View Full Version here: : CD solar spectroscope?
08-05-2011, 03:37 PM
I'm in the process of writing a high school (Year 11/12) teaching unit on the Sun, with the emphasis on observing and experimenting rather than looking up images. Is it possible for school students to see absorption lines reasonably well using a CD spectroscope? I've tried making one with no success (at seeing the lines): just a bright, glaring image surrounded by continuous spectra. Nothing like the images on the net. Any ideas would be helpful.
08-05-2011, 05:19 PM
There's a chapter on CD/DVD spectroscopes in Tonkin's "Practical Amateur Spectroscopy" - I'll have a look later...
I think the resolution is pretty low and the absorption lines may be difficult to record.
A reflective needle and a cheap "students" grating may be an easier way to go...
08-05-2011, 09:38 PM
I am also a high school teacher and have students doing investigations into spectroscopy. Is there any way to produce a spectrum of a star that may show spectral lines for a relatively cheap outlay. I have seen where you can print off a diffraction grating and put it onto an Overhead Transperancy that can be put over the front of a telescope. of course I can not find where my mate told me to look for this. Any suggestions.
09-05-2011, 12:54 AM
Unless the design (and your printer) can give at least 100 lines/ mm - then all you have is a glorified Ronchi screen. The resolution will be very poor.
IMHO the easiest and fastest way is either a good prism infront of a telelens (say 135mm etc) or one of the P-H student gratings and use either the needle trick or mount a pencil sharpener slit in the end of a tube/ box about 1400mm long - use a mirror if necessary to shine the sunlight onto the slit and image it with a grating on the front of the lens.
This will definately show the Fraunhofer and Telluric (Atmosphere) lines etc etc. The set up can also be used to show the spectra from various lamps (Flouro/ Neon/ Incandecent etc) as well as the effects of various filters....Some neat projects there....
Geoff, see my post on Merlin66''s thread : ''SOLAR SPECTRUM WITH BAADER GRATING''
09-05-2011, 03:03 PM
Thanks Rob and others. I made a spectroscope using a DVD with similar dimensions to the one Rob described on another thread, using a cereal box and closely spaced masking tape (~0.2mm slit). I cannot seem to get the hang of seeing anything other than a continuous spectrum. No lines are visible. Any ideas?
Hi Geoff, did you split the DVD in half?
I usually cut it down the middle, twist it a bit and begin to carefully pull apart the two halves(remember DVD's are dual layer, even the blank ones).
If you do this carefully you should be able to cut a decent pie wedge from a part that isn't damaged(which happens)
Now, your continuous spectrum : happened to me also.
I realised that for some reason the ''higher order'' spectrum(to the right of the main one was where all the detail was???......that was with a CD though.
When I replaced the CD wedge with a DVD wedge, I mucked around with the angle of the wedge while looking through the view hole and suddenly, there it was............bright and detailed. The image was more to the wide, top end(closer to the view hole) of the wedge.
A couple of things to check: the wedge has to be cut from the CD as you would a ''pie''.
That wedge is mounted directly under the view hole with the wide end at the top and the small end stuck down in to the tube.
The position and angle of that wedge of DVD is important and you may need to fiddle with it a few times.
Also, the slit must be horizontal, NOT vertical in relation to the wedge and view hole.
Keep trying mate............it's a bit fiddly, and a P.I.T.A., but you should see the detail sooner or later. Keep moving your wedge and your eye around and it will pop in to view.
10-05-2011, 05:18 AM
Thanks Rob. I'll give it another try.
I spoke to a uni technical officer yesterday, a particularly smart and very experienced physicist with a very practical approach, and he was doubtful that absorption lines could be seen in solar spectra. If I'm successful, he will be particularly pleased.
I'll let you know how it goes.
Can't argue with an educated and fully qualified physicist can we? :
Also, see my new photos on the other thread.
12-05-2011, 06:07 AM
don't misunderstand my comment about the physicist: like most I know, he will definitely and genuinely be pleased to learn something new. He is a very fine teacher and is always open to new ideas and willing to learn. That's why he's so good at what he does.
