WFC3 Space Telescope Analysis Newsletter - Issue 20, March 2015
- 1 Cycle 23 Phase I Deadline Reminder and Updates to APT
- 2 WFC3/UVIS Two-Chip Photometry
- 3 A New Infrared Sample Sequence: SPARS5
- 4 New Apertures for WFC3
- 5 Updates to the WFC3 Instrument Handbook for Cycle 23
- 6 WFC3 Talks and Posters Presented at 2015 AAS Meeting
- 7 Pixel Bundles: Facilitating PSF Analysis in the FLT Domain
- 8 New Documentation
Cycle 23 Phase I Deadline Reminder and Updates to APT
M. Bourque, K. Peterson
Solicitation of Hubble Space Telescope Cycle 22 Phase I proposals is currently open. Cycle 23 runs from October 1, 2015 to September 30, 2016. Users who wish to submit Cycle 23 Phase I proposals must submit through the Astronomers Proposal Tool (APT) before the April 10, 2015 8:00 PM EDT deadline. For more information, please visit the Cycle 23 Announcement page, the HST Call for Proposals for Cycle 23, the STScI Phase I Roadmap, and the HST Primer. Observers needing assistance are encouraged to contact the STScI Helpdesk by sending an email to email@example.com.
Additionally, users planning to submit Phase I proposals are required to install the latest version of APT. The Cycle 23 release of APT contains the following updates/changes:
- Updated the version of Aladin - In order to support new features in APT (see below), we have updated to Aladin 8.1. The basic functions of Aladin for visualization are the same, but there are two tasks - zooming and changing the opacity of the aperture overlays - that are different. (See the Aladin training video: Using Aladin with APT in Phase II for details.)
- Labels now available in Aladin - It is now possible to overlay labels in Aladin for targets, exposures, and observations. You can also display the current orientation angle, as well as the POS TARG XY axes. (See the Aladin training videos: Using Aladin with APT in Phase II, Measuring/updating orientations and Creating Pos Targ offsets for details.)
- Changes to Hubble Legacy Archive access - Access to HLA data is now through an obvious button on the left hand side of the Aladin Server Selector (it used to be more hidden under the Archives button). More recent HLA data is available, and the data downloads are now JPEG images rather than FITS data (to speed the download times). (See the Aladin training video: Using HLA with Aladin for details.)
For more information, please visit the APT website.
WFC3/UVIS Two-Chip Photometry
Motivated by the very different quantum efficiencies of each of the WFCE/UVIS e2v detectors, WFC3 UVIS photometric calibration is now determined independently for each of the CCD detectors in the WFC3/UVIS channel. Flatfields for non-quad filter+detector have been created, normalized to the median value of each detector. New zeropoints have been calculated for these filters. The data products remain the same. Three new keywords are added to the image headers:
- PHTFLAM1 = inverse sensitivity for UVIS1 + filter
- PHTFLAM2 = inverse sensitivity for UVIS2 + filter
- PHTRATIO = PHTFLAM2/PHTFLAM1
And two new calibration switches are implemented:
- PHOTCORR = PERFORM PHTRATIO is calculated and keywords are populated in the header
- FLUXCORR = PERFORM Chip 2 is scaled to Chip 1 (i.e. UVIS2*PHTRATIO).
Sometime in the 3rd quarter of this year, the new calibration will be incorporated into the pipeline. In the meantime, users who wish to use the new flats and zeropoints can download these reference files (flat fields and imphttab files) from the WFC3 website, along with the modified CALWF3.
The list of filters is: F200LP, F218W, F280N, F300X, F343N, F350LP, F373N, F390M, F395N, F410M, F467M, F469N, F475W, F475X, F487N, F502N, F547M, F600LP, F621M, F625W, F631N, F645N, F656N, F657N, F658N, F665N, F673N, F680N, F689M, F763M, F845M, F953N.
More information is at http://www.stsci.edu/hst/wfc3/analysis/uvis_2_chip/.
A New Infrared Sample Sequence: SPARS5
At STScI we are developing a new infrared sample sequence, called SPARS5, intermediate between RAPID and SPARS10 that is intended to allow the efficiency and uninterrupted time series of SPARS10 and yet be short enough in cadence to isolate better a target exoplanet-host star from a nearby stellar companion in spatially-scanned observations using an IR grism. Better isolation may enable the observer to prescribe wider orient ranges, which may increase scheduling opportunities, which are often rare for these time-constrained observations. Given the available sample sequences, we anticipate that SPARS5 will be advantageous especially for G141 or G102 grism observations of stars brighter than approximately 7th magnitude in H band, scanned at approximately 1 arcsec per second or faster, although observers should make their own individual assessments.
