HET Project Feasibility

The purpose of this page is to give HET proposers some guidelines about what kind of projects are best suited for the HET to carry out productively, and conversely what project design characteristics should be avoided. The following software can enlighten you as to how your program realistically fares as an HET scheduling exercise:

Object Observability and Feasibility Calculator

Types of projects with the HET --

  • Survey programs with a lot of targets well dispersed all over the sky are the ideal type of program for the HET, as the queue scheduling flexibility is maximal and the sidereal time loading is the most even.

  • Synoptic programs are defined as those requiring multiply repeated target visits, typically with date constraints, which often have some leeway. For synoptic programs, the tighter the date constraints, the more indispensable a high TAC priority. With medium-low priority, achieving the desired cadence can fall short, particularly with uncooperative weather.

  • Target of Opportunity programs use a generic placeholder target, which sometime during the trimester becomes "activated" as the real TOO object. By nature these require a rapid and unimpeded communication process, and for efficiency require a single contact person to deal with target submission and notification of the resident astronomers. While the resident astronomers will take timely action based upon any e-mails received by the start of the night's observing, and will accord projects their rightful due in terms of TAC priority, as constrained by sky, equipment, and scheduling contingencies, it is always a good idea to be proactive on behalf of your program as follows. For top priority not-to-be-missed opportunities, feel free to phone through to the RA desk (432-426-3689), while including fuller details in a corresponding e-mail (astronomer@het). For somewhat lower priority situations, such as the encroaching seasonal or lunar cycle putting paid to your most vital target, the contact person can usefully refresh the RA's awareness of the situation by e-mail.

  • In-depth study programs generally require very many long exposures (nearly full tracks at nearly the same sidereal time slot). This type of program is the least suitable for the HET since the tracking length limits the per-visit exposure time and many programs may be competing for your sidereal time slot or slots. One can estimate the number of achievable observations of your object thus:
    1. Determine how many nights during the month the target will be visible using the HET observability calendar, making sure to set the skybrightness constraint appropriately.
    2. Multiply by the sky transparency window: (0.8 for non-spectroscopic, 0.6 for spectroscopic and 0.3 for photometric).
    3. Multiply by the image quality window by consulting the seeing statistics.
    4. Multiply by the institutional share of the observing resource.

    Thus for a UT target observable throughout one particular 4 month period, with declination -6 and observing constraints: photometric, dark time, IQ < 2.16", one estimates 56 tracks x 0.3 x 0.45 x 0.6 = 4.5 tracks. For a PSU target observable during another particular 2 month period with declination +30 and observing constraints: spectroscopic, dark time, IQ < 2.3", one estimates 98 tracks x 0.6 x 0.6 x 0.3 = 10.6 tracks. Remember that your target will have to compete for these tracks based on your TAC priority, and it is by no means impossible for a large stronger priority program to "checkmate" an In-depth study program's sidereal time slot. The situation is somewhat less stringent than sketched so far, in the case of far northern and far southern targets, which transit within the northern or southern azimuth quadrants of the sky. Due to the shallow angles at which these transit the HET elevation window, there is significant additional scheduling flexibility, that the operational software allows the HET astronomer to take advantage of in these instances.

  • Pencil beam survey programs are the analog of the preceding In-depth study program category, but "pencil beam" implies targets that are clumped in a relatively small solid angle of the sky, under a few square degrees. It has essentially the same serious drawbacks as the former category, due to the congested sky distribution.

Common mistakes that are made with HET program design --

  1. It is important for the PI to realize that due to the design of the HET the effective collecting area changes over a trajectory. Near either end of a trajectory the HET has half the collecting area in comparison to the middle of a trajectory. As such, two medium length visits each centered on a transit (i.o.w. track middle) are sometimes more valuable than a single long visit.

  2. Be sure to not confuse the DIMM (site) seeing with the delivered image quality of the telescope. You should check the current seeing statistics to make sure that your requirements are deliverable.

  3. Due to its design, the HET has fixed limits on the maximum exposure length that can be executed for a given track. Be sure to check the track length for your target, including a reasonable setup time.

  4. Try to avoid scheduling that is crucially dependent on the initial or final 10 or so minutes of a track. To see the effect of the requested exposure time on the effective pupil, i.e. the time-integrated effective light collecting aperture, experiment with the HET Filling Factor Calculator.

  5. A very large number of (even short) visits to the same target, is strongly disfavored due to the handicaps explained above in the In-depth survey category.




Last updated: Sun, 08 Jan 2012 02:48:35 -0600 caldwell



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