CFHT, Instruments, QSO, ESPaDOnS

Queued Service Observing with
ESPaDOnS: an Echelle SpectroPolarimetric Device
for the Observation of Stars at CFHT

Saturation and Exposure Times for EEV1 and Olapa

Althought many parameters influence the final flux recorded by ESPaDOnS (Image Quality, airmass, fiber alignment for a given run, quality of guiding, star's color), CFHT has established guidelines to prevent saturating the CCD (at 65,000 ADUs). The detector is linear up to the saturation point, for all readout speeds, but saturated exposures may cause difficulty during data reduction.

Here are actual numbers for individual exposures taken with EEV1 (not for 4-exposure polarimetric sequences) from past science exposures. These are maximum exposure times recommended for EEV1; for the new chip Olapa, the maximum exposure times are about half. Under exceptional conditions, the exposure times given here might actually saturate the detector!

A star of mag V... and individual
exposures of...
gets a peak S/N
(for that single exposure)
per spectral/CCD bin
up to
with EEV1
and max ADUs per pixel
with EEV1
-1.5 (Sirius) 2s 980/1175 58,000
0.0 (Vega) 4s 820/990 48,000

For Olapa, the numbers are about half of those indicated in the table above, or about 1s maximum exposure time for Sirius and 2s for Vega.

Using the Vega number above, conservative exposure times for EEV1 (which keep individual pixels below a safe 40,000 ADU level) are presented in the next table (by simply multiplying the exposure time by 2.5 for each increment in magnitude):

A star
of mag V
Will saturate on EEV1
with individual exposure times
Will saturate on Olapa
with individual exposure times
5.0390s (6.5min)195s
6.0976s (17min)488s
7.02440s (41min)1220s
8.06100s (102min)3050s

For various reasons (cosmic rays, overall strategy), single exposures above 2400s are not recommended).
Back to the QSO Mode page for ESPaDOnS
This CFHT Web page is maintained by Nadine Manset (manset -=AT=-