The Trans-Antarctic Mountain range behind BTN
 
 
     

PILOT Performance Specifications

   

   
Sections

Introduction

This document provides a performance comparison between the capabilities of a 2m telescope located at Dome C, Antarctica (e.g. PILOT) with a 8m-class telescope located on a temperate latitude site (e.g. Gemini). PILOT is envisaged as a 2m-class telescope, capable of diffraction-limited, wide-field performance from 30 microns to less than 1 micron. Below we outline the results of calculations to estimate the performance of such a telescope based on the characteristics that have been measured, and inferred, for the Dome C site through the site testing program underway at the French-Italian Concordia Station.

Assuming equal telescope performance, the integration time needed to reach a given sensitivity limit scales as B (theta/D)2 where B is the background emission, theta the spatial resolution and D the telescope diameter. There are three different regimes that should be considered when comparing the performance of Antarctic and temperate-latitude telescopes:

  1. Diffraction Limit, when theta is proportional to 1 / D and so t is proportional to B/D4. This generally applies in the mid-IR, when the diffraction-limit exceeds the seeing at a site.
  2. Extended Source, when theta is the same between two sites. Then t is proportional to B/D2. This applies in any regime where theta is greater than the seeing or diffraction limit.
  3. Seeing Limited. Here the performance comparison depends on the achievable spatial resolution, which varies significantly with wavelength and site, as well as on the performance of the adaptive optics system used. It also depends crucially on the isoplanatic angle within which any wavefront correction may be made. For instance, in the near-IR, diffraction-limited operation can be achieved at both sites, but the isoplanatic angle is significantly larger at Dome C. In the optical regime, performance close to the diffraction limit can only be obtained with an AO system on an Antarctic telescope, meaning that a 2m telescope at Dome C with an AO system can be more sensitive than an 8m at a temperate site (where an AO cannot operate in the optical).
There is also a fourth regime which applies:

  1. In wavebands where the atmospheric transmission makes observation virtually impossible to undertake at the temperate site, but feasible from Antarctica, then completely new kinds of investigation are possible. This applies in the mid-infrared regimes, from 17-30 microns, and in the far-infrared terrahertz windows, especially near 200 microns.

We examine the performance of the two telescopes below, first comparing the atmospheric transmission and emission, and then the achievable angular resolutions. The comparison shows that there are three separate regimes that should be considered where PILOT can conduct front-line science:

  • High spatial resolution imaging at visible wavelengths (0.6-1.0 micron),
  • Wide-field, near-diffraction-limited observations at near-IR wavelengths (1.0-2.5 microns),
  • Wide-field, diffraction-limited observations at thermal-IR wavelengths (3.0-30 microns),
  • Terrahertz astronomy, from 200-450 microns.
Sky Transmission

The three plots below compare the transmission of the atmosphere, from 2.5-5 microns, 5-30 microns and 30-500 microns, at Dome C (3,268m - blue) to Mauna Kea (4,200m - red). The low temperatures in Antarctica prevent the atmosphere from holding significant amounts of water vapour, leading to increases in atmospheric transmission. This has the most dramatic effect at mid-infrared and sub-millimetre wavelengths, but is also noticeable in the near infrared.

 

Sky Background

The extremely low temperatures of the atmosphere above Dome C results in the infrared sky emission being significantly lower than at mid-latitude sites. This is demonstrated in the figure below, showing models derived from the LBLRTM radiative transfer code, for the sky background in Janksies per square arcsecond at Dome C (blue) and Mauna Kea (red), for 2.4-5 microns, 5-30 microns and 30-500 microns. A typical background drop of a factor of ~20 occurs between Mauna Kea and Antarctica for all wavelengths beyond 3 microns. From 2.27-2.45 microns the drop is ~50 times.

