Determing the instrumental zero point

The zero-point of an image can be used to determine the magnitude of an object. An example of the how the zero-point can be found is given here

A python code find_zeropoint.py can determine the zero-point of an image (either a single image, or a stacked image). This codes calcualates the zero-point from every star in the image that has a match in the all-sky APASS catalog, and then returns the sigma-clipped average. To determine the zero-point of an image, carry out the following steps:

For an image for which you want to know the zero_point, run the code:
```
python /mnt/64bin/find_zeropoint.py 
```
e.g.
```
python /mnt/64bin/find_zeropoint.py amosaic_gaia.fits 
```
The code returns the equation needed to determine the magnitude of any objects. e.g.
```
APASS9_V = 23.820 - 2.5*log10(counts)  (GAIA) 
```
In this example, the zero point is 23.820, calibrated against the APASS star catalog. Note that every image (or stacked image) will have a different zero-point (because the weather in Durham is variable, and so from image to image 1 count will correspond to a different magnitude).
This zero-point can be used in e.g. GAIA to determine the magnitude of an object (e.g. Gaia-->Image Analysis-->Aperture Photometry-->Results in magnitudes and enter the zero-point).

Back to the Home Page ams 2019-Feb-09 08:59:11 UTC