On the night of Feb 20/21, 2024, under good conditions, I acquired images of the occultation of a background star by the asteroid (173) Ino. A very brief summary is that I measured the background star to disappear for about 9.7 seconds, as this animated GIF of my images shows.
I also acquired another set of images with my Nikon camera and 300mm lens, as practice for the solar eclipse.
Contents:
This occultation was predicted by the team at asteroidoccultation.com, with webpages provided by Steve Preston. You can find the information they provided to observers at
Our observatory
RIT Observatory: longitude -77.66423 -77:39:51
latitude +43.07538 +43:04:31
altitude 172 m
was predicted to lie close to the path center, so I decided to try to observe it.
The target star is TYC 684-00016-1 at
RA = 04 56 54.95 Dec = +09 17 31.7
was faint enough that I decided to use our ASI6200MM CMOS camera to take the data. Specifically, a one-second exposure time of a small sub-frame region would provide adequate signal-to-noise ratio and a reasonable time resolution, with a sequence of
Here's the field of the event. The target lies in the crosshairs near center. The field of view of this finding chart is about 15 x 15 arcminutes.
I defined a small sub-region on the camera just 198-by-206 pixels in size, large enough to encompass the target star, the bright star HD 287340 to its southwest (lower right), and two fainter sources to its northeast (upper left).
I started acquiring images at 09:40:58 PM EST = UT 02:40:58, about four minutes before the predicted time of occultation, and continued until 09:48:24 PM EST = UT 02:48:24, well after the event. The skies were clear during the observations, as far as I could tell.
I reduced the images in the following manner:
I didn't divide by a flatfield frame because the field was very small and close to the optical center of the field, and because the background sky levels were very low -- just 7 or 8 ADU, not much larger than the readout noise.
There are two possible problems with the times I have associated with each measurement of the target star's brightness.
Let me discuss them briefly in that order.
First, the round-off issue. I used the program MaximDL (version 6.16) to operate the camera and save images to my computer's hard drive. The program read the computer's system clock, set via NTP, and placed a value in the FITS header like this:
DATE-OBS= '2024-02-21T02:40:58' /YYYY-MM-DDThh:mm:ss observation start, UT
Note that the time is "starting time" for the exposure. Since all exposures were 1-second long, it was easy to add 0.5 seconds to yield the time of mid-exposure (which I did, a bit later in the process).
But note also that the time is recorded only to the nearest integer second. Since the dead time was about 0.6 seconds between successive exposures, this mean that a sequence of images might have times recorded as follow:
DATE-OBS= '2023-09-28T07:06:14' /YYYY-MM-DDThh:mm:ss observation start, UT DATE-OBS= '2023-09-28T07:06:16' /YYYY-MM-DDThh:mm:ss observation start, UT DATE-OBS= '2023-09-28T07:06:18' /YYYY-MM-DDThh:mm:ss observation start, UT DATE-OBS= '2023-09-28T07:06:19' /YYYY-MM-DDThh:mm:ss observation start, UT DATE-OBS= '2023-09-28T07:06:21' /YYYY-MM-DDThh:mm:ss observation start, UT DATE-OBS= '2023-09-28T07:06:22' /YYYY-MM-DDThh:mm:ss observation start, UT DATE-OBS= '2023-09-28T07:06:24' /YYYY-MM-DDThh:mm:ss observation start, UT
These do NOT accurately reflect the actual starting times of the images. So, I fit a linear function to the times recorded in the FITS headers for the entire dataset (275 images, covering about 7.5 minutes), then interpolated using this function to find the mid-point of each image. I then used that interpolated time -- which shows a smooth, linear progression -- in all subsequent analysis.
The second issue is that there may be some fixed systematic offset that affects all the recorded times, if the computer's clock was offset from the actual time. I saw no obvious offset when I compared my iPhone's clock display to that of the computer, so any offset couldn't have been large.
I conclude that it is likely, at least, that the recorded times have no zero-point offset larger than +/- 1 second.
Here's how I measured the brightness of the target and three comparison stars in each image. First, for each image independently, detect all stars, measure their positions, and then perform aperture photometry at those positions, using a local sky background based on annuli around each star. I used parameters
I used the XVista software package to carry out all these reductions and measurements.
Next, I performed inhomogeneous aperture photometry to bring all the images to a common photometric reference point. Because star "A" was brighter than the others, it was given most of the weight in this procedure. The graph below shows the zero-point adjustment made to each image; points higher in the graph correspond to images in which all the stars appeared fainter.
The first outlier was the result of my slewing the telescope to push the stars closer to the center of the little sub-frame, and the second was due to a particularly violent gust of wind (I think).
One of the results of the ensemble photometry is an estimate of the uncertainty in each star's average (instrumental) magnitude. The graph below shows those estimates for -- reading left to right -- stars "A", target, "B", and "C". The typical uncertainty in measurements of the target star was about 0.07 mag.
Finally, here are the light curves of the four stars based on this analysis. I've shifted all the instrumental magnitudes so that star "A" has a value of 12.021, which matches the APASS9 value of V = 12.021.
First, all the measurements ...
... and now, a closeup of the region around the occultation.
The target star disappeared in the following manner:
frame_index mid-exp_UT mid-exp_JD target_star
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120 02:44:12.18 2460361.6140299 full brightness
121 02:44:13.81 2460361.6140487 0.6 mag below normal
122 02:44:15.44 2460361.6140676 1.1 mag below (full occultation)
125 02:44:20.32 2460361.6141241 1.1 mag below (full occultation)
126 02:44:21.95 2460361.6141429 0.3 mag below normal
127 02:44:23.98 2460361.6141618 full brightness
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