1. NIRSpec Time Series Observations
Stephan Birkmann, European Space Agency
on behalf of the NIRSpec SOT
Enabling Transiting Exoplanet Observations with JWST
STScI, July 2017

2. The Near Infrared Spectrograph (NIRSpec)
Built for ESA by Airbus Defence and Space, Germany
Part of Europe’s contribution to JWST (also Ariane 5 launcher, ½ of MIRI, 15 scientist & engineers for operations support)

3. NIRSpec Observing Modes
Target Type
Aperture Mask
MOS: Multi-Object Spectroscopy
Rich fields or extended objects
Selectable from ~250,000
0.2” x 0.46” micro-shutters
IFS: Integral-Field Spectroscopy
Moderately extended objects
3.0” x 3.0” IFU
with 0.1” spaxels
FSS: Fixed Slit Spectroscopy
Single (compact) object
0.2 x 3.2 slits (3)
0.4 x 3.65 slit
1.6 x 1.6 aperture
BOTS: Bright Object Time Series
Transit/eclipse spectroscopy
NIRSpec’s Focal Plane Assembly (FPA) consists of two closely spaced HAWAII-2RG sensor chip arrays with 5.3 mm cut-off wavelength and ~100 mas pixels on the sky

4. NIRSpec Aperture Plane
Detector gap
S200A1
S200A2
S400A1
S1600A1
S200B1
(backup only)
S1600A1:
(1.6” ☐) aperture for small slit losses and good radiometric stability
small enough that background and “spoiler” sources are not an issue

5. NIRSpec Spectral Configurations
All dispersers are available in all NIRSpec modes
R ~ 1000
R ~

6. NIRSpec BOTS Observations
Bright Object Time Series Observations have dedicated template in Astronomer’s Proposal Tool (APT)

7. NIRSpec BOTS Observations
After Template is selected, remaining options become available:
Target Acquisition (TA filter, readout pattern, subarray)
Science Parameters (subarray, disperser/filter, exposures, readout mode, number of groups and number of integrations)
For more details on BOTS template (and all others) refer to the JWST user documentation at

8. NIRSpec BOTS Observations
Target selection is next. Visit splitting distance will depend on target position (availability of guide stars)

9. NIRSpec BOTS Observations – Subarrays
Size
Frame Time
Comment
SUB2048
32 x 2048 s
Full wavelength coverage for gratings
SUB1024A
32 x 1024 s
Partial coverage for gratings (low/hi)
SUB1024B
Partial coverage for gratings (center)
SUB512
32 x 512 s
Full wavelength coverage for prism
SUB512S
16 x 512 s
No pixels outside aperture

10. NIRSpec BOTS Observations – Disperser/Filter
1.27 mm
0.97 mm
1.87 mm
1.66 mm
3.12 mm
2.87 mm
5.18 mm

11. NIRSpec BOTS Saturation Limits
Use the ETC ( to check for saturation of your target
Sources below lines can be observed without saturation anywhere in science band
SUB2048
SUB512

12. NIRSpec BOTS Observations – Exposure Setup
Number of exposures: use 1 single exposure with many integrations (max 65,535) if possible (best for detector stability); should be possible for all but the longest observations of very bright targets with the PRISM using SUB512(S) subarrays

13. NIRSpec BOTS Observations – Exposure Setup
Readout pattern: use NRSRAPID Groups/Int: governs integration time (together with subarray and readout pattern); use ETC to determine number of groups before saturation Integrations/Exp: use as many as needed to cover full event, plus ~15 minutes (detector stabilization) For exposure lengths >10,000 sec will get warning (High Gain Antenna might move during exposure); maximum exposure duration is sec (2 days)

14. NIRSpec BOTS Data Many integrations (up to 65,535) per exposure
One spectrum per integration
PRISM/CLEAR example (SUB512 subarray, ground testing):
G235H/F170LP example (SUB2048 subarray, both detectors):
0.6 mm
1.0 mm
2.0 mm
3.0 mm
3.5 mm
4.0 mm
4.5 mm
5.0 mm
5.3 mm
1.7 mm
1.8 mm
1.9 mm
2.0 mm
2.1 mm
2.2 mm
2.3 mm
2.4 mm
2.5 mm
2.6 mm
2.7 mm
2.8 mm
2.9 mm
3.0 mm

15. NIRSpec BOTS Data (Ground Test)
600 integrations per temporal bin – 15 bins
Data taken during ISIM CV3 campaign
9000 integrations with 3 groups in SUB512 and PRISM/CLEAR (~0.9 s per integration)
OSIM source showed jitter and drift (~5 times higher than expected in orbit) and some flux variation
After correction, came down to expected noise floor (photon + readout noise model)
Systematic noise sources (due to e.g. intra-pixel variations, detector persistence) likely <200 pm

16. NIRSpec BOTS SNR Estimates
SNR per spectral pixel for 1 hr transit – 2 hr baseline (3 hour total)
Use PandExo ( to do your own SNR calculations
See also APT and PandExo presentations by Denise Taylor and Natasha Batalha

17. Target Acquisition APT 25.2
NIRSpec is very sensitive in imaging mode (used for TA)
Targets brighter than ~12 ABmag (~11 Jmag) are too bright for TA
APT 25.4 will support Wide Aperture Target Acquisition (WATA) for NIRSpec:
will allow use of fainter reference sources (~12 to ~26 ABmag) for TA
Reference target will need to be within visit splitting distance (same guide star)
Use the ETC ( to determine
suitable filter / subarray / readout pattern for TA

18. Timing BOTS (e.g. Transit) Observations
Timing constraints are specified using special requirements

19. Summary
NIRSpec offers a dedicated template for (bright object) time series observations
0.6 to 5.3 mm wavelength coverage with different spectral resolutions
For fainter targets (>10.5 Jmag), PRISM covers full range in one go
Bright targets (~6-7 Jmag) can be observed with the high resolution gratings and acquired using a reference target