The Panstarrs Telescopes

The optical design of the telescope is relatively straightforward. It is essentially a Ritchey-Cretien telescope with a 3 element corrector. There are the usual challenges with aspheres, baffling, etc., which we believe should not be major problems. The image size budget is driven by the excellent seeing on Mauna Kea (free air median seeing of 0.5-0.6 arcsec), and should not degrade the PSF significantly. The filters are large (500 mm diameter), and will comprise the standard SDSS g, r, and i filters, with a z filter which differs in that it is cut off at 915 nm, and a y filter which extends from 965 to 1025 nm. The filter changer is a simple stack of three slides, each carrying a filter at each end and an open aperture in the middle. The University of Bonn is fabricating the large shutter.

The focal plane is 400 mm wide and will be curved with a radius of 470 m (see Fig. 1). We expect to build an atmospheric dispersion compensator (although PS1 will initially be built without an ADC), which is a drop-in replacement for the first lens of the system. The ADC consists of a prism which rotates between two other wedged lenses, all of which are immersed in Siloxane oil to suppress ghosting. We will not depend on the telescope to maintain a good collimation for long periods of time. The primary mirror will be actuated to compensate for offsets in the vertical direction as well as a 12-point astigmatism correction. The secondary will be mounted on a hexapod for all five degrees of freedom.

We are designing a calibration system, which should enable us to reach 1% photometric accuracy. This system comprises a flatfield screen which we can be illuminated with monochromatic light and monitored with a calibrated photodiode. Thus we should be able to characterize the throughput of the entire optical system on a weekly basis. This will also give us flatfield images that we can match to any spectral energy distribution of interest. In particular we should be able to simulate the night sky emission and build an artificial fringe image for subtraction. We will also have a sky monitor telescope which will give us a minute-by-minute measure of the transparency. It will have a spectroscopy channel to monitor atmospheric absorption and emission as a function of wavelength on a minute-by-minute basis in the direction that Pan-STARRS is pointing. The low resolution spectra from the monitor telescope will provide a high resolution theoretical model of the atmospheric behavior.

Pwi-STARRS Feed Surf»

Pwi-STARRS Feed Surf»

Figure 1. The Pan-STARRS focal plane with detector layout, 3 deg circle of good focus, hexagonal tile, and 1.65 deg circle for wavefront sensing.

The focal plane will be equipped with two wavefront sensors. A deployable Shack-Hartmann sensor illustrated in Fig. 2 can be brought out over the focal plane to perform a detailed analysis of wavefront tilts when a bright star is steered onto its entrance aperture.

There will also be continuous wavefront sensing available from an apparatus just outside of the 3 degree field of view. This curvature sensing apparatus consists of a converging lens and block of calcite which makes simultaneous above- and below-focus images of all the stars present. These donuts are very sensitive to wavefront errors, and can be analyzed to reveal errors in secondary displacement and tilt. We expect to read out these curvature sensing images every 30 seconds along with the science data, and have information immediately available about the state of focus and collimation of the telescope. With proper choice of the optical axis of the calcite, we can arrange for the two star images to be adjacent to one another at a convenient separation. The introduction of the calcite would ordinarily put the balanced pair of donuts below the nominal focal plane, and since we want this apparatus to be parfocal with the rest of the field of view, we introduce a converging lens (see Fig. 3).

Figure 2. The Pan-STARRS Shack-Hartmann wavefront sensor folds the light in such a way that it is parfocal with the science images. The sensor unit is deployed above the huge focal plane which serves as its detector. The apparatus also provides an out-of-focus pupil image and carries LEDs and calibrated photodiodes for purposes such as monitoring QE or window fog.

Figure 2. The Pan-STARRS Shack-Hartmann wavefront sensor folds the light in such a way that it is parfocal with the science images. The sensor unit is deployed above the huge focal plane which serves as its detector. The apparatus also provides an out-of-focus pupil image and carries LEDs and calibrated photodiodes for purposes such as monitoring QE or window fog.

Figure 3. This schematic diagram shows how a calcite block and converging lens can provide above- and below-focus images which provide focus and collimation information. We will employ at least two of these units above the Pan-STARRS focal plane, just outside of the 1.5 deg radius.

Was this article helpful?

0 0
Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment