July 2021 STAN
This STAN offers a set of tips and tricks for preparing and submitting Phase II Proposals ahead of the Cycle 29 Phase II deadline. Additionally, this STAN also provides an article on the defringing of G750M/G750L spectra, offering several examples of the new Python defringing tools applied to STIS spectra.
Submitting STIS Phase IIs: Tips and Tricks
The Phase II files submitted by Guest Observers (GOs) are the detailed descriptions of the observations to be carried out for each accepted program. Before any planned observation is executed, it must first undergo a technical feasibility review and, for programs using one of the STIS MAMA detectors, a safety review. Providing incomplete information increases the time and resources needed for STScI to clear observations for scheduling and could ultimately result in losing scheduling windows.
Health and Safety:
Because the MAMA detectors can be irreparably damaged by overlight conditions, it is vital to consider the contribution of all ultraviolet (UV) light sources that will or could illuminate the detector. Detector safety must be proven for (1) the science target, which must address the maximum UV brightness of time-varying sources, (2) physically associated objects (e.g., hot companions to cool stars or unresolved UV bright regions in galaxies), and (3) incidental field sources.
Information for points (1) and (2) should already be present in the Phase I material. Per the Call for Proposals, "proposers must address the safety of their targets and fields with respect to the appropriate count-rate limits of the photon-counting detectors." GOs should provide this information to their Contact Scientist (CS) as soon as possible if it is missing or incomplete in the Phase I.
The Astronomer's Proposal Tool (APT) provides a Bright Object Tool (BOT) for identifying potentially unsafe field objects (point (3) above) with a 5’’ buffer around the science aperture. All unknown or unsafe objects identified by the BOT as well as unidentified bright sources visible in the DSS or GALEX images (e.g., bright extended sources) must either be cleared for safety or avoided by the use of an ORIENT constraint or changes to the instrument setup. See Sections 7.7.6 and 12.4 of the STIS Instrument Handbook for more details. While GOs are ultimately responsible for ensuring the safety of their observations, they should consult with their CSs if they need additional guidance.
Furthermore, special considerations have been defined to verify the safety of M dwarfs, which flare stochastically in the UV. STIS ISR 2017-02 details these procedures. As announced in the November 2018 STAN, Changes to STIS Technical Review Procedure, the STIS team will provide a spreadsheet that GOs should fill out and return to facilitate clearing their M dwarf observations.
Supplying Exposure Time Calculator (ETC) IDs:
GOs should run ETC calculations for both scientific and acquisition exposures to ensure sufficient S/N and to avoid saturation or overlight conditions. These ETC IDs should be copied into the corresponding places in the APT file. As announced in the November 2018 STAN Changes to STIS Technical Review Procedure, CSs will request this information from GOs if it is missing. GOs are reminded that for target acquisitions, the exposure times need to be calculated using the STIS Target acquisition ETC.
Acquisitions and Target Coordinate Precision:
The STIS field of view (FOV) for acquisitions is small (only 5''x5'' for point source acquisitions). GOs are responsible for providing precise enough coordinates and proper motions to ensure the acquisition target (which may or may not be the science target) falls within the FOV during the blind pointing stage. Because of the small FOV, neglecting to specify even modest proper motions can place the intended target well off-center even with perfect pointing. Furthermore, the acquisition centering algorithm will center the brightest object in the scene. It is the GOs responsibility to verify that the brightest object is the acquisition target. Check out the November 2018 STAN Article Common Acquisition Errors for some examples of how the algorithm behaves in more complicated scenes.
STIS coordinates are required to be in the ICRS reference system (see Table 3.3 in the Phase II Proposal Instructions). When using the Simbad target generation tool in APT, the “Reference Frame” is auto-filled with “Simbad,” and GOs must manually update this to “ICRS” after verifying the reference frame.
Additionally, we advise GOs to double check that the coordinates, proper motions and units are correct and consistent, by confirming that the target is in the cross hairs on the target confirmation chart. We refer GOs to the July 2020 STAN Article for additional information on the verification of Gaia coordinates for targets with proper motions in APT.
Targets of Opportunity (ToOs):
To help facilitate efficient safety and technical reviews of ToOs by CSs, GOs should provide early and complete information about their planned observation setups and properties of the anticipated science targets. For disruptive ToOs using one of the MAMA detectors on STIS, it is imperative that GOs provide a representative spectral-energy distribution (SED) of the type of object to be observed before the ToO is triggered, as well as a clear explanation of how the absolute scaling of the UV flux will be determined once a ToO object is identified. If the class of objects has emission lines in the UV passband of observation, GOs must also communicate how bright these lines are expected to be.
STIS Coronagraphy Visualization Tool:
The STIS team is offering a preliminary version of the coronagraphic aperture visualization tool to aid users in visualizing their scientific scenes using the various STIS coronagraphic configurations. The tool is available on the STIS website software tools page. More details on the software and examples can be found in the March 2021 STAN.
