New transcient, perhaps a nova

Information about outbursts of eruptive stars, Be activity, ...

Re: New transcient, perhaps a nova

Postby Tim Lester » Wed Aug 02, 2017 6:38 pm

Aug 2 - Balmer lines have increased significantly!
Here are my spectra so far converted to aprox. flux using AAVSO data.

novasct2017_20170726_086_tlester.png

novasct2017_20170726_086_Halpha.png
novasct2017_20170726_086_Halpha.png (40.61 KiB) Viewed 4197 times

novasct2017_20170726_086_Hbeta.png
novasct2017_20170726_086_Hbeta.png (44.25 KiB) Viewed 4197 times
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Re: New transcient, perhaps a nova

Postby umberto sollecchia » Wed Aug 02, 2017 9:29 pm

Bon soir à tous, je place deux profils, respectivement du jour 31/07/2017 et 01/08/2017
Umberto

Profilo nova flux.png
Profilo nova flux.png (11.75 KiB) Viewed 4190 times


Profilo nova flux.png
Profilo nova flux.png (10.9 KiB) Viewed 4190 times
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Re: New transcient, perhaps a nova

Postby Christian Buil » Thu Aug 03, 2017 11:56 am

My two latest spectra taken from "OHP station" (now we are at Juan les Pins ;) ) - the nebular phase quick coming :

Image

Image

I have a important question for the observers who calibrate spectra in absolute value. Are you using a narrow slit or a slitless slit (large photometric slit) ?

As you know, the presence of atmospheric chromatism (the nova is not very high above the horizon from France) with a narrow slit can cause significant errors on the shape of the continuum.

In my case I use a refractor, which adds the problem of instrumental chromatism (although the FSQ106ED is apochromatic). The telescopes equipped with focal reducers can know also difficulties (less in general, but be careful ...)

For me these two phenomena and the setup produce high spectral distortion in the form of the continuum for the nova. For example, the ratio of a slit spectrum and a slitless spectrum (Alpy600 180 microns photometric slit) of the same star nearly the same time. The aspect of the continuum is very changing between the two situations !!!

Image

To correct this dramatic (but ultimately classic) problem, I intensively exploited the photometric slit during the OHP run. For information, here I describe quickly the adopte protocol, I named "flash photometric correction" :

Step 1 : observation of a standard star (Miles) by using narrow slit (here 18 microns wide and a relatively short exposure time -> standard processing -> check and/or extraction of wavelength calibration function ("S0" spectrum)

Step 2 : observation of the same standard star by using the photometric slit (180 microns wide) - important point : no response correction applied (nearly the apparent spectrum view by the detector). Relatively short exposure observation (i.e. "flash"), typically 2 to 5 minutes exposure time. The result is the "S1" spectrum.

Step 3 : calculate the ratio S1 / "MILES" ("MILES" = the expected true profile is from the Miles data base). The result is the "PSR" profile (Photometric Spectral Response Profile).

Step 4 : observation of the target objet (here the nova) by using the photometric slit - important point : no response correction applied at this stage - "flash" procedure (short exposure time, the idea is to obtain the global acceptable form of the continuum and eventually absolutes flux infos). Result is the "S2" spectrum.

Step 5 : compute the ratio S2 / PSR -> "S3" spectrum. The S3 is the probable true spectral profile of the target (but some noisy because short integration time and degraded spectral resolution because use of a slitless mode).

Step 6 : observation of the target object (here the nova) by using a normal method : narrow slit and long exposure for the desired SNR. Do not applied here response correction. The result is the intermediate "S4" science profile of the target.

Step 7 : Compute a spectral correction term, the SSRT (for Slit Spectral Relative Transmission) : SSRT = S4 / S3.

Step 8 : smooth the SSRT profile (ISIS "continuum" tools for example -> remove hight frequency artifact) give the SSRT2 function.

Step 9 : Compute the "S5" final science profile : S5 = S4 / SSRT2.

The final equation is :

S5 = S4 x smooth[ (S2 x MILES) / (S1 x S4) ]

The procedure may seem long and tedious, but this is not really the case and it can be automated (you have to observe S1, S2 and S4 spectrum). This is the rather rigorous method that allowed me to obtain relatively correct spectra of nova during the "OHP campaign" by using a small refractor (but remember, a SCT telescope can be also affected).

The subject is important and deserves probably an exchange of opinions.

Christian B
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Re: New transcient, perhaps a nova

Postby umberto sollecchia » Thu Aug 03, 2017 7:46 pm

Bonsoir Cristian ,

pour mes observations, je l'ai adopté la méthode décrite dans cet article:

http://spectro-aras.com/forum/viewtopic ... 897&p=4044

Je suis coordonné avec Paolo qui observe en même temps l'étoile en prenant les médias magnitude V.

Pour reprendre le spectre, j'utilise la norme fente dell'Alpy 600, orientée le long de l'axe de déclinaison.

Umberto
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Re: New transcient, perhaps a nova

Postby Christian Buil » Sat Aug 05, 2017 12:06 am

Umberto c'est comme cela qu'il faut procéder en effet (en fait il y a je pense plus simple pour convertir un spectre de référence en flux absolu via l'onglet Spectre 3 de ISIS, puis la fonction "Conversion en densité de flux").

Le problème que je soulève est celui d'un effet géométrique avec la fente. Il a déjà fait l'objet de discussions sur ce forum, mais l'observation de cette nova est l'occasion d'actualiser la chose.