We anticipate the SPARS5 sample sequence to be available sometime in the first half of HST Cycle 23. If SPARS5 is not ready as anticipated, affected observers may have to delay their observations or substitute one of the existing sample sequences. Appropriate calibration reference files (notably darks) will be obtained on orbit as part of the nominal calibration plan for WFC3 organized by STScI. Those>necessarily will be obtained after the SPARS5 sample sequence has been tested on orbit and may delay the best analysis of any observations made with it.
Table 1 compares the readout times for the SPARS5 sequence with the associated values for RAPID and SPARS10, specifically for the apertures of greatest interest to exoplanet spectroscopy (GRISM256 and GRISM512). After readout 1 and respectively for GRISM256 and GRISM512, the SPARS5 cadences are 2.35 s and 2.92 s, intermediate between the RAPID and SPARS10 cadences (0.278 s and 0.853 s for RAPID, 7.35 s and 7.9 s for SPARS10).
Table 1: Comparison of sample times for the new sample sequence SPARS5 (bold columns; subject to change) with RAPID and SPARS10, for GRISM256 and GRISM512 apertures. Adapted from Appendix A of Petro & Wheeler (2006).
New Apertures for WFC3
Two new UVIS apertures are being defined for cycle 23 to enable observers to automatically place a small target near a readout amplifier to reduce CTE losses. (See Section 6.9 of the WFC3 Instrument Handbook for a discussion of CTE losses and mitigation strategies.) UVIS2-C512C-CTE and UVIS2-C1K1C-CTE will place the target ~ (257, 257) pixels and (512, 512) pixels, respectively, from the C amplifier. This amplifier has been chosen for the reasons discussed in Section 6.4.4 of the WFC3 Instrument Handbook. These new apertures will use the same target placement as the subarray apertures UVIS2-C512C-SUB and UVIS2-C1K1C-SUB (Figure 6.2 and Table 6.1 in the WFC3 Instrument Handbook), but will read out the entire UVIS detector. They should generally be used instead of the subarray apertures for exposure times >348 sec, which can accommodate parallel readout of the entire detector.
Updates to the WFC3 Instrument Handbook for Cycle 23
Observers who are proposing programs for cycle 23 should use the latest edition of the WFC3 Instrument Handbook for information relevant to WFC3 observations. Updates to the handbook for cycle 23 include:
- Strategies for dealing with zodiacal light, earth-shine, and air glow in planning WFC3/UVIS exposures (Sections 9.7.1 and 9.7.2).
- Updates on the evolution of the UVIS dark current (Section 5.4.8) and UVIS hot pixels (Section 5.4.9).
- Updates on the evolution of CTE losses for point sources and plans to implement an empirical pixel-based correction algorithm in MAST (Section 6.9.3).]
- Analysis of UVIS pixels which have reduced sensitivity for one or more anneal periods, especially at the bluer wavelengths (Section 5.4.3).
- Analysis of UVIS “sink pixels”, which under-report the number of electrons generated in them during an exposure because they contain a number of charge traps (Section 6.9.3).
- Measurement of small photometric drifts in UVIS filters (Appendix A).
- Derivation of zodiacal light maps for six WFC3/IR filters and the G141 grism using WFC3/IR exposures (Section 7.9.5).
- Analysis of the He I airglow line at 10,830 Angstroms, sometimes strong even at high earth limb angles (Sections 7.9.5 and 9.2).
- Strategies for dealing with zodiacal light, earth-shine, and air glow in planning and reducing WFC3/IR exposures (Section 7.9.5).
- Evolution of the WFC3/IR blobs, identified in new time-dependent reference files (Section 7.9.6).
- Confirmation of the capability of the WFC3/IR detector to observe point sources as bright as V~0 mag using the grisms in spatial scanning mode (Section 8.6).
WFC3 Talks and Posters Presented at 2015 AAS Meeting
Four WFC3 posters were displayed at the 225th American Astronomical Society Meeting that was held on January 4-8 in Seattle, Washington. The abstracts are shown below. Electronic copies of the posters are available at the conference poster webpage. Please feel free to send any questions that you have to firstname.lastname@example.org.