 

Sensitivity

The low infrared background implies that a Dome C telescope should be at least an order of magnitude more sensitive than a mid-latitude telescope of the same size. For different sized telescopes, the relative sensitivity changes depending whether point source or extended objects are being observations. It also depends on the capabilities of the AO systems.  The left graph below shows the limiting magnitude per arcsec2 for extended-objects and the right graph the point-source limiting magnitude. In both cases the SNR=10 in one hour of observation at R=5. Three telescopes are compared: an 8 m telescope on Mauna Kea (e.g. Gemini) (red lines), PILOT - a 2m telescope at Dome C (green lines), and an 8m telescope at Dome C (blue lines). Other assumptions made are that the pixel scale is chosen to be half the greater of the relevant diffraction or the seeing limit of the telescope / site for point source imaging, with the object assumed to be summed over 25 such pixels. The telescope emissivity has been taken as 3% with a temperature of 0C at Mauna Kea and -60C at Dome C. An overall system efficiency of 50% is assumed in all cases, together with a spectral resolution appropriate for the filter bandpass (see Table below).

The table below provides these figures in tabular form. It also includes the effective spatial resolution that could be obtained, combining the seeing with the diffraction limit.

At short wavelengths the larger telescope is clearly more more sensitive, but the gain is less for extended objects than for point sources. In the thermal infrared PILOT has comparable sensitivity to Gemini, and can exceed it. An Antarctic 8m telescope is more sensitive in all wavebands.

 

Band
Lambda
Width
MK8m
Ant 2m
Ant 8m
MK 8m
Ant 2m
Ant 8m
MK 8m
Ant 2m
Ant 8m
 
(microns)
(microns)
 
PILOT
   
PILOT
   
PILOT
 
     
Point Source (magnitudes)
Extended Source (mags/arcsecond2)
Spatial Resolution (arcsec)
V
0.55
0.09
26.3
25.5
27.1
26.5
25.0
26.5
0.50
0.25
0.24
R
0.65
0.15
25.9
25.1
26.7
26.1
24.6
26.1
0.48
0.24
0.23
I
0.80
0.15
25.1
24.3
25.9
25.3
23.8
25.3
0.46
0.24
0.23
J
1.25
0.26
23.0
22.5
24.1
23.1
21.9
23.4
0.42
0.24
0.21
H
1.65
0.29
21.8
21.4
23.2
21.8
20.9
22.4
0.40
0.26
0.20
K
2.16/2.30
0.22/0.23
21.2
21.3
23.3
21.1
21.0
22.5
0.38
0.30
0.19
L
3.8
0.65
16.7
16.3
18.6
16.6
16.3
17.8
0.35
0.42
0.19
M
4.7
0.24
14.4
14.9
17.4
14.3
15.1
16.6
0.34
0.52
0.20
N
11.5
1.0
10.7
9.1
12.0
10.7
10.2
11.8
0.40
1.2
0.32
Q
20
1.0
7.2
5.5
8.5
7.6
7.3
8.8
0.57
2.1
0.53

 

Spatial Resolution and Isoplanatic Angle

The median seeing at Dome C is 0.27" in V-band, and for 25% of the time is better than 0.15" in V. These are extraordinarily good numbers, more than a factor two better than conditions at the best temperate sites. In simply the natural seeing, without any wavefront correction, a 2m telescope can therefore achieve resolution better than much larger facilities at temperate sites. Of course, since seeing varies as (Wavelength)^-0.2, eventually the larger facility will provide superior resolution on account of its small diffraction limit. Comparing a 2m at Dome C to an 8m on Mauna Kea, this will occur for wavelengths longer than 3 microns. The figure below illustrates the seeing-limited resolution attainable at Dome C and Mauna Kea as a function of wavelength for 2m and 8m-sized telescopes.

Seeing limited resolution for Dome C in median seeing and best 25% seeing conditions, compared to the resolution attainable on Mauna Kea. The resolution is the sum, in quadrature, of the seeing and the diffraction limit at the relevant wavelength, site and aperture combination. The diffraction limited performance for a 2m and an 8m telescope is also shown. For the best 25% conditions, near-diffraction limited performance for a 2m telescope at Dome C is obtained for wavelengths greater than 1.65 microns (H-band), whereas in median conditions this occurs for wavelengths greater than 2.2 microns (K band). An 8m telescope on Mauna Kea has superior resolution to a Dome C 2m for wavelengths greater than 3 microns (L band), when the diffraction limit is greater than the seeing.

