HET Performance Characteristics


The HET meets its specifications and points to better than 7 arcseconds rms.


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, although a limited range of non-sidereal guiding is possible by offsetting the guiding fiducial to compensate for the motion of the target (limited to the field of view of the guide cameras, about 20 arcseconds).



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 to match the wavelength of the science instrument in use (g' for LRS2-B and VIRUS, r' for LRS2-R, i' for HPF).

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.

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.


Focus is maintained by two Wave Front Sensor probes (WFS) which are placed on point sources brighter than about 15th mag. These are generally centered up and their focus loops closed in the first few minutes of any given observation. For short observations there may not be enough time to acquire a WFS star and focus will be maintained by visual assessment of the FWHM of the guide star.

The WFS can achieve constant focus of less than 0.02 mm for a full trajectory unless the temperature is changing rapidly in which case the WFS tends to lag behind the changes in focus by up to 0.06 mm. When the Telescope Operator is required to keep focus manually (for short observations or if no WFS stars can be obtained) then the focus is generally kept to 0.05 mm.

Global radius of curvature of the primary mirror array is maintained by looking that the DMI during a trajectory when the telescope is in 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 target.

Observing Status

Last updated: Fri, 29 Dec 2023 12:06:25 +0000 stevenj


The Telescope

Technical Overview

Object Observability


Non-sidereal observations



LRS2 Summary

LRS2 Details

LRS2 Fiber Layout and Position Angle

LRS2 Throughput

LRS2 Observing details


VIRUS - Summary

VIRUS - Setting up on targets

VIRUS - Misc details

VIRUS - Throughput and sensitivity

VIRUS - low surface brightness sensitivity

VIRUS - Dithers, IFUs, Tiling

VIRUS - Tiling observations

Habitable Zone Planet Finder (HPF)

HPF Details

HPF Setting up on targets

HPF Throughput and Exposure Meter

HPF Data Reductions

HRS-2 (in development)

HRS-2 Summary

HRS-2 Details

HRS-2 configurations

HRS-2 exposure meter

HRS-2 Position Angle and Fiber layout

HRS-2 Throughput

Old Instruments

HRS - old

LRS - old

MRS - old

Program Preparation

Web Management System