Splitting Double Stars with your Digital Camera
by Gregory A. Pruden

Splitting double stars, with a telescope, is popular with many amateur observers. Splitting double stars with a digital camera, attached to a telescope, can also be rewarding and act as photographic documentation of the observation. The ability of each telescope-eyepiece-camera combination to split double stars is different and dependant on the pixel size and the effective focal length which is computed from the focal lengths of telescope (Ft), eyepiece (Fe), and camera (Fc) as in the formulae for afocal astrophotography below:

Feff = Ft x Fc ÷ Fe

To successfully split a double star with Digital Camera Astrophotography the resolving power of the telescope-eyepiece-camera system must have a considerably higher resolution than the angular distance between, or separation of, the stars. In some cases, the zoom in the camera can be powerful enough to split the stars where otherwise they would still appear as one star. In this case, Fc varies from the widest to the most telephoto zoom.

Resolving Power = (206265*Pside)/Feff
where Pside is the size in mm of a side of a single pixel

The resolving power above is the piece of sky in arcseconds that falls on a single pixel in your digital camera. Using an arbitrary number of 15 pixels between stars for a imagable separation the ability of the telescope-eyepiece-camera system to split doubles can be roughly predicted using the resolving power and effective focal length formulae above. By multiplying the resolving power by 15 you can estimate the minimum separation that can be split under ideal seeing. See the example to the right.

The same issues arise when splitting double stars photographically as visually and stars that are of similar magnitude and color will split easier than those with wide ranges of color and magnitude. However, some of the more stunning images of multiple star systems are those with striking color differences. In addition, a type of blooming can occur as the exposure time is increased. If exposures are too long the stars can grow and run into one another effectively negating the split. If your camera has ISO settings use the highest value for example 400 or 800 with the shortest possible exposure. Many of the photos in this article were taken at ISO800 and using a 1/2s shutter speed. As the resolving power is changed to by increasing the camera focal length (zoom) the shutter speed may need to be adjusted for slightly longer exposures to avoid false color and underexposed images. The camera settings discussed too are dependant on the telescope-eyepiece-camera system so your mileage may vary.

Splitting of close doubles are subject to seeing just as with visual observations but photographing in high-power poor-seeing situations poses unique problems for photography. To overcome the effects of less than great seeing images should be taken quickly one after another and stacked if possible to reduce noise and increase the chance of getting a few good images.

Don't trust your camera's LCD alone when evaluating a split. The camera reduces the image significantly in order to display it on such a small screen and this has both a sharpening effect and gives the impression that a split has not occured. To overcome these effects use the camera's digital zoom, which blows-up the pixels, to confirm that a split has taken place. In addition, after a picture has been taken some cameras have the abilility to blow-up the picture in preview mode and in the Nikon 995 this abilility is 6x where the digital zoom is only 4x. Another important tool is an external monitor, such as a small TV or even better a playstation monitor, connected to the digital camera's video out port. This will further expose the actual image and reduce other problems assocaited with the small dim screen.

Digital zoom and the external monitor should also be used to focus any astronomical image as the sharpening effect discussed above will disguise an out of focus image and only when the photographs are downloaded to your computer will you see that they are not focused. To focus set the camera to manual focus at infinity and use the telescope to focus the image while viewing the object, a double star in this case, in full digital zoom. Then after you are confident that you have crisp points back out of the digital zoom and take the photograph only in optical zoom.

We will classify double stars that are split using the zoom within the camera as split "In Camera" and those that are already split when the camera is in the widest zoom setting as "Split Wide." Stars that are not split can be classified as "Unsplit" or "Elongated" but images of this type usually end up in the recycle bin.

To create animations of stars that are split in camera take a picture at each optical zoom step from wide to telephoto. Then using a program like Animation Shop or Fireworks create an animated gif file with frames from wide to telephoto and telephoto to wide. It is best to work first with the star split in telephoto to ensure that you have split it and to get the correct exposure. Correct exposure is critical on the split star but less so on the unsplit or elongated frames.

Happy Splitting!

What is a double star?

Most double stars appear to the unaided eye as a single star but when they are viewed with a telescope or binoculars their true nature is revealed and they resolve into two or more components. Some of these components may be near or in orbit around each other (physical) and other components may be simply in front of one another from our perspective here on earth(Optical). Physical double stars that have a known orbit are called Binaries and binaries that we can see from earth with a telescope are Visual Binaries. The act of resolving double stars into their components is called splitting.

Example:

A Meade LX90 203mm SCT
with focal length of 2032mm is used with a 24mm eyepiece and a Nikon 995 digital camera with wide focal length of 8.2mm,telephoto focal length of 31mm, and a 3.45 x 3.45 micron pixel.

