PSU HET Primer  

Original by Jason Wright. Last updated by Mike Eracleous on 06/12/2025

The Proposal Process

The HET operates via a queue,  meaning that time is awarded in hours and observations are executed by an experienced  staff astronomer according to constraints set by the proposer and priorities set by the  Penn State Time Allocation Committee.  

Observations are proposed, executed, and delivered in three phases. Phase I proposals for  HET time are solicited three times per year; the Penn State TAC awards time based on proposal merit. In Phase II, successful proposers submit their requested observations to  the HET resident astronomer. In Phase III, proposers manage their observations and  retrieve their data.  

Time Allocation

All time is awarded with an associated priority, which determines an observation’s place in the observing queue. The highest priority observations (lower priority number) will be  executed as soon as constraints permit; lower priority time (higher numerical priority)  will be executed when there are no higher priority options.  

Proposers should read the call for proposals carefully and note any special restrictions for  a given trimester. In particular, proposers should consider the total amount of Penn State  time available at each priority, and what fraction of that time they are requesting (a  request should not, for instance, be for more time than Penn State may allocate!)  

Priorities  

There are five priority levels. The TAC frequently uses its judgment to assign priorities that are different from those requested, so proposers should consider their priority request  carefully and not propose for higher- or lower-priority time than their science warrants. That said, typical proposal pressure for high priority time means that the TAC will rarely increase the priority of requested time.

P0.– These observations take precedence over all other observations. This time will  generally be awarded only for highly time sensitive observations, such as transient phenomena (e.g. GRB afterglows, SNe, etc) or very time critical observations (e.g., simultaneous observations with space telescopes). Penn State may allocate ~13% of its time at  P0.  

P1.– These observations are only overridden by P0 observations. This priority is best used  for transient phenomena, urgent observations that should be executed early in the trimester, and highly meritorious science. Penn state may allocate 20% of its time at  P1.

P2.– This priority is best used for time-insensitive observations (e.g. spectra of constant  sources). In general, most P2 observations will be executed. Penn State may allocate  33% of its time at P2.  

P3.– This is the lowest regular priority. P3 observations have a reasonable chance of being  executed, depending on pressure from higher priority observations at the same sidereal  times. Penn State may allocate 33% of its time at P3.  

P4.– This special priority is best used for observations that can be performed in marginal conditions (i.e. during thin cirrus or very poor seeing) and programs that are not  particular about which subset of a large target list are observed. P4 time is also sometimes  executed to fill in “holes in the queue” that develop at sidereal times where all higher  priority targets have been exhausted, to prevent the telescope from sitting idle until P0-3  targets become available.  In principle PSU may allocate an arbitrary amount of P4 time, at the discretion of the TAC. However, P4 programs will compete with each other for scheduling, and the completion fraction will depend on the target lists and weather conditions.

Unlike the other priorities, P4 time is charged without regard to overhead (i.e. slew and  setup time). Because P4 time is designed for times with lower atmospheric transparency,  only half the actual open shutter time is charged (so an award of 1 hour of P4 time could  generate up to a total of 2 hours of open shutter time, regardless of the number of targets  actually observed). An ideal proposal for P4 time is one with a large number of targets spread out across the sky, any number of which might be usefully observed (i.e. a survey with a large target list but no need for a high completion rate) and with loose constraints  on signal to noise ratio and moonlight contamination.  

As an example, over the three 2019 trimesters, PSU-averaged P2 completion fractions varied from 48%  to 86%, while P3 completion fractions varied from 33% to 98%. P0 and P1 completion  fractions appear to have been driven primarily by the ability of accepted programs to  provide targets with high observability. Programs that were able to provide P0/P1 targets  with good observability seem to have had >90% completion, as expected. P4 programs  had on average 40% completion due to several programs that achieved > 80%  completeness; typical P4 programs saw 33% (median) completion.

Target-of-Oportunity (ToO) Observations

Because of its queue scheduling, the HET is well suited for many types of time-sensitive observations, including ToO observations. Proposers can apply for ToO observations and request the priority that is most suitable for the response speed required to achive the scientific goals of the program. For example, rapid response observations may require P0 time so that they can override other programs and be executed at the highest priority after the request is received by the resident astronomers. Programs that do not require immediate response may be served better by P1 or P2 time, for example. Programs requesting P0 time must specify the observing conditions carefully to achieve the desired rapid response. For example, the requirement of rapid response (that is, a target be observed on the first night it is accessible after the request is made) may compete with requirements/constraints on observing conditions (e.g., background level, seeing, transparency). Therefore, for the response to be as rapid as possible, the requirements on the observing conditions should be correspondingly lax. 

In evaluating ToO proposals the TAC considers scientific merit and program design (including feasibility), demand for high-priority time by other programs, and the probability that the requested ToO time will be actually used. In other words, the TAC scrutinizes ToO proposals closely and is reluctant to allocate substantial time to programs that are unlikely to use it.

Target Observability and Feasibility of Observations

As a fixed-elevation telescope, HET has idiosyncratic restrictions on when and where it  can observe. Roughly speaking, it can observe objects in a ring on the sky centered on the  zenith (see the figure at the top of this page). This results in most objects being observable for about 1 hour on  the “east track” as they rise and another hour on a “west track” as they set. Objects near  the northern and southern declination limits of the telescope are observable during a single (potentially long) northern or southern track. The shape and illumination of the  telescope pupil and telescope throughput are variable across a track, being worst at the  edges and best near track center. There is also a tower used for calibration that vignettes  the pupil at certain azimuths.  

All proposers should check the observability and feasibility calculator on  the HET Observability Page to see the track lengths of their targets, the available exposure times, and the accessibility of the targets during the trimester (see the useful scripts and plots in the second half of the page). 

Available Instruments

A great deal of additional useful information on the current instrument and telescope  properties and performance is available at the HET Overview page. Below you will find an executive summary of the main instruments.

LRS2. – This is a low-resolution IFU spectrograph based on the VIRUS design. It covers the wavelength range 3700–10500 Å in two parts, each part observed with a different spectrograph units (LRS2-B and LRS2-R) that cannot observe the target at the same time.  The spectral resolving power varies between 1100 and 1900 depending on the spectrograph unit and arm. More details about this instrument are available at the LRS2 Instrument Page.

HPF. – A high resolution, extremely stable near infrared (YZJ) spectrograph. It is  available as a facility instrument in high resolution mode only. Proposers interested in  doing precise radial velocity work with HPF should include a credible plan for deriving velocities from HPF data products and include HPF team members as co-Is.  Documentation for HPF, including an exposure time calculator, is available at the HPF instrument team’s web site.  

VIRUS. – The spectrograph suite used in the HETDEX survey is available for  use as a guest observer instrument. While VIRUS is a mature instrument, observations are still considered on a shared risk basis because commissioning is formally ongoing as new units are being installed.  Any proposer wishing to use VIRUS should carefully read the Observing With VIRUS page.