photo:astrophotography
astrophotography
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see also:
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photographing the moon:
on the web:
Astrophotography:
tip: if using
film, always start a roll of film with a normal daylight or flash photo so that the film processors know where to start as astrophotographs can make their job very difficult!
tip: the best film cameras to use are those that can take long exposures without introducing light or vibrations, and for connecting to a telescope, it should be a SLR. A popular film camera is the Olympus OM-1n as it has a mirror lock up and self timer to minimise vibrations and does not need batteries which often fail in cold weather.
tip: telephoto lenses for stars or comets will need guiding for exposures more than a few seconds. For comets, guide on the comet not stars as some comets move quite fast relative to the background stars (eg. 3/4 arcminute per minute).
tip: the best comet images are often taken using f/2 to f/2.8 rather than f/1.4, and avoid placing the comet head to near the edge where there tends to be aberrations.
tip: some lenses produce soft images as each star is surrounded by a unfocused blue halo. When using film, this can be reduced by using a light yellow filter such as Wratten 2B or 2E, but if using BW film, can use a stronger yellow filter.
tip: to accentuate a comet's blue gas tail, if using film, use a Wratten 47A filter, to accentuate the reddish dust tail, use a Wratten 21 filter;
determining the effective F ratio:
prime focus method:
camera without lens, mounted to telescope without the eyepiece
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nb. if a Barlow lens is used, this method is referred to as negative projection method
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afocal method:
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camera with lens, mounted to telescope with eyepiece
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EFL = telescope objective focal length x projection magnification
where, projection magnification =
Barlow magnification x camera lens focal length / telescope eyepiece focal length
f ratio = EFL / telescope objective diameter = f ratio of telescope x projection magnification
positive projection method:
camera without lens, mounted to telescope with the eyepiece
if A = distance between the center of the eyepiece elements to the focus of the objective,
and B = distance between the center of the eyepiece elements to the film plane
then projection magnification = B/A, and,
f ratio = projection magnification x telescope objective focal length / telescope objective diameter
determining the effective focal length of system:
calculating the film image size:
example approximate subject angular diameters:
comet with tail are often 5-10° long (ie. 200-300mm focal length is ideal), but faint tail may extend to 50°
nebulae are often 1-2° (ie. 900-1200mm focal length) eg. orion nebula = 1.1°
sun = 0.5333°
moon = 0.5116°
jupiter = 0.0111°
saturn = 0.0116°
venus = 0.0055°
mars = 0.0022° = 18-25 arc seconds at opposition (ie. approx. 100x smaller than the moon)
uranus = 4 arc seconds at opposition
what digital sensor for your telescope?
atmospheric extinction:
degrees altitude | extinction (stellar magnitudes) | ISO correction factor |
0.75 | 8.78 | 0.00032 |
1 | 6.58 | 0.0024 |
1.5 | 4.39 | 0.018 |
2 | 3.29 | 0.049 |
5 | 1.32 | 0.298 |
7 | 0.94 | 0.423 |
10 | 0.66 | 0.546 |
15 | 0.44 | 0.671 |
20 | 0.33 | 0.739 |
determining the correct exposure:
for point sources of light:
telescope objective diameter is more important than f-ratio of focal length in determining correct exposure
a lens with twice the usable diameter will require half the exposure time
light recording power of a system = objective radius2 / f ratio2
thus comparing a 300mm f/2.8 on a full frame camera with equivalent magnification as a 150mm f/2.8 lens on a Olympus dSLR, even though they would give the same magnification and f ratio, the former would have 4 x the light gathering power.
however, you can get a 150mm f/2.0 in the Olympus which has practically the same light gathering power as a 300mm f/2.8.
eg. omega centaurus:
400ASA film 6“ diam f/7 40min

DSLR f/6 1.5 to 24min see pic

and
2
for non-point sources of light:
photography problems:
motion blur:
this may be caused by:
camera shake if shutter speeds < 1/focal length are used hand held
telescope vibration - esp. if windy, or camera mirror creates vibrations, consider using a self-timer
rotation of earth:
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the greater the magnification, the greater the effect of the rotation of the earth to cause “star trails”
a 50mm lens on 35mm film shows star trails if exposures are greater than 8secs
maximum exposure times are inversely proportion to effective focal length, thus 25mm lens can expose for 15secs
this can be minimised by tracking with either:
vignetting:
this is when the edges are blackened without image
prevent this by ensuring the camera lens diaphragm is positioned at the point of focus of the image
alternatively one can use the SLR camera to determine how close the camera must be to the telescope for optimum image coverage
new eyepieces such as the MaxView 40 (1.25”) and MaxView II (2“) are designed to reduce vignetting with digital cameras
minimise vignetting by:
Use an eyepiece with long eye relief.
Couple the end of the camera lens as close as possible to the eye lens of the telescope eyepiece.
Set the digital camera at macro mode rather than infinity.
Use digital camera at full optical zoom.
Purchase lenses specifically designed for the type of digital camera used.
If possible, use a camera lens with a focal length longer than the eyepiece focal length.
excessive contrast:
reciprocity law failure:
films tend to become less efficient at long exposures (minutes) and thus corrections need to be made for this effect
normally, the same exposure can be obtained by doubling the exposure when the f ratio is altered by one f-stop, but as exposures exceed a certain duration for a given film, this relationship fails, thus one may need to quadruple the exposure time when the exposure time is 30 secs and wish to change f ratio by 1 f-stop!
for a great pic of stars in a dark sky, the following are approx. equivalent exposures on an f/2 lens:
see film characteristics for degree of failure.
fortunately this is not a problem with digital photography
photo/astrophotography.txt · Last modified: 2020/08/28 10:01 by gary1