WFC3/UVIS Photometry of HST standards: Encircled Energy and Spatial Stability with Wavelength
Ariel Bowers, Jennifer Mack, Sylvia Baggett, Susana Deustua, Derek Hammer (STScI)
We present encircled energy (EE) measurements for the UVIS channel derived from observations of HST photometric standards over several years. These white dwarf and solar analog standard stars were observed in all 42 filters at multiple positions across the detector and allow us to characterize wavelength-dependent structure of the point-spread function (PSF). These measurements allow us to compute the average sensitivity ratio per chip for all UVIS filters, which are important for deriving the new chip-dependent zeropoints. They are also ideal for quantifying the accuracy of the flat fields by comparing the observed photometry at various locations across the detector.
WFC3: Instrument Status and Advice for Proposers and Observers
John MacKenty and the WFC3 Team (STScI)
The Wide Field Camera 3 continues as the most used HST instrument after over five productive years in space. We summarize its basic performance characteristics including our analysis of its stable and time variable calibrations. Key recent improvements in our calibrations and instrument characterizations will be discussed including the development of improved libraries of Point Spread Functions, better models of persistence in the infrared detector, and advances in our understanding of sources of variable infrared backgrounds. Basic calibration improvements include the adoption of a CCD specific QE curve for the UVIS channel together with improved flat fields, an ongoing effort to include more filter elements with high precision astrometric calibration, and time dependent calibration of the UVIS photometric zeropoints. Recent lessons learned regarding the use of the spatial scan technique to enable extremely high precision photometric and astrometric measurements will be presented. The calibration program for Cycle 22 will also be summarized.
WFC3 UVIS Detector Performance
Heather Gunning, Sylvia Baggett, Catherine Gosmeyer, Matthew Bourque, John MacKenty, Jay Anderson, and the WFC3 Team (STScI)
The Wide Field Camera 3 (WFC3) is a fourth-generation imaging instrument installed on the Hubble Space Telescope (HST) during Servicing Mission 4 (SM4) in May 2009. WFC3 has two observational channels, UV/visible (UVIS) and infrared (IR); both have been performing well on-orbit. Since installation, the WFC3 team has been diligent in monitoring the performance of both detectors. The UVIS channel consists of two e2v, backside illuminated, 2Kx4K CCDs arranged in a 2x1 mosaic. We present results from some of the monitoring programs used to check various aspects of the UVIS detector. We discuss the growth trend of hot pixels and the efficacy of regular anneals in controlling the hot pixel population. We detail a pixel population with lowered-sensivity that evolves during the time between anneals, and is largely reset by each anneal procedure. We discuss the stability of the post-flash LED lamp, used and recommended for CTE mitigation in observations with less than 12 e-/pixel backgrounds. Finally, we summarize long-term photometric trends of the UVIS detector, as well as the absolute gain measurement, used as a proxy for the on-orbit evolution of the UVIS channel.
Updated Calibration and Backgrounds for the WFC3 IR Grisms
Norbert Pirzkal, Gabriel Brammer, Russel Ryan (STScI)
We present new and improved calibration of the WFC IR (G102 and G141) grism mode. These new calibrations were generated by combining data obtained over six observing cycles and include a better sampling of the field of view. The result is a calibration of the spectral trace that has been improved to better than 0.1 detector pixel. A new fiducial wavelength reference spectrum is now used to calibrate the wavelength dispersion of the grisms and we show that the rms of the solution has been reduced to approximately 7 and 14 Angstrom for the G102 and G141 grisms, over the entire field of view. Overall, both the trace and wavelength calibration have been improved by about a factor of two and the G102 and G141 solutions are in better agreement at wavelengths where the two grisms overlap. We demonstrate that the grism calibration can be extrapolated for objects that are outside of the field of view but still result in dispersed spectra on the WFC3 detector.
We also present new master sky images that can be used to improve the sky background subtraction from grism exposures. The individual components of the new background model include the zodiacal continuum and a strong He I emission line at 1.083 microns from the upper atmosphere. We find that fitting science exposures with a linear combination of these two background components enables modeling of the WFC3/IR grism background with an accuracy that is better than ~0.01 electrons/s/pix across the detector.
Pixel Bundles: Facilitating PSF Analysis in the FLT Domain
This article presents a pilot investigation into a new kind of data product. We explore a new way of "bundling" together information from FLT images so that users can do their analysis closer to the original data domain.