Je pense Umberto que vous avez fait le meilleur choix (best choice) en orientant la fente parallèle à la déclinaison compte tenu du fait que l'on observe plus fréquemment au voisinage du méridien. L'illustration suivante montre la situation au méridien justement (l'étoile en couleur est une vraie étoile et montre le vrai chromatisme lorsque l'astre ce trouve à environ 15° au dessus de l'horizon, j'ai utilisé pour la prise de vue un petit ETX90 et une caméra couleur ASI224MC) :

Image

On voit que si la fente est horizontale (au méridien) on biaise le contenu spectral. C'est encore plus clair sur l'autre exemple suivant, toujours avec le vrai chromatisme atmosphérique constaté d'une étoile observée avec le ETX90 et la caméra couleur :

Image

Suivant la position de la fente dans la ligne du bas, on favorise le bleu ou le rouge. A ceci s'ajoute le chromatisme instrumental... D'où une procédure qui exploite la fente photométrique pour tenter d'amenuiser tout ceci, en particulier lorsque l'angle parallactique augmente.

Sur le stage OHP j'ai vu pas mal d'observateurs travailler avec l'axe de la fente le long orienté suivant l'axe AD, faisant valoir un meilleur guidage. Douai, pas convaincu, et surtout, l'argument du chromatisme atmosphérique est un killer dès lors que l'on souhaite bien travailler.

Je vous livre ici une astuce que quasiment personne ne met en oeuvre : la caméra de guidage est utilisé sans filtre dans la plupart des cas. Comme ces caméras sont bien sensibles au proche infrarouge, en fait on guide sur la partie rouge du spectre, et donc en présence de chromatisme atmosphérique (quasi toujours présent en fait) on décale l'image stellaire sur la fente du coté rouge sans que l'on s'en aperçoive, ce qui ajoute une erreur photométrique systématique. La solution consiste à mettre juste à l'avant de la caméra de guidage un filtre "IR cut". En plus les images seront plus fines et vous focalisez mieux sur la fente.

A tip: adopt a IR-cut filter just in front the guiding camera -> the center of gravity of stellar image on slit is better centered on the central part of the spectrum in presence of atmospheric chromatism -> better photometric quality (relative and absolute) + better focusing of stellar image on the entrance slit -> better efficiency.

Christian B
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Re: New transcient, perhaps a nova

Postby Paul Luckas » Sat Aug 05, 2017 1:19 am

Christian Buil wrote:My two latest spectra taken from "OHP station" (now we are at Juan les Pins ;) ) - the nebular phase quick coming :
.
.
I have a important question for the observers who calibrate spectra in absolute value. Are you using a narrow slit or a slitless slit (large photometric slit) ?
.
.
The subject is important and deserves probably an exchange of opinions.

Christian B


Hello Christian,

Some (myself included) acquire simultaneous V-band photometry with a second telescope (or the same telescope with multiple light paths) and then use the flux calibration feature in ISIS. However, it would be good in a future update of ISIS if it was possible for users to add their own V filter transmission curves as an option in the flux calibration feature, as there is currently some (small) discrepancy between the available Bessel option, and the transmission characteristics of my specific V filter.

Paul
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Re: New transcient, perhaps a nova

Postby Christian Buil » Sat Aug 05, 2017 2:11 pm

Paul, the question concern two points : (1) absolute calibration, (2) true continuum restitution.
The use of a separate V-mag measure is a good option (I understand your suggestion about specific filter spectral profile) for calibrate one point of the spectrum. We can note also the possibility to use photometric slit as a multicolor photometer - see RR Lyrae example here :

http://www.astrosurf.com/buil/alpy600/photometric_slit.htm

...two instruments in one (but I admit, the photometric slit (i.e nearly slitless mode) is not always very easy to use because wavelength calibration difficulty... little bit of experience is recommended).

The true continuum aspect restitution is a distinct problem.

Christian B
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Re: New transcient, perhaps a nova

Postby Paolo Berardi » Sun Aug 06, 2017 10:32 am

Hi all, I have the 31 micron slit AR oriented. Except a couple of cases, I used the 1200 l/mm grating. I hope the continuum error is not too large.

Very useful the advice on using an IR-cut with guider camera. I have a Lodestar model (very sensitive to red-IR) but it's directly connected to the Lhires guider port and there is no room for a standard filter. To understand how to do...

My last observations.

H-beta and Fe II (42) region on Aug 3:

Image

H-alpha region on Aug 5 (tellurics removed):

Image

The latter is not fluxed because Umberto didn't observed the nova with Alpy 600 during the session (I rescale the continuum to the low-res flux one)

An animation to show the great evolution of the spectral profile in just a few days around maximum:

Image

Paolo
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Re: New transcient, perhaps a nova

Postby Robin Leadbeater » Sun Aug 06, 2017 1:03 pm

Hello Paolo,

Paolo Berardi wrote:I have a Lodestar model (very sensitive to red-IR) but it's directly connected to the Lhires guider port and there is no room for a standard filter. To understand how to do...


There is a colour version of the Lodestar. I wonder if that uses an IR block which could be fitted into the mono Lodestar? (I suppose it would be difficult to remove though when you need maximum sensitivity on faint targets)

Cheers
Robin
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Re: New transcient, perhaps a nova

Postby Paolo Berardi » Mon Aug 07, 2017 8:50 pm

Thanks for the tip, Robin. Yes, so often targets or guiding stars are faint and the IR cut filter could make it difficult to guide (i.e. red targets). It would be better to find a solution which allow to take it out easily.

Paolo
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