Monday, July 16, 2012

Midnight in the Garden of GOODS and AEGIS

I just could not resist the pun on John Berendt's best-seller as a title for the following tale of recent CANDELS behind-the-scenes adventures with Hubble Space Telescope ("HST") scheduling.  Much of my CANDELS effort, thus far, has been devoted to leading our Observations Planning and Scheduling Working Group.  In the metaphorical garden-of-science that will be the CANDELS observations, I fancy myself as both landscape architect and master gardener.  I do hope the kind reader will abide, as the explication of the title's strained pun will prove useful to the narrative.

CANDELS burning low...

At the time of writing, the HST observations that comprise CANDELS are roughly two-thirds finished. What still remains? Two target regions of our five total, the two northern-most in the sky:
In short: GOODS and AEGIS (see title, above).  I hasten to emphasize that absolutely no moral judgement whatsoever is to be inferred by this pun — both surveys have produced good science by good people! Indeed, large elements of both survey teams (including yours truly, from GOODS) merged HST proposals to form the CANDELS collaboration.

Anchorage in the the Spring

In mid-June, I had the pleasure of attending the American Astronomical Society ("AAS") semi-annual conference.  This time around, the AAS convened way up north, in Anchorage, Alaska. Along with a good bit of sight-seeing beforehand/afterward, I and some other team members showcased early CANDELS science results (see recent blog post).  The northern portion of Alaska, well north of Anchorage, resides above the Arctic Circle in the so-called "Land of the Midnight Sun". So called, because anywhere north of the Arctic Circle, for a portion of the year, the Sun will never set.  Translated into astronomy jargon that will re-appear later: the Sun will never enter occultation by the Earth. 

For most inhabitants of this rotating orb, the always-daily occurrence that the Sun enters occultation by the Earth (and also rises) is unremarkable, the natural order of things.
However, this is a conspiracy of triple-coincidence! Namely:
  1. The Sun has not yet tidally locked the Earth, which is rotating at 0.000696 RPM.  (slightly faster than once per 24 hours!)
  2. The axis of that rotation is only moderately tilted, by 23.4 degrees of arc, with respect to our orbit around the Sun.  Thus the Sun's coordinates in the sky are always oscillating within 23.4 degrees of the Celestial Equator.  Still enough wandering to drive our seasons, though!
  3. Virtually all people (and virtually all else) are living more than 23.4 degrees of latitude away from the North and South Poles.
The sky behaves very differently at the Poles (both North and South) — nothing sets daily: the Moon sets monthly; the Sun sets yearly.  All of the stars in the sky at the Poles never set: they wheel around the horizon at their same elevations, perpetually in view, and visible continuously throughout the months-long intervals when the Sun is far enough below the horizon for a dark sky.  During winter at the Poles, one may say that the entire sky is a "continuous viewing zone", where stars never set and are never hidden from view by the sunlit sky. 

The AAS conference ended just a week short of the Northern solstice (or "summer solstice", for us northerners), when the Sun reaches its farthest point north of the Celestial Equator, and daytime is at its longest in the Northern Hemisphere.  In Anchorage, 19 hours and 20 minutes of daytime, to be exact. I can report firsthand that the five-ish sunless hours of mid-June Anchorage only darkened the sky to a twilight before the sunrise.  Alas for us astronomers, all those non-setting stars wheeled around while hidden by the ever-too-bright sky during the several-day conference. 

Midnight in the Garden...

In June, midnight in the (Alaska Botanical) Garden is almost
this bright.  [Credit : Barbara Miller]
What is bad for astronomers in Alaska (nighttime never dark) is probably good for plants in Alaska.  I did some nature sightseeing around Anchorage, but unfortunately was unable to visit the city's lovely Alaska Botanical Garden.  I gather from their website that, unlike most of the rest of Anchorage, the various Garden exhibits remain open all through the "night". 

Had bright-night insomnia (and/or jet lag) been a problem for me, I could thus have spent many a midnight in the Garden, perhaps reading a best-selling novel, perhaps contemplating GOODS and AEGIS overhead, continuously viewable but for the twilight sky.