It is well known that the pixels in the FLT images provide the only real constraints we have on the astronomical scene. They represent a hard measurement with a well-understood error of exactly how many counts were registered within a certain pixel boundary over a known period of time. In addition, the point-spread function (PSF) in the raw detector frame can be well characterized so that stars can be fit with high-precision and non-stars can be analyzed with an understanding of the convolution that the telescope and pixelization have performed on the scene.
If we could just take a single image of the scene and do science on that, then some aspects of image analysis would be easier. Unfortunately, because of cosmic rays, the detector's undersampling and artifacts, a single exposure at a single pointing is not able to give us all all the information about the scene that is delivered to the telescope. We need to dither to recover resolution and remove bad pixels. This dithering process is critical, but it makes it much harder to do our analysis on the FLT pixels, since one must use elaborate transformations to inter-relate the constraints from different exposures. These transformations are complicated by several factors: (1) the complicated distortion solution and (2) the fact that HST's pointing is not perfect. The distortion has been solved for in layers (lithographic pattern, filter dependence, polynomial, etc. (See WFC3 ISR 2014-12)), which makes it hard for non-experts to do the forward and reverse mappings transparently. Also, even within an orbit, HST's pointing can have errors of a several hundredths of a pixel, making the exact inter-relation of pixels fuzzy.
Astrodrizzle has been designed to provide one way for users to combine together a dithered set of data into a single, easy-to-use image. It has an accurate model for the distortion and can use "tweak-shifts" to improve the alignment among the input images. Astrodrizzle's main focus is to preserve flux and S/N, and it does this rigorously. It also improves the resolution. Unfortunately, there is no unique, perfect way to combine dithered images into a single image with regular sampling. As such, the pixels in the output image are somewhat "fuzzy" constraints on the scene, in that they have imprecise light-centers and often have unknown correlations among them. This makes it difficult to use them for high-precision analysis, such as PSF-fitting.
This is one of the reasons that even though HST's PSF is relatively stable and modelable in FLT-space, not much effort has been invested into modeling it, since the products that most users have access to have been resampled and are not amenable to such an analysis. Increased attention to inter-image transformations has made it easier to relate the FLT-pixels to one another so that we can use them as simultaneous constraints. This then makes possible a proper PSF-type analysis and justifies an effort to understand and model the PSF better.
To this end, the WFC3 team has recently gone through the entire WFC3/UVIS archive and extracted every image of a star with S/N of 200 or greater (a total of more than 5 million star images taken over five years). These star images have been studied to understand how breathing affects the PSF and how we can predict the PSF in a given exposure. The results are encouraging, and several ISRs are forthcoming.
In anticipation of having access to better PSFs, we have begun to attack the second aspect of the challenge: the ability to relate pixels to one another to set them up for detailed PSF analysis. An ISR by Anderson (WFC3 ISR 2014-24) describes a tool that can be used to identify an object in a set of images that have been aligned with the routine hst2galign (see WFC3 ISR 2014-23) and package together the pixels in a raster centered on this object into a "bundle". This bundle contains all the information in the FLT images (the pixel values, the error array, and the data quality array) along with the mapping of each pixel into the reference frame and the inverse mapping from the reference frame into each local pixel raster. Finally, the tool also provides the PSF appropriate for the particular location in the exposure. The current PSF model for a given filter is static, but we hope in the future to be able to provide PSFs that are appropriate for the particular focus state of the telescope.
The goal of these bundle products is to encourage the user community to start thinking about how they can make use of the "hard" constraints provided by the FLT pixels, by means of accurate PSFs and mappings between the detector and sky frames. Not all projects will benefit from going back to the original pixels, but several projects could benefit from a lower-level analysis, such as: high-precision photometry and astrometry of stars, point-source subtraction, and weak-lensing studies. As mentioned above, this project is an exploratory pilot study and there are several issues that need to be addressed before a more automated data product is possible. However it is time for potential users to start thinking about how they might make use of such data products, and how such data products could be improved and analysis tools developed to better meet their needs.
ISR 2014-21: Infrared Blobs: Time-dependent Flags - P.R. McCullough, J. Mack, M. Dulude, B. Hilbert
ISR 2014-22: Flagging the Sink Pixels in WFC3/UVIS - J. Anderson, S. Baggett
ISR 2014-23: hst2galign: an Automated Galaxy-based Alignment Routine - J. Anderson, S. Ogaz
ISR 2014-24: Local Bundles: Bringing the Pixels to the People - J. Anderson
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