As you may have surmised from the email to Supercircuits that I CC'ed to you yesterday, I purchased a PC-164C video camera a couple of weeks ago and have been trying it out. I was amazed the other night how well it would pick up the new Ikeya-Zhang comet with a relatively fast C-mount lens. You really should try this if you have a suitable lens. (I have purchased a couple of lenses on Ebay, along with 2 monitors.)

Anyway, one of the things I have recently undertaken is to compare the sensitivity of the PC-23C and PC-164C with regard to stellar magnitudes. Here's the method I used:

After getting a rough idea of the of the sensitivity of the cameras, I chose a patch of sky in the Milky Way with a good number of stars in the desired magnitude range roughly 6 to 9. I had previously used the Hyades and Pleiades for this purpose for the PC-23C, but they're getting rather low in the west, so I chose an area near Eta Geminorum (where Jupiter currently is - very easy to find). I then printed out a star chart out to about mag- nutude 11 from MegaStar version 3 (which Dave Hasenauer gave me after upgrading to version 4). With a couple of tries I found that an area about 8 x 10 degrees worked well for the Angenieux 25mm f0/.95 lens I was using. Then I set up the camera and tripod with monitor on our back patio and aimed it in the right direction. With the chart and monitor in front of me on a picnic table, I methodically marked with a highlighter on the chart the stars that I could see on the monitor. (In some cases, "barely see" would be a better description!) Then I went back to my computer and, running MegaStar and Excel simultaneously, entered each star's spectral type, visual magnitude, and SAO number into Excel under either a "visible" or "invisible" column. Finally, I produced a chart in Excel, indicating the visible and invisible stars with different symbols, for each camera.

The result is the attached Excel file found at the end of the article.

1. The increased red sensivitity of both cameras means that the magnitude limit is higher for reddish K and M stars than for bluish B and A stars, as much as one full magnitude over the full range of spectral types.

2. There is some overlap of the "visible" and "invisible" data realms. This undoubtedly stems from my own variability in ascertaining whether a very faint dot is or is not really visible on the screen. The brightness and contrast settings on the monitor were similar for both sessions, and I did adjust them for what I felt gave the best visibility of faint stars. There are probably some errors in the MegaStar database; I ran across a couple of data items that couldn't possibly be correct. Also, with a fairly large star sample, there are certainly going to be some variable stars.

3. These surveys were done when Santa Ana wind conditions made the sky quite clear (for urban Southern California!), but a darker sky might improve things, particularly for the more sensitive PC-164C. I had noticed previously that with such a fast lens, the camera was picking up skyglow (from light poluution) near the horizon. The star field was probably at about 45 degrees elevation at the time of my surveys; the visual magnitude limit was probably only about 4, which is about as good as it ever gets here. That's a guess based on my prior experience, I wasn't dark-adapted at all.

Conclusion: With this lens under good local conditions, the PC-23C can detect A-class stars down to about Mv=7.5, and K-class stars down to about Mv=8.5. For the PC-164C, it's almost exactly 1 magnitude fainter (Mv = 8.5 to 9.5). I didn't do any regression fitting, but I'd say this is at about the 50% confidence level. For a higher confidence level of say 90 to 95%, decrease these by about 0.5 magni- tude.

John Isenberg
john@opticalres.com
Pasadena, CA

EXCEL FILE DOWNLOAD: Supercircuits_Camera Sensitivity