Feb 022015
 

We are pleased to announce the Version 1.0 release of Epoch 1 of MACSJ1149.5+2223, after the completion of the first 70 orbits of ACS and WFC3/IR imaging on this cluster and its parallel field from our Frontier Fields program ID 13504 (PI: J. Lotz). These v1.0 mosaics have been fully recalibrated relative to the v0.5 mosaics that we have released during the course of this epoch from November 2014 to January 2015. For ACS, the v1.0 mosaics incorporate new bias and dark current reference files, along with CTE correction and bias destriping, and also include a set of mosaics that have been processed with the new selfcal approach to better account for the low level dark current structure. The WFC3/IR v1.0 mosaics have improved masking for persistence and bad pixels, and in addition include a set of mosaics that have been corrected for time-variable sky emission that can occur during the orbit and can otherwise impact the up-the-ramp count-rate fitting if not properly corrected. Further details are provided in the readme file, which can be obtained along with all the mosaics at the following location:

More general information about the data for this project can be obtained from our Frontier Fields MAST archive website:

as well as our main Frontier Fields HST project website:

and our Frontier Fields astronomer’s blog and other social media:

We hope that these high-level science products are useful for your research, and we welcome any suggestions or questions that you may have about them.

 Posted by at 10:19 pm
Jan 102015
 

We are pleased to announce new v0.5 HST mosaics for the cluster MACSJ1149.5+2223, from the observations of this target from our Frontier Fields program ID 13504 (PI: J. Lotz). The observations for this epoch are now complete at a total of 70 orbits, which we have combined into mosaics on the main cluster field observed with WFC3/IR (F105W, F125W, F140W, F160W) and on the parallel field with ACS (F435W, F606W, F814W). We note that these v0.5 mosaics do not yet include older archival data from CLASH or from other programs; this is to help facilitate study of transient phenomena, for which the data from our program 13504 can be compared with other datasets. All other archival data will be combined together with the Frontier Fields data when we release the full-depth v1.0 mosaics. The current v0.5 mosaics have all been processed beyond the default pipeline calibration. For ACS, these mosaics incorporate CTE correction and bias destriping, while the WFC3/IR mosaics have been masked for persistence, bad pixels, and other detector defects, and also include a set that have been corrected for time-variable background emission in the IR. All the mosaics are aligned to a common pixel grid, as well as to an absolute astrometric frame based on pre-existing catalogs of this field. Further details are provided in the readme file, which can be obtained along with all the mosaics at the following link:

More general information about the data for this project can be obtained from our Frontier Fields MAST archive website:

as well as our main Frontier Fields HST project website:

and our Frontier Fields astronomer’s blog and other social media:

We hope that these high-level science products are useful for your research, and we welcome any suggestions or questions that you may have about them.

 Posted by at 2:32 pm
Jan 102015
 

We are pleased to announce the Version 1.0 release of Epoch 1 of MACSJ0717.5+3745, after the completion of the first 70 orbits of ACS and WFC3/IR imaging on this cluster and its parallel field from our Frontier Fields program ID 13498 (PI: J. Lotz). These full-depth v1.0 mosaics also include archival ACS and WFC3/IR data in the Frontier Fields filter set, from programs 9722 and 10420 (PI: H. Ebeling), programs 10493 and 10793 (PI: A. Gal-Yam), program 12103 (PI: M. Postman), program 13389 (PI: B.Siana), and program 13459 (PI: T. Treu). These v1.0 mosaics have been fully recalibrated relative to the v0.5 mosaics that we have released during the course of this epoch in October and November 2014. For ACS, the v1.0 mosaics incorporate new bias and dark current reference files, along with CTE correction and bias destriping, and also include a set of mosaics that have been processed with the new selfcal approach to better account for the low-level dark current structure. The WFC3/IR v1.0 mosaics have improved masking for persistence and bad pixels, and in addition include a set of mosaics that have been corrected for time-variable sky emission that can occur during the orbit and can otherwise impact the up-the-ramp count-rate fitting if not properly corrected. Further details are provided in the readme file, which can be obtained along with all the mosaics at the following location:

More general information about the data for this project can be obtained from our Frontier Fields MAST archive website:

as well as our main Frontier Fields HST project website:

and our Frontier Fields astronomer’s blog and other social media:

We hope that these high-level science products are useful for your research, and we welcome any suggestions or questions that you may have about them.

