On the night of July 8/9, 2022, under good conditions (but very bright skies), I measured the brightness of Titan and the star SAO 164648 as Titan moved in front of the star and blocked its light. I wasn't able to measure the re-appearance of the star some five minutes later, as the nearing sunrise increased the sky brightness too much. This page contains a quantitative record of my measurements near the end.
For more information about this event, see
The equipment used for this observation was
For a description of the KIWI-OSD unit, see
On this morning, Saturn was surrounded by its moons in the following arrangement, with Titan sitting to the east-southeast.
A CCD image (not taken with the Watec video camera!) I took a few hours before the event showed a very similar field ... but with one difference. There's an extra object sitting just to the west (right) of Titan.
That other object, which is slightly fainter than Titan, is the star SAO 164648 = HD 207123. Over the course of the next few hours, the star would drift slowly to the left, disappearing behind Titan at about UT 2022 July 09 09:15:36 and re-appearing at UT 2022 July 09 09:20:00. From my observation location, this was a difficult event to observe for two reasons: first, the Sun was only 4.6 degrees below the horizon at the time of disappearance, and only 3.9 degrees below at re-appearance; hence, the sky was very bright, and growing brighter with every minute that passed. Second, Saturn and Titan were low in the sky, about 29 degrees above the horizon to the southwest.
The setup for the measurements was a bit complicated. The video camera was mounted at prime focus of the telescope. The video signal was fed to the KIWI-OSD, which added a timestamp to every frame of the video -- one example is shown below.
From the KISI-OSD, the signal was sent into a video-to-digital converter unit, the Canopus ADVC110, which captured and digitized each frame. The digital output from the Canopus went was sent via Firewire into a 2011 MacBook Pro running iMovie, which recorded and saved the data as a (raw) Quicktime video file.
Finally, in order to perform numerical measurements, I converted each frame of the video file into an 8-bit digital image using FFmpeg. In particular, I used a command line like this:
ffmpeg -i clip.mov -vf format=gray -qmin 1 -q:v 1 output_dir/img_%05d.pnm
The pnm file was then converted to an 8-bit FITS image via the ImageMagick suite of programs. I then turned the 8-bit into a 16-bit image in order to avoid overflow/underflow in the subsequent processing, and used my own image processing tools in XVista for the subsequent analysis.
As an example, here's the FITS image corresponding to the sample video frame shown above:
The seeing varied quite a bit from frame to frame, due to the large airmass, but it was typically around 4 pixels = 5 arcsec. The radial profile of Titan in the sample frame above is shown below; the units on the horizontal axis are pixels.
About 90 minutes before the event, I acquired a video clip of 121 frames with the lens cap covering the camera. I used this clip to create a median dark image.
I subtracted this median dark image from all of the images used in the occultation measurements.
I started acquiring a video record for the occultation at UT 09:09:54, roughly 5 minutes before the prediction for disappearance. This video recording continued through UT 09:22:12, about two minutes after the predicted re-appearance. The GPS unit lost its satellite lock around 09:19:48, slightly before the time of re-appearance, but the KIWI-OSD continued to count the video frames accurately.
In order to calculate the time corresponding to each digitized image, I did the following.
seconds after UT 09:14:00 = 54.809 + (frame_index - 9000) * 0.033369
How to measure the brightness of the merged Titan+star image? The rising sky brightness, and possible terrestial clouds, could cause errors in any simple measurement of sources in the images. Ideally, one would use another point source as a reference, allowing one to compute a differential brightness or magnitude; unfortunately, the sky at this time was so bright that the other moons of Saturn could not be detected in the video frames. I therefore turned to a less-than-ideal source to serve as a photometric reference: Saturn itself. The planet and portions of its ring system were obviously saturated, but it was the only game in town.
The measurement process was:
I examined the resulting light curves and determined that the best results came from the smallest Titan aperture: 4 pixels (= 4.8 arcsec) in radius. Photometry based on that aperture is shown in the figures below, and provided in the datafile linked a bit further down.
The light curve before and during the disappearance shows just how noisy the measurements were. The uncertainty in each measurement of brightness is about 0.20 mag, or about 20 percent. One can see a clear decrease in brightness starting at about 70 seconds (= UT 09:15:10) and continuing until the predicted time at which the star would vanish behind the limb of the moon (96 seconds = UT 09:16:36). That indicates that the atmosphere of Titan was dimming the starlight for about 26 seconds.
During the time when the star's light was struggling through the atmosphere of Titan, these measurements show a set of three or four "events" -- brief spikes in brightness. In order to show these more clearly, the figure below includes a version of the light curve which has been smoothed with a boxcar filter of size 4 frames (= 4 x 1/30 sec = 0.13 sec). This figure also zooms in a bit on the time when the star was shining through Titan's atmosphere.
Are the spikes real? Several other observers have noted brief increases in brightness, but it's not clear to me if the size and duration of these events matches those other reports. It's quite possible that these are simple random coincidences of several randomly high values in a row.
The measurements are given in two formats in the files linked below. Each has a simple ASCII header which explains the format of the data; I'll show it here, too:
# Measurements of combined light of Titan and SAO 164648 # during the occultation of UT 2022 July 9. # Data acquired at the RIT Observatory with 12-inch telescope, # Watec 902H3 video camera, with KIWI-OSD timestamps on each frame. # Effective exposure time is 1/29.9679 of a second, no filter. # # Aperture photometry with radius of 4.8 arcsec centered on the # combined light of Titan and the star, subtracting a local sky value. # The instrumental magnitudes are compared to those of a # much larger aperture around Saturn, and the values tabulated here # are difference of the instrumental magnitudes. # # Created by Michael Richmond, 7/20/2022 # # The columns are: # # 1) time since UT 2022 July 09 09:14:00, in seconds # 2) row position of Titan centroid in frame (pixels) # 3) col position of Titan centroid in frame (pixels) # 4) differential magnitude of Titan + SAO 164648 vs. Saturn # 5) uncertainty in differential magnitude #