NRAO Home  >  Green Bank  |  Wiki Topic:    GB > PTCS > MustangEngSpecs

Changes | Index | Search | Statistics | Go

PTCS Scientific Specifications for Penn Array Demonstration Science

Introduction

This page lays out PTCS scientific requirements for fall/winter 2005/2006 commissioning and demonstration science with the Penn Array. The PTCS scientific requirements derive from the demonstration science goals. Three classes of demonstration science are anticipated:

• Imaging reasonably bright, extended galactic objects such as star forming cores
• Photometry on (ie measuring the flux density of) a series of known point sources with flux densities from a few mJy down to a few microJy
• imaging nominally "blank" fields deeply, to obtain long-wavelength maps of high-redshift star forming galaxies (which appear as point sources at this resolution) and possibly discover higher-redshift galaxies than those seen by existing mm and sub-mm maps.

These are discussed in detail below. The goal in all cases is to do unique, interesting science with the Penn Array on the GBT, in an expert-user mode, in an amount of time that is at worst comparable to what can be done with existing instruments (ideally significantly faster).

The following can be assumed about the Penn Array itself:

I've used as a starting point Jim Condon's original scientific specifications for the PTCS from PTCS Project Note 27 (PTCS/PN 27), to be found at: http://wwwlocal.gb.nrao.edu/ptcs/ptcspn/ptcspn27/ptcspn27.pdf However, Jim's original specification is more general; in particular many specs are driven by single-beam receivers.

Antenna Performance Specifications

To simplify the specifications we introduce the concept of a CALIBRATION CYCLE, which comprises: not less than 30 minutes of astronomical observing; a slew of up to 5 degrees on-sky to a nearby calibrator; up to 10 minutes spent doing calibration observations; and a return slew of approximately the same length. The calibration observations themselves will nominally consist of peaking and focusing on a point source, although more sophisticated procedures could be done if recommended. Fancier routine calibration procedures may require more coordination between the PTCS team and the Penn Array team. See the section on "Observing Modes" below for more details.

Specifications:

• minimum surface efficiency . As a point of reference our current estimate of the surface RMS is 390 um from Q-band efficiency observations, corresponding to a 90 GHz surface efficiency of 11%.
  • Driver: needed for demonstration science to be scientifically visible and exciting.
• stability of peak forward gain (including for instance surface and focus tracking):
  • within 10% over one calibration cycle, for 95% of calibration cycles. So for instance in the absence of significant focus errors. This corresponds to about a 10 micron change in the Ruze-equivalent RMS over that time period. (driver: relative calibration of a single target)
  • stable within 15% , 95% of the time, over a 15 minute timescale that includes a slew of up to 180 degrees in azimuth and 50 degrees in elevation. (driver: absolute calibration , i.e., transferring calibration from a primary calibrator source)
  • It is acceptable for these requirements on the peak forward gain to be evaluated after applying a known gain curve (for instance, a function of elevation) to the data.
• Beam width stable to 10% in each orthogonal direction over one calibration cycle.
  • Driver: image fidelity & calibration of extended sources images.
• Pointing reconstructable to within a 2D RMS of assuming a linear trend in pointing is removed over one calibration cycle
  • Driver: image fidelity & dynamic range. Note that a pointing error drift in one direction is roughly equivalent to a 15% error in the FWHM in that direction.
• Pointing commandable to within 95% of the time, again assuming a linear trend in pointing is removed every 30 minutes.
  • Driver: placing a given detector on a given spot of the sky, so that we can plan our final maps to have approximately uniform sensitivity.
• Blind pointing is good to within for 95% of the time
  • Driver: being able to easily place a calibrator within the field of view.

Performance envelope: The above capabilities are desirable within the following performance envelope.

• Wind: Nominal target-- winds < 2.5 m/s. However since good high frequency time is scarce, a higher wind limit if reasonably feasible is desirable. One should bear in mind that reconstructability of telescope pointing is the goal not commandability of telescope pointing; given this perhaps limits significantly higher than Jim Condon's 2.5 m/s target are possible. An idea of what that wind limit is, with the reconstructibility spec, would be helpful to have when observing.
• Thermal environment: clear cold skies and no sun can be assumed. It is acceptable to wait ~2 hours after sundown to begin observing.
• Telescope motion: as described in the section on Observing Modes, the Penn Array will typically collect astronomically useful data while the telescope is moving at speeds of up to 1 arcmin/sec with accelerations up to 0.1 arcmin/sec^2 (which follows from our 1/f timescale ~10 sec, and our typical map size, ~5'). For some scan patterns (eg: the box scan) significantly higher accelerations may occur at times, but we are happy to not use that data for astronomy. Initial science will be bright stuff and point sources. Longer term we should think about a simple chopping tertiary if we want really good large area imaging (eg: SZ Effect maps).

Observing Modes

Bright Source Tests: The goal is to look at a half dozen bright sources in a night and verify that the optics and calibration,are stable, see how bad the atmosphere is, see what the internal noise source looks like, etc. This is probably one of the first things we'll do. The intent in doing "large" daisies (as opposed to the small daisies which keep a given source in the field of view and optimize photometric efficiency) is to provide some blank sky off-source data so we can also look at how the noise integrates down, look for the error pedestal beam, etc.

