[because pan-STARRS is not accessible for me, I observe some faint comets presently visible.
The spectra are taken in urban conditions (the surfacic magnitude of comets is only a very small fraction
of sky background). The telescope aperture is also modest (200 mm) but luminous (f/3.9).
The result show only very discrete comet signature (SNR = 4 - 6). But with a 400 mm f/4 telescope in darkness,
the Alpy spectrograph (also LISA probably) can be a comet killer in the range V=13-16, or more faintest - an ideal
configuration - really ! ]
La comète pan-STARRS étant définitivement impossible depuis mon obervatoire (urbain) je me
suis rabattu sur d'autres objets de ce type, mais bien plus faibles...
C''est une observation difficile pour moi car je travaille dans un milieu très pollué (de plus en plus !) par des lampadaires et
que pour réussir, j'ai utilisé le spectrographe Alpy 600 (toujours en phase de test) sur un télescope
extrémement lumineux, ouvert à f/3,9 (un CN212 Takahashi). Du coup, le fond de ciel monte
vite dans l'image malgré l'usage d'une fente étroite.
Pour donner une idée, la brillance surfacique de la comète C/2011 J2 (LINEAR) - voir ci-après -
represente moins de 5% de la brillance du fond de ciel. L'objet est complétement perdu dans
le fond parasite.
En plus, le télescope utilisé est de petit diamètre, 200 millimètres, donc cet un handicap
supplémentaire. Aussi, ll faut être indulgent devant le résultat présenté
A présent j'extrapole sur ce résultat. Une configuration du genre Alpy 600 sur
un tube Newton du type Dobson de 400 mm à f/4 et placé sous un ciel, doit être une
configuration proche de la perfection (couplage surement idéal spectro/télescope).
Celui ou celle qui aura la possibilité de mettre une telle configuration va faire un malheur.
Les comètes dans le domaine de magnitude 13 à 15 sont alors des objets faciles, je le garanti.
C'est un autre pan de la spectrographie amateur qui s'ouvre.
En attendant voila ce que j'obtient depuis Castanet-Tolosan (voyez les images de guidage
pour apprécier la faiblesse des objets par rapport au fond de ciel), avec en plus
un fort vent d'Autan.
D'abord C/2006 J3 (LONEOS) vers la magnitude 13 :
Puis C/2011 J2 (LINEAR) vers la magnitude 15 :
Une fois de plus, il n'est pas fait justice ici au spectrographe Alpy 600 du fait de
la situation de l'observatoire et du télescope utilisé (diamètre). Ce type d'objet
est aussi à la portée d'un LISA je pense.
Christian
Faint comet observations
-
- Posts: 1431
- Joined: Mon Sep 26, 2011 6:59 pm
- Contact:
-
- Posts: 191
- Joined: Thu Sep 29, 2011 7:40 am
Re: Faint comet observations
Christian.
Very nice.
How are you guiding the comets?
I am unable to guide on dim objects like these on the slit so have to use a field star. The comet is moving compared to the field star so will move off the slit.
Do you just hope for the best and reposition the slit after each exposure or try to guide on the comet itself?
Cheers
Terry
Very nice.
How are you guiding the comets?
I am unable to guide on dim objects like these on the slit so have to use a field star. The comet is moving compared to the field star so will move off the slit.
Do you just hope for the best and reposition the slit after each exposure or try to guide on the comet itself?
Cheers
Terry
Terry Bohlsen
Armidale NSW
Australia
Armidale NSW
Australia
-
- Posts: 1431
- Joined: Mon Sep 26, 2011 6:59 pm
- Contact:
Re: Faint comet observations
Terry,
I guide on a field star (2 seconds exposure with
a Atik314, nearly V=11 mag.), but... many time, during the elementary 10 minutes exposure, I adjust
the guiding crossair by checking the comet position on the slit with 30 seconds
exposure (standard imagery mode with Audela software).
It is a very manual procedure (it is possible also to compute the
theoritical comet trajectory and correct the autoguider target position,
a possible procedure with Audela).
Christian
I guide on a field star (2 seconds exposure with
a Atik314, nearly V=11 mag.), but... many time, during the elementary 10 minutes exposure, I adjust
the guiding crossair by checking the comet position on the slit with 30 seconds
exposure (standard imagery mode with Audela software).
It is a very manual procedure (it is possible also to compute the
theoritical comet trajectory and correct the autoguider target position,
a possible procedure with Audela).
Christian
-
- Posts: 321
- Joined: Fri Sep 30, 2011 12:23 pm
Re: Faint comet observations
Christian - Fine work in capturing these results. I know I am fairly new to all this but I find it imposible to tell what is signal and what is noise in such spectra. Don't we need to start estimating and including with the spectragraphs the signal to noise ratio?
EDIT - I just noticed you had estimated the S/N at 4 - 6 so apologies for missing that. However, at that level how do the spectrographs differ from normally distributed noise? I experimented with plotting such noise and looking if I can't see much difference and I must say I am at a loss to see much difference apart from an increase in the S/N at longer wavelength and some very broad shift. Please don't take this as a critisism of your work I am just trying to get a better understanding.
Andrew
EDIT - I just noticed you had estimated the S/N at 4 - 6 so apologies for missing that. However, at that level how do the spectrographs differ from normally distributed noise? I experimented with plotting such noise and looking if I can't see much difference and I must say I am at a loss to see much difference apart from an increase in the S/N at longer wavelength and some very broad shift. Please don't take this as a critisism of your work I am just trying to get a better understanding.
Andrew
Last edited by Andrew Smith on Wed Apr 17, 2013 8:10 pm, edited 2 times in total.
-
- Posts: 191
- Joined: Thu Sep 29, 2011 7:40 am
Re: Faint comet observations
Christian
That makes sense. I use CCDSoft for my guiding as Audela won't allow me to use the direct guider connection for my STi guider. I could do a similar thing with CCDSoft.
Cheers
Terry
That makes sense. I use CCDSoft for my guiding as Audela won't allow me to use the direct guider connection for my STi guider. I could do a similar thing with CCDSoft.
Cheers
Terry
Terry Bohlsen
Armidale NSW
Australia
Armidale NSW
Australia
-
- Posts: 578
- Joined: Thu Sep 29, 2011 10:51 pm
Re: Faint comet observations
Christian, these are incredible results! Comets, although small, are extended objects (it's clearly discernible in the guiding camera frames). Push the limit under dark OHP sky this summer! Many compliments to you, Francois and Olivier for Alpy 600, a little jewel that I hope will contribute to spread spectroscopy within amateurs.
Concerning present observations, I note both comet spectra are very similar (apart a different SNR). It would be interesting to understand the origin of the most evident features. I wonder if, far from the sun, comets show a spectrum rather different from that we observe when they are close.
Ciao
Paolo
Concerning present observations, I note both comet spectra are very similar (apart a different SNR). It would be interesting to understand the origin of the most evident features. I wonder if, far from the sun, comets show a spectrum rather different from that we observe when they are close.
I am convinced that you'll observe it very soon! The comet will move toward north pole (all night visible) and its faint magnitude not seem will be a problem for you!because pan-STARRS is not accessible for me, I observe some faint comets presently visible
Ciao
Paolo