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Table of Contents > Overview > The Telescope > Performance

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Observing Status

HET Acknowledgments

HET Performance Characteristics

Pointing
Tracking

Acquisition
Auto-guiding

Focus
Observing Statistics

Image Quality
Sky Brightness

The HET meets its specification and points to better than 30 arcseconds. The pointing is set within the mount model and is updated on a roughly monthly schedule. This is done by pointing to 8 stars at different azimuths with the tracker near the center X=0, Y=0. Because the HET is not designed for pointing more accurately than 10 arcseconds, all targets except the brightest, most isolated ones should have a finding chart to avoid any setup ambiguity.

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. It is now believed to be understood and is a feature of the rail-curl pointing software corrections. A software fix has been developed. For most targets this drift is completely compensated by the guider corrections.

For moving targets the only tracking compensation that is presently available is to either

For bright targets: Guide sufficiently quickly on the target to keep it in place.

For faint targets: Guide on a brighter star and move the guide box once a minute or half minute to keep the moving target in place. A PI must supply an RA and DEC tracking offset rate (arcseconds/minute).

Acquisition

LRS:

LRS imaging is used for acquisition with a limiting magnitude of approximately 23, largely based upon sky brightness issues such as haze, dust and clouds.

For optically faint targets, the slit can be rotated such that a brighter target is also on the slit. Notwithstanding the extra step, this tactic can yield a net shortening of the setup duration, IF the accurate position angle is supplied to the RA. For the faintest targets blind offsetting onto the slit works well; three or more medium bright reference stars nicely bracketing the target must be indicated and xy coordinates supplied for all.
HRS:

A photomultiplier is used to locate the fiducial position of the optical fiber probe from the spectrograph. Limiting magnitude is approximately 16.

For objects fainter than 16th magnitude but brighter than 19th magnitude the object can be placed on the fiducial marker which marks the fiber location.

For objects which can not be seen in the acquisition camera (fainter than about 18th magnitude) LRS imaging can be used along with the photomultiplier to center on the fiber. The photomultiplier is used to peak up the fiber center using a bright star in the field and then an image is taken using LRS. The location of this bright star is recorded and the faint object moved to that location. This method adds about 5-8 minutes to the setup time.

Auto-Guiding

The science target need not be the guide star for either the LRS or the HRS but the guider camera field of view is about 40% the field of view of the LRS science chip.

For the lrs guide camera (LRS science) the absolute limiting guide star magnitude is 18.5 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. The field of view of the guider is 2.4 x 2.4 arcminutes approximately centered on the LRS slit center.
For acquisition mode (HRS science) the absolute limiting magnitude for guiding is 18.5 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. The field of view of the guider is 2 x 2 arcminutes approximately centered on the LRS slit center.
It is possible to obtain spectra of a moving object while guiding on a stationary one. By supplying the rate at which the object moves (i.e. RA and DEC offsets in arcseconds/minute), the lrs guide camera can be programmed to correct for these offsets.

It is important that targets be pre-screened to ensure that a suitably nearby guidestar exists. Omitting this can result in wasted HET time if none turns out to be available. It is sometimes feasible to offset the target along the slit, or rotate the field rho angle, and thus shift a suitable star into the guide camera field. In order to check for guidestar availability, one useful resource is the APM finder chart facility at http://www.ast.cam.ac.uk/~mike/apmcat/interface.html, which will list the magnitudes and positions of all objects within a specified field of view around a coordinate. We suggest all proposers screen their targets for guidestar availability prior to submitting Phase II materials.

The focus model for the HET is determined by tracking a star all the way through its trajectory and recording focus corrections with the LRS. This is done for a number of declinations in both the east and west. The focus dependence map is well determined over most of the range of trajectories, but cruder near the edges especially at the beginning of the track.

Because of this focus model the focus only needs to be adjusted once in a trajectory IF the temperature and SAMS is stable. At the present time no temperature-focus compensation is modeled.

LRS: Focusing is done by making a rough first guess with the guide camera and then refined by doing a focus curve with the LRS. Focus curves use a default of 0.1 mm steps and software solves a parabolic fit for the best focus estimate, simultaneously on up to six focus spot sequences. During an observation focus corrections can be made using the FWHM of the guide guide star only if the star is sufficiently bright (< 16th mag). Field location is not critical since the lrs guide camera differential focus pattern has been accurately determined.

HRS: Each bright run we set the zero point between the guide camera and the HRS instrument using the photomultiplier which is placed in the HRS optical path. During the rest of the month focus is done by optimizing the FWHM of the guide star with the guide camera. At the present time the focus offsets are usually limited to 0.05 mm focus offsets. During an observation focus corrections can only be made using the FWHM of the guide guide star if a sufficiently bright, < 16th mag.

Observing Statistics

Observing statistics were compiled after each science run. With our current stacking procedure and with SAMS, little engineering activity is necessary, and it can be slotted in with the least disruption to science. For this reason, observing statistics are now compiled after each HET trimester. The link below will take you to the observing statistics.

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.

HET FWHM and DIMM IQ Data

HET EE50 and DIMM IQ Data

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.

HET Sky brightness Data

Last updated: Thu, 03 Mar 2011 11:49:43 -0600 caldwell

The Telescope

Technical Overview

Object Observability

Instruments

HRS Instrument Summary

HRS Configurations

HRS Wavelength Calibration

MRS Instrument Summary

MRS Configurations

MRS Wavelength Calibration

LRS Instrument Summary

LRS Configurations

Multi-object Unit

LRS Spectral Resolution

LRS Wavelength Calibration

Visitor Filter Information

LRS Position Angles

Blind Offsetting

Guide Stars Limits

LRS Update from Gary Hill

Program Preparation

Web Management System

Other HET Documentation

Table of Contents > Overview > The Telescope > Performance

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