The turbulence profile of the atmosphere above Dome C is unique, being confined to a layer close to the surface. Combined with a lack of high altitude winds, it results in significant improvements in performance for an Adaptive Optics(AO) system on a telescope placed at Dome C, compared to a typical mid-latitude site. Based on measurements made with a MASS and SODAR instrument in the first 6 weeks of the 2004 winter at Dome C , atmospheric turbulence models can be constrained to indicate the range of capabilities that tip-tilt and low order AO systems would have on the PILOT telescope. Below we show the calculated Strehl Ratio (the ratio of the peak flux to the value obtained with diffraction limited imaging) as a function of angular size for a simple tip-tilt correction system, for wavebands from optical (V) to the thermal-IR (L). The left-hand plot shows the values in the median seeing conditions, the right-hand plot in the best 25% conditions.

From this plot it can be seen that at Dome C tip-tilt correction recovers most of the diffraction limit in the J, H and K bands (1.2-2.4 microns) over fields of view of a few arcminutes - ie over the entire field of view of the likely cameras. Tip-tilt correction is also efficient over nearly 100% of the sky, since there will invariably be guide stars available for any given source.

AO systems can provide even better spatial resolution, albeit over smaller fields of view. The performance of an AO system is a function of many parameters, including the number of actuators in use, the size of the telescope, the brightness of the guide star being used for correction and its angular distance from the object under study, in addition the site seeing and isoplanatic angle. In the figure below we compare the performance of an on-axis AO system with 45 actuators on a 2m telescope at Dome C (in median and best 25% conditions) to a similar sized telescope on Mauna Kea (median seeing conditions). The Strehl ratio is shown as a function of the magnitude of the on-axis star being used for the correction; ie a star effectively within the isoplanatic angle, which is 6" at V for Dome C and 2" at Mauna Kea. Correction is possible for stars brighter than 10th magnitude within 6" at Dome C, whereas at Mauna Kea a star brighter than 8th magntitude, less than 2" from the source, is required. With a ground layer AO system, however, correction should be obtainable in the visible over a wide field of view at Dome C (Ragazzoni, 2004, SPIE, in press).

Strawman Instruments

PILOT would have a cassegrain focus, together with two Gregorian-fed Naysmyth foci, one of which could be used to fed a subsequent interferometer. Based on the above resolution and sensitivity calculations, and the detector technologies associated with different wavebands, we suggest six instrument concepts where PILOT provides unique capabilities that could be exploited using simple camera systems:

  • high spatial resolution in the visible (V, R & I bands ) with a low-order AO (45-element) system,
  • wide-field optical imaging (V, R & I bands), fully-sampled for the natural seeing conditions, so also providing good spatial resolution (~0.25"),
  • near diffraction limited imaging in the near-infrared (J, H & K bands) over modest fields of view, with a tip-tilt system,
  • wide-field, diffraction-limited near-infrared (J, H and K bands) imaging over the whole sky, with a tip-tilt system,
  • high sensitivity, wide-field imaging in the thermal part of the near-IR (K, L and M bands) (no wavefront correction needed),
  • (relatively) wide-field, high sensitivity imaging in the mid-infrared (N and Q bands) (no wavefront correction needed).

In addition, to exploit the opportunity to observe in the 200, 350 and 450 microns windows, there would be

  • a terrahertz frequency, single-element, spectrometer

The table below gives strawman specifications for four imaging cameras that take advantage of these capabilities. Two optical (VRI) and two near-infrared (JHK) cameras are suggested. In each of these cases, one camera is designed to fully-sample at the Nyquist limit (for V- and J-bands, respectively) and the other provides a wide field of view (sampled at half the seeing limit in the optical, and under-sampled in the near-IR to give an exceptionally wide field of view, but still with excellent spatial resolution). The cameras for the thermal-IR (KLM) and mid-IR (NQ) are Nyquist sampled at the diffraction limit for the central wavelength. The largest commercially available detectors are considered for each case. The choice of wavelength coverage and the eventual instrument design will be driven by the science case and the budget.

