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Old 02-09-2014, 07:06 PM
l3gendluk3 (Luke)
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Solar Spectroscopy

I am interested in taking a spectra of the sun with a Meade lx200 and a sbig sgs, will I need a solar filter (I have one) and if so, does it need to be a special type? Also will I need any other equipment?

Thanks,

Luke
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Old 02-09-2014, 07:17 PM
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Merlin66 (Ken)
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Luke,
Interesting question....
Using any solar filter (Baader solar film etc.) will reduce the throughput by at least 1/1000....
IMHO you can easily obtain solar spectra without even using the telescope!
Just point the entrance slit of the spectroscope towards the bright daylight sky and you'll easily record the solar spectrum. (I assume you're forced to use an ol' ST8 camera with the SGS....)
This is by far the easiest and safest way of doing it.
I can supply links to detailed solar spectra for comparison.
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Old 02-09-2014, 07:59 PM
l3gendluk3 (Luke)
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Unfortunately my research project involves measuring the red shift/blue shift of the sun so I would need a direct view on the edges. I could bump up the exposure as my mount could easily handle it although its not going to be a full spectra, I will be selecting 3 or 4 samples lines and measuring the difference between those and lab samples.

The sample spectra would be great as it could give me a look on where the lines are supposed to be.

Thanks,
Luke
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Old 02-09-2014, 08:28 PM
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sheeny (Al)
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Any normal white light solar filter should do for that. I had considered a similar project for measuring spectral changes around sunspots. (But since my job has improved and got a lot more technical, I'm leaning towards less technical pastimes at home).

You'll just need to identify some distinct spectral lines, to get your red/blue shifts.

Al.
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Old 02-09-2014, 08:38 PM
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sheeny (Al)
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I don't know a lot about the SGS, but it looks to me like the slit is full sensor height. Guiding on a star, the appropriate spectrum is easy to identify, but you might experience some interesting challenges with spectra from the limb of the sun... I guess you just go for the maximum red or blue shift, is that the plan? Otherwise the spectrum should be full sensor height, with various red/blue shifts in the Y direction...

Al.

Last edited by sheeny; 02-09-2014 at 08:38 PM. Reason: typo
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Old 02-09-2014, 09:48 PM
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Does it have to be the sun?

I did a similar thing with Saturn and was able to measure the Doppler shift from each limb. See the graph attached.
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Click for full-size image (Saturnspectra.png)
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Old 03-09-2014, 06:38 AM
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For solar Doppler measures....depending on the wavelength you want to use, I'd stop down the aperture to around 100mm and include suitable narrow filter - Ha or OIII type..
This should reduce the heat loading on the scope and on the spectroscope slit plate.
A similar but different problem exists when designing a spectroheliograph.. generally these use achromatic refractors with a focal length to suit the slit height...
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Old 03-09-2014, 07:16 AM
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Quote:
Originally Posted by l3gendluk3 View Post
Unfortunately my research project involves measuring the red shift/blue shift of the sun so I would need a direct view on the edges. I could bump up the exposure as my mount could easily handle it although its not going to be a full spectra, I will be selecting 3 or 4 samples lines and measuring the difference between those and lab samples.

The sample spectra would be great as it could give me a look on where the lines are supposed to be.

Thanks,
Luke
Hi Luke,

an interesting project

Have you checked the wavelength shift you are trying to measure ? A back of envelope calculation suggests the Doppler shift from solar limb to limb will be tough to detect using the SGS. Even with the higher resolution LHIRES a measurement would be marginal.

If you do decide to go ahead, I suggest placing the slit horizontally along the equator so you can measure the velocity profile across the width in one go. This allows differential messurements to be made which gets round the problem of stability between measurements which will be poor with solar measurements due to temperature effects and the need to use a lab lamp reference.

See here for a similar measurement on Saturn and Jupiter. The shifts for the sun will be significantly smaller though
http://www.shelyak.com/dossier.php?id_dossier=16

To measure the very small shift I would recommend using cross correlation of the whole spectrum rather than measuring individual lines. You will be able to detect smaller shifts this way. You will still need to use the lamp spectrum to define and correct for any inherent slant/curvature in the lines.


Cheers
Robin

Last edited by robin_astro; 03-09-2014 at 07:37 AM.
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Old 03-09-2014, 09:38 AM
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There are a couple of good solar spectrum references:
http://fermi.jhuapl.edu/liege/s04_0000.html
http://bass2000.obspm.fr/ephem.php

http://bass2000.obspm.fr/solar_spect.php
Hope this helps.
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