 Posted by at 2:30 pm
Sep 242014
 

We are pleased to announce the Version 1.0 release of Epoch 2 of MACSJ0416.1-2403 after the completion of all the ACS and WFC3/IR imaging on the main cluster and parallel field from our Frontier Fields program
(13496, PI: J. Lotz). These v1.0 mosaics have been fully recalibrated relative to the v0.5 mosaics that we released regularly throughout the course of this epoch during August and September 2014. For ACS, the v1.0 mosaics incorporate new bias and dark current reference files, along with bias destriping and correction for CTE losses, and also include a set of mosaics that have been processed with the new selfcal approach to better correct for low-level dark current structure. The WFC3/IR v1.0 mosaics have improved masking for persistence and bad pixels, and in addition include a set of mosaics that have been corrected for time-variable sky emission that can occur during the orbit and can otherwise impact the up-the-ramp count-rate fitting if not properly corrected. Further details are provided in the readme file, which can be obtained along with all the mosaics at the following location:

More general information about the data for this project can be obtained from our Frontier Fields MAST archive website:

as well as our main Frontier Fields HST project website:

and our Frontier Fields astronomer’s blog and other social media:

We hope that these high-level science products are useful for your research, and we welcome any suggestions or questions that you may have about them.

 Posted by at 2:52 pm
Sep 162014
 

UPDATE — Light Sabers have Earthly Origin!

24 September 2014

New investigations of these artifacts have determined the root cause.  They are not due to internal light scattering. These so called light saber are the result of bright earth flat fields that left persistence, which was then imprinted on our Frontier Fields data. Some, but not all of the persistence was flagged, thereby resulting in the remaining persistence appearing in our final image products.

Now that we have identified the root cause, we are DOING SOMETHING BETTER…..

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

16 September 2014

A new undocumented artifact appeared in the Frontier Fields exposures of the MACSJ0416.1-2403 cluster on Visit 94. As seen in Figure 1, the artifact appears almost as a string of satellite trails that appear strongly on the left side of the images and diminish greatly towards the right side of the images.

Scattered light from a bright star just outside the field of view produced artifacts, affectionately called light sabers.

Figure 1:  Scattered light in the MACSJ0416 field of view, thought to be from a bright star just outside the field of view.

Due to their unique appearance, we call these artifacts “light sabers,” following the theme of the WFC3 “Death Star” artifact. Light sabers are most apparent in the first exposure of the F105w filter, and less intensely so in exposures 2-4 for F105w and in exposures 1-4 of the F160w filter. The suspect image is the first exposure in F105w, with the presence of the artifact in subsequent exposures and in the F160w images regarded as persistence of the initial light scattering.

The main cause of the light sabers artifact is considered to be internal scattering of the light from the bright star located at 04:16:6.9752, -24:05:43.01 (RA,DEC). The main light saber is marked in a purple (Figure 2) with the rest of sabers (thought to be harmonics of the primary) marked in other colors. The dashed green line seems more likely to be due to a satellite trail.

Figure 2:  Highlighted light saber artifacts, believed to be caused by the bright star just outside the field of view.

Figure 2: Highlighted light saber artifacts, believed to be caused by the bright star just outside the field of view.

To determine if a unique position on the detector would cause the effect, the centroid of the bright star was mapped onto WFC3 detector coordinates. The ACS drizzled image of MACS0416 from 2014-Jan-10 in F435W was used to determine the centroid of the bright star and obtain the precise RA/DEC coordinates. Drizzlepac’s SKYTOPIX was used to map the coordinates of the star to WFC3 and the results are shown in the plot.

Figure 3:

Figure 3:  The location of the bright star as determined in separate exposures in different filters, shown in WFC3 sky coordinates.

The light saber effect is caused by the exposure indicated with the red circle in Figure 3, with the gold points representing images showing the artifact via persistence. A well-documented and associated glint caused by the bright starlight is also indicated in green. The results seem to indicate that the light saber effect is primarily seen when the bright star is positioned very close to vertical center on the detector and -55 pixels off of WFC3’s frame. This position matches closely with the noted position of the instrument baffle as indicated in Tom Brown’s 2007 ISR 2007-16.

An example of the glint can be seen in Figure 4 inside the red circle.  Additional information and examples are available in Brown’s ISR 2008-06.  The associated glint is thought to be caused by bright starlight scattering off the knife-edge of the baffle, and we suspect the light saber effect to be caused by the same.

