Information for New Users
Please contact your local HET or TAC representative to find out how your institution runs its Phase I proposals and TAC allocations. Most questions can be submitted to the HET observations/science Google Group ( groups.google.com/g/het-obs-sci ), but for any other questions please contact the HET resident astronomers at astronomer(at)het.as.utexas.edu.
The HET is a telescope with a unique design which often causes new users a fair amount of problems. Below we list some of the common mistakes or misconceptions that new Users often have in hopes that you will be able to design an efficient observing experiment.
This set of slides has been used to educate new users on the basics of observations, and may be helpful: HETsci_intro_20200724.pdf
- Priority system
The HET is fully queue scheduled so each target is given a priority:
- Priority 0 (P0) is this highest priority and usually reserved for time critical targets or perhaps for targets disapearing into the west at the start of a trimester.
- Priority 1 (P1) is the highest regular priority time. More than 95% of targets given P1 time are completed. This is important for narrow field surveys (lots of targets in one part of the sky), synoptic programs or programs with tight observing constraints such as dark time, photometric conditions or very good seeing.
- Priority 2 (P2) targets are completed 92% of the time. This is good for grey time objects.
- Priority 3 (P3) targets are completed 75% of the time. This is good for bright time targets, targets that that lots of visits during the observing period, or programs with very flexible observing conditions such as poor seeing or non-photometric conditions.
- Priority 4 (P4) is filler time and to encourage people to make use of this time we do not charge any overhead and only half of the nominal exposure time is charged. This means that if we double the exposure time submitted by the PI because of clouds the PI still only gets charged for half of the requested exposure. This can be very efficient for short exposure time objects which may be observed in good conditions between two high priority objects instead of having the telescope idle.
A PI may request a mix of priority time and may submit the same target at different priorities levels. A very clever gray time strategy is to submit all of your targets at your lowest priority time you were given and then see what gets observed and then if a target has lots of competition from other targets at higher priority to "junk" that entry and submit a new Phase II at higher priority.
- maximum exposure length
The HET has a fixed primary and tracks objects in the sky by moving the secondary. The range of motion of the secondary is limited by the size of the upper hexagon support structure. We allow trajectories to move +/- 8.5 degrees of motion from the center of track. This creates a 17 degree annulus field of view on the sky. The amount of time that it takes for a celestial body to cover that 17 degree depends on the Declination of the object, longer in the North and shorter in the south. This can be visualized by looking at the 2nd figure on the Object Observability page or computed using our track time calculator.
- throughput changes as a function of exposure length
As the track follows the object across the 17 degree annulus the amount of the mirror that is seen by the tracker changes. This means that the collecting area changes as depending upon what part of the track is being utilized. The Resident Astronomers strive to make sure that all exposures are centered on the middle of the annulus (called center of track) where the collecting area is maximized. For a given exposure time the collecting area will change and this will be a function of the Declination of the object. To see how the collecting area changes please use our HET Filling Factor Calculator, which will give you a fractional throughput with respect to the maximum collecting area at the center of the track.
- object observability and the number of visits that can be obtained at any one part of the sky.
Because of the unusual nature of the HET, on a given night an object may be visible for at most a few hours. Thus, if an experiment requires several hours of integration this may have to be spread over multiple visits. In addition, if several targets are close together on the sky then only one of those objects might be observable at a time and the PI must be aware of the total number of visits that can be achieved at a given location on the sky. To assist with this we have created a feasibility calculator located on the Object Observability page. Using this calculator will allow the PI to see how many visits may be completed given their partner share, weather losses and observing constraints (transparency and seeing). If a the PI finds that the number of possible visits they need in a specific part of the sky is within a factor of 2 of the requested visits or if the number of possible visits is less than 10 the PI should ask for high priority time for that target. If the number of possible visits for all of their targets in a specific part of the sky greatly exceeds the number of requested visits then lower priority time could be used.
- observing with IFU on an Alt Az telescope
The HET observes with the orientation of the instruments at a fixed angle during a single trajectory. This angle is equal to the paralactic angle at the center of the trajectory. This angle can be pre-computed using our calculator located on the Object Observability page. This is of particular importance if the object of interest is not a single point source and is going to be centered on the LRS2 IFU or VIRUS IFUs. For this type of observation the orientation of the object will be different for an East track than for a West track, thus it would not be possible to simply co-add observations taken in the East with those taken in the West. As such special planning for the observations must be made and this might impact the project feasibility and the number of possible tracks over which an observation set might be made. When completing a Phase II a PI can make a special grouping type called a "pool" which will allow a target to be observed in either the West or the East.
Last updated: Tue, 12 Jan 2021 23:28:36 +0000 stevenj