Orientation Requirements in APT:
GOs interested in specifying the orientation of the STIS slit on the sky will need to make use of the ORIENT special requirement. We remind users that the ORIENT parameter gives the orientation of the HST focal place on the sky, and if the orientation of the long slit is set to be the position angle (PA) X, where X is in degrees east of north, then the ORIENT value is given as X+45 or X+225 degrees. We refer GOs to the STIS Instrument Handbook Section 11.4 for more details on setting the ORIENT parameter.
Lastly, users can visualize the specified ORIENTs in APT Aladin, by selecting the “Orient Ranges” control.
Defringing STIS G750M/G750L Spectra
Interference between multiple internal reflections in the thinned STIS CCD detector produces significant “fringing” at wavelengths longward of about 7000 A in STIS G750L and G750M spectral images. This fringing severely limits the S/N achievable in the corresponding 1D extracted spectra at those wavelengths (Goudfrooij et al. 2018a; STIS IHB 7.2.6). The fringing structure seen in the raw spectral images differs for the two gratings: for G750L, the variations are mostly in the dispersion direction, with maxima separated by 15-20 pixels and amplitudes up to +/-15% (Figure 2); for G750M, the fringing structure is more irregular and two-dimensional. For both gratings, the fringing structure is somewhat smoothed and reduced in amplitude for extended sources (including the internal lamps) observed through the wider slits. While the fringing can be effectively reduced/removed using a suitable “fringe-flat” exposure with the onboard tungsten lamp (STIS IHB 11.2.3), the proper application of that fringe flat in any individual case can require some interaction. Defringing of the spectra is therefore not performed in the current automated calstis pipeline reductions. Software for defringing has been available as an IRAF/pyraf package (Goudfrooij & Christensen 2018b), however, and those routines have recently been converted to a new Python package (defringe) by several members of the STIS team (2020 July STAN). See the defringe documentation pages for detailed instructions and examples of its use.
For effective removal of the fringing, it is necessary to match the illumination of the CCD detector as closely as possible (science target vs. fringe flats). This requires obtaining the flats as close in time to the science observations as practical (ideally within the same orbit), with no changes in the grating or wavelength setting in between. Section 11.2.3 of the STIS Instrument Handbook includes a table with recommended apertures to use for the fringe flats, for various choices of the aperture used for the science observations:
- For point source science targets observed at the default central position on the CCD, the fringe flat exposures are usually obtained through one of the short, narrow slits (0.3x0.09 or 0.2x0.06), to mimic the point spread function of the science target.
- For targets observed at offsets along a long slit (including faint targets placed at an E1 pseudo-aperture to minimize charge transfer inefficiency effects), one of the long, narrow apertures (52x0.1 or 52x0.05) should be used for the fringe flats.
- If there are multiple science objects contained in a single exposure through one of the long slits, one should obtain two sets of fringe flats: one through a short slit (for the object at the central position) and a second through a long slit (for the other, offset objects).
- If the science target exhibits extended structure, the fringe flats should be obtained through a long slit of the same width. We note, however, that structure in the source on scales smaller than the slit width (e.g., a point source superposed on an extended background) may make an exact match of the fringing pattern difficult. For such complex cases, it may be useful to take fringe flats with different aperture widths.
Again, the fringe flats must be taken at same grating/wavelength setting, and as close in time to the science observations as possible. In many cases, the flats can be obtained during the occultation immediately following the science observations. The default flat field exposure times should be fine for many applications, but one can also take deeper fringe flats for cases where very high S/N is desired (e.g., for spatially scanned spectra; see below). Users interested in this option should consult with their contact scientist and will be asked to submit a scientific justification subject to approval.
The new defringe package has now been successfully used in several applications:
- defringe was applied to STIS G750L spectra of the standard star AGK+81 266 that have been used to monitor the spectroscopic sensitivity of that mode (Hernandez 2021). The resulting time-dependent sensitivity trends are consistent with those previously obtained from the uncorrected spectra, both at the nominal central position and at E1. The fringing was also effectively removed for archival G750M/8561 spectra of BD+75 325. An appendix to the ISR lists the Python code used for the processing (including the differences required for G750L and G750M).
- The ULLYSES program (Roman-Duval et al. 2020) has been obtaining UV/optical/NIR spectra of a number of Galactic T Tau stars. Custom processing of the spectra released to the community includes defringing of the STIS G750L spectra (Figure 1). For the primary targets, the offsets between the science exposures and the usual short-slit fringe-flat exposures are generally <0.5 pix along the dispersion, and the scaling factors for the fringe amplitudes are typically between 0.8 and 1.2. Spectra of other stars located within the 52x2 science slit (in a few cases) have been successfully defringed using additional contemporaneous fringe flats obtained through the 52x0.1 aperture (Proffitt et al. 2021).
- Special calibration programs 15383 and 16442 have obtained many spatially scanned G750L spectra of the exoplanet host 55 Cnc (2020 Sept STAN; Welty et al. 2021). In order to achieve the very high S/N potentially available for those scanned spectra, multiple long fringe flats were obtained during each occultation following the stellar exposures (STIS IHB 12.12). After initial processing and removal of cosmic rays, the observed sub-array scans were placed into a full frame format, then defringed. The defringing both reduces the scatter in the de-trended longer wavelength fluxes and will be increasingly important for exploring trends over smaller wavelength intervals.