Spectra of the sun

[dark star]

HI, I would like to get some spectra of the sun using a Rainbow Optics grating and a 70mm Lidl refractor.

I can probably construct a slit to go over the refractor's objective, but would this be enough to protect the objective, the grating, my Canon 20D DSLR camera and my eyes from the power of the sun?

I can't just use my camera as I can't figure out a way to connect the grating to the lens that came with the camera (EFS 18-55mm lens).

I can connect the objective lens from a 50mm finder scope to the camera and construct a slit, but again I am worried about the power of the sun. Thanks in advance for any advice.

Most spectra from the sun seems to be obtained using a reflection rather than transmission grating.

[Merlin66]

You CANNOT use the grating with your scopes for solar work.

You need to provide a suitable slit; then focus the camera on the slit.

There are a couple of option I use:

A slit tube...basically a length of PVC pipe about 1.5m long with a coverplate and a slit at one end (you can use pencil sharpener blades and a piece of fridge magnet material to hold them) and fit the grating infront of your camera lens ( see later) - point the camera into the tube, focus on the slit and align it with the grating.

This set up can be used not only for the sun ( a small mirror can be used to reflect the sunlight into the slit) but also to get the spectra of various lamps (neon/ fuoro/ incandecent) as well as testing astronomical filters etc etc.

Then there's the reflective sewing machine needle - this forms a reflective slit and works very well on the Sun. Again, mount the grating on a camera lens (a longer focal length gives best results, I use an Ol' Zuiko 135mm lens) and focus on the reflection from the needle.

To fit the grating to the front of the camera, the easiest and cheapest is to buy a spare lens cover and drill/ file a 28.5mm diameter hole in it. The grating can then be "screwed" into the cover and the cover clipped on the lens - the cover can then be rotated to align the grating. I have a smilar set up where I use filter stepping rings from the 58mm down to 52m and then glued a T thread insert into the filter ring - a 1.25" to T thread adaptor then screws in.....

You can also use this "objective grating" set up to record wide field spectra - better resolution than when mounted on the scope!

HTH

[dark star]

Thanks a lot, I will buy a lens cap, I bought a lens hood before which is not what I need at all!

I will probably buy a second hand 135mm lens at some point, I just need to figure out which adaptor I need to fit an old lens to the Canon EOS 20D.

I will experiment with my 18-55mm lens and a PVC or cardboard tube, I will post the results when/ if I succeed!

[Merlin66]

You'll find almost any lens to EOS adaptor on ebay - I think I paid $2 for a Zuiko to Canon....

Note: On the Canon zoom lenses the front part rotates during focusing, so you need to get focused first before aligning the grating.

[dark star]

This is some spectra of the sun I took as it was setting today.

Using a Rainbow Optics grating, Canon 20D and a 50mm objective from a finder scope (focal length around 80mm I think), plus long cardboard tube with a slit in the end made from two pencil sharpener blades. The grating was in a lens cap with a hole made in it on the finder scope objective.

I could focus on the slit OK, but focusing on the spectra I had to point the camera a bit to the side and couldn't get all of the spectra in the picture. I think I need a longer focus camera lens.

Just something to do while waiting for the clouds to clear at night so I can take some stellar spectra!

[dark star]

Not too bad I think for a first attempt, but could be a lot better! Next time I will try focusing on a needle to get the sun's spectra.

[Merlin66]

Very promising!

A couple of things:

The focal length of your finder lens is probably more like 160mm

The RO200 grating has a deviation angle of about 7 degrees

So a shorter focal length would help keep the spectrum on the CCD, also tilting the camera by 7 degrees would reduce field caurvature and help with the focus.

[Merlin66]

Have a look at Samir's shoe box set up. Something like that may work for you?

Spectrograph

[sharkmelley]

The SA100 can work very well.  Here is my set up:

The entrance to the long cardboard tube is a slit created by the sharp edges 2 Stanley knife blades.  The lens is a 200mm lens with the diffraction grating attached to the front.  It is essential that the cardboard tube is long enough for the lens to bring the slit to focus.

Here is a spectrogram produced using the above setup showing the response of the RGB pixels on my full spectrum Sony A7S using a "white cloud" sky as the baseline (of course I substituted the Sony for the Canon for this test!):

To be honest, I wish I had bought the SA200, then I could have used a shorter focal length lens.

Regards,

Mark