Panning upward from this pastoral scene, consider now the view from the Hubble Space Telescope some 550 kilometers above, racing along a full circuit of its low-Earth orbit every 96 minutes.  This remote-controlled satellite-telescope must keep pointed well away from the Sun (greater than 50 degrees) at all times, to avoid overheating its sensitive ultra-cooled internals. For the same reason, it must also keep pointed out away from the limb of the Earth, by at least 20 degrees if daytime there, or by at least 6 degrees if nighttime. (Not to worry, as HST apparently has multiple close-cousins with no such restrictions against peering downward.) Despite all these profound differences in perspective, between HST on high, and that garden up north, there are surprising parallels to be drawn.  Particularly as regards our current CANDELS observations of the GOODS-North field. 

Continuous viewing, Hubble-style…

It turns out that HST has its own Continuous Viewing Zone ("CVZ"): specific regions of the sky where, for specific intervals during the year, the Earth never gets in the way, orbit after orbit.  Moreover, the Sun's position in HST's sky is slowly oscillating throughout the year, just like its yearly-oscillating height above and below the polar horizon.  Even better, HST's view of the firmament does not wheel around once per day, like it does for observatories down here on terra firma.  One might suspect that observing targets in Hubble's CVZ would make the most efficient use of HST's precious time, never needing to pause picture-taking because the Earth has blocked the shot.  That pause can be lengthy: typically 40 percent of every orbit suffers from occultation. 

The CVZ efficiency was a prime motivation for deciding the coordinates of the original Hubble Deep Field ("HDF") — 342 separate HST exposures taken during 10 consecutive days in 1995.  The HDF proved to be such a smashing success that the intervening 17 years have witnessed many deep-sky surveys, from a wondrous variety of telescopes on the ground and in space, blossoming all around the HDF.  These include the HST surveys GOODS and now CANDELS. 
The CANDELS GOODS-North region is located within Hubble's
northern Continuous Viewing Zone, as shown by this diagram. 
[Credit : NASA]
One of the primary objectives for both GOODS and CANDELS science has been the discovery of supernovae at extreme distances. We find these supernovae by comparing sensitive HST images of the same spot of sky, separated in time by several weeks. Because CANDELS is using a newer, infrared ("IR") light-sensitive camera on Hubble, we can hunt supernovae even farther away than had been possible with GOODS.  The brightening and fading of these cosmic fireworks appear slower with increasing distance, so the cadence of the CANDELS time-lapse photography is longer: 7.5 week intervals, versus 6.5 week intervals for GOODS. 

By happy coincidence, the opportunities for Hubble CVZ observations at the location of CANDELS GOODS-North also recur every 7.5 weeks! Lest you be overcome with irrational exuberance, I will spend the remainder of this post elaborating upon several complexities of our Hubble CVZ observations.  Some are generic to any CVZ-desiring program; some are exceptional headaches for CANDELS.  Surmounting these difficulties is an ongoing challenge, to be sure, but hopefully will prove worthwhile in added science returns from the venture.

Earth never gets in the way, but it is never far from view...

As can be seen from the CVZ orbit diagram above, the geometry is such that Hubble always points along a grazing incidence to the Earth.  In fact, the CVZ orbits flirt with the forbidden zone below 20-degree separation from the daytime Earth limb.  And fully half of every CVZ orbit will be looking past the edge of daytime Earth, with that annoyingly bright sky.  CANDELS is searching for exceedingly faint galaxies and supernovae by stacking multiple long-exposure HST images.  When those images are pointing near to the sunlit Earth, it is akin to midnight stargazing in mid-June Anchorage — an exercise in frustration.  So how does one make effective use of that bonus time in CVZ orbits, then?

Here on the ground, when confronted with an excessively bright sky, people resort to sunglasses that near-completely block the ultraviolet ("UV") portion of the scattered sunlight.  Up on Hubble, the CANDELS team has adopted a nearly opposite solution: during the bright-Earth exposures of our CVZ orbits, we use light-blocking filters that transmit only UV through to the camera.  The intensity of scattered UV light from the daytime Earth is low enough to avoid polluting our long-exposure images of the UV cosmos.  Rather than putting the 40 percent CVZ bonus time into glared-out optical or IR exposures, we obtain a sensitive UV survey complementing our optical/IR survey at no extra cost! 

Continuous Viewing Zone opportunities are not so plentiful...and not so continuous...

