OMI and the Canada France Hawaii Telescope

Built in the late 1970's and designed using late 1960's and early 1970's technology, the CFHT has managed to stay on the forefront of astronomy. This is mostly thanks to its instrument designers who have maintained the CFHT on the leading edge of advanced telescope instruments in the world. The flagship instrument is the MEAGACAM a 0.90 deg² 340 megapixel mosaic camera of 36 CCD's. This camera can reach magnitude 26 (r') in 30 minutes.

We wanted to show how the OMI compares to a world class 3.6 metre wide field telescope.

CFHT KEY PARAMETER

• 3.6 m aperture
• 1.5 m obstruction
• 0.8" FWHM seeing
• 85% QE (r')
• 0.9 deg² FOV (0.94 deg x 0.96 deg)
• 40s download time
• 0.187" PSF
• 21.40 Sky brightness (v)
• 5e- read noise
• -130ºC cooling (negligible dark noise).
• ugriz filter set
• 90s filter change time

OMI KEY TECHNICAL SPECIFICATION

• 1.0 m aperture
• 0.38 obstruction
• 1.25" FWHM seeing
• 94% QE (r')
• 4.94 deg² FOV (2.22 deg x 2.22 deg)
• 1.4s download time
• 0.76" PSF
• 21.82 Sky brightness (v)
• 4e- read noise
• -100ºC cooling (negligible dark noise).
• ugriz filter set
• 10s filter change time

The OMI has has a few advantages over CFHT to make up for a smaller aperture. Namely a much faster download time (1.4s vs 40s) and more importantly a FOV some 5.4X bigger (4.94 deg² vs 0.90 deg². Although the CFHT collects light 10X faster than the OMI, the OMI can see a much broader section of the sky and can download the sensor array much faster.

Below is graph that shows how the OMI compares to the CFHT taking into account the much wider field of view and the faster download times:

Immediately evident is that the OMI can actually outperform the CFHT in terms of absolute exposure up to magnitude 22. This is entirely due to CFHT's long download times, which are much longer than the exposure times. The chart is calculated for a s/n of 5 and for 0.8"-1.0"-1.25" seeing for the OMI. The better seeing definitely improves the performance of the OMI.

For wide fields of 5 deg² what's most shocking is that the OMI can dramatically outperform the CFHT up to magnitude 23 (r') with 1.25" seeing and magnitude 24 with 1" seeing. This is mostly due to the much wider field of view and faster download times of the OMI.

In real terms what this means is a wide field image, 5 deg², would only take 1.5X longer reaching 24th magnitude (s/n=5, 1.25" FWHM) with the OMI. If the OMI seeing is 0.8" then the OMI could better CFHT up mag 25 and equal at fainter magnitudes.

Science that requires a large field of view that is done on the CFHT could definitely be done on the OMI. Keeping certain considerations in mind such has a more northerly latitude, degraded seeing (1.25" vs. 0.8") and PSF (0.76" vs 0.187"/pixel) for the OMI.

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