I have seen the website you suggested and this is the design I made up the other day. I could not see the absorption lines, however. I'll try again. I assume the secondary spectrum is the one to look at? The primary is too glaring to see anything. Should I use reflected light? Do I focus on infinity or on the CD?
Geoff, I replaced the DVD wedge with a CD wedge instead.
The design shown has an angle of 60 degrees which is way out compared to my version.:shrug:
I have an approximate angle which looks more like 30 degrees.:question:
While experimenting with the angle of the CD/DVD, I noticed that the spectra(and the dark lines!) can be blurred and distorted if the angle is not right on!
Also, the viewing hole is about 12mm diameter, not a huge chunky opening like the design indicates.:rolleyes:.......the interior image needs darkness!
Yes, you need to use reflected light : off a white painted building/sheet/paper or similar.
You should be able to see in the reflection on the CD, with your eye right up against the viewing hole(like an eyepeice), the primary spectra to the left, and a fainter secondary spectra to the right.
You might need to move your eye slightly to the left and right to see these.
It's hard to tell whether the focus is at infinity or not. I had my camera set to normal and macro........both worked.......so it seems?
I would imagine that the focus point would be the slit itself.................someone correct me if I'm wrong.
Please show your physicist freind the photos below:
All taken with a DVD as a grating!
Merlin66.....................any comments or input on this?
12-05-2011, 05:39 PM
I've just finished making a spectroscope following your advice and the result, at least for artificial light sources, is far superior to the version using a box I've tried before. On incandescent bulbs there is a continuous spectrum, but on fluoro tubes and compact fluorescent bulbs, there are several bright lines. I assume these are the emission lines specific to this design of light? Unfortunately, I will have to wait until tomorrow to check out the Sun. Can't wait.
Ironically, this is the first time I can remember wanting the night to end!
p.s. Here is another site that shows the construction quite well.
12-05-2011, 05:54 PM
Sorry guys...caught up rebuilding an observatory!
The CD has about 600 l/mm (equivalent) (PAS, Chapter 5)
They recommend using it at a grazing angle of around 3 degrees, with the camera lens (50mm fl) almost touching the surface of the disk. Focused on infinity.
When the CD is used with a slit - the camera should be focused on the slit.
EDIT: Should add that the 2nd order spectrum sits about 20 degrees from the surface (based on 3 degree incoming); 1st order about 40 degrees.
12-05-2011, 10:28 PM
If you're using a Neon or a Fluoro lamp for testing, Christian Buil has a fully annotated spectrum of both on his website:
It's all a bit technical still for me, but I'm beginning to understand....slowly.
What is ''PAS chapter5''?
Also, what do you think about the photos Merlin?
Geoff, .......congrats mate!........you're nearly there by the sounds of it!
13-05-2011, 02:24 PM
PAS - Practical Amateur Spectroscopy - Ed. Steve Tonkins
I think the images may be slightly out of focus, but I'll try to load them into Vspec - see if we can pull some data....
I noticed that the images on this thread can not be enlarged:confused2:
On your solar spectrum thread, they can.
If you are going to closely examine the photos, it would be better to use those and not the ones here.
Quite possible that a couple of the shots have poor focus due to the ''autofocus'' function that can not be overidden on a standard digital camera.
From as far as I could tell, the camera was trying to focus on the slit:question:
Anyhow, please let us know what eventuates.
Hi Merlin. One of the two Amazon reviews of Tonkin's Practical Amateur Spectroscopy claims the book is riddled with errors and misprints, etc, and recommends that propective purchasers wait for a corrected edition. Care to comment?
13-05-2011, 09:59 PM
Don't know if this might be of interest:
14-05-2011, 12:45 AM
Practical Amateur Spectroscopy like many books is not perfect - sure there are some typos and errors but I have a errata available. Notwithstanding the poor press it is a very good introduction to spectroscopy and compliments well my new "Astronomical Spectroscopy for Amateurs" - recommended.
