spectro-aras.com

Hello all,

I encounter a recent issue with my last AG Dra spectrum.
Harware 14" Meade ACF telescope f/d 7 + Alpy 600 + Atik460EX camera
Software : ISIS-5.8.0
Target AG Dra symbiotic star 6x600s
Reference star HD 145454 4x80s

When comparing my spectrum to other spectra (ie Umberto spectrum), i see rather large differences in the blue part continuum.
It is rather surprising as i always use the same harware, software and reference star (HD 145454)
So my instrumental and atmospheric response may be wrong.
I computed AG Dra with two different database stars (Miles star HD 146624 and Pickles A0V).
Here are the two responses :
They seem quite similar and the final spectrum :
So do you see what is causing that issue ?
What reference star do you use for AG Dra and what is your database star for instrumental response ?

Thank you for your help.

Cheers,

Hello Jacques,
I am not expert to provide help on that subject, but your input is very interesting. On a low definition spectrograph as Alpy600 (large frequency band) I also found significant variations in the correction for “instrumental response”. It looks to be a complex function with multi parameters depending of the instrument and air mass at time of observation. The ideal would be to reach a point where the instrument response is well identified and the only adjustment variable would be the AOD (aerosol optical depth) as described in the excellent work done by C.Buil : http://www.astrosurf.com/aras/extinction/calcul.htm
Most Alpy 600 users are equipped with a very limited range of Atik camera (460EX, 428EX, 314L) it could be interesting to share our data to better understand the true instrument response (instrument only) . Starting with this fix point for a given setup and will be then much easier to identify the atmospheric effect. This may be an unrealistic proposal, it will interesting to get advice from the experts…
Anyway, to come back on your AG Dra subject, I did a quick survey of the last 11 low resolution inputs on Aras data base. It shows significant variation (object or measurement variation ?). This bring an other question : how to characterize the measurement error (or accuracy level) of the “instrumental response” ? Based on what I see in that example we may have to consider the possibility of a 10% error level in a large frequency band (3800-7200) with low resolution spectrograph.

Cheers,
Daniel

If you are interested in knowing how accurate your response correction is I suggest testing your equipment by measuring a range of MILES stars with known spectra and comparing them with their known spectra. For example as I did here recently on the BAA forum.
https://www.britastro.org/node/8153
This is the direct link to the report
https://britastro.org/sites/default/fil ... 161105.pdf

Also this earlier exercise which shows the effect of changing telescope focus on the instrument response when using an achromatic telescope
http://www.threehillsobservatory.co.uk/ ... _stars.pdf

Note that if you use a flat correction, the response of the camera, grating and spectrograph optics are all removed by the flat correction. The only contributors to the instrument response are the flat lamp spectrum and anything that happens before the flat lamp. (telescope optics, selective wavelength sampling at the slit (due to chromatic aberration and atmospheric dispersion) and atmospheric extinction. Note also that if you use ISIS, this removes an assumed black body curve for the flat lamp (of 2700K I think) which flattens the instrument response compared with some other programs.

The greatest errors are at the blue end below 4000A which is a difficult area to correct for a number of reasons. (Try rescaling all the AG Dra spectra above to 1 at 5500A. This will help show better where the differences are)
The effects of atmospheric extinction, atmospheric dispersion and chromatic aberrations are greatest in this region. (The sensitivity of the spectrograph and output of the flat lamp are also lowest here which can make the noise worse and fitting the response curve more difficult. A large number of flat lamp spectra should be averaged.)
Selective sampling at the slit is the most difficult to control and the effect can be severe. Christian Buil has a page on this here
http://www.astrosurf.com/buil/dispersion/atmo.htm

I use PHD2 for guiding which naturally moves the target backwards and forwards across the slit. This is bad for guiding and efficiency but good for instrument response as it averages the parts of the star image entering the slit. A better alternative if you are looking for high accuracy in the continuum shape is to use a wide (spectrophotometric) slit (which lets all the light from the star through) to measure the instrument response.

Cheers
Robin

Hi Jaques,

As I understand it, you are using a reference star with a catalogued spectral type A0v, used with a library spectrum of an AOv star (Pickles or MILES). This is risky if you are looking for high accuracy as we do not actually know what the spectrum of your reference star really is. I suggest first measuring your reference star spectrum using a nearby known (eg MILES) star as a reference to see if it really is a typical A0v star. Only then can it be used as a reliable reference for high accuracy work.

Cheers
Robin

Un point important : choisir des étoiles de référence suffisamment faibles pour avoir des poses élémentaires > 20 secondes ( effets de bord sur la fente)

Daniel: AG Dra est en phase calme, on n'attend pas de fortes variations du continuum d'un jour à l'autre

Il est vrai qu'il y a certaines différences dans la base
Mais, dans l'ensemble, c'est correct, voire trés correct !

Le "bon" continuum doit être, à peu près:
J'avais mis dans la précédente lettre une comparaison des spectres de T CrB obtenus par Fran Campos, Tony Rodda, David Boyd, Woody Sims, Jacques Montier
Verdict : parfait !
Ce qui pourrait être intéressant, comme propose Jacques, c'est que les collègues qui ont produit les "spectres de référence" expliquent leur protocole
(Etoile de référence, heures d'acquisition, hauteurs, type spectral ...)

On peut aussi envisager, pour le suivi des symbiotiques, proposer une liste d'étoiles de référence. Une par cible. Un avantage majeur : s'il y a une erreur, c'est toujours la même et elle est facilement corrigeable.






François