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Heads up - Occultation of bright star by asteroid 16th December 2021


lukebl
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On Thursday evening at around 21:14, 17th magnitude Asteroid 11395 will occult a relatively bright 5.5 mag star TYC639-1485-1 for around 7 seconds, visible in East Anglia and parts of Southern England.

It's not often such a bright star is occulted, and should be easy to see although the bright nearby moon will hamper things a bit. Worth a look. I'm on the centre line, although the weather forecast isn't too promising.

2021_12_16-11395-1998-XN77.gif

51742180424_7ed56f4ce9_h.jpg

 

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Interesting.

Clear Outside is predicting 100% cloud cover for Thursday night, Metcheck is predicting completely clear. Accuweather and BBC Weather also predict cloud.

We'll see. 🤔

51744203389_4a7fc133b9_b.jpg

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I have clear skies and saw an announcement for the occultation on a Swedish Facebook page. Atm I am imaging ldn 1374, but will interrupt that sequence and try to catch the event. For me that will be at 22.13 local time, since I'm just North of Stockholm. I'm not sure I will succeed with this, because I have a few trees to deal with.

Does anyone know how best to capture the event with a reflector and a mono cmos (190MN and ASI294MM)? The moon is bright, but I thought of shooting a sequence of 3 - 5 seconds exposures.

Edited by wimvb
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1 hour ago, Phil Fargaze said:

If you can, the ideal way is to capture a video sequence with a fast frame rate. I usually record for about four minutes centred around the predicted time. 

It`s cloudy at the moment so I`m not over confident of capturing it. 

 

32 minutes ago, gorann said:

Good luck Wim!

Unfortunately, either I missed the occultation, or the asteroid did. I did a test run of a SER sequence first, but didn't like it. So I opted for a sequence of 200 3 s exposures, starting at 22.08 (5 minutes before the occultation). But I couldn't see any change in the star's intensity in the previews. According to Aladin, I'm looking at the right star. I'll inspect the images in PI and maybe make a short time lapse.

Edited by wimvb
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Clouded out here. 

17 minutes ago, wimvb said:

I'll inspect the images in PI and maybe make a short time lapse.

Have a try at inspecting the capture at about 21:13 UT. That was the event time for near your location.  There has been a positive result from Sweden, which shows the  actual event hasn`t deviated from the prediction by too much.

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Bingo! Had a positive occultation here.

Despite bright moonlight, and fog so thick that no stars were actually visible to the naked eye, I captured a seven second occultation.

Amazing to think that a mere 60 km asteroid, far out in Jupiter's orbit, should cast a shadow from a distant star briefly over my observatory. Mind boggling. Here's the light curve. The green and yelllow curves are of the only other stars which were visible in the field of view at the time.

RunCam Night Eagle Astro Camera, 200mm f/5 Newtonian.

11395 occultation Light curve.jpg

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2 hours ago, Phil Fargaze said:

Clouded out here. 

Have a try at inspecting the capture at about 21:13 UT. That was the event time for near your location.  There has been a positive result from Sweden, which shows the  actual event hasn`t deviated from the prediction by too much.

I ibspected frames taken between 21.09 and 21.16 and didn't see anything. How wide was the occultation zone?

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Not the most exciting video, by any means! Here's a short capture of the 7 second occultation!

The occultation starts about 10 seconds in.

 

Edited by lukebl
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40 minutes ago, Stu said:

Very cool @lukebl, nicely done! Struggling to get a grasp on how fast the asteroid would have been passing by if it were visible. Presumably very slowly to make a 7 second occultation with a mere 60km diameter?

Thanks! Well, the asteroid is a Trojan orbiting way out in Jupiter's orbit, so I guess it's moving fairly slowly and it's quite big.

I still can't quite get my head around the fact that I was lucky enough for everything to be aligned for me to be under the shadow of that distant star!

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43 minutes ago, lukebl said:

Thanks! Well, the asteroid is a Trojan orbiting way out in Jupiter's orbit, so I guess it's moving fairly slowly and it's quite big.

I still can't quite get my head around the fact that I was lucky enough for everything to be aligned for me to be under the shadow of that distant star!

Yes, quite an amazing achievement I reckon 👍

I checked on SkySafari and it’s very slow moving as you suggest. I guess a 60km asteroid subtends a tiny angle from this distance so it makes sense it had to be moving slowly for a 7 second occultation.

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@lukebl Beautiful obs., well done and I envy your clear spell :) total cloud here :(
So I was reduced to armchair playing with your result :-

I copied your plot into gimp and measured the entry/exit , mid and total durations in pixels, and also the 10sec width(308px)  :-
238/308x10=7.73sec
222/308x10=7.21sec
216/308x10=7.01sec
Give or take a pixel or two it looks like you were on or near the max center line !

The trojans @Stu are in the same orbit as Jupiter ( at the L4 and L5 Lagrange points 60deg ahead and behind ) round the sun so it should have the same orbital velocity as Jupiter (mean 13.06km/s according to wiki).
So I should be able to get from that to an estimate of its diameter.