Wide:
Feff = 2032 x 8.2 ÷ 24 =694
Resolving Power= 206265 x .00345 / 694= 1.02 arcseconds
Min. Sep. = 15 x 1.02 = 15"

Telephoto:
Feff = 2032 x 31 ÷ 24 = 2625
Resolving Power = 206265 x .00345 / 2525= .27 arcseconds
Min. Sep. = 15 x .27 = 4"

Zoom = 15" to 4"

In addition, by multiplying the resolving power by the number of pixels in both width and height the piece of sky that appears on the camera's ccd or focal plane, the "Field of View", can be estimated. For the Nikon's most telephoto focal length and using the camera's FINE resolution the FOV is:

FOVw = .27 x 2048 = 553" = 9.2'

FOVh = .27 x 1536 = 415" = 7'

FOV = 9.2' x 7'

 

What is position angle or PA?
In a double star observation the position angle is the relationship or angle between the components in degrees from celestial North.

Almaak - g 1 And
Distance: 362.71 light years
J2000.0: 2h3'53.95'' +4219'47''
Magnitude: 2.170 MK: K3-IIb

Visual Binary
Separation: 10"
Position Angle:63

Component: Gamma 2 And

Observation
Animation 5 frames
10/19/2001 02:40:33 UT
Field: ~13.15' x 9.86'
To Field: ~
3.39' x 2.54'
f/9 to f/35 1/2 sec @ISO800
Meade LX90 203mm SCT

Split in Camera

In Arabic Almaak is "weasel-like mammal." Gamma 2 is another binary system with a separation of .5 arcseconds.  
Polaris - a Umi
Distance: 440.92 light years
J2000.0: 2h31'49'' 8915'51''
Magnitude: 2.000 MK: F7:Ib-II

Visual Binary

Separation: 18.4"
Position Angle: 218
Orbit Period: 44.0 years

Observation
08/29/2001 02:25:20 UT
Field: ~11.78' x 8.84'
f/5.42 35.9 seconds
Discovery Obsession 15" Dob.

Split In Camera

Amateurs with polar aligned scopes or goto systems observe this star a great deal, but many do not know that it is a, sometimes difficult to split, double star.

 
Achird - h Cas
Distance: 19.84 light years
J2000.0: 0h49'6'' 5748'55''
Magnitude: 3.450 MK: F9V


Visual Binary

Separation: 12.9"
Position Angle: 315
Orbit Period: 480 years

Observation
Animation 3 frames
09/20/2001 03:11:06 UT

Field: ~10.80' x 8.10'
f/11 60.1 seconds
Meade LX90 203mm SCT


Split in Camera

Cassiopea was banished to the heavens for being vain and spends half the time upside-down.
 
Albireo - b Cyg
Distance: 394.01 light years
J2000.0: 19h30'43'' 2757'34''
Magnitude: 3.080 MK: K3II

Physical Double
Separation:34.4"
Position Angle: 54
Mag. Difference: 2.0

Observation
09/02/2001 03:30:29 UT
Field: ~26.56' x 19.92'
f/4.5 60.1 seconds
Meade LX90 203mm SCT

Split Wide

"..55 Solar Systems could be lined up, edge-to-edge, across the space that separates components of this famous double!" -- Robert Burnham, Jr.

 

Struve 919 - b Mon
Triple system
J2000.0: 6h 28' 48" -6° 58' 0"

Magnitude: 4.7,5.2,6.1

Visual Triple
A-B Separation: 7.2" PA: 132°
B-C Separation: 2.9" PA: 108º

Observation
01/05/2002 02:33:43UT
Meade LX90 203mm SCT
William Optics and
Meade 8.8UWA eyepieces

Split in Camera

Herchel's Wonder Star - William Herschel, who discovered it in 1781, thought it was one of the best he had ever seen.  

Castor - Struve 1110
Double-Double-Double
J2000.0: 07h 34.6 +31° 53
Magnitude: 2.0, 2.90

Visual Sextuple
A-B Separation: 4.5" PA: 270°

Observation
01/21/2002 02:18:33UT
Meade LX90 203mm SCT
Meade 8.8UWA eyepiece

Split in Camera

Castor the twin of Pollux is itelf made of three sets of twins. While this image shows only the AB components there are also AC, BC doubles.  

Double Star Links

All photographs were taken with a William Optics 28mm Digital Camera Eyepiece or Meade 8.8UWA eyepiece and a Nikon Coolpix 995 Digital Camera.