Collecting Efficiency
Beginning in mid 2022 we are developing a new quantity called "Collecting Efficiency" (CE) to better quantify the combined effects of sky transparency and telescope pupil illumination. Eventually this will replace the TSL keyword SKYTRANS which allowed PIs to request sky transparency in ranges of >90%, >50% or >0%.
The HET's unique design means that both the sky transparency and the real-time telescope pupil illumination can be equally important in determining how much flux is being delivered to its focal surface.
Currently our expectation is that PIs will be able to request a minimum value of CE with a TSL keyword, in the same way that they can request minimum Image Quality (IQ) and minimum sky surface brightness (SKYBRIGHT). The following information is intended to help PIs select appropriate values of CE for their targets.
The Collecting Efficiency (CE) is defined in terms of the Sky Transparency (TR, ranges between 0 and 1) and the Telescope Pupil Illumination (PI, ranges between 0 and 1), as follows:
CE = TR * PI
The sky transparency is routinely measured by the guider cameras during all observations, by comparing the observed brightness of a guide star to its magnitude in the PanSTARRS catalog. The guider cameras use a set of g/r/i SDSS filters which are selected based on which instrument is used (g for LRS2-B/VIRUS, r for LRS2-R, i for HPF). Extensive documentation of sky transparency and other quality measurements in each filter are provided on this page.
The pupil illumination (PI) is calculated using hetillum (link?), based on a model of the telescope optics and relevant shadows/obstructions. A more detailed description of object observability on the HET is included on this page and a tool to calculate the pupil illumination ("filling factor") is available on this page. The PI ranges from 1.0 (center) to 0.48 (at the edge of the track). The maximum value of PI=1.0 corresponds to a 10.0m aperture with a 5.2m obscuration, equivalent to a 57 sqr-meter collecting area or a 8.5 meter unobscured primary.
Throughout a science observation we typically use guider camera exposure times of 5-10s. Each exposure provides an estimate of the sky transparency (TR) and we also calculate the pupil illumination (PI) at that time. All of the measurements obtained during a science observation are averaged together to generate the TR and PI for that target, and those average values are used to calculate the CE for that target.
Statistics
To give some background on the range of CE values we typically observe, the following graphs and statistics have been generated on all accepted science observations from May 2020 through July 2022, which includes ~25,000 exposures.
The histograms below show the distributions of sky transparency (TR) and pupil illumination (PI), based on the average values for each science exposure.