Wavebands

Array size

Detector

Pixel Scale (arcsec)

FOV (arcmin)

 

 

 

 

 

VRI

High Resolution

4K x 4K

Si

0.025

1.7

VRI

Wide Field

4K x 4K

Si

0.1

6.8

JHK

High Resolution

4K x 4K

HgCdTe

0.08

5.3

JHK

Wide Field

4K x 4K

HgCdTe

0.30

20

KLM

1K x 1K

InSb

0.23

4

N

0.5K x 0.5K

SiAs

0.7

6

200-450

1

NbN

25-60"

0.5-1.0

 

 

 

 

 

Wide Field Surveys

As a relatively small telescope, wide-field instruments can be built for PILOT cheaply, at least in comparison to instruments with the same field of view on 8m class telescopes. Combined with the sensitivity in the infrared due to the low background and the high spatial resolution due to the good seeing, this makes PILOT a particularly powerful facility for undertaking wide-field infrared surveys. The field sizes that can be achieved easily surpass those obtainable on larger telescopes without large financial investments. In the thermal infrared, while the sensitivity cannot compare with surveys conducted from space, the spatial resolution is several times higher than can be obtained with, for example, the space infrared telescope Spitzer. No wide-field, high spatial resolution surveys are planned for the thermal-IR L and M bands, and so while the depths reached with PILOT would be shallower than at K-band, this area of parameter space has so far barely been explored. Near-IR surveys conducted from space, for instance with NICMOS on the HST, suffer from small fields of view. Hence PILOT can readily probe large parts of parameter space not being tackled with other facilities, between the pencil-beam, but narrow field surveys being conducted with some 8m telescopes, and the shallow, low-resolution all-sky surveys conducted using 2MASS from 1-2.2 micron.

In the Table below are listed some prospective surveys that could be undertaken using PILOT with the strawman instrument suiteand sensitivities listed above. These surveys are still relatively modest in observing time, requiring 1 month of data (assuming a 25% data gathering efficiency over the period). Depending on the importance assessed to a particular science objective, it would be relatively easy to extend them for additional instrumentation costs, for instance by providing dichroic beamsplitters and additional arrays to survey at two wavelengths simultaneously, or by butting several arrays together to increase the field of view. Or simply by conducting the survey over an entire observing season, rather than just one month, to increase the survey area.

Band
Wavelength
Resolution
Mags
Jy
Mags/arc2
Jy/arc2
Frame Time
Area Month
 
µm
arcsec
5sig
5sig
5sig
5sig
Min
Sq. Deg.
JHK High-Resolution, 0.08" pixel scale, 5.3' FOV
J
1.25
0.24
21.0
8.E-06
20.4
1.E-05
1
90
     
23.3
1.E-06
22.7
2.E-06
60
1
H
1.65
0.26
19.9
1.E-05
19.4
2.E-05
1
90
     
22.2
2.E-06
21.7
3.E-06
60
1
K
2.3
0.3
19.8
8.E-06
19.5
1.E-05
1
90
     
22.1
1.E-06
21.8
2.E-06
60
1
JHK Wide Field, 0.3" pixel scale, 20' FOV
J
1.25
0.24
21.0
8.E-06
20.4
1.E-05
1
1200
     
23.3
1.E-06
22.7
2.E-06
60
20
H
1.65
0.26
19.9
1.E-05
19.4
2.E-05
1
1200
     
22.2
2.E-06
21.7
3.E-06
60
20
K
2.3
0.3
19.8
8.E-06
19.5
1.E-05
1
1200
     
22.1
1.E-06
21.8
2.E-06
60
20
KLM, 0.23" pixel scale, 4' FOV
L
3.8
0.42
14.8
3.E-04
14.8
3.E-04
1
50
     
17.1
4.E-05
17.1
4.E-05
60
1
M
4.8
0.52
13.4
8.E-04
13.6
4.E-04
1
50
     
15.7
1.E-04
15.9
5.E-05
60
1
NQ, 0.7" pixel scale, 6' FOV
N
11.5
1.2
7.6
3.E-02
8.7
1.E-02
1
100
     
9.9
4.E-03
11.0
2.E-03
60
2
Q
20
2.1
4.0
8.E-01
5.8
2.E-01
1
100
     
6.3
1.E-01
8.1
2.E-02
60
2

Sample Science Programs for PILOT

Some sample science programs that could be conducted using PILOT are described here. We invite you to add to this list by providing further examples.

Key Papers

While a complete listing of all papers connected with Antarctic astronomy can be obtained from the JACARA bibliography, we have extracted those particularly relevant to the discussion on PILOT, and its role as a pathfinder for future Antarctic facilities, such as ELTs or interferometers, and placed them at the following link: Key Papers.


Jon Lawrence and Michael Burton

12/10/04