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Thalestris24

The Lowspec spectrometer

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Posted (edited)

Valerio,

Ran your data through BASS Project...

Pretty good, the resolution is around R=971 in the red.

You can see the calibration is limited to the red region - this means the calibration is not so good in the blue. More reference lamp exposure time might pull out the emission lines around 4200A

The #1 exposure as you can seen doesn't appear to be over-exposed and the focus is pretty good.

The response curve of your camera is obvious, the jump around 5700A is typical! This can easily be corrected by preparing an Instrument response curve and using that to "correct" your spectrum.

Comparing with the solar reflected light (G2v) from Mars you've recorded a wealth of lines!

Well done.

You just need to open and use BASS Project to replicate these results.

Onwards and Upwards

Valerio_mars 051020_ref.jpg

Valerio_mars 051020_composite.jpg

Valerio_mars 051020_composite_G2v.jpg

Valerio_mars 051020_composite_annotated.jpg

Edited by Merlin66
info added

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I have just been clamping a CFL Lamp to the end of the scope. Can the same be done with a neon to obtain calibration spectra ? 

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Yes. but it sometimes needs more than one bulb to give sufficient light.

I've seen set-up were a ring of four (or more?) bulbs was placed in front of the objective.

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That's a plan.  I'll take 3 other irons apart then. Need a small solid state 12 to 240 inverter now. 

 

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Ken,

First of all thank you for your great help. I know, now I have to study BASS project and replicate your graphs. If I understood well you just imported a Sun (G2V) spectrum as a reference to see the absorpion lines in my spectra, correct? Are these spectra available in BASS project?

I will improve the calibration with RELCO lamp using 4/5 lamps and install them in fromt of my scope.

I see now that the spectrum #1 is not overexposed, but is there some kind of rule of thumb to determine the best exposure of the spectrum to avoid overexposing? I think that it must be more some kind of "visual" rule, but (considering my 4 spectra) lighter spectra in general are better ones?

To ceate the Instrument response curve I can use a known star spectrum and divide my spectrum of that star by that one, to obtain the Instrument response ... but once I get it is it always valid with any other object  and in every weather condition?

thanks

Valerio

 

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Valerio,

BASS project has all the tools you'll need. It has a comprehensive collection of star spectra (Pickles and Miles catalogue) built-in and available for comparison.

Before adding any more RELCO lamps, just check what exposure you need to bring out the lines down at 4200A.

The "best exposure" is the one that gives you the maximum ADU without overexposing - best SNR. Once you have taken some target spectra, you'll get a better feel for the capabilities of your set-up and be able to establish a routine. I've gone for a sub exposure of 300s and depending on the SNR required, may stack 20 or so subs.

I use Astroart to control all my cameras and pre-processing. This allows me to quickly do a "profile" along the sub exposure and confirm the  maximum ADU.

Once you have a good Instrument response curve it can be used for all your spectra. On the proviso that your using exactly the same set up. There will be an atmospheric effect to consider - it's generally accepted that the reference star spectrum (an A type star for convenience) should be taken at/ close to the same altitude as the target spectrum to minimise this effect.

To get started, and for practise in the processing, you can use the Pickles/ Miles reference spectrum, then move on to an actual reference star spectrum.

 

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On 29/09/2020 at 16:32, Merlin66 said:

Greg,

Thanks for the images, they show promise.

Not sure I understand this comment..

Are you using the DSLR body mounted on the LowSpec, if so the design back focus should allow the DSLR to get a focused spectrum.

You could double check that the collimating lens is positioned correctly and focused on the slit gap. If this is misplaced then you'd see a difference in the imaging focus position.

(Your lamp images shows some tight lines which says you're not to far away)

 

Sorry, wasn't checking for a bit.  The 100mm FL lens from Surplus Shed was apparently a little short if cemented together and/or the adapter I have is a little thick.  I put in a 108mm fl lens and it's too long but a short extension fixed that.  I am going to try a 60mm FL lens in the mouth of the T-mount adapter to see if I can compress things a bit for the small pixels I have, just playing around.

Yes, the photo was compressed, quite a bit in fact.

 

Greg

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Greg,

OK.

It sounds like you have a SS doublet pair, these are usually air spaced. I've used small aluminium foil strips in the past.

