Re-calibration: What exactly are the steps involved?
Re-calibration: What exactly are the steps involved?
There are many things done to properly
calibrate our instrument. Here are the main steps involved in creating
the data plots you see on these web pages.
Each of these is described in more detail below. Putting it all together, though, it goes something like this:
- Whenever we observe something, make cross-scan measurements.
These are "on-source" minus "off-source" measurements, which allow us to
eliminate most unwanted signals from the sky, the Earth, and our own
equipment. (In the plots you see on this web page, the subtraction has
already been made, so you don't see the raw numbers.)
- Very rarely (perhaps once a year), observe astronomical sources
of known brightness all over the sky, in order to determine how the
telescope's antenna efficiency changes from place to place on the sky.
- Observe a flux calibrator (an astronomical object of known
brightness) at least once a day to determine the end-to-end system gain.
This is a factor that converts the power we measure in our detectors
(measured in micro-Watts) to a flux coming from space (in Jansky). It is
valid only for one place on the sky, however, and for one moment in
time.
- Make frequent (roughly every 20 minutes) mini-cal measurements to track changes in how our receivers respond to signals.
- Make observations of the unknown source you wish to measure
(using cross-scans, of course!). Apply the following correction factors
to convert the power measured in our receiver to a flux from the sky:
- Multiply by the end-to-end system gain found in step 3 to convert micro-Watt to Jy.
- Apply a correction based on the mini-cal data (step 4)
to account for changes in our electronics between the time the flux
calibrator was observed and the time our target was observed.
- Apply a correction factor based on our antenna
efficiency data (step 2) to account for changes in the antenna
performance between the location of the source we are looking at and the
location of the flux calibrator used to get the end-to-end gain.
Once you've done these things, you know the flux from your target!
Sometimes we like to use different units than Jy to measure how bright
our target is. For information on that, please refer to our Brightness
Units F.A.Q. entries.