Image Scale

 

Image Scale

This following table has details of image scales for the various combinations of cameras, eyepieces and telescope f-ratios that I use. Image calibration is important for measuring the angular size of objects in the images and in the case of satellites for working out their actual size. 

System Details Image Size Typical Pixel Size Image Scale Reference Image
Philips ToUCam Pro at f10 4.92' x 3.69'   640x480 0.4613"/pixel  
Philips ToUCam Pro at f6.3  6.17' x 4.63'  640x480 0.58"/pixel  Venus and Mercury conjunction 
Philips ToUCam Pro at f3.3        
HX516 CCD Camera at f10   660 x 500    
HX516 CCD Camera at f6.3 8.5' x 6.4' 660 x 500 0.77"/pixel  
HX516 CCD Camera at f3.3 16.8' x 12.7' 660 x 500 1.52"/pixel   
JVC GR-DVL30E Digital Camcorder at f6.3, 18mm orthoscopic, x1 zoom   720x518    
JVC GR-DVL30E Digital Camcorder at f6.3, 18mm orthoscopic, x10 zoom 2.827' x 2.034' 720x518 0.236"/pixel  
Finder Camera with 135mm f2.8 lens. Uses the 1004XA board camera with a 1/3" BW Sony EXviewHAD chip. 0.003 Lux  125.4' x 94.0' 384 x 288 19.58"/pixel  
35mm SLR with 55mm lens 36° x 24°      
  • The reference for many of these scales is Albireo (Beta Cygni) RA 19h28.7 Dec +27°52' (NSA1950.0) Separation 34.6" at PA 55°
  • The camcorder images are initially 720x576 pixels but the pixels are not square and the height needs to be reduced to 90% of its original value

HX516 Details

CCD Type Sony ICX084AL HyperHAD with 7.4um square pixels. Format 660x494 pixels. Chip size 4.9mm x 3.6mm

Response curve

Satellite Sizes 

Size = Range x tan(numberPixels x imageScale)

Assuming capture from the CamCorrder at x10 zoom and that for small angles tan(theta) = theta in radians

Size (m) = range (km) x numberPixels x 0.001142

Telescope Resolution

The Rayleigh resolution limit in radians for a telescope of diameter D (metres) and light of wavelength lambda 

alpha = 1.22 lambda /D 

For a wavlength of 550nm (corresponding to the eye's peak sensitivity) and resolution in arcseconds this corresponds to 

alpha = 0.1384 /D. 

So for the 10" LX200 the theoretical resolution is 0.54 arcseconds.

Typical Angular Sizes

A table of the angular sizes of typical objects taken from Norton's Star Atlas 1978 edition. Angular sizes for the planets are at their mean opposition distance. A British penny has a diamater of 20mm. The table shows how far away the penny would have to be to appear the same size as the object.

Object Diameter (km)  Angular Size  Distance for Penny (metres)  
Moon 3476 1/2° 2.3
Hadley Crater  6 3.22" 1281
Jupiter  142800 46.86" 88
Mars  6790 17.87" 231
Saturn   119300 15" 275
Uranus  47100 3.1" 1331
ISS main solar panel at 483km 12 metres 5.3" 778
CCD frame diagonal at f6.3   10.64' 6.46
CCD frame diagonal at f3.3 21.06' 3.26

 

 

 

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