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Telescopes · Moving / rotating · PSU

PSU moving telescope: DISH222

The active research telescope. It can steer to any part of the sky, which a fixed dish cannot.

From fixed to moving

Why the project moved to a steerable dish

A fixed telescope cannot choose where to look. A steerable one can.

The PSU and PNU fixed telescopes point straight up and let the sky drift through the beam. To map chosen regions of the Galactic plane, repeat observations and point at calibration sources, the project moved to a motorised dish that steers in azimuth and elevation: DISH222, now the active research telescope.

More: the fixed-to-moving story · project background · lecture slides 12–17

DISH222

How DISH222 observes

A conceptual overview for students. It explains the measurement, not how to operate the telescope.

A steerable dish

DISH222 is a 5 m parabolic reflector on an azimuth/elevation mount. To observe a sky position, software converts its celestial coordinates (right ascension and declination, or Galactic longitude and latitude) into azimuth and elevation for the current time and site. A motor controller then drives the dish there, and the dish reports its position back. The observing software keeps scans above 30° elevation.

Receiver chain

Radio waves collected by the dish reach the feed and a low-noise amplifier, then a HackRF One software-defined radio locked to an external 10 MHz reference. In the 26 Aug 2026 session the receiver was tuned to 1419.9 MHz with 2.4 MHz of bandwidth. The computer turns the sampled signal into 1024-channel spectra and averages many of them.

Only these averaged power spectra are stored, not the raw radio samples.

Pointing

The beam is about 3° wide, so pointing errors of a degree matter. A Sun-based pointing calibration on 25 Aug 2026 reduced the residual error to about 1.1°. Later software added an automatic Sun cross-scan check, and comparing the observed ridge of Galactic H I with b = 0° provides an independent test.

Ways of scanning the sky

  • Raster scan: visit a planned grid of sky positions, wait at each one and record. The 26 Aug outer-Galaxy map was made this way (313 positions).
  • Area scan: cover a rectangular box of sky, such as the 2 Sep Galactic-centre region.
  • Drift scan: hold the dish still and let the Earth's rotation carry the sky through the beam, as the fixed telescopes do.

Current role. DISH222 produces all data in the research archive and is the reference instrument for the calibration work described on the Calibration page. New, independently tested DISH222 software is being designed starting with offline analysis tools. The receiver's sample format and the motor-safety behaviour are to be verified independently before any new software controls the telescope.

Data

Data and archive for this telescope

The Data Library lists every DISH222 dataset with its processing level, calibration epoch and limitations.

The DISH222 archive groups the products by type Available on request. Placeholders mark products not yet connected.

Pointing

From celestial position to spectrum

Moving-system concept

For moving systems, a sky target in RA / Dec can be transformed into azimuth / elevation, sent to telescope pointing, and then measured as a spectrum.

  1. Sky target
  2. RA / Dec
  3. Coordinate transformation
  4. Azimuth / Elevation
  5. Telescope pointing
  6. Spectrum

Instrument record

What is documented about DISH222

Generated from the project's instrument registry. Details that are not documented are listed as open questions rather than guessed.