14-05-2011, 01:13 AM
OK I ran one of the images (052) through IRIS cropped and saved as a .fit file - imported into Vspec and selected what I thought were "real" features; the G band at 4308A and the Na doublet (not resolved) at 5895A to give a first calibration - this was then compared with a G2V spectrum....
The attached result shows the change in intensity across the spectrum due to the Bayer Matrix (Blue curve) and some absorption lines - the purple curve is a G2V comparison.
Yes, there is some detail there, but go back and compare this with the Baader grating/ needle results.....
Certainly an interesting start! (I'm glad I marinated those daks!)
14-05-2011, 05:14 AM
Thanks so much for your advice, Rob. I tried the spectroscope out on the Sun Friday morning by using it to look firstly at a piece of white paper taped to a wall facing the Sun: no luck, just a continuous spectrum. The I turned the spectroscope to the reflection of the Sun in a glass door and the bright (almost too bright, but tolerable) reflection was replete with dark lines! I compared them as best as I could with the Wikipedia "map" of lines and thought I could identify the major ones. I was stunned at how prominent the lines were, like the gaps between tracks on an old LP (now you know how old I am), only more frequent and more irregularly spaced. I cannot think that these dark lines, running parallel to the arcs of the background spectrum, can be anything but dark absorption bands. Is that right?
The needle spectroscope will be much, much easier for the students to make and use, so I'll get to that this weekend. The image looks like it might be of poorer quality to the DVD spectroscope, but I need a method that is simple and easy for teenage students. Thanks so much for the tip.
p.s. Also tried the spectroscope on sodium lamps on the way home last night: brilliant! Interesting, amid all the bright transmission lines was one, lone absorption line. Any ideas?
14-05-2011, 03:19 PM
Nyeh, bodgeyed up a 'sewing-needle spectrometer' this afternoon and gave it a go, using a Star Analyser grating & Canon 400D. Not really successful - a few faint absorption lines showing though. :shrug:
Too much cloud & the Sun's low in the trees now. Might have another go with it tomorrow, with a CD.
14-05-2011, 03:33 PM
Actually that's not too bad a result!
What lens were you using?
You'll get a bit more dispersion with a longer focal length telelens - 135mm or even a 200mm.
Have you tried to obtain a profile in Vspec yet?
14-05-2011, 09:18 PM
Thanks Ken. I was using an 18-55mm lens at 55mm. This was a rough job, LOL - I was holding the cardboard sheet at arm's length while looking through the viewfinder of the camera. In my other lens (55-200) I couldn't get focus at 55mm, let alone 200mm!
Might have to actually set this up, rather than have things waving in the wind. :lol:
Anyway, I did graph it and it doesn't come up all that well. Some of the main Fraunhofer lines are there, but not others. Montage attached.
15-05-2011, 12:49 AM
Not sure about that calibration...
BTW why the pink region in the blue?
The standard Canon UV-IR filter "dies" around 380nm and 720nm
15-05-2011, 12:14 PM
Good question Ken, no idea why the pink - and agree about the calibration. :thumbsup:
Anyway, for the purposes of the exercise I had a shot with a CD today. It did throw a spectrum of sorts - to photograph it I had to use a little auto point-&-shoot camera to get close enough. Autofocus snapped on the CD so I had to get focus on the needle then pan the camera across & shoot. Not ideal & focus could have been better but nonetheless did get something - see attached.
So feasible to get Fraunhofer lines in a solar spectrum with just a needle, a bit of cardboard, a CD and an ordinary p&s camera.
Also had another go with Star Analyser & 400D - more lines showing. Each spectrum has the full frame above, to give an idea of what the set-up was (rough!!!). :lol:
15-05-2011, 04:53 PM
Still poor resolution with the CD - would be interesting to see your SA100 results with a 100mm+ lens...
I think the line in the green is the G band and the line in the red the atmospheric Telluric...this would then give you (?) the Na lines in the orange.....
I have to stress that what is seen with the naked eye on a CD(the DVD was even better) spectroscope,is extremely clear, sharply defined dark lines :
1 in violet
2 in blue
6 in green
3 in yellow
2 in red
These are the obvious..............but there a numerous fainter ones.
As per Rob_K's post(link on post number 20), the image of the last solar spectrum of three with the lines labelled, is EXACTLY what I see visually with a CD spectrum;).............so what gives here guys.......there is no ''hashy'', rough as guts ''dirty'' looking spectra........just a smooth one with many, many lines????:rolleyes:
This is all with a reflective(not transmissive) grating at an an odd angle with a very dodgy slit, and viewing hole.......no lenses or cameras, needles:rolleyes:
The camera attempt to take an image was a complete joke as I didn't have access to a top of the line Canon DSLR, and it's a surprise that I captured anything at all.The images I posted were not only out of focus, but also distorted due to the angle required to see the spectra within the camera(thus the ''uneven'' focus also)
16-05-2011, 02:53 PM
Sounds like an exciting option for showing the visual spectrum. Could be that to obtain usable images you'll need a more robust set-up.
Yes, definately Merlin.
It's all fun to see how much can be obtained from a crappy CD, but it's not a serious grating if I decide to point it at the stars.
Oh well, maybe in the near future?:thumbsup:
17-05-2011, 12:09 AM
Yep Rob, sorry, I wasn't casting nasturtiums on or ignoring your method - I intend to go there when the Sun comes back! :thumbsup:
The reason for going the needle first was that it acts like a slit and is highly reflective, like a mirror. Mind you, the needle I chose was ROUGH!!! It would work a lot better with a smoother needle. I'd still be a little worried that a surface like the side of a white shed might or could introduce absorption lines of its own, depending on the surface composition. And that might vary from white-shed to white-shed. But those could easily be weeded out. I was super-impressed with what you put up!
Hey no problem or offense taken Rob_K..........none at all.:)
It's interesting when you mention different types of white sheds/walls and their possible influence on the spectral lines........never thought of that:doh:.
White paint has : Titanium dioxide, calcium, silica, zinc oxides, talc,binders etc.(all depending on what type of paint is used and the condition or oxidisation level).
So, spectroscopes are designed to reveal these elements and any others that make the so called ''white light''.
A reflective needle will be far truer towards ''solar light'' than a white painted wall no doubt:question:
I will repeat this little exercise with a shiny needle and try to photograph the results using a CD based spectroscope.
17-05-2011, 05:27 PM
The amateur spectroscopes we're talking about are not sensitive enough to do the sort of analytical stuff being discussed.
Most surfaces will re-radiate the solar energy into the IR; you could take a spectrum and then compare it with a standard G2V to see any differences...this method is successfully used to show the methane absorption bands in the gas giant planets.
18-05-2011, 06:20 AM
I am a complete new-comer to this business, so appreciate everyone's expertise. Thanks.
Regarding the problem of reflections off walls, initially I used a reflection off a window to reduce the intensity. Yesterday I used a (new, glass) petri dish as a small reflector held in front of the slit of a DVD spectroscope (the design mounted inside a cardboard tube). The response was promising: easier than standing in the glare of a reflected solar image in a window. My goal is to mount the petri dish at 45 degrees to the slit, so that whatever the orientation of the Sun in the sky I am looking perpendicular to the line of sight. I have also started mounting the device on a tripod with slow motion controls for easier use.
It has also been suggested to me that many of the absorption lines are due to substances in Earth's atmosphere, and that these would need to be identified to distinguish them from those in the solar atmosphere.
18-05-2011, 09:04 AM
The atmospheric or Telluric absorption lines caused by H20 and O2 are well mapped. Yes, these do have to be corrected for in the final analysis.
Spectral processing software like Vspec or RSpec have a very comprehensive library of these Telluric lines. (BTW they come in handy at times for calibration purposes)
18-05-2011, 09:54 AM
OK, had a go with Rob's design this morning, modified to use a needle rather than aiming at a wall or other white surface. Didn't have a ready-made tube, and not willing to wait for the Glad Wrap to be used up I rolled up thin card with a sheet of black paper. Added a black paper 'shield' to cut down extraneous light entering the tube.
With the shield on, you simply tilt the "spectroscope" (ha!!) down at about 45-deg to the incident sun rays, look through the peephole and rotate the tube slightly until the Sun reflects 'squarely' off the needle. Awesome, very clear spectrum with so many fine strong absorption lines showing!! The needle used was finer & smoother than the one I used previously.
Photography is problematic & I won't even bother trying - this is just a simple, cheap way of introducing yourself (& students?) to the solar spectrum (and other bright light sources - good job on various lights around the house last night).
Great job, thanks Robz! :thumbsup:
18-05-2011, 10:57 AM
Excellent idea, Rob_K: I copied your replacing the slit with a needle and the spectroscope is far easier to use on the Sun. Very fine absorption lines. It is less effective than the slit design on lights, however (at least mine is). This means I now have a design I can get students to make and use as an introduction to solar spectroscopy.
Credit to you guys, of course. May I use your names? It is for a teaching unit I am developing for uni, all not for profit; just sharing ideas on how to teach Year 11-12 students about the Sun from an observational approach. I like to give credit where credit's due.
18-05-2011, 11:25 AM
Same here Geoff, because the lights are so much less intense than the Sun. But still shows the emissions OK.
No credit due here Geoff, all down to Robz I reckon. And when it all boils down, all these 'designs' (and probably anything else you could possibly think of!) have been played with for aeons!! :lol:
Weakened and tried to get a photograph of the spectrum but it nearly drove me mad! :rolleyes: Did get the attached but it just doesn't do the visual view justice, there are so many more fine lines that didn't come out.
Geoff and Rob_K..............it gives me immense satisfaction that you have both achieved to get such great results:thumbsup:
CD's CAN show some detail in solar spectra eh?;)
I hope that students and others following this thread do indeed find some fascination in what they see.
Light is a fantastic gift to the universe..............full of mystery and surprises.
No credit needed Geoff............just glad you got to see the elusive lines in the end:thumbsup:
18-05-2011, 03:12 PM
Cheers Rob, and thanks again!
Geoff, something you should take into consideration - any observing of the Sun comes with risks. You should prep your students in the dangers of observing the Sun directly, which can lead to severe eye damage or blindness without suitable solar filters. There'll always be some clown who wants to use the "spectroscope" as a telescope! It's all about safe observing by indirect means (reflection of a tiny portion of the Sun's light and splitting that into an observable spectrum). No doubt you already have that in hand though.
Good luck with it, and I'd love to hear how it all goes! :thumbsup:
Great reminder Rob_K :thumbsup:
And to think that Galileo waited for a very low, reddish sun, through haze/cloud before he could view it directly through his telescope:eyepop:.................. ...........what a LEGEND!!!!!!:rofl:
18-05-2011, 05:22 PM
I tried the needle-DVD spectrometer with one of my students this afternoon: it took her about 8 seconds to see the absorption lines for the first time.
I made one more modification to the design. Frustrated by not being able to see lights clearly, I replaced the needle with the tube from an old eyepiece. This gives just as sharp solar absorption features, is significantly easier to use on the Sun since the image is so much brighter and clearer, and it works on lights. It should also work on flame tests.
So, looks like the first leg of my journey into spectroscopy is almost at an end. Thanks guys, you're outstanding teachers and mentors. I've learned so much in the last week or so and will now pass on this design to students who can see Fraunhofer lines for themselves. And you're right, of course, Rob_K: students do do silly things, so it's a matter of weighing the risks and benefits of anything to do with science teaching, particularly in such a litigious society. Still, that's teaching for you. But what if it inspired a good kid to take up science professionally...? Yep, worth the risk.
All the best,
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