• Large daisy scan (r=1', t=30 sec) on a primary calibrator (point source)-- peak vel 0.1'/sec, peak accel 0.01 arcmin/sec^2
• Long slew
• Numerous large daisy scans on one source, which is bright enough to determine pointing.

Imaging Square Fields: The goal is to make a picture of a Galactic star forming region, or a deep map of a high-z galaxy field (also test of instrument noise properties)

• Small daisy scan (r=10 arcsec, cycle time=10 sec) on bright (1 Jy) primary calibrator.
• Large slew (across half the sky: 10 minutes)
• Small daisy scan on secondary calibrator near target field (same parameters)
• Billiard Ball scan on a single field (roughly 10 arcmin x 10 arcmin in size). The scan speed is approximately constant in the relevant part of the map (1 arcmin/sec); peak accelerations can be close to instrumental limits (a few arcmin/sec^2) but only at the map edges where the data can be thrown away.

Photometry: The goal is to measure the flux density of a dozen or so point sources in one night.

• Small daisy scan (radius=10 arcsec, full diameter traversed every 10 sec) on bright (1 Jy) primary calibrator. Peak velocity in this scan is 0.05 arcmin/sec or 8.3e-4 deg/sec; peak accel is 0.015 arcmin/sec = 2.5e-4 deg/sec^2. Multiply these by two for the maximum required in the azimuth direction to allow for full execution up to 60 deg elevation (on-telescope vs on-sky).
• Large slew (across half the sky: 10 minutes)
• Small daisy scan on secondary calibrator near targets (same parameters as per the bright calibrator scan)
• 15 small daisy scans, each of 2 minutes duration, on 15 separate sources (same parameters). These may be scattered within a field of several square degrees on a side and are not bright enough to determine telescope pointing.
• Small daisy scan on nearby calibrator; go to previous step (15 2 minute scans) and continue cycling.

Further notes:

Here is my "competitiveness" calculation which drives the surface efficiency spec. The IRAM 30m delivers 2 mJy rtsec per detector at 1.2mm, and the JCMT 90 mJy rtsec per detector at 850 um. Scaling that to 3mm for a point source with a dust opacity (beta) of 1.35, the JCMT equivalent sensitivity is 1.3 mJy rtsec and IRAM, 2 mJy rtsec. Our old nominal Penn Array sensitivity figure was 343 uJy rtsec but that assumed 12 GHz bandwidth whereas we now have 8 GHz; also allow a 20% contingency for an overall Penn Array of 0.5 uJy rtsec. Both of these numbers assume 36% surface efficiency (comparable to Jim Condon's "usable" 3mm surface efficiency specification). Suppose we want to do photometry a factor of four faster than JCMT at 850 um, then we need 0.65 uJy rtsec with GBT at 3.3mm, which implies a minimum surface efficiency of 28% or 300 um RMS (Ruze).

The revised "maximum slew to calibrator for referenced pointing" was derived as follows. 16 fields randomly spaced around the sky north of the celestial equator were searched for nearby calibrators using Jim Condon's "calfind2" program (at gb:/home/astro-util/pointing/calfind2). These sources need to be bright enough to determine telescope pointing assuming we achieve the above surface efficiency, but aren't necessarily in the class of "whopping bright 1 Jy sources" that you'll first look at with the GBT. We chose 100 mJy as a reasonable realistic minimum flux density for a pointing calibrator, and find that all 16 fields have calibrators with 90 GHz flux densities greater than 100 mJy for a search radius of . With this flux limit, 7 of the 16 fields did not have calibrators within (the old PTCS offset pointing slew of) 3 degrees radius. A limit of was adopted as a round number which provides 20% more sources than the radius thus allowing some margin, for instance, if the extrapolated fluxes in the catalog are systmatically low.

Degrees of Freedom: that can be assumed. These aren't specific proposals, but just to give you an idea.

• Our pointing solutions can be linearly interpolated between calibration cycles, if this is useful.
• Some form of ``Antenna Active Surface peak-up'' could be done every calibration cycle to take out astigmatism and other surface errors (note: we currently have the ability to "dial in" Zernicke polynomials on the GBT surface). We would need to coordinate efforts on the data analysis if this capability is assumed.
• PTCS could be provided with several concrete trajectories that we desire (position versus time). See boundary issues below.
  • We will do this -- Brian will attach several concrete trajectories to this web page soon, which are the baseline cases that we shoot for good performance on.

PTCS/Penn Array Boundary Issues: that should be given careful attention over time

• Coming up with specific scan trajectories that do what we want without placing unreasonable demonds on the servos or QD solutions, for instance.
• Development of special-purpose observing procedures and analysis routines.
• We are discussing an auxilliary mirror unit, probably for fall 2006/winter 2007; this would greatly improve performance for SZ and other extended source measurements, and make less urgent many servo improvements we might consider. The goal is to rapidly slew the FOV in a fixed direction (eg: cross-elevation) and increase the rate at which low spatial frequencies on the sky are sampled. If Phil can pay for parts and construction, Simon Dicker can probably design it.

-- BrianMason - 29 Dec 2004

(RMP editted this 23rd October 2007 to pop it back up the "changed" index)