HET Performance Characteristics


The HET meets its specification and points to better than 20 arcseconds rms. Because the HET is not designed for pointing accuracy better than 10 arcseconds, all must have finding charts (most will be generated automatically by the Night Operations Software). The PI should specify how accurately and what method is used for setup on their targets.


At the present time the HET has a tracking error of roughly an arcsecond per minute. This error is azimuth dependent and variable in magnitude. For most targets this drift is completely compensated by the guider corrections.

For moving targets no tracking compensation is currently implemented.



Auto guiding is done on either the Guide camera 1 (gc1) or Guide camera 2 (gc2). These guide cameras have several selectable filters (B,g',r',i',clear) and a field of view of about 15". Unless there is a compelling science case the guide stars will be selected automatically by the Night Operations Software.

The absolute limiting magnitude for guiding is 20.0 Under 1.7 arcsec FWHM seeing for a dark sky. . At this magnitude the guide star is just a small detection above the background. No focus corrections can be made with such a faint guide star.

It is standard operating procedure to save images from the guide camera during all science trajectories. These can be used to determine the image quality and transparency during a trajectory.


The tracking height model for the HET is determined by minimizing the changes in he DMI during trajectories. Focus is both a combination of the height of the tracker above the primary mirror and the global radius of curvature of the mirror. Thus the DMI can not be used to maintain focus. At the present time focus is maintained by the Telescope Operator by looking at the FWHM of the guide camera images.

Global radius of curvature is maintained by looking that the DMI during a trajectory when the Telescope Operator has good focus. If this distance deviates by more than 200 um from the "ideal" distance then a GROC correction is applied at the end of the current trajectory. This GROC correction takes less than 4 minutes to apply and thus can be done while setting up on the next targets.

Observing Statistics

The observing statistics below were compiled before the Wide Field Upgrade was started. As new statistics are compiled we will remove these older stats. The link below will take you to the observing statistics.

Image Quality

Starting in the first period of 2001 we began collecting HET fhwm, ee50 and ee80 image quality data with the LRS "pre" images as reported in the night reports. Beginning in 2002 we started also measuring the FWHM in the LRS "pre" images. Site seeing data are gathered using the Differential Image Motion Monitor (DIMM). DIMM data are reported in the night report for every science object when the DIMM is operational since the first period of 2002. The entire DIMM data set is archived and will be made accessible on the web. In the third period of 2003 we began measuring the FWHM for HRS observations from the FIF bent prime guider. Thus as of the first period of 2004 FWHM data now comes from two source LRS "pre" images and the FIF guider while ee50 data comes exclusively from LRS "pre" images. The link below will give you the most up-to-date statistics.

Sky Brightness

HET sky brightness data are derived from g1 and g2 spectra and entered in the night reports. During dark time an observer can expect the sky brightness per square arcsecond to be between 21.1 and 21.4 mag.

Last updated: Tue, 31 May 2016 09:54:43 -0500 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