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