When CANDELS was first proposed, our intention was to exploit the matching cadences of the GOODS-North CVZ and the supernovae-searching by conducting ten successive search epochs of 15–16 HST orbits apiece.  The HST Time Allocation Committee approved this strategy, but the implementation quickly ran afoul of the realities of HST scheduling.  In the immortal words paraphrased from Moltke the Elder, "No battle plan survives contact with the enemy." Due to the vagaries of the HST orbit, the windows of opportunity for GOODS-North CVZ have been very narrow in our first few epochs.  Three to four days, tops; sometimes as narrow as two days. Doing the orbits-per-day math with 96 minute orbits (= 14–15 orbits per day), you might conclude that even two days is more than ample for our 15–16 orbit epochs.

Alas, there are several devils-in-the-detail that prevent CANDELS from observing during more than five-ish orbits on a given day, CVZ or otherwise. Foremost among these complications is the forebodingly-named South Atlantic Anomaly, or "SAA".  Unsure why, but this term always reminds me of the paranormal Bermuda Triangle.  Nothing paranormal about the SAA, though, which is much larger, and well to the south.  It is simply the point of closest approach to Earth of the irregularly-shaped Van Allen radiation belts girding our planet.

As you might guess, HST battens down its proverbial hatches every time its orbit passes through, or even near to, this quasi-stationary radiation storm.  The SAA is so large that 7–8 consecutive HST orbits per day are buffeted as the Earth (including the South Atlantic and its Anomaly) rotates underneath.  These SAA-impacted orbits are no good to us: CANDELS has way too much picture-taking jam-packed into every orbit to pause for a radiation storm. 

As for the six-ish remaining orbits per day that are clear sailing, CANDELS must share these together with all other HST observing programs, and HST station-keeping duties.  Amazingly enough, CANDELS is not the only HST observing program with stringent calendar constraints for the HST schedulers to juggle.  The end result is that CANDELS cannot expect all its "CVZ-length" orbits to fit within the available windows — sometimes up to half the orbits spill out.  The silver lining is that GOODS-North CVZ windows are usually lined by a day or two of "almost-CVZ" orbits.  These are still much longer than normal, and perfectly fine for CANDELS.

Snake in the garden…

Despite wanting entirely non-occulted orbits for the fullest amount of UV picture-taking, CANDELS never actually has HST stare at the same spot for more than one orbit. Every new orbit is targeting a new, nearby location, in order to map out an area of sky 15–16 times larger than the HST camera's (small) field of view.  Sliding Hubble's boresight and taking aim at a new target are actions that require several minutes of down-time between picture-taking.  With careful advance planning, we can arrange for this down-time to coincide with occultation, so as not to lose any picture-taking opportunity.  At worst, we need to jettison one of our two nice, long UV exposures to fit these sub-CVZ orbits. 

One noteworthy complication of using close-but-not-quite CVZ orbits is that we need to plan the orbit differently, depending on whether the HST schedulers have slotted us before or after the true-CVZ window.  For CANDELS purposes, all occulted HST orbits are defined to "start" immediately after the prior occultation ends.  For GOODS-North, the orbits immediately preceeding the CVZ window are starting with the bright Earth below, and transitioning over to dark Earth during the orbit.  The exact opposite is the case for orbits immediately following the CVZ window — the orbit starts over dark Earth and ends over bright Earth.

As you may guess, some care is needed with these near-CVZ orbits to ensure the optical/IR exposures and the UV exposures are commanded in the proper order! Hopefully the following diagram may clarify the situation better than another thousand words.  First, a shout-out to CANDELS post-doctoral fellow Marc Rafelski, for creating this highly useful graphical representation of CANDELS CVZ scheduling.  The three panels march along through the hours of one single day: in this case, 22 July 2012.  The undulating ribbon through the middle of the panels is the so-called "limb angle" of the Earth with respect to HST when pointing at our CANDELS GOODS-North region.  Each undulation represents one full orbit of HST around the Earth (taking 96 minutes).  The limb angle cannot dip below 20 degrees, or the target is considered to be occulted by the Earth.  The play of colors along the ribbon may remind you of a poisonous snake in the garden, but is intended to denote bright Earth limb (yellow), dark Earth limb (black), and twilight Earth limb (purple).  You can see that the snake is hacked into many pieces by the Earth occultations (light gray bars) and the SAA passages (salmon-colored bars).  Only the first half of the day is true-CVZ (no occultations), and one of those orbits is spoiled by an SAA passage.  The goal is to precisely align our CANDELS observations with the largely or completely uninterrupted orbits, placing down UV exposures during "day" and optical/IR exposures during "twilight/night".  Toward the end of this particular day, you can see that the occultation zones have grown quite large.  They will remain large until the next GOODS-North CVZ window approaches, in another 7 weeks. 

