Apparent magnitude drop, Eurybates occultation
Oct 20th 2021
Occultation by 3548 Eurybates
The apparent magnitude drop that we’ll be able to detect

As Vadim Nikitin points out (see his diagram below) the target star is actually a very close double star, separation 5", that we will unlikely be able to resolve. So instead of an occultation dmag. of about 3, that component, only slightly fainter than the target, will not be occulted and the real dmag. will be about 0.8. Note that this is in addition to another component, a little fainter than the target, about 15" away that is resolved in our images. The close component is also evident on the zoomed in Aladin image, but seems to be fainter there.

But Dave Herald provides further information:

In the next version of Occult a new capability will be present in Occult (it is currently still under test). Occult will automatically identify situations where there is one of more stars within 15 arc sec of the target, which are brighter than the target star, or up to 1 mag fainter.

In this case no such stars are identified. The reasons are:
Target star V mag =13.5  [UCAC4 563-008709]
Closest star V mag = 15.6, separation  about 5”  [UCAC4 563-008708] – much fainter than the target
Next closest  V mag = 13.7, separation  about 18”  [UCAC4 563-008706] – more than 1” from the target

If your system can’t separate these stars, the combined mag will be 12.8, and the occulted magnitude will be 13.7 – so a mag drop of 0.9, which is quite observable.

However if your resolution is good enough to exclude the star at 18” (which is most likely), the mag drop will be from 13.4 to 15.6, or 2.2 mags.

So, the bottom line is that the mag. drop that we’ll actually be able to detect for the Eurybates occultation should be somewhere between 0.8 and 2.2, probably closer to 2 than to 1, as long as the target (and the star 5” away) can be resolved from UCAC4 563-008706, which has been our experience in all the tests we have done so far.


FWIW, since there was a disagreement between the Stellarium chart that Vadim prepared and the information Dave Herald provided, I thought I’d send the attached image that confirms Dave is correct, and the Stellarium database apparently has an error.

I couldn’t travel, so I observed from home in southern California on the slim chance of picking up a satellite. Not surprisingly, I had a miss. To get the best S/N possible, I used my C8 at f/10 (~2000mm fl) to split the wide pair, and ran the camera at x8. I’m assuming most observers used a shorter focal length so might not be able to resolve the close pair, even with more aperture, leaving the question about the companion unanswered.

The image is a stack of 400 frames, stretched, and the faint close companion is just barely visible, so Dave’s suggestion that the mag drop should be about 2.2 looks right.

Sorry I couldn’t help out in the path.

Bob Jones