PhoSim Tutorial 19: How to Simulate a Drift Scan Telescope

PhoSim automatically corrects for the rotation of the sky relative to the telescope due to the Earth's rotation. PhoSim also can add a residual error due to the tracking imperfections. However, PhoSim can also turn all of this off for systems that do not use tracking, but instead clock their detectors to effectively cancel the rotation. This is known as drift scanning.

To demonstrate these capabilities we start with a simple catalog like the following.

rightascension 0.0

declination 0.0

filter 2

obstime 10.0

stars 15.0 30.0 0.2

galaxies 15.0 30.0 0.2

Using following command,

phosim catalog -c examples/nobackground

We obtain the following image.

Note that the wiggling is due to the atmospheric turbulence. To turn off the telescope's field rotation, we add the rotationtracking command to the catalog.

rightascension 0.0

declination 0.0

obsid 9999

filter 2

obstime 10.0

rotationtracking 0

stars 15.0 30.0 0.2

galaxies 15.0 30.0 0.2

This results in the following image.

Now to clock the detector, we can use either the driftx or drifty command. This drift clocks the pixels during the exposure by a certain number of pixels per second. This value could be negative. In this particular example, we use the following catalog.

rightascension 0

declination 0.0

obsid 9999

filter 2

obstime 10.0

drifty 29.0

rotationtracking 0

stars 15.0 30.0 0.2

galaxies 15.0 30.0 0.2

This results in the following image.

This largely corrects the vertical blurring. Note that the image is now longer than the default size, since there are additional rows that will be generated by reading out during the exposure.

An interesting application of the ability to turn off rotation tracking is to take a long exposure near the celestial poles. This will then produce the familiar arcs seen in astrophotography. The following catalog accomplishes this using the Yale bright star catalog for a 1 hour exposure.

rightascension 0

declination 90.0

filter 0

obstime 3600.0

rotationtracking 0

includeobj examples/ybsc5

This catalog is done using the human eye instrument, which naturally has a large field of view.

phosim catalog -c examples/nobackground -i eye

The following image is produced (after a very long computation time).