Figure 4:  An example of the glint.  See ISR 2008-06 for more information.

Figure 4: An example of the glint. See ISR 2008-06 for more information.

A mask has been created for light sabers and any images affected have been appropriately reduced.  All pixels impacted by this effect in the current epoch were fully masked, so that the final combined mosaics of MACSJ0416.1-2403 do not include any of the affected pixels.  Upcoming observations of MACS0717 are found to not be affected by this particular artifact, and future Frontier Fields observations will be positioned appropriately and taken with the effect of light sabers in mind.

Harish Khandrika – Frontier Fields Science Data Products Team Member – WFC3/IR

 Posted by at 11:55 am
Sep 082014
 

We are pleased to announce the Yale Frontier Fields Workshop November 12-14 in New Haven, CT.

This workshop will be an opportunity for observers and theorists to discuss their work on Frontier Fields data obtained by Hubble, Spitzer, and other telescopes, and to prepare for the remaining data yet to come.  Topics will include high-redshift galaxies, galaxy cluster lens modeling, cluster masses and dynamics, intracluster light, supernovae, and future science.

Please submit abstracts by September 20 to yalehffworkshop2014@gmail.com.  Include your name, institution, position, presentation title and abstract, and preference for a talk or poster.  There is no registration fee, but space is limited.

For more details, please see:
http://www.astro.yale.edu/yale_frontier_workshop/

FF-Yale-2014-logo

And please circulate to colleagues and students.

We hope to see you November 12-14 in New Haven!

-Priya Natarajan and Dan Coe (SOC chairs)

 Posted by at 10:33 am
Aug 022014
 

Epoch 2 observations for the MACSJ0416.1-2403 cluster and its parallel field have been scheduled and began executing yesterday.  This epoch places WFC3/IR on the main cluster and ACS/WFC on the parallel field.

These observations will continue through late August and probably early September.  You can watch the progress of the scheduling and execution of these observations on our status page:

http://archive.stsci.edu/prepds/frontier/obs-status/

The large exposure numbers listed in late August (at the time of this writing) merely indicate the total number of exposures during that observing window and that those exposures have not been planned on specific days yet.

Keep checking back and watch the observations add up!

 Posted by at 1:38 pm
Jul 282014
 

We are thrilled to announce the v1.0 release of Abell 2744, Epoch 2, which completes all the Frontier Fields observations of our first cluster!

The Abell 2744 cluster with all observations from both Frontier Fields epochs.

The Abell 2744 cluster with all observations from both Frontier Fields epochs.

This second epoch consisted of 70 orbits on the main cluster with ACS (in F435W, F606W, F814W) and on the parallel field with WFC3/IR (in F105W, F125W, F140W and F160W), thereby complementing the 70 orbits obtained in Epoch 1 with WFC3/IR on the main cluster and ACS on the parallel field, in the same filters.

Abell 2744 parallel field, with observations from both Frontier Fields epochs.

Abell 2744 parallel field, with all observations from both Frontier Fields epochs.

The v1.0 mosaics for this epoch are completely reprocessed and include the following:

  • recalibration of all exposures using the latest, up-to-date reference files for dark current, read-noise and bad pixel masks
  • high-quality astrometric alignment, generally achieving accuracies within 0.01 pixel for the mean offset measurements of exposures relative to one another.
  • for ACS data, an additional “self-calibration” step is included, which improves especially the low-level noise in the images by means of a more accurate treatment of the underlying dark current structure
  • for WFC3/IR data, improved flagging of pixels affected by persistence and detector defects, as well as correction and removal of issues related to time-variable sky background.

Together with the previous Epoch 1 release, the full dataset achieves final depths of ~28.7 – 29th ABmag across all filters. Further details on these science-ready mosaics can be found in the Readme file,
along with the full mosaics, which are all available at our MAST webpages:

http://archive.stsci.edu/prepds/frontier/

All of these mosaics are immediately ready for science!  What will you discover?

 Posted by at 3:31 pm
Jun 262014
 

Analysis of the IR channel observations of Frontier Fields data has revealed that some of the observations contain a time-variable background signal.

We know that the background signal rate is changing with time thanks to the way in which the IR channel data are collected. Each IR observation is produced using multiple non-destructive readouts of the detector. In the case of Frontier Fields, we usually have 14 – 16 readouts (or “reads”, in WFC3 jargon) of the detector for each exposure. With this method, we can watch the signal in a given pixel grow with time throughout the observation.

