The Occultation by Nina

Aug 15, 2016

Bright star, but tough; only 18 degrees from a 90%+ moon and only 23 degrees above the SSW horizon in Aquila. Needs clean skies

Preston page and map

UT is 10:09:23 for 122W longitude. Mag drop is only 0.58 for max of 11.8 seconds. Radnovich Ranch is only about 3 mile south of Derek's path, and other spots above the fog are even closer, so likely I'll end up at Radnovich Ranch parking lot.


Post-Occultation DeBriefing...

Drove up to Summit Store, arriving about 2:25am with 45 minutes to set up, getting above the fog about a half mile before hitting Highland Way. The Summit Store parking lot was slightly deeper into the asteroid shadow but lights and trees made it unsuitable. I went back to the original plan; to drive to the parking area for Radnovich Ranch, where the start of the Skyland 10K was a couple of years ago. Perfect! Good horizons all around, and no barriers to entry. Air felt reasonably dry, and although hard to be sure, I didn't see any clear evidence of the smoke from the Sobrantes Fire which was blowing over Santa Cruz during the day. Now, the following day, it's blowing a different direction. I don't think smoke was a problem; the stars all looked pretty normal to the naked eye. However, it's possible there was still smoke causing variable extinction during the 2+ minutes of recording. I would have thought that given a low angle near full moon, that any variable smoke in the sky would show up fairly obviously. The Milky Way was fairly clearly visible; no smoke, though I did try hard to see evidence of it.

At the scope

To more precisely document the scope's location on Google Earth



Set up as per usual, 10" Meade SCT with f/3.3 and Watec 910hx, set on 4x (2 frames integration). There were no comparable stars in the FOV for photometry, but since the target star was 10.9 and the asteroid was 10.5, so we had a bright photometry/tracking object in the Nina itself. Therefore, only "Object A" is in this LiMovie dataset. The seeing was rather poor. The moon was setting, 90% sunlit and behind the hillside. But there was noticable twinkling apparent while watching target the LCD. I used 5,7,25 pixel radii, as I usually do, to insure all light from the star was in the aperture. 4x or 2 frame integration, examining the lightcurve by frames gives an offset = 1 for Occular analysis.

LiMovie light curve

Watching on the LCD screen, I thought it looked like a clear step and then final stepdown, but the lightcurve looks less convincing. The poor seeing leads me now to think it's perhaps noise. Let's see what Derek's light curve shows.

The LiMovie screen, showing the parameters selected

The Occular solution on the light curve, showing the parameters.

Expanded view of the Occular light curve solution, with figure of merit plotted as the "hill". Here, each point is the average of each 2-frame block.

The Occular "Final Report"

The error histogram, nicely Gaussian

My observing site, marked with the cross, in the parking area for Radnovich Ranch on Highland Way.


The Occular light curve output .csv file is here, in case others want to try any noise filtering or other acrobatics. The LiMovie photometry output .csv file

Here's Derek's light curves


Derek and Tony subjectively believe there is a Fourier peak at around a tenth or two of a second in my light curve. About what you'd expect for seeing variations, which are pretty random. However I disagree, and my reasoning is this:

1. I see no difference in such appearance between Derek's close up of the event, and my Occular lightcurve
2. My frame-by-frame chart contains an artificial signal due to the 4x setting on the Watec video camera (2-frames integration), so the adjacent light levels of the two points will be very similar but not identical.. This gives the impression of statistical significance which isn't there. When you instead look at the blocked light curve and compare with Derek's, I see no subjective evidence of a Fourier peak in either.
3. Realize that if the actual brightness is a constant over some seconds, that when there is a noise excursion then reversion to the mean is MORE likely than not, and so an oscillating point to point signal is to be expected, and the eye will try and make it significant when may not be.
4. There was no AC current in any component. The 12" LX200 was powered by a 12VDC battery, converting to 18VDC internal to the telescope. AC however may be generated in running the internals. A test could be to somehow keep a fixed light source in the vide camera and then run the RA or Dec drives and see if some oscillating signal happens. However, the fact that there is no oscillating significant signal noise in mine vs Derek's light curves close up makes me highly dubious there is a spurious oscillation here, beyond normal twinkling.

Tony George did a Fourier analysis on my light curve and finds evidence of scintillation noise only, so no evidence of any sharp Fourier peaks. However, his software has the ability to take out a certain amount of scintillation noise, at least to remove the signature of stonger high frequency power vs low frequency power, turning the power spectrum much closer to white noise. Here's the results

The Fourier spectrum of my original light curve. More power at the small end is typical of scintillation noise (bad "seeing") which is no surprise. The surprise, for me, was that there is a module in Tony's software for removing at least the imbalance in the power spectrum

He used a parabolic filter to remove the scintillation noise and then calculated a new Fourier spectrum, which is now much quieter, with little power at any frequency

The resulting light curve, applying the scintillation filter. Now the question is - does this alter the D and R timings significantly?

My submitted timings, with Derek's, were analyzed and produced this projected asteroid outline.