NIRSpec Operations Concept

Michael Regan(STScI), Jeff Valenti (STScI)

Wolfram Freduling(ECF), Harald Kuntschner(ECF), Robert Fosbury (ECF)

NIRSpec Optical Layout

Fore-optics CollimatorCamera

Micro-Shutter Array Grating/Prism/Mirror

WheelDetector

ArrayFilter

Wheel

Pick-offOptics

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 x, y, and roll• Offset pointing and roll to correct location

Contemporaneous Calibrations• After target acquisition • Switch to a long pass filter • Configure MSA for observation• Take a short direct image

– This will help pipeline processing

• 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

T2 T2 T2 T2T2

Samples

Groups

Reset

TIME

Sig

nal

Lev

elBaseline Readout Mode

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