Objective Prism Spectroscopy

chiltonstar · (#1)

Thought you might like this (seasonal) image - Xmas tree on local church tower, using an objective prism.

Chris

Merlin66 · (#2)

Interesting!
What set-up did you use?

jsandse · (#3)

Nice one Chris.

After tuning my spectroscope to the red and infra-red I tried getting a spectra of Rudolphs red nose on Christmas Eve but he was just too fast as he flew across the sky

Have a great New Year - lets hope we get plenty of interesting spectra.

cheers

John

Jack Martin · (#4)

Chris,

Nice image how did you obtain it?

Regards,

Jack

chiltonstar · (#5)

Quote:

Originally Posted by Merlin66

Interesting!
What set-up did you use?

Nikon D300 + 90 mm telephoto, with a front lens flange to which a spectrograph prism (Hilger 0.5 M, skipped by my former boss - optics rescued by me) had been bluetacked.

I'm going to try it on the front of my ED80, if I can work out how to point the scope at specific targets! The prism is about 5cm x 3cm, so it will cut down the aperture quite a bit.

Chris

Merlin66 · (#6)

Chris,
You can mount the prism at an angle - about 1/2 the apex angle to give "best" performance....
If the prism is orientated say in Dec then the dispersion angle will be in RA you can do a couple of trial shots to confirm the angular off-set then re-set your finder ( or re-syn your GOTO) to frame the shot.

Jack Martin · (#7)

Chris,

Follow Merlin66 advice and you should get a good result.

Keep us informed.

Regards,

Jack

chiltonstar · (#8)

First attempt with the prism and a star!

Sirius, 1 sec exposure (no tracking), 300mm Nikon prime lens with D300 DSLR. Some of the Balmer series visible......Some way to go!

Chris

Merlin66 · (#9)

Well done!
A great start, look forward to seeing more spectra.
Check out Walker's Spectral Atlas....
Richard Walkers Page

chiltonstar · (#10)

Quote:

Originally Posted by Merlin66

Well done!
A great start, look forward to seeing more spectra.
Check out Walker's Spectral Atlas....
Richard Walkers Page

Many thanks for this ref Ken - I shall have to dust off my German to understand it all though!

I recently acquired an SA100 as well - I shall try comparing simple spectra both with the prism and the SA100, before I decide whether to go ahead and make a "proper" spoectrometer.

Chris

Merlin66 · (#11)

Chris,
Down the bottom rhs of Richard's page, you'll see the english version of the Atlas.
(We don't directly link to this as he's constantly updating it!)

The SA100 works very well as an "objective grating"
Janet S. uses it with a 85mm Canon prime for some excellent "survey shots" - easily gets down to > 9 mag.

chiltonstar · (#12)

Quote:

Originally Posted by Merlin66

Chris,

The SA100 works very well as an "objective grating"
Janet S. uses it with a 85mm Canon prime for some excellent "survey shots" - easily gets down to > 9 mag.

Interesting! It must cut the f number of the lens down a lot though.....

I was thinking more of putting it in the traditional place 70mm or so in front of the sensor of my DSLR, and using my ED80, for a bit more light gathering. Any ideas of a supplier for a suitable length nosepiece (t-thread to filter thread)??

Chris

Merlin66 · (#13)

"In the converging beam" you can use a standard 1.25" to T thread nose piece..the grating just screws into the front.
This together with the depth of the camera body will give you around 80mm distance.
A good starting point.
Just remember to focus on the absorption lines, not the zero star image.

Jack Martin · (#14)

Chiltonstar,

Nice spectrum of Sirius I look forward to more spectra of different stars.

Regards,

Jack

Jack Martin · (#15)

Merlin66.

Please explain why you said,

'Just remember to focus on the absorption lines, not the zero star image'.

Regards,

Jack

chiltonstar · (#16)

The night before last, I had the chance to ring the changes and try a few options; I got the best result with the SA100 in the conventional position 70mm in front of the DSLR sensor. Hypothermia set in so I wasn't able to get sharper images though!

eg Alcor/Mizar and spectra,

Chris

Merlin66 · (#17)

Good catch. Have you done any proceesing ie calibration and profiles yet?
Jack,
The best focus for the spectrum varies with the telescope optics (refractor/ reflector) and the distance between the grating and the CCD.
The "best" focus is definately not when the zero order star is in focus!
Usually you start by getting the cleanest, narrowest spectrum you can, then tweaking the focus to bring out the absorption bands..at "optimum" focus the lines will be well defined and give maximum contrast.
Doug West did a write-up on the process:
Resolution Calculation for a Slitless Spectrograph

chiltonstar · (#18)

[QUOTE=Merlin66;2105583]Good catch. Have you done any proceesing ie calibration and profiles yet?)

I've had a try with RSPEC, which seemed to work ok, although it may take more than the trial period to get to grips with!

I've had several tries with VSPEC, but couldn't load the program properly, maybe a Win 7 issue. When I had another try yesterday, I found the link seemed to go to a junk website - maybe there is a new link?

I have some worries about artefacts when using the suggestion in the SA100 manual to compress vertical height to 1 pixel and then re-expand to 30. if I do that with the Alcor and Mizar spectra above, I suspect some of the detail is artefact as the resolution (ie linewidth) appears to exceed the 10nm or so limit??

eg

Chris

Merlin66 · (#19)

Chris,
The latest version of VSpec is available here.
Vspec - Download
(If you have further problems, drop me a PM and I'll email a copy to you.)

Yes! I think there's a 100% chance of introducing artifacts doing it that way.
You don't need to reduce and then re-expand. Just get the spectrum level across the screen and select the spectrum....
(I use IRIS to remove the background sky...in the latest Vspec Valerie has included this option, but I haven't tried it yet)
Onwards and Upwards..

chiltonstar · (#20)

Ken

Many thanks - VSPEC appears to install ok in this version, but when attempting to run the program, the error message "ComponentCOMDLG32.OCX or one of its dependencies is not correctly registered, a file is missing" appears

which was the erropr message I got before when I attempted to install earlier versions.

Chris

Merlin66 · (#21)

Hmmm,
Are you installing as Administrator and running it as Administrator?

chiltonstar · (#22)

Quote:

Originally Posted by Merlin66

Hmmm,
Are you installing as Administrator and running it as Administrator?

Yes, I've just checked to be 100% sure.

Chris

Merlin66 · (#23)

Hmmm
I've just re-installed last week the latest version V4+ on my Win7 machine - no problems. I did however have the old V3.8 somewhere on the HD.
(This needs to be installed, I think first, before going to the V4?)
I'll have a look and get back to you.
EDIT: Yes the V3.8.8 needs to be installed first and then upgraded - also see the Warnings-
Warning 4: It seems that VSpec has issue when installed in another drive than c:/

Warning 5: install in admin and run Vspec in admin (right click and select open as admin) if you have message telling some ocx are not properly registered.
This is from the download page:
http://astrosurf.com/vdesnoux/download1.html

Hope it helps.

chiltonstar · (#24)

Ah! Many thanks - it does run ok if I right click and open as administrator.

How do I get my files (jpeg, tiff, .nef etc) into the file formats for VSPEC?

Chris

Merlin66 · (#25)

Vspec needs fits files as an image input.
Just run them through either IRIS or your image capture software and save them as 16 bit fits.