Added an explicit mention of the timing for the first epoch. It already appeared in the APT file, but those timing specifications don't appear in the proposal PDF.140022289Assigned ID: 9
----- Attempting Submission 1 (Fri Mar 22 19:24:32 GMT 2019) -----
HST Phase I Proposal 9 (tbrown_plax.apt) successfully submitted.
Receipt: # 9-1
----- Attempting Submission 2 (Thu Mar 28 17:27:02 GMT 2019) -----
HST Phase I Proposal 9 (tbrown_plax.apt) successfully submitted.
Receipt: # 9-2
----- Attempting Submission 3 (Tue Apr 02 00:04:16 GMT 2019) -----
HST Phase I Proposal 9 (tbrown_plax.apt) successfully submitted.
Receipt: # 9-3
----- Attempting Submission 4 (Fri Apr 05 14:36:07 GMT 2019) -----
HST Phase I Proposal 9 (tbrown_plax.apt) successfully submitted.
Receipt: # 9-4Resubmission with failed Visit 01 rescheduled as Visit 51. Scheduling constraints on Visit 51 are slightly wider than those on Visit 01. Visits 02 and 03 are unchanged. Note that the reason for the Visit 01 failure is still unclear at this time. This program may need to be adjusted to avoid the same failure. To maintain consistency between visits, that adjustment may require changes to Visits 02, 03, and 51.
02/18/2020 - ROYLE - Opened orient on visit 03 to allow scheduling.
02/19/2020 - ROYLE - Further opened orient on visit 03
02/21/2020 - ROYLE - Updated ORIENTs on visits 02 and 51 based on expected ORIENT for visit 03 (U3 302.1)
02/21/2020 - ROYLE - Temporarily expanding ORIENT on visit 51 to keep window while ORIENT issue is examined
02/24/2020 - ROYLE - Expanding ORIENTs to allow for +/- 2D from preferred ORIENT
02/26/2020 - ROYLE - Setting up ORIENTs on visits 02 and 51 to match the scheduled roll for visit 031586430495----- Attempting Submission 1 (Fri Jul 12 07:52:33 GMT 2019) -----
----- Attempting Submission 2 (Mon Sep 09 14:20:30 GMT 2019) -----
----- Attempting Submission 3 (Tue Feb 18 13:43:57 GMT 2020) -----
----- Attempting Submission 4 (Wed Feb 19 15:13:29 GMT 2020) -----
----- Attempting Submission 5 (Fri Feb 21 21:20:54 GMT 2020) -----
----- Attempting Submission 6 (Fri Feb 21 22:02:59 GMT 2020) -----
----- Attempting Submission 7 (Mon Feb 24 18:42:02 GMT 2020) -----
----- Attempting Submission 8 (Wed Feb 26 14:38:20 GMT 2020) -----These warnings were also present in GO-13817, GO-14336, and GO-14773. To ensure all of the spatial scans are in the same frame, the exposures must be grouped in a way that avoids multiple GS acquisitions, but it produces these warnings in APT. Note that Visit 03 no longer produces such warnings, so presumably there is a fix once the proposal is processed.Completing the HST Parallax Legacy: M4There are only two globular clusters where a modest investment of HST time can yield a high-precision measurement of trigonometric parallax: NGC6397 and M4. The parallax to NGC6397 has been measured successfully with HST to 2% uncertainty. An important HST legacy would be the completion of this pair, by measuring the M4 parallax while HST can still do so.
In the context of the Galaxy's globular cluster system, these two clusters are complementary. NGC6397 is ancient (13 Gyr) and metal-poor ([Fe/H]=-2.02), while M4 is younger (11.5 Gyr) and of intermediate metallicity ([Fe/H]=-1.15). NGC6397 is a frequent fiducial for stellar population models, while M4 hosts a rich set of RR Lyrae stars that can be used to better calibrate the period-luminosity relationship. The utility of each cluster as a population template and calibrator depends critically upon the distance uncertainties. The scarcity of population anchors in this regime is in contrast to the parameter space at younger ages (< 10 Gyr) and higher metallicities ([Fe/H] > -1), where many open clusters have high-precision parallaxes.
In a broader context, these two clusters represent an essential independent check on the parallaxes from the Gaia mission, which currently suffer from significant systematic errors on global, intermediate (20 degrees), and small (1 degree) scales, hampering distances to the Galaxy's globular clusters. If these errors are eventually resolved, HST parallaxes to globular clusters, independent of Gaia with distinct methodology, will provide an important validation. The observations we propose here will help anchor the distances to the entire globular cluster system of the Milky Way.Our proposal obtains a high-precision (2% 1-sigma uncertainty) distance to M4 using the spatial-scanning capability of WFC3/UVIS. Given the technical challenges, we have already done all of the preparations for the observations, thus ensuring feasibility. These preparations include selection of the best field and orientation, taking into account the number of cluster and field stars with useful scans (uncontaminated by neighbors), appropriate sampling of the parallax ellipse, schedulability, and scan position angle relative to the parallactic motion at the time of each observation. We have simulated the spatial scan analysis of this optimal field, using previous experience with spatial-scan observations as a guide. The signal-to-noise ratio in each 350-second spatial scan has been estimated using the extant photometry (Stetson+2014) and the throughput of the F606W filter. Our test Phase~II proposal indicates that in each epoch, we can execute 4 spatial scans in the F606W filter (forward, reverse, forward, reverse) of 3600-pixel length, plus unscanned images binned 2x2. These binned images will employ the F336W, F467M, F547M and F850LP filters. The resulting 4-filter photometry, when combined with ground-based spectroscopy of the field stars, will enable the required constraints on the absolute parallax reference frame. We will obtain multiple images in each filter for the entire scanned area, enabling cosmic-ray rejection and increased dynamic range. We also require one orbit (during the first epoch) dedicated to direct imaging; previous spatial-scanning programs were able to pack all of the direct imaging into the same orbits as the spatial scans, but recent shortening of the HST visibility windows, and the need to provide time when the telescope is not scanning (see the Call for Proposals), make it impossible to include all of the necessary imaging within the spatial-scanning orbits.