Onwards and Upwards

 

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Hi

I tried to get the spectrum of 2 bright stars and analyze them with BASS project. Still learning but improving :)
First one Capella: I was able to calibrate it and try a response correction. Not sure if I went through the best steps for the response correction. After the calibration with the RELCO lampI used a G5III ref spectrum and divided my spectrum bt this one, then I used that curve as
the correction response curve.

Capella_and_refs_chart_1D.thumb.jpg.fb2428b1b7ab329de80cac2062f87835.jpg

Capella_chart_1D.thumb.jpg.2505e3b90a5611bbf5f11f25ae1a72b7.jpg

This seems pretty similar to the spectra I found online. I couldn't find a way to add elements lines with the Element name as Ken did in a previous post... just the lines

 

I then tried with Aldebaran. I thought that I could use the same response file, but it doesn't seem to work, it transforms my spectrum too much I think...
Aldebaran_chart_1D.thumb.jpg.62e392b2776a6cab01c981e47d1e8739.jpg

This is the original spectrum, only with calibration

 

1435130753_Aldebaran_withCapellaresponse_chart_1D.thumb.jpg.8da15e2590c9363fb81e5e6353f76900.jpg

the purple one is with Capella response curve

So I tried with a reference spectrum K5III (the same as aldebaran) and I got this curve, that seem more similar to the reference K5 spectrum but it seems it has less "resolution"

1081970827_Aldebaran_withK5IIIresponse_chart_1D.thumb.jpg.6b7f8f6a1355915b17a56be2e10cb786.jpg

 

A lot to learn but something nice is happening 🙂

Valerio

 

 

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Valerio,

Well done mate! Good to see you moving along. You've done well to produce your first 1D calibrated profiles.

Capella (#1) - Your reference lamp looks a bit out of focus. The calibration is close, but I as I mentioned previously the lack of good reference lines down around 4200A reduces the accuracy.

I'd recommend changing the scale to the more common Angstrom ( Chart Settings/ Advanced/ Wavelength unit)

When you compare to the reference spectrum you can see the mismatch between features. (look at the region around 5200A) A longer reference lamp exposure and use of the blue lines would help.

In the meantime if you want to study a particular region you could "nudge" the calibration. Right click on the target image, Profile Properties/calibration - at the bottom of the window you'll see a box "Lambda offset (nm)" you can enter a suitable number and see the impact on the registration of the two curves (ref spectrum/ target spectrum). Try -0.1 (one angstrom) for starters then try again to improve further. (Note: pressing Apply will show you the impact but will not "lock it in" until you press OK, giving you time to play!)

After calibration, normalise the profile to make the numbers easier (!) Select a quiet region in the spectrum (say around 6400, 6400-6410) and set this to 1 ( Image tab/ normalise flux scale)

Response Curve: You don't show the response curve you used....The process is detailed in the BASS User Guide (press F1 to open) Section 2.15 gives all the info you need.

Labels/ Lines - See Section 2.26 of the Guide. Once you've labelled your spectrum you can save it for future use. I attach some label files which may be useful.

Other than the effects of different atmospheric absorption (altitude difference between ref and target spectra), if the same set up is used it should work reasonably close....

I think the calibration of the Aldebaran is sightly out, but obviously the registration between your Capella response and the target is well out! (Look at the bump in the target around 6000A in the target and the bump around 5700A in the corrected version could be an indicator)

The last profile shows that your response curve is not working. If you had a good response curve the result would be a spectrum very close to the shape of the ref spectrum (for a K5III). It's always a good sanity check to compare the corrected profile against a reference profile......

 

You're doing well. Once you've mastered these basics they will be used over and over again on future spectra.

 

 

Sample TypeF.lbl Sample TypeG.lbl Sample TypeM.lbl Sample TypeB.lbl

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Very bad weather these past days so I couldn't get more spectra. I hope soon... and I hed no time to look deeply into your great explanation. Next week end I think I will try something

Thanks!!!

 

 

Edited by Valerio

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Valerio,

Thanks.

Yes, the weather everywhere seems to be limiting us.

Hopefully things will improve soon.

