RR Lyr campaign 2013

Time resolution capacity of Alpy 600 spectrograph on a small telescope

Christian Buil - Castanet-Tolosan Observatory

For a presentation of 2013 RR Lyrae photometric and spectrometric campaign, see:

See also 2012 Wiki page: http://rr-lyr.ast.obs-mip.fr/dokuwiki/doku.php?id=rrlyr2012

and my personal 2012 spectrographic observations report: http://www.astrosurf.com/buil/rrlyr2/rrlyr_campaign.htm

Setup used for the present evaluation

Telescope : Takahashi CN212 used at the direct f/4 newton focal plane (212 mm aperture)

Spectrograph : Alpy 600 (+ guiding module + calibration module) -
see hee for more details

CCD camera : Atik 460EX model

Observatory : Castanet-Tolosan near Toulouse town, suburban conditions

Comparison of a sequence of three RR Lyr spectra taken with a time resolution of 2 x 150 seconds. The camera is used in 2x2 bining mode
(9.08 microns equivalent pixel size)


Detail of blue and UV spectral region.

Comparison of a sequence of 8 RR Lyr spectra taken with a time resolution of 300 seconds. The camera is used in 1x1 binning mode
(4,54 microns pixel size)


Detail of blue and UV spectral region.

Comparison of bin 2x2 and bin 1x1 spectra of star HD184875 (type A2V, reference star recommended for spectrographic campaign)
Note the better resolved UV region in bin 1x1 mode.


The spectra obtained in 1x1 binning are noisier than in 2x2 binning, but the difference is quite small. The measured SNR in 1x1 binning mode is 175. It is 200 in 2x2 binning mode. All for a 300 seconds equivalent exposure.

The sampling noise is stronger in 2x2 binning, which can cause false alarms.

With Alpy 600 specrograph , I recommend for this campaign to use the camera in binning 1x1 mode (4.54 micron pixel for an Atik460EX camera, 6.45 microns pixels for an Atik314L + camera, for example).

Note that the spectrograph Alpy 600 can observe in the ultraviolet at least to 375 nm, while the exposure time is relatively short (300 s), the star is faint (V = 7.5) and the telescope diameter is very modest (D = 200 mm).