A New Video Method to Measure
Double Stars
28thAnnual IOTA Meeting December 4, 2010
Richard Nugent
WHY MEASURE DOUBLE STARS ?
 Position
angle (θ) and separation (ρ) allow
computation of orbits
 Orbital periods  Kepler’s 3rd law  Actual
separations/distance
 Distance  absolute luminosity, masses, radii
 Absolute quantities is the basis for the distance
scale of the Universe
CURRENT METHODS TO MEASURE DOUBLE STARS
Micrometer/Visual
Fig. 1
Fig. 2
drift
CCD Camera Method
 Take CCD image
 Measure positions of
double star and
reference stars
 Perform astrometric
reduction
 With (RA, DEC) of
components, compute
position angle (PA)
and separation (Sep)
Video Camera Methods
 Individual frames are
chosen and stacked
for an astrometric
reduction
 East-West direction is
derived from drift from
two widely spaced
frames
New Video Methods
• Both methods use “LiMovie” software written by Kazuhisa Miyashita and
require GPS time insertion
1) Artificial occultations - Uses LiMovie custom version to create an
artificial occultation
Details here: http://weblore.com/richard/double_stars.htm
2) X-Y Video Drift across the Field of View
• LiMovie Basics
LiMovie calculates brightness of stars for individual frames using for up to 3
adjustable apertures = 3 individual stars
Apertures can remain fixed or drift to follows stars
Results are exported for all frames to Excel CSV file for analysis: brightness
levels, noise filtering, diffraction, FFT, etc.
LiMovie’s grid size is 635 x 475 pixels, origin is upper left corner of FOV
At 30 frames/sec recording rate, a 1-2 minute
video can produce 1,000’s of (x,y) data pairs to
analyze.
With LiMovie’s (x,y) position output for each star,
separation and position angle are computed as follows:
(user must determine quadrant)
Camera/telescope orientation
drift angle
Drift does not have to be perfectly aligned East-West, a drift
angle is easily calculated using least squares:
3-D contour plots
RESULTS
WDS
X,Y
WDS
X-Y
Star
Sep
Sep Diff
PA
PA
Diff
Observer Notes
--------------------------------------------------------------------------------------------------------------------------------61 Cygni
31.1
31.1 0.0
152
153.1 1.1 RN
Questar
WDS 2346-1841
6.9
6.5
0.4
136
127.4 8.6
RN
Questar
STF 2280
14.2
14.2 0.0
182.9 183
0.1 RN
Questar
15 AQL
40.5
39.7 1.8
209
209.7 0.7 RN
Questar
SF 2848
10.8
10.8 0.0
56
56.4
0.4 RN
Questar
WDS 2353+1155
19.3
19.2 0.1
282
281.5 0.5 RN
Questar
WDS 0013+2659
18.0
19.0 1.0
224
224.8 0.8 RN
Questar
WDS 0128+0758
69.1
67.8 1.3
100
105
5.0 RN
Meade 14”
WDS 0153+1918
7.5
7.3 0.2
356
358
2.0 RN
Meade 14”
WDS 0203-0020
43.0
42.6 0.4
194
196
2.0 RN
Meade 14”
WDS 0213-0224
16.6
16.7 0.1
234
235.7
1.7 RN
Meade 14”
WDS 0218+4017
11.4
10.9 0.5
294
284.7
9.3 RN
Meade 14”
WDS 0227+1034
73.6
73.2 0.4
32
31.5
0.5 RN
Meade 14”
WDS 0230+0504
27.1
26.5 0.6
328 310.7 17.3 RN
Meade 14” - Video very noisy/bad seeing
WDS 0231+0106
13.8
13.4 0.4
220 219.9
0.1 RN
Meade 14”
WDS 0243-2017
19.7
19.0 0.7
164 157.8
6.2 RN
Meade 14”
WDS 0257-0034
8.9
8.6 0.3
192 195.8
3.8 RN
Meade 14”
WDS 0302+0005
8.0
7.8
0.2
139 147.2
8.2 RN
Meade 14”
WDS data 10 yrs old
WDS 0303-0205
8.9
8.7
0.2
223 222.6
0.4 RN
Meade 14”
WDS 0319-1834
8.4
6.9
1.5
120 124
4
RN
Meade 14”
star images merged
WDS 0345-1320
19.8 20.0
0.2
324 324.5
0.5 RN
Meade 14”
stars out of focus
Potential Sources of Error
• Precession/proper motion – can usually be neglected for data < 10 yrs
compared to WDS positions
• Abberations and distortions of your optical system – can be
neglected since the effect should be identical for both components
• f/10, f/6.3, f/3.3 – Use the least amount of glass between the stars and
your video camera
• Gnomonic projection - can be neglected for small (< 100) separations
• Seeing/turbulence effects – are averaged out for the large numbers of
video frames and effect both components simultaneously
Other Considerations
• Scale factor – must be re-computed for each run. Slight instrument/
flexture changes and differential refraction.
The scale factor should be computed for each star and averaged
Watec 902H – f/10 Meade 14 LX-200 = 0.6/pixel
f/14 Questar 3.5,
= 2.0/pixel
• LiMovie’s CSV (x,y) output is the “Tracking (x,y)” data:
• Doubles with large magnitude differences > 3 or very faint companions cause
LiMovie’s apertures to jump around too much  use linked tracking
From Kazuhisa Miyashita:
In operation, LiMovie examines the value of each Pixel in a StarTracking
(aperture) circle set by the ‘radius’ setting, before doing photometry. Any
pixel with a value of at least 50% of the maximum value in that is assumed
to be part of the "star image", and the centre of gravity of the pixels is
recorded as a centre of the star.
When you set the "StarTracking Radius" small, the centre of gravity can be
decided more accurately. However, when the movement of the star grows,
LiMovie cannot track the star because the searched range becomes
smaller than the movement. The StarTracking radius needs to be set to
accommodate the movement of the star. In general, it should be set to be
the same as Aperture, or 1-3 pixels larger than the Aperture radius."
LX-200 Users – No need to turn Scope off for Drift
= motor drive OFF
= motor drive sidereal rate
Conclusions
• X-Y drift video method produces excellent results for PA and separations
• This technique uses a feature of LiMovie that was previously overlooked
• Large # of (x,y) data pairs is unprecedented in the data analysis
compared to any other double star measurement technique
• Occultation observers now have a new pastime in between occultations
• Web page : http://weblore.com/richard/double_stars_video.htm