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12 High-Precision Differential Photometry
Some observatories have sky temperature meters that measure the temperature of the
sky by looking at its infrared brightness. A warm sky is a key indicator of high cirrus
clouds (although not always if they are very high). Likewise, many observatories
have a sky brightness meter, which is effectively a small telescope with a camera
pointing at the meridian and measuring the sky count for precision photometry, you
should try observing only on nights with low sky brightness.
However, as noted previously, phenomena such as exoplanet transits tend not to be
at optimal times where there are good photometric sky conditions. Given that you are
unlikely to be able to move your telescope away from city lights or to a high altitude,
the only real way of dealing with sky noise is to choose the right photometric filter.
Sky noise is relatively flat across the optical part of the spectrum, except for some
strong lines from sodium and other street lighting sources, but your target is going to
be brighter in some bands than others (which can be checked in SIMBAD). Imaging
in the brightest band will improve the signal to sky noise ratio. So for example, if
your target is an M-dwarf, you should image in the R band (or a similar red end
band).
12.6 Lucky Imaging and Adaptive Optics Photometry
Lucky imaging was introduced in Sect. 8.8.1 as a process whereby very high angular
resolution imaging may be accomplished using a modified commercial webcam or
an astronomical webcam. These standard webcams are unsuitable for photometry, as
they have high noise levels, especially read noise. However, a new generation of lucky
imagers based on electron multiplying CCDs, or EMCCD, are now being deployed.
More often used on high-magnification shallow depth of field microscopes, EMCCD
use a photomultiplier to increase the signal without increasing read noise.
By examining the noise within the image and the point spread function of the
stars therein, it is possible to reduce the images taken to those with low scintillation
and sky noise. Stacking these to form single images with a cadence similar to that
produced by a traditional imager should produce images with lower noise and hence
lower uncertainty.
EMCCDs are heavy compared to traditional imagers and therefore can be difficult
to mount on a small telescope. Their very high frame rate (the author has used an
EMCCD at a frame rate in excess of 1000 frames per second) means that standard
computer interfaces are too slow to cache the data, and special interface cards are
needed. Also, the rate of caching can exceed the transfer rate to the computer’s hard
drive.
An additional problem with EMCCDs is that they can be damaged when pointed
at bright sources. This includes the Moon, most planets, and bright stars.
Lucky imaging photometry is out of the range of small observatories and is still
in its infancy. The signal-to-noise gain to cost is high, although for crowded field
and PSF photometry, lucky imaging may provide advantages over standard imaging
systems.
12 High-Precision Differential Photometry
Some observatories have sky temperature meters that measure the temperature of the
sky by looking at its infrared brightness. A warm sky is a key indicator of high cirrus
clouds (although not always if they are very high). Likewise, many observatories
have a sky brightness meter, which is effectively a small telescope with a camera
pointing at the meridian and measuring the sky count for precision photometry, you
should try observing only on nights with low sky brightness.
However, as noted previously, phenomena such as exoplanet transits tend not to be
at optimal times where there are good photometric sky conditions. Given that you are
unlikely to be able to move your telescope away from city lights or to a high altitude,
the only real way of dealing with sky noise is to choose the right photometric filter.
Sky noise is relatively flat across the optical part of the spectrum, except for some
strong lines from sodium and other street lighting sources, but your target is going to
be brighter in some bands than others (which can be checked in SIMBAD). Imaging
in the brightest band will improve the signal to sky noise ratio. So for example, if
your target is an M-dwarf, you should image in the R band (or a similar red end
band).
12.6 Lucky Imaging and Adaptive Optics Photometry
Lucky imaging was introduced in Sect. 8.8.1 as a process whereby very high angular
resolution imaging may be accomplished using a modified commercial webcam or
an astronomical webcam. These standard webcams are unsuitable for photometry, as
they have high noise levels, especially read noise. However, a new generation of lucky
imagers based on electron multiplying CCDs, or EMCCD, are now being deployed.
More often used on high-magnification shallow depth of field microscopes, EMCCD
use a photomultiplier to increase the signal without increasing read noise.
By examining the noise within the image and the point spread function of the
stars therein, it is possible to reduce the images taken to those with low scintillation
and sky noise. Stacking these to form single images with a cadence similar to that
produced by a traditional imager should produce images with lower noise and hence
lower uncertainty.
EMCCDs are heavy compared to traditional imagers and therefore can be difficult
to mount on a small telescope. Their very high frame rate (the author has used an
EMCCD at a frame rate in excess of 1000 frames per second) means that standard
computer interfaces are too slow to cache the data, and special interface cards are
needed. Also, the rate of caching can exceed the transfer rate to the computer’s hard
drive.
An additional problem with EMCCDs is that they can be damaged when pointed
at bright sources. This includes the Moon, most planets, and bright stars.
Lucky imaging photometry is out of the range of small observatories and is still
in its infancy. The signal-to-noise gain to cost is high, although for crowded field
and PSF photometry, lucky imaging may provide advantages over standard imaging
systems.