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Prism spectroscope: A slight problem.


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Hello everybody,

a few weeks ago, a friend of mine, who used to be an engineer at Zeiss/Jena, sent me a nice present: an eypiece spectroscope made by Zeiss/Jena.

Long story short (a phrase the origins of which, according to Abe Simpsons, are complicated ramblings) I set to work adapting it to a cooled CCD camera and then I went about taking some spectra. I started off with Albireo. The workflow is as follows (I implemented it in a 20-line python script):

  1. Plot the raw image with a logarithmic scale to be able to see faint features more easily.
  2. Rotate the raw image to have the spectra horizontal.
  3. Take two non-overlapping, rectangular regions from the spectra of Albireo A and Albireo B (hopefully).
  4. Then take the sum over each column of these regions and finally plot these sums.
  5. Do a median filter (median over four neighbouring pixels) to get rid of hot pixels and low-pass filter the results. Divide the smoothed spectrum with one that has been smoothed to a very extreme degree (median over 200 pixels), to identify and match features in both spectra. Then shift the spectra with respect to each other.
  6. Plot the spectra over each other and scale the fainter up to better compare them. Mind you, the lines for "Halpha" and "Hbeta" are just semi-educated guesses. I am very probably wrong, here  ;-)

Now, the only thing missing (and the most important thing, too, I admit) is the spectral calibration which I'll set about doing shortly.

But there is one problem I have: Where the Dickens do these oscillations in the spectra come from?
They are in none of the reference spectra I found online. So is this real physics, or is there a conceptual error on my part?

Please, could someone who has more experience than I do comment on what I did so far?

Thanks a lot & all the best,

Kai.

 

PS I did use an ATIK 426 and an f/6 apochromatic astrograph of 70 mm aperture, if anyone wants to know ;-)

albireo_log.jpeg

rotated.jpeg

Regions.jpg

sums.jpeg

Linienfinder.jpeg

Spectra_shifted_to_match.jpg

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Hello Kai,

Yes the waves are artifacts. (The processing is exagerating them perhaps. The H alpha beta gamma lines are very obvious in the spectrum image but less so in your processed spectrum)  They are quite common in spectra and can be from a number of sources.  

They may be from the processing.  For example  rotation can produce artifacts, so I suggest trying with the spectrum horizontal to start with.

They may also be generated in the  spectrograph.  Reflections between closely spaced surfaces for example can produce interference fringes. Is your camera colour? If so the IR filter can cause this effect

The camera sensor is another possible source. Some sensors have small scale variations in the sensitivity with wavelength that produce these sorts of effects.

If you cannot get rid of them then you will need to make sure you include them in your instrument response so they divide out when you make the instrument response correction.

Cheers

Robin

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Hello Robin,

thanks very much for the quick answer!

The camera is b/w. And, if you look into the raw image, you can already see these oscillations. Might actually be the chip window: there is nothing else in the way. I will try measuring the spectral response of the instrument, next.
I plan to take a very bright spectrum with as few lines as possible and calculate both the spectral calibration and the instrument function from this.

Vega may be a good candidate: very bright, very high up in the sky, and with a spectrum almost void of any lines that I could resolve with this tiny spectroscope, except the Balmer series of hydrogen. Then, I should almost get a black body spectrum, with only the hydrogen lines (something like the image attached). I'll keep this up to date as I go along.

Thanks and have a good weekend,

Kai.

blackbody_vega_balmer_marked.png

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Hello Kai,

The CCD windows are known to give wave problems but the spacing of the waves is much narrower than you are seeing here (Only a few Angstroms apart in the spectrum so are only seen in high resolution spectra). Interference in the sensor structure itself  can cause these waves at the sort of spacing you are seeing and can be quite severe with some sensors.  The Sony CCD are usually not too bad, perhaps 5% peak to peak, so I was a bit surprised to see them that intense in your spectrum. (An interference pattern between the faces of the (Amici?) prism perhaps?) A flat correction would remove them with a slit spectrograph but that does not work with slitless setups unfortunately. 

Cheers

Robin

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Hmm..  All nice stuff.  I was thinking of making a spectroscope from a standard plastic project box plus diffraction grating (500/mm) plus webcam.  What do you think?  Would this work or does it need to be a prismatic spectroscope.

Regards

Steve

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Hi Steve,

That is effectively how the Star Analyser works.  You dont even need the project box :-)    At its simplest, you either mount the grating in front of a camera with lens or for fainter objects,  between a telescope and camera without a lens. provided the target appears reasonably point like in the image, you will get a spectrum

500 l/mm is ok for mounting in front of a camera lens but you are better off with a coarser grating between telescope and camera as you get aberrations which blur the spectrum which increase with finer gratings in this configuration.  (The Star Analysers are 100 or 200 l/mm.)   Any transmission grating can be used but many cheap ones are very inefficient so limited to the brightest objects (prism spectrographs are very efficient but have various drawbacks which means gratings are more often used these days) 

Here are some  of the examples of gratings being used from my website to give you a flavour of the sort of things that  are possible.

100 l/mm Star Analyser in front of DSLR   http://www.threehillsobservatory.co.uk/astro/spectroscopy_11.htm

200l/mm Star Analyser in front of astro camera and 50mm lens  http://www.threehillsobservatory.co.uk/astro/spectroscopy_17.htm

100l/mm Star Analyser in front of webcam with 30mm lens (solar eclipse)  http://www.threehillsobservatory.co.uk/astro/spectra_27.htm

100l/mm gratings in front of web/video cameras (this gives a very short spectrum with a wide field lens. a 500l/mm grating would be better here)  http://www.threehillsobservatory.co.uk/astro/spectra_20.htm

100l/mm grating between telescope and webcam   http://www.threehillsobservatory.co.uk/astro/spectra_12.htm

100l/mm grating between astro camera and telescope http://www.threehillsobservatory.co.uk/astro/spectra_42a.htm

Cheers

Robin

 

 

 

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1 hour ago, robin_astro said:

Hi Steve,

That is effectively how the Star Analyser works.  You dont even need the project box :-)    At its simplest, you either mount the grating in front of a camera with lens or for fainter objects,  between a telescope and camera without a lens. provided the target appears reasonably point like in the image, you will get a spectrum

500 l/mm is ok for mounting in front of a camera lens but you are better off with a coarser grating between telescope and camera as you get aberrations which blur the spectrum which increase with finer gratings in this configuration.  (The Star Analysers are 100 or 200 l/mm.)   Any transmission grating can be used but many cheap ones are very inefficient so limited to the brightest objects (prism spectrographs are very efficient but have various drawbacks which means gratings are more often used these days) 

Here are some  of the examples of gratings being used from my website to give you a flavour of the sort of things that  are possible.

100 l/mm Star Analyser in front of DSLR   http://www.threehillsobservatory.co.uk/astro/spectroscopy_11.htm

200l/mm Star Analyser in front of astro camera and 50mm lens  http://www.threehillsobservatory.co.uk/astro/spectroscopy_17.htm

100l/mm Star Analyser in front of webcam with 30mm lens (solar eclipse)  http://www.threehillsobservatory.co.uk/astro/spectra_27.htm

100l/mm gratings in front of web/video cameras (this gives a very short spectrum with a wide field lens. a 500l/mm grating would be better here)  http://www.threehillsobservatory.co.uk/astro/spectra_20.htm

100l/mm grating between telescope and webcam   http://www.threehillsobservatory.co.uk/astro/spectra_12.htm

100l/mm grating between astro camera and telescope http://www.threehillsobservatory.co.uk/astro/spectra_42a.htm

Cheers

Robin

 

 

 

Hi Robin,

I love your website and the stuff you do.  I read the article about the exo-planet you confirmed.

The issue I have with the analyser is just the price.  The grating I have cost about £5, but the analyser is $200.  The other problem is that many of the items we look at are large and round, a slit would be nice to avoid the muddying of spectral lines.

So I thought project box, slit, grating and camera (offset) to get a clean set of lines.

How do you do the intensity graphs? I guess open the image and read the pixels.

Regards

Steve.

