Affiliation
American Association of Variable Star Observers (AAVSO)
Sun, 07/14/2019 - 13:39
There is a very bright dwarf nova in Cygnus now:
http://www.cbat.eps.harvard.edu/unconf/followups/J21040470+4631129.html
I wonder if this could be a nice beginner's object for low-res spectroscopy with modest equipment (Star Analyser and upwards).
Thoughts?
Anyone else trying this and willing to share experiences?
CS
HBE
Yes this should be an interesting object for the Star Analyser with strong H alpha ~10A FWHM. The other Balmer/He emisison lines might be tougher to see at typical Star Analyser resolution.There are some high resolution echelle and an R ~1100 LISA spectrum on the ARAS forum here
http://www.spectro-aras.com/forum/viewtopic.php?f=39&t=2355
Hi Robin,
As I write this I am using a SA 200 ( I am one of the 6000 and am in Wilderness, Virginia, USA) on a meade 12-inch LX-200 to obtain spectra of SS Cyg which is currently fading from a recent outburst. I have been observing the star nightly since September 24 and see the changes mentioned in this thread where the H emission lines fade as the star rises to maximum. As of tonight they have not reappeared as my spectrum images are featureless. However, I have a nagging belief that part of that may be that my focus isn't optimal. Am I correct in thinking I should set focus position that gives the narrowest spectrum? In this position the zero order star mages are decidedly out of focus. If I focus the stars to their best, then the spectrum appears much broader and the features all but dissapear. Any thoughts on this would be much appreciated. I am very new to spectroscopy and am in awe at the work I see posted here and on the ARAS forum! As an aside, I use the zero order image in the Star Analyzer for differential photometry ( which I am also very new to) but it seems to work. My light curves show flickering of SS Cyg relative to the check and reference stars just before it began its fast phase of the outburst- which my curve shows the star rising at about 0.1mag/hr.
Myron Wasiuta
Yes with the Star Analyser, the resolution and visibility of the spectrum features is very dependent on the focus and seeing. Because of the diffraction angle, the focal plane is curved so the best focus in the spectrum is slightly in from the best focus of the zero order and varies slightly along the spectrum. (This is more pronounced for the SA200 compared with the SA100). Depending on your focal ratio, the zero order will look significantly out of focuswhen the spectrum is in focus. Focusing for the narrowest spectrum will get you near but there is some astigmatism so for best results you need to focus on spectrum features. The way I normally do this is to first pick a brightish main sequence A star (which also serves as a calibration reference) and bring the strong Balmer absorption lines to good focus before moving to the target in the knowledge the focus is good. The program RSpec can be useful here as it can show the spectrum in real time). Another technique (particularly if you have a digital focuser) is to note the offset in focus between the zero order and spectrum focus.You can then apply that offset to any target after focusing on the zero order.
Because H alpha is at the far end of the spectrum the focus change from the zero order is largest and the H alpha absorption line is not very pronounced to focus on with A stars so if you are specifically looking for H alpha, you could chose a star with a known strong H alpha emission line to focus on initially. Gamma Cas or P Cygni are good targets for me at least at this time of year. This would also give you confidence that seeing conditions are good enough on the night to be able to do good spectroscopy.
Using the zero order for differential photometry is a cool idea. Be aware though that the spectral content is rather odd (the inverse of the grating spectral response so predominantly light from the blue and red ends with the middle wavelengths missing) so it would be very difficult to transform the results to a standard photometric system (best to colour match your comparison stars as close as possible)
Cheers
Robin
Robin,
Thank you for the detailed advice on focussing the SA 200! I found it very useful-especially now that I know the focal plane is curved it makes so much more sense in analyzing some of my images. That is whynsome spectra show broadening o one end and naroowing on the other-with the narrower end showing ore spectral features! I find my best results are when the spectrum is slightly tapered at each end. I will also incorporate your advice on concentrating on specific features in the spectrum.
In experiments I did with a color CMOS camera, I noted the zero order star images to be amber colored.You are right in that this would be difficult to correlate with other observations. however, I am interested in constructing a light curve that shows interesting changes like brightening or dimming, and then being able to use that light curve to pinpoint possible changes in the spectra of the event with that of more quiescent phases of the light curve. I do this with flare stars and believe I have captured possible rapid changes in the blue end of the spectrum of EV Lacerta. Ill post some images of my SS Cyg project once I finish and thank you again for the help!
Yes the Star Analyser is a good potential way of catching and analysing fast transients. (Difficult to do quantitatively with conventional spectrographs because you dont collect all the light with a narrow slit). I used a Star Analyser to follow one in a T Tauri star here
http://www.threehillsobservatory.co.uk/astro/spectra_42a.htm
Since then I further analysed the event and was able to derive V and B mag values direct from the spectrum. I talked about this at the joint BAA/AAVSO meeting last year. The video is on the BAA website here but only for BAA members.
https://britastro.org/video/13862/14769
I dont know if it is on the AAVSO website somewhere.