The proposal provides five single-orbit epochs of scans, at six-month intervals that sample the times of maximum parallactic motion; we request that the first epoch be scheduled between 24 Aug 2019 and 8 Sep 2019, a month before the formal start of Cycle 27, but in the period when the earliest Cycle 27 visits will begin executing. Our test Phase II indicates that we would have at least a week of schedule availability for each epoch, at the appropriate orientation and date. Note that we selected the "increase scheduling flexibility'' option in our Phase I proposal, which trims 6 min off each visit, but that is excessive; our test Phase II demonstrates that we only need 2 min of unused visibility to ensure scheduling. The orientation and cadence would each be constrained, but a single orbit within a 1-week period, repeated every 6 months with exact 180 degree flips in orientation, should not present a burden to the HST scheduling system. Each spatial scan is nearly along the y-axis of the detector (0.05 degree offset), minimizing the effect of charge transfer inefficiency, but providing subsampling of the point spread function in the resolution direction (perpendicular to the scan).
Observations at each epoch result in a one-dimensional position estimate for each of the target stars at that epoch. In order to estimate the (relative) parallax for each star, a multi-epoch solution requires three parameters per star: mean position, parallax amplitude, and proper motion in the resolution direction. In addition, a number of global parameters are needed to register observations at different epochs. Besides a shift and rotation, a polynomial fit is necessary to fully match the reference systems for different epochs and orientations. At least four epochs are needed to constrain the registration parameters as well as measure relative parallaxes. However, five epochs are strongly preferred, for several reasons. First, the additional epoch greatly increases the redundancy of the stellar measurements, therefore improving the constraints on the registration parameters. Second, having the first and last observations at the same parallax phase -- with equally spaced observations in between -- provides better separation and lower covariance between parallax and proper motion, resulting in an improved parallax estimate by ~20%, better than the simple increase in signal-to-noise ratio. Third, the extra epoch facilitates the identification of outliers; current experience suggests that ~10% of the targets have anomalous residuals, for technical (e.g., cross-contamination of trails) or astrophysical (e.g., astrometric binaries) reasons.
Impact of reduced gyro mode: If HST were to enter reduced gyro mode, the reduced field of regard (approximately half of the current field of regard) would impact the scheduling. Currently, we are scanning at an angle nearly perpendicular to the parallax ellipse at times of maximum parallax offset, repeated every 6 months with 180 degree flips. In reduced gyro mode, those particular orientations will likely not be available at those particular times of the year, so we would need to explore other combinations of orientation and scheduling to see if the program is feasible, perhaps with a larger uncertainty on the parallax (using less optimal sampling of the parallax motion, and losing the ability to have the epochs at each end of the parallax motion mirror each other).Our team has a strong background in both general HST observations and specifically in spatial-scanning parallax measurements. We successfully proposed and executed the only program to date that obtained a spatial-scanning parallax for a globular cluster (NGC 6397). Tom Brown has expertise in a variety of stellar populations programs with HST. Stefano Casertano and Adam Riess have HST spatial scanning expertise from the NGC 6397 program but also a series of Cepheid programs. Dave Soderblom has expertise in stellar populations and a variety of investigations of stellar cluster distances. Jay Strader has expertise in stellar populations and access to ground-based facilities to support this program. Don VandenBerg has expertise in theoretical modeling of stellar populations.Stellar Populations6falsefalsefalsefalsefalsefalsefalseSUPPORTEDSTELLAR CLUSTERGLOBULAR CLUSTERJ2000J2000WFC3/UVISImagingNoNoF606WWFC3/UVISImagingNoNoF336WWFC3/UVISImagingNoNoF467MWFC3/UVISImagingNoNoF547MWFC3/UVISImagingNoNoF850LPWFC3/UVISImagingNoNoF606WWFC3/UVISImagingNoNoF336WWFC3/UVISImagingNoNoF467MWFC3/UVISImagingNoNoF547MWFC3/UVISImagingNoNoF850LPWFC3/UVISImagingNoNoF606WWFC3/UVISImagingNoNoF336WWFC3/UVISImagingNoNoF467MWFC3/UVISImagingNoNoF547MWFC3/UVISImagingNoNoF850LPWFC3/UVISImagingNoNoF606WWFC3/UVISImagingNoNoF336WWFC3/UVISImagingNoNoF467MWFC3/UVISImagingNoNoF547MWFC3/UVISImagingNoNoF850LPWFC3/UVISImagingNoNoF606WWFC3/UVISImagingNoNoF336WWFC3/UVISImagingNoNoF467MWFC3/UVISImagingNoNoF547MWFC3/UVISImagingNoNoF850LPThe scheduling data was generated by CASM 27.2.CASM was run in Phase II mode.1566804782000120.89999999999998(Tic-Visit
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