mirror to put the loadstar view away from the hole gives me plenty of other stars to guide on but how on earth do you ensure the wanted star is fully through the hole and hitting the grating?

So at the moment I have gone back to my original plan and am saving for a new 60/40 semi-silvered mirror for the first element, I found one from a company that supplies semi-silvered mirrors for semiconductor manufacturing and optical research, it is costly, $270 plus import duty and VAT for a 30mm x 30mm x 0.8 mm size mounted in a titanium holder, but passes the full visible and ir bandwidth used at 45 degrees to the incoming beam and as it is the most costly item in the whole build I guess it is a small price to pay overall as long as I don't break it!

I have also scrapped the original single piece Ali' milled casing, it was much too heavy, nearly 3kg with all the parts installed, and will begin milling a carcass construction of solid milled frame with Ali' sheet covers once the warmer weather returns and I can get to work in the shed, should be able to get the weight down by a half.

When the project is complete I will post a step-by-step journal here but I'm thinking it probably won't be much before the end of this year as I haven't  even begun the software yet to read the output from the photodiode/Arduino and convert it to a spectrum to view on the Mac, plus I am in the middle of a new observatory build....and I thought retirement was going to be a nice long holiday...

William

 

 

[Oddsocks]

I forgot to say above, this book gave me all the information I needed for the physics of the spectroscope construction and different methods gratings/slits/prisms etc as well as actually understanding the waveforms captured.

http://www.amazon.co.uk/Astronomical-Spectroscopy-Amateurs-Practical-Astronomy/dp/1441972382/ref=sr_1_21?s=books&ie=UTF8&qid=1455564959&sr=1-21&keywords=spectroscope

Title: Astronomical Spectroscopy for Amateurs

Author: Ken M. Harrison

Publisher: Springer

Range: Patrick Moore's Practical Astronomy Series

ISBN: 978-1-4419-7238-5

[Merlin66]

William,

A guide "module" built from a Vixen flip body and a Surplus Shed beamsplitter 60/40 (or even just a microscope slide 90/4) and used with Al's Reticule can provide a good solution. See ASfA, p 220.

Neil,

Have a look at the fundamentals - Google "diffraction grating spectroscopes", or ASfA p 175>

http://h2physics.org/?cat=49

I think the grating you have may be a Holographic film (rather than photographic) I think you'll find the quality and performance will be poor. A better solution would be to purchase one of the quality, blazed transmission gratings from Paton Hawksley.

http://www.patonhawksley.co.uk/diffractiongratings.html

In the 1970's Watkis came up with a neat folded design which can be adapted to a webcam body etc.

https://books.google.com.au/books?id=ZKGAqlM5wEgC&pg=PA100&lpg=PA100&dq=watkis+BAA+spectroscope&source=bl&ots=qqnIYs-XkD&sig=wvbIhbiRWXDwXO9_VpIcXO9xypU&hl=en&sa=X&ved=0ahUKEwjwg_ux8PrKAhXIGpQKHbIrAH8Q6AEIIjAB#v=onepage&q=watkis%20BAA%20spectroscope&f=false

Do some reading and get back to us.

Ken

 

 

[Oddsocks]

2 hours ago, Merlin66 said:

William,

A guide "module" built from a Vixen flip body and a Surplus Shed beamsplitter 60/40 (or even just a microscope slide 90/4) and used with Al's Reticule can provide a good solution. See ASfA, p 220.

 

 

Hi Ken,

Thank you for taking the time to read through my long post and your reply, I hadn't thought about looking at Surplus Shed for the beam splitter, the referenced document looks helpful too. I have an unused Meade flip mirror sitting in a cupboard so I will look at adapting that for an external guide module, it would certainly simplify the design and give me more space inside the spectrometer housing.

William.

[Stub Mandrel]

Hmmm... thanks all for your detailed thoughts, however, I think you're expecting me to walk before I can run. Plus I would rather something very basic as proof of concept that will only cost me a few quid before I start thinking of investing real cash. My cheap grating is supposed to be better than DVD/CDs and there are plenty of people getting results with DVDs.

This is the sort of thing I had in mind, something very basic mused with 'spectral workbench':

https://astrophotovideo.wordpress.com/astrospectroscopy-with-a-webcam/

However I am a bit confused as there appear to be several different choices:

Collmating the incoming light: using a scope; a slit; a camera lens; a combination of two of these.

Creating a spectrum: reflecting the light off the grating or shining it through the grating.

Imaging the spectrum: using a lens focused at 8 or 9" but placed nearer the grating; projecting the spectrum directly onto the webcam.

 

Playing around using my fingers to make a slit, I seem to get a good spectrum very close to the grating. Best results appear to be using the grating transmissively  at ~45 degrees to the beam.

If I use a lens for collimation, where should the grating be relative to the focal length of the lens?

 

 

[Merlin66]

Hmmm..........

The light to the grating should be collimated - either by looking at a very distant object (street light etc.) or by using a lens focused on the light source (or a slit) This lens should sit close to the the front of the grating. If a slit is used, the lens should be at it's focal length from the slit - this will give a parallel collimated beam to the grating. Note: the length of the slit should be parallel to the direction of the grooves on the grating.

The spectrum as you've found will be produced at an angle around 40 degrees...to image the spectrum a webcam with no lens - i.e. the body only, can be used behind an "imaging lens" the distance from the lens to the grating should be minimal. The webcam is then placed close to the focus of this imaging lens and should allow you to obtain a spectral image.

You said you were about to use a transmission gratin, so the light goes through it like the "stripped" DC example....the more professional gratings are reflective and reflect the spectrum at an angle back towards the imaging lens.

 

[Stub Mandrel]

Thanks, it's beginning to make sense.

There seem to be two approaches:

1 collimate the light from the slit and focus the webcam at infinity.

2 don't collimate but focus the webcam on the slit

Presumably 1 is more efficient from a light gathering point of view. The use of a 1345mm telephoto lens seems a bit OTT, I suspect a cheap longish f/l eyepiece will do for starters.

I found an interesting idea for a slit - cut a slit in rubber and use two bits of paper to keep it open, seems a very easy way to create an accurately spaced slit.

[robin_astro]

I assume your grating is a reflection grating?   If you are planning to use it on stars, you could initially run without a slit which gets round the problem of locating the star on the slit and keeping it there. Christian Buil's website is a good source of design ideas.

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

 Here for example is simple classical design you could kick off with using  surplus camera lenses

 http://www.astrosurf.com/buil/us/spe2/hresol.htm

 

Robin

[robin_astro]

If the diffraction angle is small (ie the grating is coarse, say <200 l/mm) you can get away without  collimating the light at the grating and still get a reasonable result. eg as in the designs here on my website using a slit and a transmission gratings

http://www.threehillsobservatory.co.uk/astro/spectroscopy_4.htm

http://www.threehillsobservatory.co.uk/astro/spectroscopy_18.htm

This arrangement will not work well with a 1000 l/mm grating though as the aberrations caused by the non collimated beam will be severe so I would definitely recommend a design where the light is collimated at the grating.  Here is a collimated version using a transmission grating

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

and another one using a transmission grating from Christian Buil 

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

 

Robin

 

 

[Stub Mandrel]

Lots of useful stuff there.

My grating is 1000 l/mm and seems to work best as a transmission grating (a crude test with sunlight gives a feeble reflected spectrum and a bright transmitted one).

I think for a simple start I will try and collimate using a ~40mm f/l lens group and then use a webcam at infinity and see if I can get anywhere using spectral workbench.

If it works, I will see if I can fit my modded DSLR instead to make it sensitive enough to try on stars.