I had a look at SS Cyg H alpha/He 6679A last night at a resolution R of 5500 with my LHIRES. H alpha is strong now, almost up to quiescence strength so it should be visible with the Star Analyser
Cheers
Robin
Wow what an interesting concept- being able to obtain B and V magnitude estimates from the spectra! Makes sense though. I would like to learn more about that and will research the link you provided. I think i might be joining the BAA.
Hi Myron,
Looking at the light curve I think SS Cyg has a few more days to go before it gets back to quiescence. comparing with spectra in the BAA database at roughly the same phase, H alpha emission should now be visible but the other lines could still be lost in the absorption lines eg
https://britastro.org/specdb/data_graph.php?obs_id=387
In a few days though the lines should be back to full strength
https://britastro.org/specdb/data_graph.php?obs_id=136
Cheers
Robin
Hi Robin,
Thank you for the nice spectrum of SS Cyg in the H Alpha region you posted on my behalf yesterday.I images SS last night as well but still not convinced I can detect the H alpha emission yet. Im posting two images from my analysis in RSpec-one from last night which doesnt really show the H emission yet and one from the 25th of september, 2019 which shows clear emission features. I think they just may still be below my resolution capability. Tonight looks to be clear so I will continue my efforts.
Hi Myron,
I had a quick look at SS Cyg tonight with my very low resolution faint object ALPY200 (a slit spectrograph with similar resolution to the Star Analyser (R~130 so ~50A at H alpha). Attached is the raw uncalibrated spectrum. I can see H alpha and H beta clearly, similar to your first spectrum so as you say, it could be just your current conditions (The Star Analyser is very sensitive to seeing)
Cheers
Robin
Hi Robin,
Yep-I see them tonight as well! Seeing is very good and my spectra are very thin and sharp. Clear H alpha and beta visible tonight. SS must be getting close to quiescence again. Thanks for all your help over the past few nights-I have alot of data to mull over and will be correlating (mostly for my own interest) the light curve of this recent outburst with all my spectra.
Warm Regards,
Myron
Hi, Myron,
I've attached a new pdf that discusses focusing with a Star Analyser.
Thanks for the encouragement, I'd like to briefly report back:
With just an 8" scope, probably not the ideal placement of the Star Analyzer 100 (I just put it in the filter wheel) and way too few photons collected (clouds...), I still got something that looks like a consistent spectrum on July 15th. With my setup I get about 8 Angstrom per pixel, not great, but ok-ish I guess for purposes of just learning spectroscopy basics at the moment. Working with the Star Analyser and RSpec software is quite some fun for the buck, I highly recommend it, especially because of the excellent tutorial videos for Rspec!
CS HB
Glad you were able to get something (The SA100 is my baby. I developed it back in 2005 and there are now almost 6000 of them out there somewhere). Yes that is pretty much as I expected. The Star Analyser spectrum resolution in a typical configuration is around 40 Angstrom. (The maximum theoretical resolution is the dispersion (A/pixel) * The size of the in focus star image (in pixels), though the actual resolution can be lower because of the simple converging beam configuration.) The hot continuum and the H alpha emision line is clear here, but if you look at the higher resolution spectra on the ARAS forum, you will see why the other lines are difficult to see because they are made up of a narrow emission line embedded in a broad absorption which at the Star Analyser resolution combine to give an almost flat result. "Proper" classical novae (and supernovae) give better results with the Star Analyser as the lines are broader because of the higher velocities involved. With an good astro camera and a longer total exposure, it is possible to go much fainter with the Star Analyser even with 8 inch aperture, to around mag 14. There are some examples on my website
Cheers
Robin
www.threehillsobservatory.co.uk
Ah right, I remember! Impressive success for the product, and I think it's now even more useful than "back in those days" with those hi-res, fast-readout, low-noise CMOS monochrome astro-cameras that are really affordable now.
CS
HB
I am not familiar with star analyzer spectra and I am impresed that you can see the H-alpha line. Very nice! Could you try to observe SS Cyg? It is at 12 mag now, a nice target for your setup, and I am curious to see what you can get ...
Thank you in advance!
Best wishes - clear skies,
Stella.
PS: please remember to upload yoru spectra in our database
Stella and any other interested peoples,
It is not only possible to see the hydrogen alpha line with the SA100 but also the H-beta and H-gamma. I published a paper in the Vol 47 No. 1 of the JAAVSO using the SA100 to observe Miras. While that paper doesn't include the particular Mira in which I observed the three H emission lines, the paper should show how useful adding an SA100 to a photometric observing programme can be in obtaining useful data on them.
