August 11, 2016

Spotting planets and unseen objects in our solar system and beyond with current and near future telescopes

Adam Crowl describes the magnitude (telescope visibility) of new planets in our solar system. Currently the Pan-STARRS1 Sky Survey is trying to capture everything that can be seen from Hawaii down to a magnitude of +22.

Eventually Pan-STARRS hopes to push down to +24 in magnitude, which *might* capture Planet Nine, if Pan-STARRS sees its part of the sky.

Planet Nine averages a distance of 700 AU from the Sun and is about Neptune size. In that case it’s 13.7 magnitude points dimmer than Neptune’s current +8 at about +22. If it’s near its aphelion (furthermost position from the Sun) at a distance of ~1,200 AU, then it’d be ~40^4 times dimmer than Neptune, with a magnitude of +24. At its closest approach to the Sun (about 270 AU) then it’d be 9^4 times dimmer at magnitude 17.5.

Every doubling in distance, the light intensity decreases by 2^2 = 4 fold.

Astronomical Visual Magnitude is a strange scale – it goes up by 5 for every 100-fold *decrease* in observed brightness.



Large Synoptic Survey Telescope could reach +28 magnitude or 60 times less bright objects

In 2022, the Large Synoptic Survey Telescope (LSST) should begin operating. It is a wide-field survey reflecting telescope with an 8.4-meter primary mirror, currently under construction, that will photograph the entire available sky every few nights. The telescope uses a novel 3-mirror design which delivers sharp images over a very wide 3.5-degree diameter field of view, feeding a 3.2 gigapixel CCD imaging camera, the largest digital camera ever constructed.

From its mountaintop site in Chile, the LSST will image the entire visible sky every few nights, thus capturing changes and opening up the time-domain window over an unprecedented range of timescales for tens of billions of faint objects. Each sky patch will be visited 1000 times during the survey with a pair of exposures per visit. The LSST data will enable qualitatively new science. Tens of billions of objects in our universe will be seen for the first time and monitored over time. Thirty trillion photometric measurements will be made.

The speed with which you can survey an area of sky for objects of a given faintness is proportional to throughput (collecting area times field of view in meters squared degrees squared). The LSST enables totally new windows on the universe because it has such a high throughput, or "etendue." The etendue of LSST is 320 square meters square degrees. A primary mirror diameter of 8.4 m (effective aperture 6.7 m due to obscuration) is the minimum diameter that simultaneously satisfies the depth (24.5 mag depth per single visit and 27.5 mag for coadded depth) and cadence (revisit time of 3-4 days, with 30 seconds per visit) constraints. Above a throughput or "etendue" of 200-300 square meters square degrees, many different surveys can be done using the same wide-fast-deep survey data—a large multiplex advantage.

Some of the science can be done on a smaller telescope in a longer time, but consider the numbers: The speed with which you can survey an area of sky for objects of a given faintness is proportional to throughput (collecting area times field of view in meters squared degrees squared). The LSST enables totally new windows on the universe because it has such a high throughput, or "etendue." The etendue of LSST is 320 square meters square degrees. A primary mirror diameter of 8.4 m (effective aperture 6.7 m due to the tertiary mirror area in the middle of the primary-tertiary mirror, and some obscuration) is the minimum diameter that simultaneously satisfies the depth (24.5 mag depth per single visit and 27.5 mag for coadded depth) and cadence (revisit time of 3-4 days, with 30 seconds per visit) constraints. Above a throughput or "etendue" of 200-300 square meters square degrees, many different surveys can be done using the same wide-fast-deep survey data—a large multiplex advantage.

LSST will repeatedly scan the sky south of +10 deg Dec. accumulating ~1000 pairs of 15 second exposures through ugrizy filters, yielding a dataset that simultaneously satisfies the majority of the science goals. This concept, the so-called "universal cadence", will yield the main 18,000 square degree deep-wide-fast survey (typical single visit depth of r ~24.5) and use about 90% of the observing time. The remaining 10% of the time will be used to obtain improved coverage of parameter space such as ultra deep frequent observations, observations with very short revisit times (~1 minute), and observations of "special" regions such as the Ecliptic, Galactic plane, and the Large and Small Magellanic Clouds. For example, fifty selected 10 square degree "deep drilling" fields could be covered with 40 hour-long sequences of 200 exposures each. Each exposure in a sequence would have an equivalent 5-sigma depth of r~24, and each filter subsequence when coadded would be 2 magnitudes deeper than the main survey visits (r~26.5). When all 40 sequences and the main survey visits are coadded, they would extend the depth to r~28 AB mag.



The European Extremely Large Telescope has secured 90% of its 1 billion euro in funding and construction started in 2014. This is a 39 meter telescope and first light would be 2024

A 40-meter-class mirror will allow the study of the atmospheres of extrasolar planets.

The telescope's segmented mirror will be 39.3 meters in diameter and will gather 15 times more light than the largest optical telescopes operating at the time of its development. The telescope has an innovative five-mirror design that includes advanced adaptive optics to correct for the turbulent atmosphere, giving exceptional image quality


The 24 meter Giant Magellan Telescope should also be operating around 2022-2025. The GMT will have absolute magnitude capability of 29.




Targets for direct imaging exoplanets fall into a few distinct classes:
• Planets still embedded in their parent disks (age = 1 - 10Myr, at 30 - 150pc).
• Young (0.1 - 1Gyr), nearby (3 - 50pc) gas-giant planets, which are intrinsically bright in the near-infrared due to their on-going gravitational contraction and,
• Older (> 1Gyr) planets detectable via their thermal infrared emission or reflected light.

The GMT will provide high contrast, high resolution imaging capabilities in the near and mid-infrared enabling the detection of exoplanets in each of these categories.

One of the technical goals of the GMT is to detect objects more than one million times fainter than the host star at angular separations corresponding to 1.5 λ/D to 20 λ/D






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