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Hi Steve,

The SA100 is currently £97 direct from the manufacturers.   http://www.patonhawksley.co.uk/staranalyser100.html  

You can buy the "educational quality" equivalent, TE 218 100l/mm grating 

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

though the quality is a rather pot luck as for schools use the efficiency and quality requirement is not as high. (I started out with this and struck lucky) 

Adding a slit adds quite a bit of complexity as this needs some extra optics to focus the image on the slit, ideally collimate  the beam before it passes through the grating and then refocus the dispersed spectrum on the camera sensor.  You also have the difficulty, particularly for fainter objects, of acquiring and keeping the target on the narrow (typically ~25um) slit so some means of viewing the field with the slit superimposed makes things much easier. Home build is possible, see Christian Buil's website for ideas for example

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

and the minimum system is perhaps something like this

http://www.threehillsobservatory.co.uk/astro/spectroscopy_18.htm (Star Analyser design incorporating a slit)

http://www.threehillsobservatory.co.uk/astro/spectroscopy_19.htm (colimated design using a Star Analyser)

but for commercial spectrographs this pushes up the cost to £1000 or more.

There are various programs available for the amateur to turn images into digitised spectra and calibrate them like

Visual Spec (freeware which I used when I started out in spectroscopy and still use for a quick look)  http://www.astrosurf.com/vdesnoux/

ISIS (freeware which I use for my Pro-Am Work)   http://www.astrosurf.com/buil/isis/isis_en.htm

BASS (freeware) https://uk.groups.yahoo.com/neo/groups/astrobodger/info

RSpec http://www.rspec-astro.com/

Cheers

Robin

 

Cheers

Robin

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Hello Kai,

They were generated from spectroscopic flats taken with different low resolution  slit spectrographs (either a Shelyak ALPY or LISA)  They are effectively  measurements of  the spectrum of  a Halogen lamp. The  broad continuum shape has been removed to just show the wave features.

Note that these ripples in low resolution spectra are not caused by the camera or sensor window. They are produced in the much thinner layers of the sensor itself. See this paper for a description of this phenomenon

http://www.aphesa.com/downloads/download2.php?id=1

(section 3, starting on page 7)

You can get ripples from the sensor window but these are much finer spacing and are only seen in higher resolution spectra. See here for an analysis of these type of sensor window ripples

http://www.astrosurf.com/aras/fringing/schlatter/ripple.htm

Robin

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Hello,
just to keep you posted, this is what a rwa spectrum of Vega looks like (I still have to do the wavelength calibration - I am desperately sorry, but I have not gotten 'round to doing so).

Anyway: What we see is -most likely- the convolution of the transfer function of the chip window and the responsivity of the ccd sensor.

Vega.png

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8 minutes ago, eikonal said:

Hello,
just to keep you posted, this is what a rwa spectrum of Vega looks like (I still have to do the wavelength calibration - I am desperately sorry, but I have not gotten 'round to doing so).

Anyway: What we see is -most likely- the convolution of the transfer function of the chip window and the responsivity of the ccd sensor.

Vega.png

Hi Eikonal,

It would be great to see some pictures of your kit. 

Regards

Steve.

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  • 2 weeks later...

Hello Steve,

 

here goes... ;-)

Spectroscope with camera (ATIK 416 Monochromatic)

Spectroscope_closed.jpg

 

Exploded view of spectroscope:

Spectroscope_exploded_view.jpg

Schematics of the setup: PF - primary focus of telescope. C - camera, W - camera window

Spectroscope_schematically.jpg

The camera can be rotated by means of a ring dovetail. 
Collimation is achieved by focusing the lens in front of the camera onto a distant object (say, a tree some 100 m away). The eyepiece is then focused, so that stars can be seen through the telescope. This means that the light coming from the eyepiece is, essentially, collimated, in other words: the rays are parallel. For a check, the camera can be attached. It should give sharp images, now (the assembly of eyepiece and camera objective is a microscope of 2.5x magnification). Finally, the spectroscope drum can be inserted by screwing it onto the monocentric eyepiece. 

 

 

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