Bill Rea
Sure, wanted to try that anyway...only problem is weather :-) I'll post here when I get something but would not be surprised if someone else with an SA 100 will beat me to it.
Clear Skies
Heinz-Bernd
I'm not sure whether the resolution will be sufficient to do anything useful, but it sure is easy to spot, you cannot miss it in outburst :-).
Inverted preview image enclosed to motivate SA 100 beginners. It's the spectrum thru the middle.I'm trying 5 second exposures, to be stacked, with an 8" Newtonian for starters.
https://www.aavso.org/sites/default/files/live.png
I have not seen any SS Cyg Star Analyser spectra but it would be an interesting target to follow through an outburst. There is a nice series of spectra in the BAA spectroscopic database from 2016 by Joan Guarro Flo using a LISA at R~1100. I pulled three of them out of the database at high, low and intermediate brightness and filtered them to R~130 to simulate what might be seen using the Star Analyser. It looks like it should be possible to follow the change from emission in quiescence to absorption during outburst.
Cheers
Robin
Very nice spectral sequence on SS Cyg ... and we are in luck: SS Cyg is in outburst now! I would like to encourage anyone who can observe to take data - it would be great to cover the outburst spectroscopically while TESS is acquiring photometry!
Best wishes - clear skies,
Stella
The original R~1100 spectra
https://britastro.org/specdb/data_graph.php?obs_id=329
https://britastro.org/specdb/data_graph.php?obs_id=1220
https://britastro.org/specdb/data_graph.php?obs_id=387
Hmm...at the moment I see nothing except a very smooth ca 20000 K Planck spectrum, while some other stars in the field look quite feature-rich and in good agreement with their spectral-type prototypes from a ref library. ???? If there are features in the spectrum I don't seem to be able to resolve them.
Cheers
HBE
Yes this is quite normal for certain phases of a dwarf nova in outburst. The strong narrow Balmer emission lines at quiescence diminish relative to the continuum and are balanced by the intensity of the broader underlying Balmer absorption lines so the features disappear at the Star Analyser resolution. Compare for example these R~600 resolution spectra from Kevin Gurney prior to and during the current outburst in the BAA database
https://britastro.org/specdb/data_graph.php?obs_id=4680 (2018-08-05)
https://britastro.org/specdb/data_graph.php?obs_id=4681 (2018-08-19)
Cheers
Robin
Wooooh, what a difference!! Thanks for the info.
Just for completeness, here are two SA 100 spectra (blue and red curves) that I took yesterday night. Actually one was taken before and the other after a meridian flip. Because of the change in orientation, the spectra will fall over different parts of the field and overlap with different background stars, so it's perhaps a good control to see what is real and what is just a background star contaminating the spectrum .
https://www.aavso.org/sites/default/files/SS_Cyg_20190822.png
What is your exposure time? You should be able to see emission lines ...
Also, if you want more targets for yoru star analyzer, perhaps try to take a look at some WR stars? Some suggested targets are in the AAVSO spectroscopy observing section page:
https://www.aavso.org/spectroscopy-observing-section
I hope this helps!
Best wishes - clear skies,
Stella.
Hi Stella,
>What is your exposure time? You should be able to see emission lines ...
This spectrum was taken at maximum where the narrow emission lines are weak and tend to be lost in the broad absorption lines at Star Analyser resolution. For comparison, here is a high SNR spectrum at R~600 (~4x higher than the Star Analyser), taken around the same time by Kevin Gurney
https://britastro.org/specdb/data_graph.php?obs_id=4728
Even the H alpha emission line there is weak and only ~1/10 of the relative intensity at minimum
The emission lines are comming back again now currently as it returns to quiescence eg as currently being discussed up the thread here
https://www.aavso.org/comment/66602#comment-66602
As an aside, are the videos of the BAA/AAVSO meeting I mentioned there available to AAVSO members anywhere ?
Cheers
Robin
Hello Robin,
Thanks for your comments. To answer to your question: BAA hosted the 2018 spring meeting, and they requested for all videos to be disseminated only through their web page. Your talk was exceptional and very informative, so perhaps you can consider sharing the .ppt (or .pdf) with me, and I could post it on the AAVSO spectroscopy observing section page ... what do you think?
Best wishes - clear skies,
Stella.
Hi Stella,
There is a link to the pdf of the presentation "Pushing the limits using commercial spectrographs" including a script on my website which includes other workshop presentations which might be of use. You are welcome to link to that page
http://www.threehillsobservatory.co.uk/astro/spectroscopy_10.htm
Cheers
Robin
Splendid - there are all kinds of valuable resources on your page. I just linked it to our Spectroscopy observing section resources - thank you Robin!
Best wishes - clear skies,
Stella.