7.73x13.06  gives 100.95km
7.01x13.06 gives 91,6km
Hmmm, I think I have gone astray a bit, or maybe I need to find the min. orbital velocity.

That was fun ! quite interesting, not a million miles off the 63to71 km dia in the header of the pic in post#1 of the ground track.

Edited by Malpi12
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Somewhatlater ! :-

From https://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html
Jupiter orbital velocity (km/s)
max 13.72
min 12.44 
(12.44x701) = 87.2km

Better, but not there yet, this is very interesting, what have I not taken into account ?
The star is an ideal point source and the asteroid is far enough away to be an ideal knife-edge, they are both outside of the twinkling atmosphere and not subject to the aperture of the scope (Airy disk etc.) ?

Frequency/Bandwidth/Sample rate in the recording chain ??

Time I was in bed  :)

 

Edited by Malpi12
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11 hours ago, Malpi12 said:

entry/exit , mid and total durations

This would (only?)  happen if (a) the star was an extended source ie. not point-like or (b) the asteroid had an atmosphere (c) I have made a fundamental blunder !

7.73x12.44=96.16km
7.21x12.44=89.69km
7.01x 12.44-87.20km

So I have two problems, the spread of values, and they are all too large :(
Being too large could be due to me making an error on the velocity. Is the asteroid orbiting the L-point with a significant deviation from that of Jupiter ?

The spread I cant explain it should be there in one camera frame and not in the next at the start (oppo at end) :(

a & b improbable c very probable :) 
HELP !! my head is about to explode !!
 

Edited by Malpi12
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No luck for me.

Attempted visual observation with a voice memo recorder ready to get as good a time reference for the occultation as possible. Did not see any changes in the star’s brightness though.

However, I was observing from a position 75 km from and perpendicular to the central line passing through Uppsala, Sweden so it seemed like a longshot to begin with.

Had fun preparing and it was exciting to look for nevertheless.

Edited by davhei
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On 18/12/2021 at 02:02, Malpi12 said:

@lukebl Beautiful obs., well done and I envy your clear spell :) total cloud here :(
So I was reduced to armchair playing with your result :-

I copied your plot into gimp and measured the entry/exit , mid and total durations in pixels, and also the 10sec width(308px)  :-
238/308x10=7.73sec
222/308x10=7.21sec
216/308x10=7.01sec
Give or take a pixel or two it looks like you were on or near the max center line !

The trojans @Stu are in the same orbit as Jupiter ( at the L4 and L5 Lagrange points 60deg ahead and behind ) round the sun so it should have the same orbital velocity as Jupiter (mean 13.06km/s according to wiki).
So I should be able to get from that to an estimate of its diameter.

7.73x13.06  gives 100.95km
7.01x13.06 gives 91,6km
Hmmm, I think I have gone astray a bit, or maybe I need to find the min. orbital velocity.

That was fun ! quite interesting, not a million miles off the 63to71 km dia in the header of the pic in post#1 of the ground track.

I think that you need to take the Earth's orbital velocity (approx 30 km/s IIRC) into account in your calculations.

In fact, on the (simplistic, but not unreasonable) assumption that the line of sight (LOS) between the star and target is sweeping across the solar system at the same speed as the orbital velocity of the asteroid, I think we actually caught up and overtook the shadow!

There is also the Earth's surface rotational velocity to factor in, although this is a lot slower than our orbit.

And what about the relative motion between the Solar system and the occulted star? I'd hazard a guess that this is pretty small, but someone, somewhere, has probably included it in their calculations... ;)

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Interesting fact. If you look closely at the curve, you’ll see that the star didn’t disappear immediately. Rather, it look about 8 frames (i.e. about a third of a second) to disappear and the same to reappear. I thought that this might be a fault of my camera or, and my mind was going wild at the point, because of the actual width of the star. Could it be that I’d measured the actual diameter of the star?

No. The experts (well, Eric Frappa, THE European expert on asteroid occultations) tells me that it’s caused by something called Fresnel Diffraction, which I’d never heard of. It occurs when a light wave goes past a hard edge. You’d have to Google it.
 

Well, you learn something new every day.

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2 hours ago, lukebl said:

The experts (well, Eric Frappa, THE European expert on asteroid occultations)

Very interesting.

I had the same thought about star diameter, I didnt (yet) come to a conclusion !

Also I had worked with terrestrial radio antennas and  propagation in obstructed paths some years ago and was aware of Freznel (near field) and Fraunhoffer (far field) and had persuaded myself that they were negligible :( at these astronomical distances , it seem will have to think again ! :)

Do you have a link to the works of Eric Frappa and/or other occultation experts ?

(edit, update) Found some of his works https://ericfrappa.com/publications/  that lot should keep me out of mischief for a while  ! :)

 

Edited by Malpi12
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