• TA = target acquisition
  • TSO = time-series observations
  • FoV = field of view
  • ETC = JWST exposure time calculator
  • WATA = wide-angle target acquisition

Discussion items

General UpdateKevin
  • See slides titled "TrExWG-2018-01.pdf"
  • TA was recently brought up as a concern for TSOs of bright targets and how STScI would handle proposals/targets for which TA was not feasible
  • WG performed study of TA limits for all four instruments
    • NIRISS: no concerns, can always perform TA on science target
    • NIRCam: can achieve 0.1 pixel pointing accuracy with 4 partially and 1 fully saturated pixels (K = 6.1), brighter objects may need to use offset TA or saturate additional pixels with a decrease in pointing accuracy (~1 pixel)
    • NIRSpec: most objects will need to use offset TA, but brightness range is 11 < K < 25 therefore finding a suitable offset target should not be an issue
    • MIRI LRS: no concerns with LRS mode, can always perform TA on science target
    • MIRI Photometry: no TA for MIRI photometry, blind pointing accuracy (4 pixels at 1 sigma) is well within FoV for smallest subarray (SUB64)
  • Conclusion from TA study is that all science targets should have a viable means to perform TA
  • Potential improvements for Cycle 2
    • Narrow filter for NIRCam, fewer groups for NIRSpec, improved TA on saturated targets (software), micro-shutter assembly TA for NIRSpec
  • Other highlights include...
    • MIRI TSO photometry enabled for Cycle 1, includes full array and all subarray modes
    • Developed strategy to provide timestamps for each integration since time is recorded once every science (full frame) image, will create FITS extension with BJD_TDB times
    • Finalized "chunking" strategy for large datasets, exposures will be evenly divided into segments of <= 2 GB each; will enable faster processing of data and easier sharing on MAST
  • Everyone should update their local version of PandExo
 NIRCam TA with saturated pixelsBryan
  • See slides titled "TA_with_saturated_pix.pdf"
  • Tested TA accuracy for various levels of saturation with NICam
  • Range: K = 3.3 - 7.3 in 0.5 mag intervals
  • Pixel scale = 0.065″/pix
  • Fully saturated: saturates in Group 1
  • Partially saturated: saturates in Group 2 or 3
  • Can achieve <1 pixel pointing accuracy with up to 8 fully saturated pixels (K~3.5)
  • The "desirement" is for 0.1 pixel TA accuracy, which is possible without saturating any pixels, but we cannot specify a requirement until we're on sky and can evaluate the impact of systematics
  • Proposers should evaluate how their science goals might be impacted by reduced (~1 pixel) TA accuracy for very bright targets before determining their preferred TA strategy (offset vs saturation)

ETC vs PandExoJonathan
  • See slides titled "Comparisons_between_Pandeia_and_PandExo.pdf"
  • ETC and PandExo produce consistent results (SNR) when the # of integrations is 1
  • ETC produces a significantly lower SNR (compared to PandExo) when the # of integrations is large
  • The is because the ETC includes the flat field uncertainty for every integration, which is correct for non-TSO data, whereas TSOs are a relative measurement that don't depend on a precise flat fielding, so long as the pointing jitter is small
  • ETC and PandExo do agree when we use the ETC SNR for 1 integration and then bin down assuming "root N" statistics
  • CONCLUSION: We should encourage people to use PandExo for their TSO planning

  • See slides titled "jan18_updates.pdf"
  • APT has gone through a series of updates over the past few months
    • NIRCam: max NGROUPS for TA = 65
    • NIRSpec: optional TA, added WATA
    • MIRI: optional TA for LRS
    • NIRISS: only NISRAPID permitted in SOSS template
    • ALL: TSO special requirement added, NINTS <= 65535, warning (not error) for exposures > 10,000s
  • Ran out of time to discuss JDox


Action items