THE ONE METRE INITIATIVE (OMI) AND THE CANADA FRANCE HAWAII TELESCOPE (CFHT) WIDE-FIELD COMPARITIVE CAPABILITIES
Frank Roy, Elektra Observatories
(frank.roy@elektraobservatories.org,
http://elektraobservatories.org)
Abstract. The One Metre Initiative (OMI) is a wide-field telescope (5 degrees²) designed to be an extremely efficient imaging platform. The OMI is compared to the CFHT Megacam; Canada’s premier wide field instrument in terms of throughput and wide-field imaging performance. The OMI will be the only large étendue (WA) instrument in the world offering a seamless (continuous) image due to its monolithic CCD sensor.
1 Introduction
The performance of a wide field imaging telescope has several key parameters that determines its overall performance. These include the field size, aperture, download times, filter change times, CCD imager topology (monolithic vs. mosaic), seeing and telescope overhead (position to position time, etc.). We will show that the OMI can outperform the CFHT for objects brighter than mag=23.5 for a 5 degrees² FOV (r’, s/n = 5). The comparison assumes similar filter transmission, mirror reflectivity and lens transmission. Both the CFHT and the OMI use the u’, g’, r’, i’, z’ filter set. In addition the OMI will employ an ultra wide-band, 420-870 nm filter reaching 0.65 magnitude deeper when compared to the r’ filter, useful for rapid detection when this is the overriding requirement. A monolithic image sensor will give the OMI a distinct advantage when compared to other large étendue telescopes.
2 CFHT
The CFHT Megacam covers a FOV of 0.90 degrees² (0.96º X 0.94º) with a clear aperture of 3.6 meters and a central obstruction of 1.5 meters (17.5% areal obstruction). The CCD imaging array uses thirty-six 2048 x 4612 sensors arranged in a 4 x 9 pattern. The mosaic has 3 X 9 gap grid with 15”/12” gaps between the sensors with two 85” connector gaps. Thus a dithering movement is required to fill in the gaps. The download overhead is 40 seconds and filter change times are on the order of 90 seconds. Typically an overlap is required to join the fields (~10%, 0.1deg, 5% per side) for wider area surveys.
3 OMI
The OMI covers a FOV of 4.97 degrees² (2.23º X 2.23º) with a clear aperture 1.016 meters and a central obstruction of 0.42 meters (17% areal obstruction) The CCD image sensor is monolithic with a 10560 x 10580 array, thus no gaps. Download times are on the order of 1.4s and filter change times are smaller than 10 seconds. The OMI has less than 6% vignetting on the extreme edge of the field (corners). The monolithic image sensor offers a much simpler calibration and geometric correction, and removes the need for dithering with more consistent QE throughout the array and 100% fill factor. Although the OMI captures 10X less light per unit time than the CFHT the overall throughput (area/unit time) can be much higher for objects brighter than magnitude 24 due to the CFHT’s large overheads. Indeed, due to the OMI’s larger FOV it has an extra efficiency gain because a smaller percentage of the image area is committed to overlap.
4 CFHT and OMI Comparison
The CFHT and the OMI key parameters (see Table 1) highlight the important differences. In terms of imaging efficiency, the important overhead differences are download times (40s vs. 1.4s), filter change (90s vs. 10s) and a mosaic array vs. a monolithic sensor. For m<23.5 (r’, s/n=5) the OMI can outperform the CFHT, as shown in Figure 1, for a 5 degrees² FOV. Although the CFHT has over 10X the light gathering capability of the OMI its overheads reduce the efficiency. For dimmer objects, the OMI performs at 30% the rate of the CFHT. For multicolor imaging the CFHT’s 90s filter change times are significantly longer than the OMI’s change times of 10s as seen in Figure 2. Figure 3 shows the OMI/CFHT relative efficiency for different s/n from magnitude 22 to 26 (r’), the relative efficiency diminishes with larger s/n but tops out at ~35% for a s/n ≥ 10. The OMI is seeing limited, with expected seeing of 1.25” FWHM. With similar seeing to the CFHT (0.8” FWHM) the OMI could almost match it even at very faint magnitudes for 5 deegrees² FOV, as can be seen in Figure 1 and 2. The OMI will be located at 400 m altitude and will have a cosmic ray flux ¼[1] that of the CFHT at 4200 m.
