Asteroid Occultation Timing by Amateurs: Scientific

Measurements from TeamworkSteve Conard

International Occultation Timing Association (IOTA)

May 23, 2010

Agenda

• Why Time Asteroid Occultations?• The Basics• Equipment• Software• Results• Equipment Demonstration

Why Time Asteroid Occultations?

• Measure the size and shape of asteroids and their satellites and accurate positions of the asteroids– Use times of background stars occultation by an asteroid,

measured in a number of locations across the path, to construct a silhouette of asteroid in a single rotational angle

– Critical requirement is to have multiple sites covering potential path

– These events can occur anywhere at anytime• Accurate orbits allow mass determinations from

measuring perturbations of the orbits• Very close double stars and stellar angular diameters

resolved in some cases

Why Amateurs?• Equipment needed can be relatively inexpensive and

“small”• Most amateurs have “mobile” equipment, and often

travel with their equipment• Amateurs are scattered throughout the world• As currently done, this is a “part-time” activity

– Most dedicated observers are only trying <1 event per week, on average

• Even though we are “amateurs”, our results are published, and professional astronomers request our assistance often

History

• Difficult to predict; 1st seen in Sweden in 1958• As orbits & astrometric techniques improved,

more observed in mid to late 1970’s• Predictions considerably more accurate in 1997

with release of Hipparcos space astrometry data• Geometry shown in next figure• More distant objects (Trojans, Centaurs, TNO’s)

harder to predict but more valuable

Geometry of an Asteroid Occultation

Courtesy of David Dunham

The Basics

Find Potential Occultation

Plan Observations

Perform Observations

Analyze Data, Report Results

Finding Potential Occultations

• Two primary methods employed by IOTA observers at this time– Search for occultations within a geographic

area of interest, using freeware “Occult”– Use web-based tool, “Occult Watcher” (OW),

which uses “feeds” for the area of the country you are in to look for events within your filter parameters• Location, brightness, maximum duration, etc.

Occult Asteroid Main Display

Occult Asteroid Search Display

Occult Asteroid Output

Predictions for Pottstown, PA—within 100 km of path

Occult Output Example

Occult Output Zoomed

OW Main Display

Predictions for Pottstown, PA—within 100 km of 3-sigma line, mag >12, >20 degrees from horizon, >10 km shadow width

Planning Observations• Either method for finding potential occultations will give

you critical information about the event– Chance of getting a positive from your location

• Don’t take these numbers too literally!

– Combined magnitude and delta magnitude• Can you see the target, and can you notice the occultation?

– Maximum time• Is it long enough to detect with you system? If visual, > 0.5 seconds

– Azimuth and elevation for your location• Can you see it with your horizon• Is to too high for your mount with camera attached?

– Distance to moon, phase of moon• Are you likely to be lost in lunar scattered light?

– Angle sun below horizon• Too close to twilight?

Planning Observations

• Location, location, location…– General rule is unless it is a rank 90 or more, you

should not be traveling great distances to observe it– My rule is if I’m not going to substantially increase my

chances (double?), observe it from home• Traveling increases the risk of getting a “no observation” due

to hardware issues, power problems, forgotten equipment, location problems

• A negative observation will often have value, a no observation has none other than being a learning experience!

Multiple Observations• To maximize the scientific value of the

observation, more than one positive measurement is required– A single chord gives only the smallest the asteroid

can be in a single dimension– Two well-spaced chords allow an elliptical shape to be

fit to the data– Many chords give excellent size and shape

information—examples to be shown later

• Yahoo group, email, OW used to drum up support for individual observations

Multiple Observations

• If you use OW, coordination of multiple observers is simple– Find an event you are interested in observing, and

often you’ll already see other observers signed up for it

– You can visually see where they are scattered across the path• Where they are along the path is generally irrelevant

– If you are the first to sign up for an event, often others will follow

OW Map Display

OW Station Sorts

Data CollectionBasic Requirements

• Obtain Universal Coordinated time of occultation’s disappearance/reappearance – Accuracy to ~1 second is minimum for good data quality, better

is desirable– Typical event durations are 1 to 15 seconds

• Asteroid occultations typically involve faint stars– The fainter you can go, the more events you can record– To get a reasonable number of opportunities, you need to be

able to detect occultations of 11+ magnitude stars• At least two observers in separate location must record

the event to obtain a rough diameter– More can give shape information– Observers must know their locations to a few tenths of a

kilometer

Basic Hardware• The minimum hardware would be a

telescope and a source of time signals—typically WWV radio shortwave– The observer visually watches the star for

several minutes around the predicted time, and records the time of disappearance and reappearance

– A voice recorder makes this task easier• Record both WWV and your call of “D” and “R”• Desirable to calibrate out your reaction delay

Video Hardware• In order to get data with better time accuracy,

recording of a series of images is required– This is typically video, although other options such as

a “drift scan” with a cooled CCD are also used

• For high sensitivity (faintest limiting magnitude), low-light level cameras are used (security cameras)

• A 0.0001 lux† camera, coupled to an 8” telescope, can record video to magnitude ~12.5 with good transparency† 0.00001 lux is approximately the light from Sirius

Simple Video Set-up

Telescope

te

xt

te

xt

Camcorder with Video Input

$50 and up USED!

12 vdc Power~$10

Shortwave Radio$30 and up

Star Light

SoundMicrophone

$10

Low-light VideoCamera

$90 and up

C-mount Adapter

~$30

Harnessing~$25

Advanced Set-up

Telescope

te

xt

te

xt

Camcorder with Video Input

$50 and up USED!

