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ALTAIR Component

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This page explains how to configure ALTAIR in the Observing Tool. Note also the guidelines at the bottom of the page for configuring the OT's "Target Component" when ALTAIR is being used.

ALTAIR Component

The detailed component editor for ALTAIR is accessed in the usual manner, by selecting the ALTAIR component in your science program, and is shown here:

The Atmospheric Dispersion Corrector

Currently this button has no effect as the Atmospheric Dispersion Corrector has been removed from ALTAIR. It was never commissioned.

Dichroic Beamsplitter

In general you should select the default 850 nm - 2.5 um science dichroic. New dichroics are in the process of being commissioned.

Cass Rotator

The Cassegrain rotator may be set to either "following" or "fixed" (in NGS & LGS modes). The telescope normally operates "following", with the Cassegrain rotator rotating such that the position angle of the sky stays constant on the detector (usually with North=up and East=left). For ALTAIR, this does not yield an optimal correction. This is because as the Cassegrain rotator moves, rotating static aberrations from the primary and secondary mirrors combine non-linearly (i.e. unpredictably) with the fixed aberrations from the instrument path causing (1) decreased Strehls and (2) stellar profiles that vary with the rotator angle.

For best correction, set the Cassegrain rotator to "fixed", which will cause the science field to rotate in the detector, but will give you better AO performance. With the telescope's current vibration enviornment, the Cassegrain rotator positioned at 0 degrees gives the best AO corrections. When the Cassegrain rotator is fixed in the OT, the observation is automatically set to have this Cassegrain rotator PA (position angle) value. The orientation of each individual field is recorded in the image header, so you will know where North is on each image for de-rotation and combining the images, but you will not be able to specify this ahead of time. Operating "fixed" is recommended for all imaging applications where field rotation is not critical and a stable, high-Strehl PSF is critical, such as looking for faint companions to bright stars and imaging extended structures.

It is NOT possible for the telescope to have the Cassegrain rotator fixed and for you to simultaneously specify the field orientation. Thus, "following" is required for all spectroscopy applications because keeping an object in the slit for a rotating field is not supported.

The Neutral Density filter

The Neutral Density (ND) filter is used to observe very bright stars. It is normally placed in front of either the Natural Guide Star Wave-Front Sensor (NGS WFS) or in front of the tip/tilt sensor (STRAP) for the Laser Guide Star (LGS).

In NGS mode, guide stars brighter than R = 5.5 should use the ND filter always, and the PI should set the button as such. Stars with 5.5 < R < 6.5 may need the ND filter. In this case the PI should leave the ND filter out and place a note in the observation for the observer stating that the ND filter may be needed. For stars fainter than R > 6.5 the ND filter should not be used.

In LGS mode, guide stars brighter than R = 10.5 should use the ND filter. Please note that the LGS ND filter is currently NOT commissioned. This can result in somewhat elongated images because the Altair control loop has not been optimized for the low sky background seen with the ND filter. If you need to guide on a star brighter than R = 10.5 (such as a Telluric standard) it is recommended to use the NGS mode instead, which has the exact same optics in the science path, lower acquisition overhead, and lower scheduling requirements (i.e Space Command clearance). If you do require the LGS ND filter, you must place a note in the program that says the ND filter is needed. This will alert the observer who will make sure the ND filter is in prior to observations. Failure to do so will cause STRAP to shutdown in an ungraceful manner and it potentially dangerous to the system.

Note that the ND filter only affects the Altair path, not the science path, so steps should still be taken with the science instrument to reduce flux if saturation might be an issue.

Guide Star Type

This field is used to select several Altair parameters. All science users should read the detailed Field Lens page before submitting NGS observations. The Field Lens is required with LGS due to the optical design of the LGS system.

  • Natural Guide Star mode without the Field Lens is typically used for on-axis point source observations.
  • Natural Guide Star with Field Lens provides higher Strehl ratios and sharper image quality at large distances from the guide star.
  • Laser Guide Star + AOWFS uses a nearby star for tip-tilt correction and produces nearly diffraction-limited PSFs.
  • Laser Guide Star + PWFS1 is the new "super-seeing" LGS mode using a tip-tilt star as far as 6 arcminutes away to reduce the natural seeing PSF FWHM by a factor of 2-3.

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