Observe a field in SDSS Stripe 82 at relatively low airmass (X<1.3).
Initial pixel scale and orientation information should already be contained as WCS keywords in the image FITS header.
Use ds9 to open the image from one of the two CCDs.
Once the image is up, within ds9 go to the Analysis->Image Servers->2MASS menu item and retrieve the 2MASS image.
Once the 2MASS image is retrieved and has appeared in the 2nd window of ds9, go to the Frame->Match Frames->WCS menu item.
Do the two images have the same orientiation (rotation)? If note, note the differences. One can use the Rotate and Invert items under the "Zoom" menu to attempt to get the same orientation.
Identify 2 stars that appear both in the PreCam image and in the 2MASS image.
Select the Region->Shape->Ruler.
On the PreCam image, connect the two stars with the ruler.
Click on the hypotenuse of the resulting right triangle. A window will pop up with the title "Ruler".
In the "Ruler" window, the value of the length should be in pixels (the coordinates should be labeled as "image"). Note this value.
On the 2MASS image, connect the two stars with the ruler.
Click on the hypotenuse of the resulting right triangle. A window will pop up with the title "Ruler".
In the "Ruler" window go to the "Length" menu and choose "WCS".
Also in the "Ruler" window, go back to the "Length" menu and choose "ArcSec".
The value of the length in the "Ruler" window should be in arcsec. Note this value.
The camera pixel scale is the length of the hypotenuse in arcsec divided by its length in pixels. It should be near 1.43 arcsec/pixel.