On the Frontier Fields project, one of the first data quality checks we perform is to examine the signal level versus time for each IR exposure. In order to produce high signal-to-noise plots, we typically will calculate the mean signal across the detector for each of the 14-16 reads, and plot these signals versus time. We expect to see nice, straight lines with a constant slope, such as that in Figure 1.

An example of a Frontier Fields WFC3/IR exposure without time-variable background.

Figure 1:  Examples of two Frontier Fields WFC3/IR exposures with nearly linear signal accumulation.

But instead, in some cases, we see something like that shown in Figure 2.  In this case, the curves indicate that the mean signal rate was higher for the first half of the exposure than during the second half. Comparison with other, non-varying exposures indicates that the signal rate the in the second half of the exposure is the expected signal rate. This implies that there is some amount of “extra” signal present during the early reads of the exposure, and that this extra signal slowly turns off as the exposure progresses.

Figure 2:  Bad ramps

Figure 2: An example of the mean signal rate changing during the exposure.

Further investigation has revealed that two sources are responsible for this variable background.

The first, seen in Figure 3, is a scattered light which falls along the left side of the detector at times when HST is pointed close to the bright limb of the Earth. A more detailed explanation of this scattered light is given in Section 6.10 of the WFC3 Data Handbook. This glint appears in only a small number of the Frontier Fields observations, and then usually only in the final ~100 seconds of a particular observation.

Figure 3: Scattered light on the left side of the image is due to HST pointing at the bright limb of the Earth.

Figure 3: Scattered light on the left side of the image is due to HST pointing at the bright limb of the Earth.

The second source of time variable background has been traced to atmospheric helium emission (at 10,830 angstroms) occurring along the line of sight, only when HST is above the bright side of the Earth. For more details on the orbital geometry and the emission itself, see Instrument Science Report WFC3-2014-03 by Gabe Brammer.  Given the limited wavelength range of the emission, this excess flux only affects the Frontier Field observations taken with the F105W filter. All of our other filters have a short-wavelength cutoff that is redward of the emission.

So, how do we deal with these two types of “extra” signal in our data? We have come up with a method that we believe does a good job of removing both. For each pixel, we fit a function to the signal versus time, using all of the reads.  This function models how a pixel should behave in the presence of a constant signal throughout the exposure. We then compare this model fit to the actual signal values in the exposure. In the case of a pixel contaminated by one of the two types of time-varying background signal, the resulting fit will not be very good. In this case, we then ignore the first read of the exposure, and try re-fitting the function to the signal in reads 2 through the end of the exposure. If the fit is still not good, then we ignore reads 1 and 2, and fit our function from read 3 to the end. This process is repeated until we arrive at a good fit.

With a good fit produced using only the good reads of the exposure, we can now predict what the signal should be in all of the reads. At each read then, the difference between the measured signal and our modeled prediction represents the amount of “extra” signal in that read. At this point, we could simply remove this excess signal from our pixel, but this would make for a very noisy result since most of the pixels on the detector see only the background signal and therefore have a relatively low signal to noise ratio. In order to produce a higher quality correction, we perform one more step. Once we have determined the amount of extra signal present in all of the pixels in all of the reads, we go read-by-read and smooth the extra signal across the detector. This smoothed signal is then subtracted from the original exposure, resulting in an observation which has been cleaned of the excess signal. Figure 4 shows the mean signal for the same two exposures as Figure 2, after our correction has been performed.

Figure 4:  Corrected ramps

Figure 4: The mean signal after all ramps are corrected for the variable sky in the same exposures used in Figure 2.

The corrected signal is not perfectly linear with time, but the majority of the extra background signal has been removed.

Bryan Hilbert – Frontier Fields Science Data Products Team Member – WFC3/IR

 Posted by at 12:21 pm
May 302014
 

We have just experienced our first non-acquisition of a guide star during Frontier Fields observations.  This occurred while in the midst of Abell 2744 observations.

