The following histograms show how many visits are expected during each trimester for each hour of Local (Mean) Sidereal Time. The thick line shows all opportunities when the Sun is more than 18 degrees below the horizon; the thin black line shows "dark" time when the Moon is below the horizon; the grey line shows grey+dark time (i.e., <60% lunar illumination plus dark time).
These predictions include average losses from weather, engineering time, etc. Note that nominal HETDEX observations will occupy the vast majority of the dark time, leaving dark time available only between limited LSTs (NB, this shows HETDEX targets as of mid-2021, and is not updated as the main survey reaches completion).
To determine the LST(s) at which your targets can be observed, first remember that HET has a fixed elevation of 55° and a tracking radius of 8.4°. This means that the air mass with the tracker centered will always be 1.222.
Translating to hour angle and declination, we derive the following target
accessibility locus on the sky:
Targets can be observed within the grey shaded region between the inner and outer limiting altitude
curves. For example, targets at +40° declination could be observed at HA between -3h and -1h as it rises, or between HA of +1h and +3h as it sets. If your target was at RA=4h, it would be observable from LST=1:00 to 3:00 (in the E), and from 5:00 to 7:00 (in the W). If you require dark time, check the histogram above to see what is available.
If you want to generate histograms of the possible visits for your own targets, you can download a simple Python code from: github.com/sjanowiecki/HET_observability This tool allows you to visually determine the accessibility for as many objects as you like, given their celestial coordinates and exposure times.
There is also a python tool available (https://indiajoe.github.io/pyHETobs/) to calculate the HET's pupil illumination at various positions which was created by Shubham Kanodia and Joe Ninan as part of their work on the HPF engineering team's publication: Kanodia+2021.
Other useful CGI scripts
- Please use our HETDEX Constrained Object Observability and Feasibility Calculator to determine if your targets can be completed in a specific trimester.
- Determine the longest possible track time for your target.
- To see the effect of changing exposure times in a moving aperture try the HET Filling Factor Calculator.
- To get the parallactic angle for an HET observation try the HET LRS2 Position Angle Calculator .
The maximum
track time occurs at a declination of 63°. The length of time a given object can be observed
varies with declination and telescope azimuth. The following figure shows the maximum time that
can be spent on an object at a given declination with the telescope at the optimal azimuth:
The plot below shows the relationship between the target object declination and the optimal Azimuth which the object should be observed.
Using the optimal Az from above one can calculate the parallactic angle for the center of track that will be held during an observation (so that the angle on the sky is held constant).