Wet nights are ideal for catching up with processing. ;)

 

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Hello,

still struggling a little bit on calibration. I was trying to obtain a "final" instrumental response for my telescope+spectroscope. I have read this web page that seems to be pretty clear http://spectro-uvex.tech/?p=1585&lang=en, and also something similar here https://www.horiba.com/fileadmin/uploads/Scientific/Documents/OSD/203_Instrument_Response_Corrections.pdf

So I have created with an halogen tungsten lamp (2950K) this spectrum

Light_60s_Bin1_gain120_20201031-224754_-14.8C_0001_d_FB.thumb.png.0b3081ed3a874af08c0e0326022e780b.png

that gives me this 1D profile

halogen_1D.thumb.jpg.dbc5c0ca6fb7fb87b35e1b5bad3ecc37.jpg

that I compared with a planck curve at 2950K (green). Dividing one by the other I should obtain the Instrument_response = lamp_signal / black body profile - blue line

Intrument_response_1D.thumb.jpg.282c3499c239783d81a4c794f0ed782b.jpg

that "should" be pretty universal for my setup (telescope C6 f/10+spectroscope). Of course this is not valid for the atmospheric corrections, I know, I should follow the second "long" part of the process to add that calibration, too.

What I am not yet sure is the curve of the halogen lamp. In the examples in those websites they show a "mountain-like" curve, that goes almost to zero on the right part of the spectrum, but with my setup all the spectra I get are getting more and more bright toward the red part of the spectrum, even if they should go to black I think. Is this due to the IR part of the spectrum that my camera (ASI 294MC PRO) is too sensible to? Should I use an IR-Cut filter to obtain the right spectra (in general)?Preview_30s_Bin1_gain120_20201102-211318_0C_d.thumb.jpg.c479b4ff5cc4b95bec7b663a7828cfd8.jpg

Also, not still sure which slit to use... it depends on the brightness of the source? Less bright... thicker slit?

thanks!

Valerio

 

 

Edited by Valerio

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Valerio,

It's more usual to use a tungsten lamp to prepare a spectral flat. (you can see you have some doughnuts in the red)

Using this technique and approximating to the Planck curve can obviously work (a la Buil). It's more usual to use his "long" method with a suitably placed A type star.

I assume you calibrated the lamp from an earlier reference spectrum?

You followed the instruction, very well and obtained an Instrument response.

The profile obtained looks different due to the CMOS camera you used. The pronounced dip around the 5700A (and some of the others) is due to the Bayer matrix.

The sanity check for a response curve is to divide the original by the response and see if it matches the "reference" ( i.e. the Planck curve for 2950K)

When this is done, there's a pretty good match - inferring the response curve was doing its job.

(You can try the same exercise with your solar spectrum and using a G2v reference. - I've already done this and the response curve generated is very similar!!)

Your camera is doing its job and is, as you see, quite responsive in the red - not a bad thing.

Re Slit gap

This depends on the resolution you want to achieve and the star size at the telescope focus (check out SimSpec)

Narrower gaps give higher resolution, but reduced throughput, for fainter targets it's sometimes better to loose some resolution to get better SNR.

You're doing well!

 

2950K_planck_response.JPG

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I made some more experiments with calibration and acquisition of Sun spectra and Mars spectra. I think that I am getting a pretty good calibration, which I got with a halogen lamp (Flat) and the RELCO lamp.

Mars_Sun.thumb.jpg.f0dba34c087392ae08fae62301447966.jpg

Mars_Sun_zoom1.thumb.jpg.6883b05bd73f32453bca16e27f140767.jpg

Mars_Sun_zoom2.thumb.jpg.a021665aa275e7986d16890b34cd7cb3.jpg

Mars_Sun_zoom3.thumb.jpg.ff3e5b2e3d378171ac22814854375d7b.jpg

 

Valerio

 

 

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Valerio,

Looks like you're having fun with BASS Project!!

Looks very good.

The last profile (H alpha) shows a possible issue. Notwithstanding your calibrations (RELCO), the Telluric bands are not a perfect match. (You could even have used them to add to your calibration.)

All in all, I think you should be pleased with your progress in such a short period of time.

Keep it up.

 

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Yes I am pretty happy with the results. Fun and science, a perfect match 🙂

I must say that sometimes the program does some strange things, above all with labels, and I have to reload everything and start again.

You are right the Telluric bands are not perfectly matched, it's strange because I had many good lines in the RELCO reference and I think I have put them correctly in the calibration

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