Table 1. OMI and CFHT parameters
|
|
CFHT |
OMI |
|
Aperture (m) |
3.6 |
1.016 |
|
Obstruction (m) |
1.5 |
0.42 |
|
Effective Area (m²) |
8.4 |
0.81 |
|
FOV (degrees²) |
0.90 |
4.97 |
|
Download times (s) |
40 |
1.4 |
|
Array topology |
4 x 9 |
monolithic |
|
Filter change time |
90 |
10 |
|
Read noise (e-) |
4 |
14 |
|
PSF (arcsec/pixel) |
0.187 |
0.76 |
|
QE 500 nm (%) |
85 |
94 |
|
Dark noise (e-/pix/hr) |
~1 |
~1 |
|
Sky Brightness v (mag/arsec²) |
21.4 |
21.82 |
|
Seeing (mean FWHM) |
0.80 |
1.25 |
|
Altitude (m) |
4200 |
400 |
Table 2. OMI and CFHT comparative cycle times for 5 deg² FOV for a mag=23, s/n=5 with a filter change
|
Magnitude r’ |
CFHT (0.85”) |
OMI (1.25” ) |
Notes |
|
Single Frame Exposure (s) |
9.4 |
123 |
|
|
Download (s) |
40 |
1.4 |
|
|
Equivalent FOV (frames) |
6 |
1 |
For a 5 deg², includes overlap |
|
Move to next frame (s) |
5 |
5 |
|
|
Filter change (s) (effective) |
90 (50) |
10 (8.4) |
Parallel with download |
|
Total cycle time (s) |
376 |
138 |
|
Figure 1. OMI vs. CFHT relative efficiency for a 5 degrees² FOV and varying seeing
Figure 2. OMI vs. CFHT relative efficiency for a 5 degrees² FOV in g’, r’, i’ with varying seeing
Figure 3. OMI/CFHT relative efficiency for 5 degrees² FOV with for a given s/n and limiting magnitude
5 Sky Coverage
The example given here is centered on M31, m=23 (g’, r’, i’) with a s/n = 5 and a 6º x 6º FOV with 0.2º overlap (0.1º/side) between the fields. The image requires nine OMI frames to accomplish this task which could be achieved in about 1½ hours. Similarly the CFHT would take about four hours for the same field size and require about 54 frames to cover this FOV to assure sufficient overlap between the frames (0.1º); some efficiency is lost in the overlap. Table 3 shows the sky coverage for an 8 hour period for non-overlapping and over-lapping coverage 4.97/4.41 deg² and 0.90/0.49 deg² respectively for the OMI and CFHT.
Table 3. Sky coverage in degrees² comparison for s/n=5 for an 8 hour period, includes download times and overlap (1.25” OMI and 0.85” CFHT FWHM) but does not include moving the telescope to the next position
|
Non-overlapping |
|
Overlapping (0.1º, 0.05º/side) |
|||
|
Magnitude r’ |
OMI |
CFHT |
OMI |
CFHT |
|
|
|
(deg²) |
(deg²) |
(deg²) |
(deg²) |
|
|
22 |
4800 |
610 |
4250 |
331 |
|
|
23 |
1300 |
540 |
900 |
294 |
|
|
24 |
200 |
350 |
165 |
190 |
|
|
25 |
34 |
110 |
27 |
59 |
|
|
26 |
4.97 |
22 |
4.97 |
11 |
|
|
27 |
- |
3 |
|
- |
2 |
Figure 4. 6º X 6º degrees FOV centered on M31. The OMI would need 1½ hours (9 frames) total in the g', r', i' band (1.25" FWHM) to mag=23 with s/n=5. Similarly the CFHT would take ~4 hours (54 frames) (0.85" FWHM) to image the same size field in the g’, r’, i’
Figure 5. The OMI FOV (4.97 deg²) is 5.5X the CFHT FOV (0.90 deg²). A monolithic sensor offers seamless images and higher yields, i.e. no gaps or dithering. The larger FOV requires less waste in overlap and telescope movements.