12 vdc Power~$10

Star Light

Voice NotesMicrophone

$10

Low-light VideoCamera

$90 and up

C-mount Adapter

~$30

Harnessing~$25

“Kiwi” Video Time Insertion Box

$165

GPS$85

Direct to Laptop Set-up

Telescope

Laptop PC

12 vdc Power~$10

Star Light

SoundMicrophone

$10

Low-light VideoCamera

$90 and up

C-mount Adapter

~$30

Harnessing~$25

“Kiwi” Video Time Insertion Box

$165

GPS$85

USB Video Input~$40

F:3.3 Focal Reducer

$100

Shortwave Radio$30 and up

Direct to Laptop Set-up

Experimental F:1.2 System

Pointing Options

• Two basic choices, prepoint and track– Prepoint uses recognizable stars at approximately the

same declination and (usually) to the West (lower RA) than the target• Point to those stars at a time that is approximately the RA

offset prior to the predicted occultation• Lists of prepoint stars available online

– Track uses a system with a clock drive or equivalent• Point to target from start

Pointing Trade-offs

• Prepoints are highly reliable assuming you can find the prepoint field at the right time– Uses the world’s most accurate clock drive– Shows you where the target is going to be—won’t be

surprised losing in obstructions– Don’t need a powered drive, inexpensive

• Typically, tracking uses a Go-To mount– Can verify target in advance– Can typically slew to Alt-Az to check for obstructions– If you have a Go-To, I’d recommend you use it!

Location

• Unless you live in an unobstructed area, expect to move around your yard even if you stay at home– Many events are close to the horizon

• If you travel, try to use a friend/relatives property, or safe and legal public property (schools, parks, etc)– Doing a survey on Google Earth to choose a

location is a big help

Analysis Process For Video

• Note that the remainder of this presentation will concentrate on the analysis of video data– Information on data analysis of visual data can be found in the

IOTA manual

• Simple—do a frame by frame analysis of your data– Step thru video to identify “D” (disappearance) and “R”

(reappearance), determine time of each frame—typically ~1/30 second error

• Advanced– Import to PC and use (semi) automated tools for analysis

Advanced Analysis Process

Transfer AVI to computer

Video file preparation• Convert file type (VirtualDub)• Integrate video (if required, Registax)

Data Analysis• Analyze video data (LiMovie, Tangra)• Statistical analysis (Occular)

Report Results

Transfer AVI to Computer

• 2 basic options– “Play” analog video into USB video capture

card• Capture card typically $40-50• Required software included with card

– Transfer directly via Firewire• Must have Firewire interface on both sides• Cable $10-20• Windows typically has required software included

Example Video Data-Arethusa March 09 Gamber, MD

Example Video Data-Henrietta February 09 Gamber, MD

Convert File Type

• Typically Firewire transferred video requires conversion from one type of avi to another

• Simplest way to find out is to try feeding transferred file directly into either the integrating or analysis software, and see if it bombs– If it fails, read into VirtualDub (freeware), and

convert to its standard avi output

VirtualDub

Integrate Video

• If target is very faint, it may be helpful to integrate the video to make the analysis tools work better

• Registax (freeware) can perform this task, and save back as a new avi file

• Note that time resolution is lost when this is done– Only do this if analysis fails (or you can’t find

the target in the video)

Registax

Video Analysis• The most widely used software for this step is

LiMovie (freeware)– Tracks target and reference stars– Calculates signal for each frame, subtracts

background– Somewhat fussy about tracking the target if faint

• New software, Tangra (freeware), becoming available– Much more automatic– Expected to have much better target tracking

• Output is time and signal level (csv file)

LiMovie

LiMovie Graphing

Tangra

Tangra Graphing

Statistical Analysis• CSV file is either manually analyzed or run

through Occular (freeware)• Manual analysis can use plots directly from

LiMovie to find point of “D” and “R”– Often done for events with dramatic change in signal

level

• Occular performs analysis to more accurately determine times– More importantly, determines the probability of an

event truly being detected for questionable data sets

Data Reporting

• Quick positive/negative reporting on OW, not use for analysis (information only)

• Standard spreadsheet report form is sent to regional IOTA coordinator– Partially pre-filled version available on coordinator

website, or as output of OW

• As reports come in, the coordinator performs analysis of all chords, post results

• Quarterly, results are published in the Minor Planet Circular, with credit to observers

Report Form

Example of Analyzed Data

Example of Analyzed DataMore Interesting Shape

The Good and Bad

• Positives– Amateur contribution to real science– “Team sport”, gets you out observing more often– Impressive array of freeware available

• Negatives– Weather will break your heart

• Not only do you need clear sky, but you need clear sky at a specific time and location!

– Moderate cost of hardware to record video

Other Uses for Video System

• Total lunar occultation timing– Double star discovery/measurement

• Lunar grazing occulations– Lunar profile

• Lunar meteor impact recording

Resources

• Websites:– http://www.poyntsource.com/New/Global.htm– http://www.lunar-occultations.com/entersite.htm– http://www.asteroidoccultation.com/ *– http://www.asteroidoccultation.com/observations/Resu

lts/– http://iota.jhuapl.edu

• Book– "Chasing the Shadow: The IOTA Occultation

Observer's Manual“ (free download from *)

Shameless Promotion

• Want to avoid having to hear me speak again?

• Then you’ll want to miss the Cherry Springs Star Party– "Building the New Horizons LORRI Imager: A

20 cm Ritchey-Chretien for Pluto“– Saturday, June 12, 4:00 PM