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

Ops Concept: What is it? Explain how the instrument will be used –What are the observing modes? –What types of calibrations are required? –How are observations planned? –What are the actions required to perform an observation? –Which actions are performed in flight software, flight hardware, ground planning software, and ground pipeline software? –How often will mechanisms be used?

NIRSpec Optical Layout Fore-opticsCollimator Camera Micro-Shutter Array Grating/Prism/Mirror Wheel Detector Array Filter Wheel Pick-off Optics

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(12mas 2 -pointing_error 2 )/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.

Image Stability Around 1/3 of the science will be one day per grating selection –Need to be stable on this time scale –Otherwise, will have to reacquire and recalibrate –Spacecraft roll about FGS star will need to be stable to within ~1.7” per day Smaller due to larger radius to FGS star

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

Correcting for MSA and Cosmic Ray Effects Two method for getting required positional precision –One object, multiple dithers, (slower but more flexible) –Multiple objects, one dither, (faster but more restrictive) At each location take acquisition image (up-the-ramp,3) –Form two differences (1-0, 2-1) –Flat field two differences? –Take minimum pixel value (CR reject) Add mirror offset and acquisition offset Pass delta pix to FGS Repeat if accuracy is not good enough.

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 Samples Groups Reset TIME Signal Level Baseline Readout Mode

T2 Samples Groups Reset TIME Signal Level Alternative Readout Mode (depends on noise characteristics of flight electronics & detector)

Readout Parameters Time between storing of groups on SSR = 50 sec Samples per group = 1,4 Number of groups = exposure_time/50

Other parameters Sub-array readout –Minimum 12 second exposure time is too long for many sources –Sub-array readout will be needed –Only one sub-array at a time –Readout time = 12*(number of pixels in subarray/8 million) seconds

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 K 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

Mission Lifetime Usages Assumptions –5 year lifetime –NIRSpec is used 50% of the time –70% of the time we are doing science –All observations are multi-object MSA spectroscopy

Type of projects TypeFraction of NIRSpec Time Average Time for one visit (sec) Number of visits per year Short0.103K470 Medium0.6020K430 Long K43

Usage for each visit TypeFilter Wheel Grating Wheel MSALamp Short7484 Medium6343 Long5242

Total Usage TypeFilter Wheel Grating Wheel MSALamp Short5*370*95*370*45*370*85*370*4 Medium5*330*75*330*35*330*65*330*3 Long5*33*55*33*25*33*45*33*2 Total29K13K25K13K