Transcript NIRSpec Operations Concept Michael Regan(STScI), Jeff Valenti (STScI) Wolfram Freduling(ECF), Harald Kuntschner(ECF), Robert Fosbury (ECF)

NIRSpec Operations Concept
Michael Regan(STScI), Jeff Valenti (STScI)
Wolfram Freduling(ECF), Harald Kuntschner(ECF),
Robert Fosbury (ECF)
NIRSpec Optical Layout
Filter
Wheel
Micro-Shutter
Array Grating/Prism/Mirror
Wheel
Pick-off
Optics
Fore-optics
Collimator
Camera
Detector
Array
Target Acquisition
• Need to have maximal light from science targets
going through all the slits formed by shutters
– This requires getting both the correct pointing and the
correct roll
– After acquisition both the pointing and the roll must
be held relatively constant throughout the observation.
Target Location Tolerance
• Assure that the ensemble throughput is not
reduced by more than 10% for 95% of the
observations
– Leads to a two sigma error of 25 mas.
– Therefore, one sigma we must be within 12 mas of
desired location.
– Both pointing and roll errors contribute to this error
How do they interact?
• Sin(roll_error) < sqrt(12mas2-pointing_error2)/100”
Roll Angle Acquisition
• User will be given a range of roll angles after visit has
been preliminarily scheduled
– User will select a roll and design their shutter mask
– Chosen roll angle and shutter mask will be put into visit file
– Spacecraft will use star trackers to move telescope to
required roll angle
Positional Acquisition
• Uncertainties in the locations of stars in the
GSC2 are much larger than the required
(<10mas)
– Have to take acquisition image to get an offset to the
correct location.
Microshutter Grid and Point Source
Location
• Microshutter grid will lead to
biases in the centroid of a point
source ~14mas.
– More sophisticated
algorithms can reduce this
• Only by dithering one source or
using multiple reference objects
can this be averaged out.
• With 9 targets get final error of 5
mas.
Roll requirement
• With a 5 mas positional uncertainty
– Allowed roll error is ~15 arcseconds
• Even with perfect positional accuracy
– Allowed roll error is ~20 arcseconds
• Note that this error includes the user’s
uncertainty in being able to determine the
required roll angle
• Therefore, for now, we are assuming
that roll will need to be adjusted.
Steps in a Target Acquisition
• Assume wheels at home locations or move them:
– filter wheel at closed location
– grating wheel at mirror location
• Turn on calibration lamp
• Take image of MSA plane (uncertain mirror location)
• 1D – Centroid each fixed slit
– Store away the difference between expected and
actual position
• Turn off lamp
• Open all MSA shutters [except those around bright
objects in field]
• Move filter wheel to requested acquisition filter
• Take acquisition images and centroid
• Find Dx, Dy, and Droll
• Offset pointing and roll to correct location
Contemporaneous Calibrations
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After target acquisition
Switch to a long pass filter
Configure MSA for observation
Take a short direct image
– This will help pipeline processing
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Switch to requested grating/prism
Switch to closed filter wheel
Turn on emission line lamp
Take a wavecal image
Turn off emission line lamp
Switch to filter wheel long pass filter
Begin science exposures
Detector Operations
• NIRSpec will be detector noise limited in
R>1000 modes
• Up-the-ramp/Multiaccum sampling has been
shown to be better than Fowler for detector noise
limited observations
• In addition, up-the-ramp sampling is more robust
against cosmic rays
Signal Level
Baseline Readout Mode
Reset
Samples
T2
T2
T2
T2
T2
Groups
TIME
Electronic Gain
• Goal is to have only one gain setting for
NIRSpec
– Maximum gain is set by Nyquist sampling single
sample read noise (~9e-) or ~4 e-/ADU
– Would like to be able to use entire full well ~90K -200K e– 16 bit A/D values lead to 64K dynamic range
– Saturated values can be reconstructed from early
reads in up-the-ramp.
– A single gain of 1.5 e- to 2.5 e- will work
Calibration
• Assumptions
– NIRSpec will have internal line and continuum
sources
– Line sources will reach required S/N is a 60 sec
exposure
– There will be NO parallel calibration
• Although it should not be ruled out
– Wavelength zero point calibration are required every
time the grating wheel is moved.
– MSA-to-detector calibration is required every time
the mirror is moved in.
Monitoring Calibrations
• Two types
– Parallel Capable (do not require dedicated visit)
• Dark current/read noise/gain
• Hot pixels
• Shutter throughput
• Fixed slit throughput
• Small scale flat field variations
– Dedicated (frequency depends on stability of detectors and
geometry of optical bench)
• Linearity
• Persistence
• Geometric distortions
• Large scale flat field
• Wavelength solution