Since HST is in constant motion, pointing is maintained by a set of three Fine Guidance Sensors (FGS) which find and lock on to a pair of guide stars, or a single guide star if pairs are not available.  These guide stars are selected by software based on several  criteria, including magnitude, relative position to other similar stars, position within the FGS “pickles” (Fields of View) and any  pointing constraints on the observation such as ORIENT or POS TARGs within the Phase 2 program.  Selected guide stars need to stay within the FGS pickles for the entire orbit, including all pointing changes due to POS TARGs or PATTERNs.  If an observation spans more than one visibility interval, the guide stars are reacquired after each interruption either from occultation or SAA passages.  A pair of guide stars provides the most accurate and stable pointing since they act as sort of handles for HST to focus on.  If two stars are used in two separate FGS pickles, then HST is able to maintain almost perfect pointing throughout the observations.  If only one star is used, HST may show some drift around the single star since there is not a second star to keep the telescope from rotating.  More information about the accuracy of each type of guiding can be found online at www.stsci.edu/hst/acs/faqs/guide_star.html.

In some cases, a guide star may fail to acquire or it might successfully acquire but can not be maintained.  Sometimes this is a result of a telescope problem, but more often, it turns out that a selected guide star fails to meet one of the criteria it initially appeared to pass.  This can happen in the case of a variable star, a multi-star system that previously appeared as a single star, or with the presence of a similar star (called a spoiler) nearby that confuses the FGS.  When PAIRs are used, it is possible to fail to acquire one star, but succeed with the other, resulting in observations taken with single star guiding which is often good enough for most science.  There may also be situations when a star is acquired initially but fails to re-acquire in a subsequent orbit, or lock may be lost on one star during an orbit.  This is usually due to the star itself being at the very edge of usability and violating one of the limits set by the telescope to help ensure HST knows where it is pointing.  With guide star pairs, science can usually continue as long as one of the stars is acquired.  If both stars fail (very unusual) or an observation using single star guiding fails to acquire its one star, the observations default to gyro control.  This is often problematic to the science as the observations are likely to show significant drift and rotation, or may be far enough off that the target is completely missed.

During the first Frontier Fields visit observing Abell 2744 on May 14, one of the two selected guide stars failed to acquire, resulting in the observations continuing on single star guiding instead.  As with all failures, the failed star was investigated and was found to be a bad star.  It was flagged in the database within 24 hours of the failure, such that future observations would not attempt to use the same bad star.  The second Frontier Fields visit of Abell 2744 on May 15 also failed, as it was already on the telescope and set to use the same guide star pair. Several other visits that were scheduled to execute on the telescope the following week, with the same guide star pair, were quickly reworked by the calendar-building team at STScI to use a different guide star pair.  The remaining visits in the epoch not yet put on a calendar are unaffected, since the bad star is no longer an option for our software when selecting from available guide star pairs.

HST WFC3 and ACS field of view, with Fine Guidance Sensors fields of view used to lock onto guide stars.

Figure 1:  The HST Field of View of Abell 2744, with Fine Guidance Sensors Fields of View indicated by the large, gray arcs.

The green boxes in Figure 1 identify potential guide stars.  To use guide star pairs, two stars must fall into separate FGS pickles and remain there throughout any shifts in pointing during the visit.  If two similar guide stars are too close to each other, neither can be used since the FGS could lock onto the wrong star.  Because of the multiple criteria involved and the need for precision, not all guide stars can be used for a given observation, even if the Field of View seems to show stars that could be used.

The Frontier Fields data products team carried out a detailed examination of all the data from the two visits that were affected by these guidestar issues. For the first visit (number 37), only one of the guidestars was lost, while the other star was successfully acquired and the observations were able to continue in single guide star mode. Analysis of the resulting images showed no measurable impact on the pointing or the PSF quality (consistent with our knowledge that HST is able to perform successfully with a single guide star, when necessary), and all the data from this visit were included in the mosaics.

For the second visit (number 81), the failure mode was somewhat different.  The guide stars were fine during the first two orbits of this 4-orbit visit, but began to show problems during the third orbit and failed the reacquisition for the fourth orbit. Consequently, the ACS shutter was closed at the start of the fourth orbit and the fourth exposure for each filter was not obtained.  As a result, we include only the first two exposures for each filter in our fast-turnaround v0.5 products, although we may include the third exposure in future versions. For WFC3/IR, all the exposures were obtained, and analysis revealed that the last exposure was offset by no more than a few tenths of an arcsecond compared to its expected location.  Thus, there was no significant evidence of drift during the exposures, indicating that the telescope was able to track successfully in gyro mode during these exposures.

So, it makes no difference.  Two, one, or zero guide stars – we can do great science in any case!

Patricia Royle – Frontier Fields Program Coordinator

 Posted by at 3:04 pm