HET Object Observability

Useful CGI scripts

  • Try our HETDEX Constrained Object Observability and Feasibility Calculator .

  • Or use the older original Object Observability and Feasibility Calculator .

  • See when the HET can observe your target, i.e. the first date when fully half of a target's eastern trajectory is observable just before dawn 18 degree twilight, until the last date when fully half of its western trajectory remains observable just before dusk 18 degree twilight.

  • Determine the longest possible track time for your target.

  • To get the specific dates over an entire year when a target is available try our yearlong observing calendar, or try our trimester-based observing calendar.

  • 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 Theory

    Because the HET has a fixed elevation of 55°, and a tracking radius of 8.4°, it can only observe objects that fall within a certain zenith distance. This means that the air mass with the tracker centered will always be 1.222. Translating to astronomical coordinates, we derive the following target accessibility locus on the sky:

    Objects are available within the region between the inner and outer limiting altitude curves. 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).

Last updated: Thu, 16 Nov 2017 11:04:08 -0600 shetrone


The Telescope

Technical Overview

Object Observability




HRS-2 Summary

HRS-2 Details

HRS-2 configurations

HRS-2 exposure meter

HRS-2 Position Angle and Fiber layout

HRS-2 Throughput


LRS2 Summary

LRS2 Details

LRS2 Fiber Layout and Position Angle

LRS2 Throughput

LRS2 Observing details


VIRUS - Summary

VIRUS - Details

VIRUS - Throughput

VIRUS - Fiber Layout and Position Angle

Old Instruments

LRS - old

Wavelength Calibration

Program Preparation

Web Management System

Other HET Documentation