Spatial scanning of stellar spectra upon the IR detector creates the potential for spectrophotometry of unprecedented precision. By spreading a stellar spectrum perpendicular to its dispersion (Figure 8.8
), more photons can be collected per exposure, and the exposure times can be longer without saturating the detector. The most prevalent scientific application is transit spectroscopy, in which a time series of stellar spectra are obtained before, during, and after an exoplanet transit or eclipse; examples are programs 11622, 12181, 12325, 12449, 12473, 12495, 12881, 12956, and 13021.
Results for exoplanet transit spectroscopy have been published by numerous investigators; examples include Knutson et al. 2014
,Kreidberg et al. 2014
and McCullough et al. 2014
. Knutson et al. 2014
report the most precise observations of this type as of late 2014: one-sigma precision of ~20 ppm in ~20 nm spectral bins from two transits, which suggests the mnemonic "20:20 with two eyes".
Potential benefits of spatial scanning are 1) reducing overhead for time-series of short exposures due to detector operations required before and after each exposure, 2) avoiding saturation for very bright stars, 3) improved spectrophotometry due to collecting more photons per HST
orbit. We note that the main disadvantages are 1) STScI pipelines do not work on spatially-scanned IR data so the observer will need to reduce IR data themselves even to produce simple images, and 2) astronomical sources will overlap more often than with staring-mode observations, especially for spectra.
The scan rate can be any real number between 0.0 and 7.84 arcsec s-1
. Without FGS control, referred to as “gyro control”1
, rates as high as 7.5 arcsec s-1
have been demonstrated. Under fine-guidance-sensor (FGS) control, rates between 0.0 and 4.8 arcsec s-1
are supported for exposures with a single scan line. Due to a software limitation, boustrophedonic (serpentine) scans at rates greater than 1 arcsec s-1
must be executed under gyro control.
In WFC3 ISR 2012-08
, McCullough and MacKenty recommend scan rates for WFC3 IR spectroscopy of bright stars and include formulae to predict appropriate scan rates. For stars with H-band apparent brightnesses fainter than H = 4.1 mag, G141 spectra can be unsaturated with scan rates less than or equal to 4.8 arcsec s-1
, which is the maximum achievable under FGS control. FGS control is recommended for time-series applications such as exoplanet transit spectroscopy, in order to keep the spectrum from drifting on the detector from one scan to the next, during an HST
orbit. For stars in the range 4.1 > H > 3.7 mag, WFC3 G141 spectra can be obtained with less than 25,000 DN/pixel only under gyro control, i.e. with rates between 4.8 and 7.8 arcsec s-1
. For stars brighter than H = 3.7 mag, spectral orders other than the +1st must be used to avoid saturation of the IR detector; e.g., observations of Vega in Visit 1 of HST
calibration program 12336 used the -1st order (WFC3 ISR 2014-15
), thus demonstrating that stars as bright as V~H~0.0 mag can be observed.
APT provides a diagram to assist observers planning spatial scan observations, shown in Figure 8.9
. The line connecting the green and red arrows corresponds to where the target’s direct image would appear. In grism observations, the first order spectrum appears at larger X coordinate values than the direct image (see Figures 8.4
). By design of the GRISM128, GRISM256 and GRISM512 subarrays, a POSTARGX = 0 centers the first order spectrum in each subarray in the X coordinate. Because the diagram is only approximate and because of on-going developments in APT and spacecraft operations, users of fast spatial scans (~1 arcsec/sec or faster) should consult with their contact scientist to optimize the POSTARGY value.