spectra of red stars [AN article]

[nytecam]

Despite last night's full moon I managed to recorded [below] some lo-res spectra of red stars renowned for their red appearance in the eyepiece as listed in the September Astronomy Now article. I've added some nearby bright stars like Vega, Deneb and Altair as these show strong hydrogen Balmer lines [marked] and P Cygni with emission lines of H-alpha, H-beta and helium in yellow. Normally colour is a disadvantage for spectra as it tends to mask the absorption lines but it worked ok. The very narrow yellow band, at the transition between red and green, was also helpful in aligning the spectra. I used a 30cm SCT+Rainbow grating before a SX Lodestar-C camera in exposures from ~0.1s - 30s.

Perhaps the most remarkable feature of these spectra is the near-IR radiation recorded to the right via the unfiltered Lodestar-C camera - all this radiation, covering nearly half the whole spectrum, is invisible to the eye and that probably includes H-alpha too.

The red stars themselves are virtually devoid of any 'blue light' and this contributes to their red appearance. As an experiment the 'box' to the exteme left in each spectrum is the averaged 'colour' from H-gamma in blue to H-alpha in red for each star - the warm tones mirror the absence of blue in the spectrum. The reddest star for this series is T Lyrae near Vega. One star [s Cep] @ Dec +78 proved too far north for my scope eg the camera colliding with the scope forks Hope the spectra are of interest.

[the rev]

Really great information. What program did you use to create the comparison image?

[andrew s]

Hi - Nice work and very well presented.

Just one point if these spectra were recorded with a diffraction grating then the light out beyond the very near IR will be a mixture of the real IR radiation and the second order visible light. So if the first order starts at about 350nm then by 700nm it will appear again alongside the real 800nm light.

Regards Andrew

[nytecam]

Hi - Nice work and very well presented. Just one point if these spectra were recorded with a diffraction grating then the light out beyond the very near IR will be a mixture of the real IR radiation and the second order visible light. So if the first order starts at about 350nm then by 700nm it will appear again alongside the real 800nm light. Regards Andrew

Suggest you check the full mu Cep spectrum [via file name] - the 2nd order spectrum lower right is extremely weak via the blazed grating and 'contributes' little to the 1st order

[Merlin66]

Well done, a very impressive collection.

Keep up the good work Maurice!!

("Grating Spectroscopes- How to use them" contains a comprehensive listing of "Red" stars.

Richard Walker's excellent spectral atlas, provides additional annotated comparison spectra

http://www.ursusmajor.ch/astrospektroskopie/richard-walkers-page/index.html

(bottom rhs for latest version))

[andrew s]

"Suggest you check the full mu Cep spectrum [via file name] - the 2nd order spectrum lower right is extremely weak via the blazed grating and 'contributes' little to the 1st order "

Just had another look and did a rough calibration using a ruler on the image and I make that the 3rd order on the far left.

Maybe I have done my sums wrong. I agree the 2nd order will be much weaker and may well not be significant.

Regards Andrew

[nytecam]

Hi Ken - good to hear from you back in Oz

Really great information. What program did you use to create the comparison image?

Hi - don't have in fancy or expensive IP s/ware eg capture and primary IP via SX SXV-M7C-USB supplied with cam s/ware and rotation, crop, paste and title in PaintShopPro v4.14SE [=shareware].

All the stars recorded in the Astronomy Now list were distant red giants except for nearby red dwarf EV Lac [below] - its high proper motion across the starfield fooled me as to its location and identify so a better spectrum must await

[robin_astro]

Suggest you check the full mu Cep spectrum [via file name] - the 2nd order spectrum lower right is extremely weak via the blazed grating and 'contributes' little to the 1st order

Hi Maurice,

Yes but there is no blue light to interfere in Mu Cas but what does the IR end of your Altair spectrum look like ?

For quantitative work even with blazed gratings, the contibution can be significant depending on the grating and camera response.

Some of your spectra (eg mu Cep) show an interesting phenomenon which I have seen from time to time with some optical arrangements. Generally the aberrations in the converging beam arrangement get worse as the speed of the optics increases and indeed, presumably because of your fast f3.7 optics, the defocussing across the field due to the tilt of the focal plane of the diffracted beam is particularly severe but the spectrum lines remain sharp suggesting severe astigmatism (in this case working in our favour) There is also chromatic defocussing at the blue end eg in the Altair spectrum but again the lines stay sharp. Do you have any theories as to why this is seen in some systems but not in others?

Cheers

Robin

[nytecam]

Hi Maurice,

Yes but there is no blue light to interfere in Mu Cas but what does the IR end of your Altair spectrum look like ? For quantitative work even with blazed gratings, the contibution can be significant depending on the grating and camera response. Some of your spectra (eg mu Cep) show an interesting phenomenon which I have seen from time to time with some optical arrangements. Generally the aberrations in the converging beam arrangement get worse as the speed of the optics increases and indeed, presumably because of your fast f3.7 optics, the defocussing across the field due to the tilt of the focal plane of the diffracted beam is particularly severe but the spectrum lines remain sharp suggesting severe astigmatism (in this case working in our favour) There is also chromatic defocussing at the blue end eg in the Altair spectrum but again the lines stay sharp. Do you have any theories as to why this is seen in some systems but not in others? Cheers Robin

Thanks Robin - yes it's an optical nightmare at f/3.7 but severe astigmatism strangely working here to advantage if not pretty! Likewise for red star, with little blue radiation, this is essentially absent from the 2nd order too abutting the NIR 1st order. I've no experience or theories for other systems

It's clear at dusk tonight and I may have another play on some new targets ....

[nytecam]

Clear last night at dusk and thought I'd try the red trio in Corona Borealis eg R CrB, TT Crb in same field and T CrB a little SE but it was not to be. Got spectrum of Arcturus [my scope's initial alignment star] then gamma CrB.

Then tried to see if I could recorded supernova 2012ei in NGC 5611 in Boo field through the Rainbow grating but not its faint spectrum. It certainly absorbs a lot of light needing x10 the exposure than without the 'filter' in place. Now low cloud rushed in from SW and I packed up. An hour late it cleared again but I wasn't in the mood to restart

[nytecam]

Here's Arcturus + gamma CrB from last night - the Balmer series extends to <383nm in violet and for this hot Sirius type star the blue part of the 2nd order spectrum is clearly recorded abutting the red end 1st order of its spectrum to the right - mu Cep from earlier included for comparison

[Vlad]

Very interesting. I hope you make more posts on this subject. I WILL get into it

I WILL get into it

I WILl get into it

..........................................and so on.