SNIFS Observing Sequence
SNIFS Shutter and CCD Readout Sequence
The observing sequence for any science observation needing guiding,
assuming a common shutter is as follows:
- open shutter
- (re)acquire guide star & start guiding until stabilized
- close shutter
- flush all CCD's
- open shutter
- take science observations
- close shutter
- read 3 CCD's simultaneously
- begin next observation
If guiding is not required, then the first three steps can be omitted.
For some calibration observations it is may not be necessary to read all 3
CCD's.
If we wish to allow different exposure times for the spectrograph and the
imager, then the spectrograph should also have a shutter. Then there are
two operational cases to consider:
If (t(imager) < t(spectrograph)):
- open global shutter
- (re)acquire guide star & start guiding until stabilized
- close global shutter
- flush all CCD's
- open both shutters
- take imager science observation
- close both shutters
- read imager CCD
- open global shutter
- (re)acquire guide star & start guiding until stabilized
- close global shutter
- flush imager CCD
- open both shutters
- take second imager science observation
- repeat until end of spectrograph exposure
- close spectrograph shutter
- read 2 spectrograph CCD's simultaneously
- begin next spectrograph observation
If (t(imager) > t(spectrograph)):
- open global shutter
- (re)acquire guide star & start guiding until stabilized
- close global shutter
- flush all CCD's
- open both shutters
- take spectrograph science observation
- close spectrograph shutter
- read 2 spectrograph CCD's
- open spectrograph shutter
- take second spectrograph science observation
- repeat until end of imager exposure
- close both shutters
- read imager CCD
- open global shutter
- (re)acquire guide star & start guiding until stabilized
- close global shutter
- flush imager CCD
- open both shutters
- continue
For a long sequence of observations with difference exposure times,
the ordering of these two cases will eventually
SNIFS Calibration Components
Here is an example of the types of calibration we would like SNIFS to
obtain. In making this list, I've assumed that all options are open.
The amount of calibration might be excessive, but since we won't
be reducing it by hand it can't hurt.
Non-standard calibration which might require some explaination includes:
- Spectrograph monochromatic observations so all the lenslets can be
located on the CCD's.
- Imager/spectrograph ratio observations to track sensitivity drift
between the spectrograph and the imager. This is needed so that
extinction scaling can be performed using stars on the imager.
Types of Calibration:
bias internal
dark internal
imager continuum flats dome/twilight/sky
spectrograph continuum flats dome or internal
spectrograph arcs dome or internal
spectrograph monochromatic dome or internal
spectrograph illumination correction? dome twilight
imager/spectrograph ratio dome or internal
spectrophotometric standard stars open dome
imager photometric standard star fields open dome
Note that use of dome options limits sensitive calibration to night time
unless the dome is very light-tight.
It also limits calibration to times when we have full access to the
telescope and it may limit telescope orientations which can be used/tested.
SNIFS Suggested Observing Sequence
- Afternoon:
- electronics checkout
- bias
- imager continuum dome flats
- spectrograph continuum dome flats
- spectrograph internal arcs
- spectrograph internal monochromatic
- imager/spectrograph ratio
- Sunset - instrument closed:
- dark
- periodic spectrograph monochromatic (for flexure monitoring)
- Usable night just before our time starts - instrument closed:
- routine instrument checkout
- spectrograph internal arcs
- spectrograph internal monochromatic
- imager/spectrograph ratio
- Usable night - instrument open
- star focus with autoguider
- autoguider checkout: N and S offsets - compare with historical values
- imager photometric standard star field - compare with historical values
- spectrophotometric standard star - compare with historical values
- observing loop (see observing script)
for more details)
{
- field pointing
- focus check
- field acquistion
- autoguide (with monitoring of star photometry)
- SN observations
- while(current response close to historical & guide star is steady)
{
- spectrophotometric standard star every 40 to 60 minutes
}
- while(guide star is steady)
{
- spectrophotometric standard star every 60 to 90 minutes
}
- while(guide star unsteady && monitored == FALSE)
{
- monitor spectrophotometric standard star for 10 minutes
taking ~ 10 exposures to sample cloud impact on extinction
- set monitored = TRUE
}
}
- Morning twilight - instrument open
- imager photometric standard star field(s)
- spectrophotometric standard star(s)
- imager dithered twilights
- spectrograph twilights
- Sunrise - instrument closed
Greg Aldering (galdering@lbl.gov)
last updated Feb 28, 2001