K3's AstroPhotography
"When I consider your heavens, the work of your fingers, the moon and the stars which You have set in place, what is Man that You are mindful of him?" -- Psalm 8:3,4

Example of using Nikon Coolpix 995 digital camera for astronomy purposes

Nikon Coolpix 995

CCD chip: SONY ICX252AK (Type 1/1.8")
Image size: Diagonal 8.93mm, 7.2mm x 5,35mm
Effective pixels: 2088(H) x 1550(V) ~3.24M pixels
Total number of pixels: 2140(H) x 1560(V) ~3.34M pixels
Unit cell (pixel) size: 3.45µm(H) x 3.45µm(V)
Chip size: 13.8mm(H) x 12.0mm(V)

Integrated lens: f=8.2mm-31mm / F2.6-F5.1
Sensitivity: ISO100 - ISO800
Color filter: Complementary CYGM

Wide field astrophotography - using built in lens

Focal Length f=8.2mm f=18.8mm f=31mm
Zoom Factor 1x 2.29x 3.78x
Maximum speed F2.6 F3.8 F5.1
Aperture at maximum speed 3.15mm 4.95mm 6.08mm
Resolution (limited by diffraction) 36.16 arcsec 23.04 arcsec 18.75 arcsec
Pixel resolution (full image size) 84.27 arcsec/pix 37.85 arcsec/pix 22.29 arcsec/pix
Field of view 2876.3' x 2157.2'
47.9° x 36.0°
1292' x 969'
21.5° x 16.2°
760.83' x 570.62'
12.7° x 9.5°

Note: Pixel resolution and field of view are approximate values, because in wide field of view the pixel resolution is not as linear accross the image as it is in narrower fields of view.

Astrophotography with telescope - using afocal photography

Afocal photography with 80mm F5 refractor.
Most of current cameras in the market have their lens fixed with camera body - there is no possibility to detach objective lens and use camera at prime focus.
But there is another possibility how to use camera with scope - afocal photography. The camera with objective lens works like a human eye - the CCD is like retina and objective lens like eye lens. That means that we can attach the camera to the eyepiece (just like we enclose our eyes to eyepiece). The eyepiece should be focused to required object and camera should be focused to infinity.
There are several ways how to attach camera to eyepiece. Very good description of various adapters you can find on Simon's Szykman web pages - Digital Camera Telescope Adapters and Homegrown Digital Camera Telescope Adapters.

In my case I am using standard photography adapter for prime focus and eyepiece projection for 35mm film camera. It has 42x0.75mm thread (T-thread) for attaching T-adapter specific for each camera. I ordered a special adapter for Nikon from Astromeccanica company.

All parts needed for afocal photography (from right to left):
1, Camera adapter with T-Thread
2, 25mm Plössl eyepiece
3, Additional extension ring
4, T-Thread adapter for Nikon CP995 (from Astromeccanica)
5, Homemade plastic washer (made from CD-R disk)

25mm eyepiece in camera adapter. Eyepiece sticks out camera adapter, so additional extension ring is necessary.

Camera adapter + eyepiece + extension ring + T-Thread adapter for Nikon CP995. Eyepiece should be as close to Nikon's objective lens as possible to reduce vignetting.

Plastic washer added for preventing Nikon external protective objective glass from scratching from 28mm threaded part of T-Thread adapter (it doesn't allow to screw adapter too deep)

Problem of vignetting

Field of view (FOV) of standard 35mm camera with 50mm objective is 47°. Transfocator at wide angle position has FOV even greater. Common Plössl eyepieces have FOV about 50°. As digital camera's objective is a little bit away from eyepiece, the picture created by eyepiece may not cover the whole CCD area. This phenomenon is called vignetting. By another words - there exists an area in CCD (film) - a circle, which is fully illuminated (all rays captured by objective lens/mirror and transformed by eypiece fall to CCD area. The area outside full illuminated circle becomes darker untill total black.

80mm F5 refractor + 25mm Plössl Eyepiece + Nikon @ 8.2mm (1X Zoom)
Visible vignetting

80mm F5 refractor + 25mm Plössl Eyepiece + Nikon @ 16.9mm (2X Zoom)
Only a small vignetting in corners notable

80mm F5 refractor + 25mm Plössl Eyepiece + Nikon @ 31.0mm (3.8X Zoom)
Virtually no vignetting

Calculation of effective focal length of afocal system


EFL - Effective Focal Length   FE - Effective Focal Ratio (F Number, Speed)
fO - Focal Length of Camera's Objective Lens   F - Scope's Focal Ratio (F Number, Speed)
fT - Focal Length of Telescope's Objective Lens / Mirror   DT - Objective Lens / Mirror Diameter
fE - Focal Length of Eyepiece   w - CCD chip width
M - Telescope Magnification   h - CCD chip height
      FOVEH - Result Field of View (horizontal)
      FOVEV - Result Field of View (vertical)

Effective Focal Length:

EFL = fO . fT / fE              (1.1)

EFL = fO . M                  (1.2)

Effective F Number:

FE = EFL / DT                (1.3)

FE = fO . fT / (fE. DT)       (1.4)

FE = F . (fO / fE)              (1.5)

Result Field of View (Horizontal, Vertical):

FOVEH = 2 . arctan(w / (2 . EFL))     (1.6)

FOVEV = 2 . arctan(h / (2 . EFL))      (1.7)

Practical example - afocal photography with my scopes

  80mm F5 refractor 200mm F6 Newtonian
Scope parameters
Diameter of objective 80mm 200mm
Focal length 400mm 1200mm
Focal ratio F5 F6
Focal length of eyepiece 25mm 25mm
Magnification 16x 48x
Nikon @ 8.2mm
Effective focal length 131.2mm 393.6mm
Effective focal length [35mm] 623.2mm 1869.6mm
Effective focal ratio F1.64 F1.97
Result FOV (horizontal) 3.14° 1.05°
Nikon @ 16.9mm
Effective focal length 270.4mm 811.2mm
Effective focal length [35mm] 1284.4mm 3853.2mm
Effective focal ratio F3.38 F4.06
Result FOV (horizontal) 1.53° 0.51°
Nikon @ 31mm
Effective focal length 496mm 1488mm
Effective focal length [35mm] 2356mm 7068mm
Effective focal ratio F6.2 F7.44
Result FOV (horizontal) 0.83° 0.28°

Look at M13 and M57 photos using afocal with my 80mm F5 scope.

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Last Update: 08.07.2004