AT 2108hld spectrum - a probable nova in M31

Alerts and Monitoring of Novae
etienne bertrand
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Re: AT 2108hld spectrum - a probable nova in M31

Post by etienne bertrand »

Hello Robin,

Why you don't trry exposure 1200s or 1800s for have more flux ?
Robin Leadbeater
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Re: AT 2108hld spectrum - a probable nova in M31

Post by Robin Leadbeater »

Hello Etienne,

Yes 1200s exposures might have helped a bit with such a weak signal. The total flux stays the same but the total read noise would be a bit less (14e- compared with 20e- assuming 16x 5e- read noise.

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
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Re: AT 2108hld spectrum - a probable nova in M31

Post by Robin Leadbeater »

Robin Leadbeater wrote: Yes 1200s exposures might have helped a bit with such a weak signal.
I decided to calculate the noise contribution from each source (EDIT Aaargh! why does the forum software remove the spaces in the table :roll: )

counts/bin e- noise e- %
signal 300 17.3 21
sky 700 26.5 50 (includes M31 background and local sky pollution)
read noise 20 28
thermal 13 3.6 1

total noise (RMS) 37

SNR = 8.1

Taking the same total exposure in one exposure would increase SNR to 9.4
Doubling the total exposure time (to 5hrs 20 min) would increase SNR to 12.2

Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Rainer Ehlert
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Re: AT 2108hld spectrum - a probable nova in M31

Post by Rainer Ehlert »

Robin Leadbeater wrote:Hi Rainer
Rainer Ehlert wrote: Is it visible through the imaging camera or the guide camera ? or do we have to find it using the coordinates and pointing at all the bright dots until we see a similar spectrum like you posted.
I use the goto controls (eqascom + Cartes de Ciel) to go to the coordinates published for the discovery. I check that I am in the right place by comparing the star field in the spectrograph guider image with DSS images and sometimes also by checking using astrometry.net
I then take a series of 10 second exposures from the guider image and add them to identify the "new star" target. (see attached image)
With a target this faint, I cannot see it in short exposures while guiding so I choose another star in the field bright enough to guide on and measure the offset of that guide star from the target.
I then calculate where I need to place the guide star so that the target will be on the slit and start guiding with the guide star at that position. (It is always a relief when the first spectrum appears 10 minutes later !)

Cheers
Robin
Hi Robin,

Thank you for the explanation.

saludos Rainer
regards Rainer____Observatorio Real de 14_____MÉXICO_____N 22° W 101°
Robin Leadbeater
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Re: AT 2108hld spectrum - a probable nova in M31

Post by Robin Leadbeater »

[
Robin Leadbeater wrote:
I decided to calculate the noise contribution from each source
Actually that calculation is wrong. I only included 1 pixel of read and thermal noise instead of the full height of the binning zone so the contribution of read noise is more like 60%. This means taking only 1 exposure instead of 16 would have improved the SNR per bin significantly from ~5.9 to ~9.3

Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
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Re: AT 2108hld spectrum - a probable nova in M31

Post by Robin Leadbeater »

Now confirmed as an Fe II nova by Wiliams et al from a spectrum taken using the 2m Liverpool Telescope.

http://www.astronomerstelegram.org/?read=12138

(My spectrum taken a day earlier is also referenced there)

Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
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