A day in the life of HST, while pointing at the CANDELS GOODS-North field.  The predicted angle between the Earth limb and HST is plotted over the course of a particular upcoming day in July.  During the first half of this day, there are several opportunities for HST Continuous Viewing Zone observations.  See text, for detailed description.  [Credit : Marc Rafelski]

Timing is everything...and is unpredictable...and is hard to pin down...

As mentioned above, our goal in each CANDELS CVZ orbit is to obtain UV exposures when HST is pointing out across the sunlit Earth, and optical/IR exposures when pointing out across the nighttime Earth.  If we get this wrong, we risk harm to some or all of our bread-and-butter optical/IR exposures by drowning them in glare from the bright Earth.  Minutes count, here. 

Problem 1: the timing of bright-Earth/dark-Earth transitions requires knowing exactly where HST will be, which turns out to be an impossible feat.  You might think that 550 kilometers overhead would qualify as more than "low Earth orbit", but there is actually enough tenuous atmosphere that far up to drag HST very slowly downward.  The drag on HST varies unpredictably based on the wispiness of that thin air, and HST's orientation as it plows on through.  This translates to inherent unpredictability of HST's exact location into the future.  No coincidence that the accuracy is akin to weather forecasting, according to the HST Primer: "For example, the predicted position of the telescope made two days in advance can be off by as much as 30 km from its actual position.  An estimated position 44 days in the future may be off by 4000 km (95% confidence level)." This seems like a vast uncertainty, until one considers that HST merrily rolls along its orbit at 7.5 kilometers per second.  Meaning that our timetable of day/night boundaries may be off by 9 minutes.

For CANDELS CVZ observations, a 9 minute miscalculation is bad news. One, if not two, of our five dark-sky exposures in every orbit will be "scorched" by either starting too early or ending too late.  To best avoid this unpleasantness, with each successive epoch of CANDELS GOODS-North observations we calculate exposure start-times using the most recent HST ephemeris prediction possible before those orbits' exposures must be placed on Hubble's next to-do list.  So far, this labor-intensive, real-time supply chain scheme has worked acceptably well. 

Problem 2: The HST orbit-scheduling juggernaut, which must fit together the jigsaw puzzle of thousands of orbits allocated each year to hundreds of deserving programs, needs to be fitting that puzzle together weeks ahead of actually commanding the telescope to carry out the observations.  This juggernaut is not designed to allow the end-user to lock down orbits' start-times to the minute, particularly when that desired minute is ill-defined in advance.  Fortunately for CANDELS, we have overcome this hurdle by virtue of even-deeper-behind-the-scenes legerdemain from our Program Coordinator, assigned by the Space Telescope Science Institute to ferry our program (and many others) from the drawing board to the telescope commanding. 

I refer curious readers to the Hubble Space Telescope Primer for a more thorough description of these complexities, and for an excellent overall description of HST and best practices for its use.  To conclude this rambling tale, I first cite the dire warning in the HST Primer to those who dare contemplate HST observations in the CVZ:
"There have been cases in the past (e.g. the Hubble Deep Field observations) where optical imaging has been interleaved with other kinds of observations.  However such observations are difficult to schedule and require strong science justification. ... CVZ observations are also generally incompatible with special timing requirements (e.g., timing links, special spacecraft orientations, or targets of opportunity...)."
Naturally, CANDELS is fiddling with all three of these special timing requirements in and around our CVZ observations.  So far, with two out of our ten CVZ epochs completed and the third nearing execution, we have been largely successful in scheduling these complex observations as desired.  Here's hoping that 18 months from now, when all these CANDELS CVZ epochs are completed and the images all stacked together, you will be reading blog posts about the wonderful resulting science and will have long forgotten these behind-the-scenes tribulations.

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