Instrument Observation Strategy

1
Instrument Observation Strategy

• Chris Bebek
• Lawrence Berkeley National Laboratory
• 10 July 2002

2
Photometry illustration

• Color
• K correction1
• Photo z
• Classification2

Flux
U B V R
Wavelength
Light curve S/N requirements
R
V
1z spaced B-band filters
B
1A. Kim 2J. Lamoureux
3
Spectroscopy illustration
Metallicity
SII W
SiII
4
Fixed filter concept
Note the ability to weight integrated exposure
time in a particular filter.
5
Focal plane gymnastics
6
Focal plane concept
7
Observation concept

• Repetitive program
• Step the focal plane through the observation
  field.
• N steps in each CCD filter 2N steps in each
  HgCdTe filter.
• Fixed length exposures determined by a shutter.
• The multiple exposures per filter are used
• to implement dithering/drizzle
• to eliminate cosmic ray pollution.
• S/N assists
• NIR filters have twice the area of visible
  filters
• time dilation gives more points on the light
  curve for increasing z.
• All stars see all filters (modulo scan field edge
  effects).
• Fields revisited with fixed cadence, 4 days.
• SNe automatically discovered and their evolution
  followed for 100s of days.

8
Observation concept

• Targeted program
• SNe candidates are scheduled for spectrographic
  measurement near peak luminosity.
• Light curve and color analysis done on ground to
  identify Type Ia and roughly determine z.
• Note peak luminosity is 14 days to 40 days after
  discovery for z 0.3 and 1.7 respectively.
• Star is steered into spectrograph port.
• Imager exposures can be taken during spectroscopy
  albeit with longer exposure times and
  non-standard orientation to the observational
  fields.

9
Signal-to-noise

• Input
• SNe luminosity as a function of z and epoch.
• Zodiacal background
• Detector specs
• Sum noise and signal over 4 effective pixels
• Solve for S/N
• Vary exposure time
• Number of exposures.
• Find 300 s exposures with 4 in the CCDs and 8 in
  the HgCdTe meet S/N requirements.

10
SNe flux
CCD HgCdTe
11
Zodiacal background light
12
Imager Sensors Specs
13
S/N achieved
Requirement
S/Ngt30
S/Ngt30
S/Ngt20
S/Ngt15
S/Ngt10
S/Ngt3
Dmag attenuation
14
SNe yield

• Input
• Spectroscopy time as a function of z
• SNe production rate
• Focal plane scan strategy
• 4 day cadence
• Time allocation 60 photometry, 40 spectroscopy
• Plot SNe yield per year in 0.1 z bins

15
Spectrograph Sensors Specs
With these specs, total integration time (w/ 1000
s exposures) is
16
SNe production rate
17
SNe yield
SNe discovered SNe w/ spectroscopy
2200 SNe discovered in 7.5 sq. deg. 1600 w/
spectroscopic follow-up
18
Summary

• This tool can be turned around by fixing the S/N
  at a particular z and fixing the exposure time
  and number of exposures to establish margins on
  detector performance specs.
• Better left to advanced modeling and simulation
  tools that make use of light curve fitting
  note, the study shown synthesizes S/N points on
  the light curve at the required epochs.
• This Excel spreadsheet study is superceded by
• Fisher matrix approach of Gary Bernstein
• SNAP simulators
• The simulator is best used for the specification
  margining to establish detector goals and
  requirements.

19
(No Transcript)