Is is possible to use CCDI to determine the seeing conditions, if so how

Dean

Dean,

I use CCDI to inspect conditions every night.

Here is my processes:

1. Find a star close to the Zenith.
2. Center the star on the CCD camera.
3. Focus as best you can.
4. Open CCDI FWHM monitor.
5. Select a sub-frame around the star between 50 and 100 pixels in width and height.
6. Take a 0.5 second exposure.
7. Confirm that the star is not saturating the camera. I like the peak ADU around 20,000.
8. Now start taking continues images of that star.
9. The FWHM monitor will now start measuring FWHM.
10. Right click on the number below FWHM, and change that to 'Chart Moving Average'.
11. Right click in the graphing area on the right and select FWHM.
12. Right click in the graphing area again and select Moving Average to 20.

Once you have everything setup, you can now sit back and let CCDI measure FWHM a few minutes. The number you see below the FWHM on the left (the moving average) will be very close to what you will see on any image you take that evening.

Seeing does change over the course of an evening, but if you start out at 3.5 FWHM, then don't expect sub 2 arc-second seeing.

In order for this process to work, you need to be sampled well enough, that is less than 1 arc-second per pixel.

If you are sample higher than that, then use pixels rather than arc-seconds. On a good night with an OTA at greater than 1 arc-second per pixel you should see a FWHM of 1.6-1.8 pixels. On a bad night it will be 2.5 or higher.

rb