Astronomy Adventures in the Field: Timing Occultations for

Fun and Science

Rick Nolthenius Cabrillo Astronomy Dept. Chair

For a Few Lucky Asteroids, They Get a Special Visit

But these cost 10’s to 100’s of Millions of dollars

• …and there are 10’s of thousands of known asteroids and more are being discovered all the time

• How to characterize them in enough numbers to get decent statistics on the properties that will help us understand these objects?

• And why understand them? A few may hit us, for one! But others may be home to valuable materials we may want to mine, and there’s the pure joy of scientific understanding to help us know the Universe we belong to

Asteroids are So Tiny that We Cannot Measure Even their Shape and Size Accurately with Telescopes on Earth

But an intrepid group – IOTA (the International Occultation Timing Association) - has one

solution:

Occultations!

…By Organizing a Team of Observers Equipped to Make Accurate Timings of the Disappearance and Reappearance of the Star from Behind the Asteroid, We can Outline the

Precise Shape of the Asteroid

From the Sky Projected Outline We Learn…

• The precise diameter, which, combined with the brightness, tells us the reflectivity.

• The reflectivity then constrains what the surface is made of, and how the surface chemical composition may change across that surface.

• Photometry data as the asteroid rotates can also help constrain the changes in shape and reflectivity, but require occultations to get the precise calibrated size and shape

How To Know When and Where These Occultations

Happen?

Free Software – OCCULT4

• …Will predict the paths of the occultations of distant stars by asteroids, for Windows PC’s

• Each month there are dozens of predicted occultations of faint distant stars by faint small asteroids…

• OccultWatcher, also free software for Windows, will personalize predictions for your specific location

…From IOTA Member Steve Preston’s Site. This is just 4 days of events

But the Asteroid Shadows are as Narrow as is the Asteroid’s Small Size

• And therefore, the odds of an event happening near us, is small.

• Still, there are typically one or two events per month which are favorable enough to entertain my trying to get the data on these events.

• I list these on my “Events” link on my main Cabrillo Astronomy Department web page

But Many Things Can Go Wrong

• Clouds most obviously! • But also the path may shift away from you and

you get a “miss” • Or you fail to find the target star in time • Or the electronics involved fail • Or a host of odd “gotchas” • In the end, I usually get only about 6 - 10

successful observations per year • When things go right, what is the result?...

With Only One Observer’s Chord, The best we can do is get a precise position measurement, helping us to better

predict the future occultations

With chords of one “hit” and one “miss”, we can better constrain the position of the

asteroid, assuming the expected diameter

But with 2 chords, we can fit an ellipse, getting our first shape information for

this Asteroid

Combined with Photometry, We can Get a Good Determination of the Size, Shape, Orientation, and

Select the Best Among Competing Models

So, I’ve Assembled Two Complete Occultation Recording Set-Ups

• They use the 8” Celestron 8SE telescope, which has a “Go To” motorized mount

• A Watec 910hx low-light video camera • A Focal Reducer to make finding the target easier

and mainly for concentrating the starlight onto fewer pixels to improve Signal/Noise

• A miniDV camcorder for digital recording of the video

• An IOTA Video Time Inserter which takes GPS satellite time data to stamp the precise time to 1/10,000th of a second onto each frame of video

“Unit 1” Setup last Month for the Ivar Occultation, at Upper Meadow of UCSC

Which Brings Us to … On Nov 4 Sunday Early Morning: The Asteroid Helio Will Occult a Faint Star Along a Path Across

California and Coastal Mexico

It Misses Santa Cruz, But Nicely Crosses One of the Outdoor Treasures of

California – Pt Reyes National Seashore

There are 6 Planned Observers for the Helio Event (2 are us!). You’ll be at the Hostel (scope icon on the left), and I’ll

be past Nicasio Reservoir, a half hour drive to the East

I Propose… • To reserve a 7-person van (already done) • Drive WiSE members to Pt Reyes on Saturday Nov

3. Perhaps a stop or two along the way to inspect the Fitzgerald Marine Reserve, perhaps Tomales Bay Oyster Company?

• We Pot-Luck a dinner cooked at the hostel’s fine kitchen on Saturday night

• I train you in the set up and use of the recording gear in the evening (I have a detailed PowerPoint already, and will print each page so you can reference it)

After an early To-Bed… • At 1:30 am or so, we sneak out of the bunk rooms, I help you

get a start on setting up one station at the parking area near the hostel

• Then I drive the van and perhaps a student or two who wants to be with me and assist, if desired, on to our second chord past Nicasio Reservoir and set up our second station.

• The whole event only lasts less than a minute, but takes a good 50 minutes to set up properly even if all goes smoothly.

• Take down is quicker, so we get back and to bed ASAP! • Up in the morning, I fix a nice breakfast for the gang… eggs on

toast, oatmeal and nuts w/ milk. • Then we explore, or head back to Santa Cruz, whatever you

need to do. Check out is no later than 10am from the Hostel.

The Pt Reyes Hostel Regularly Tops the List of Best Hostels. In Remote Valley, 1 mile from Limantour Beach

Limantour Beach on a Foggy Morning

Hostel’s Commons Room

I Plan to Enjoy breakfast at the Picture Window

Preparing Oysters, on a Trip Years Ago

There’s a men’s Dorm, a Women’s Dorm, and a Co-Ed Dorm. I’ve Already made my reservation ($35) for Saturday night Nov 3 at the Men’s Dorm. 48hr

cancellation w/o charge. Reserve early!

BBQ and tables on the patio

Saturday, or Sunday we can Drive to the Pt. and Walk to the Lighthouse. One of the Windiest Spots

in North America, on Average!

Hopefully We’ll Get Good Light Curve Data Like This, After Reduction Using LiMovie

Photometry Software

Then OCCULAR takes the photometry data and does Monte Carlo Simulations on it to determine the Best D and R times,

and Estimated Errors

You Can Be Involved in That, If You’d Like

• Or I can take over after the trip. I know you’re all busy with more adventures.

• But LiMovie and Occular are loaded on the Room 705 Astro/Geo/Ocean Lab classroom computers.

• I’ll first need to download the miniDV .avi video to my laptop before it can be analyzed.

• All of this analysis would take place after returning to Cabrillo on a suitable day, or again, I can just do this myself and show you the results. Your choice!

I’ll Have Hard Copies of the Star Charts that You’ll Need to Find the Target Star. The circle here is the view in the

eyepiece of the telescope. Notice that Daylight Savings Time ends just before the occultation. You MUST change your

clocks before bed to use the times below

After then removing the eyepiece and diagonal and inserting the VisualBack with Watec videocamera, the view

in the view screen of the camcorder is…

So – What Do You Need to Do to Join the Helio Team?

• Call or go online to Pt Reyes Hostel and reserve a bed in the dorm. Do that immediately! It’s a popular place and fills up on weekends.

• See my PDF on how to use the equipment before we actually train live on it, after dinner Nov 3 at the Hostel. (link here)

• Watch a video of an asteroid occultation recorded on similar equipment here