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Part 1 I
Part 2 |
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Globular
star cluster in Hercules captured with a Pentax K200D
and a Tamron
200 mm f/3.5 lens at
ISO 1600 and 15 s
Taken in the heart of the city in a sea of light pollution
- Read the first part about how below.....
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Before we plunge into the practicalities, it would be an
advantage to get a few basic things straight:
1. Stars are point-like sources of light.
This is to be understood in the sense that the stars for all
practical purposes are infinitely far away, and therefore, the
light arrives at our lens in parallel bundles - which means that
the image of a star in the first approximation mere is a mere,
single point measuring a measly single pixel on our sensor. In
practice, due to air turbulence but also because of more
fundamental optical principles the images of the stars will
become small disks, but they are basically very, very small. In
the old analogue days light was somewhat scattered in the
emulsion and activated several silver grains. Therefore, old,
analogue star photos often look more "pleasant" than the current
digital ones that may seem somewhat flat and synthetic. That
being said, to prevent disappointments.
2. Stars are point-like sources of light.
- and thus, are different from illuminated surfaces such as
moons and barn doors in that it alone (in the first
approximation) is the lens' physical opening and NOT the lens
opening / focal length, which determines how much light one
pixel element of my sensor will receive. For a given lens
diameter. only the exposure time and sensitivity of the sensor
will determine how bright a specific star will register and how
faint stars, we can catch. So I can put a 2X teleconverter on my
200 mm f/3.5 telephoto lens and spread all dimensions of the
captured image to double size - the stars will remain just as
clear or equally weak for that matter. But the dim night sky
gets darker and the contrast improves accordingly.
However, truly point shaped are the images of stars never
because the light - as it passes through an aperture (lens
opening) of a limited physical size - undergoes a diffraction
and interference, called diffraction, which causes the image of
the star to be composed of a small central spot surrounded by
alternating dark and light rings, which rapidly decrease in
strength from the centre. Consequently, more luminous stars
produce - after all - larger images than weaker.
The size of this spot (called Airy disc) decreases with the
physical size of the lens opening, but increases with focal
length - in other words: It grows proportionally with the
F-ratio. This actually means that with the same focal length the
disk will grow in size - and thus, weaken in surface brightness
- the more I stop down the aperture. And on top of that, the
more I stop down the aperture, the fewer photons I get to spread
out over a growing area!!! One may also say that you pay much
more than double for each f-stop that you stop down your lens.
This is the reason, dictated by the laws of optics, why one MUST
use as large an aperture as lens quality permits in order to
capture the fainter stars in any reasonable span of time.
3. Know your camera.
Astro Photographers usually have the greatest distrust of
in-camera noise reduction, because manufacturers surely did not
have small faint stars (which can be easily confused with pixel
noise) or weak nebulosity (which can easily be confused with
colour and luminance noise) in mind when they made their noise
reduction algorithms.
Thus, turn off any noise reduction that you can and be prepared
to do some post-processing of your images accordingly. But
beware! Even if you've turned off all noise reduction, some
noise reduction algoritms may well be runnig in the background
at high ISO. This is nothing that your manual speaks of and you
will have to make your own experiences.
Strongly amplified noise of 30 second dark exposures.
There is
more noise at ISO 800 than at ISO 1600
- even though all noise
reduction was disabled!
4. The Earth Rotates.
The sky does not, but the daily rotation of the Earth around its
axis makes the stars trail over your head as seen from your
vantage point on Earth. The closer they are to the celestial
equator the longer trails will they make on your photo (for any
given focal length and exposure time) unless you have your
camera rotating along on a so-called equatorial mount, (se more
about that in the
second part of this suite of tutorials).
I often hear fellow photographers suggest
that even with fixed tripod photography, one should use only
moderately high ISO due to the noise issues associated with high
ISO values (today, from about ISO 800 and upwards). Now, if you
want to shoot star trails under a dark sky, that may be a
reasonable advice, but if you want to record stellar and
constellation images as they look to your eye that is definitely
NOT a very well placed bit of advice for the fixed tripod
astrophotopgrapher.
In fact, knowing that the Earth rotates
once (360 degrees) every 23 hours and 56 minutes relative to the
stars which gives an angular movement of 0,00418 degrees every
second, it is quite straightforward to compute the length of
star trails on the film/sensor for a given focal length and
exposure time as:
s = cos(d)*2*F*tan(0.00418*t/2),
where
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d is the angular distance of a star from
the celestial equator (the star's declination)
-
F is the focal length of the lens used
-
t is the exposure time in seconds
-
s is the length of the star trail in the
focal plane (on the film/sensor)
For example taking a 30 second exposure
with a 24mm lens of a star near the celestial equator (d = 0
degrees) will produce a star trail of about 0.05mm.
Surely, that is not much, but with a
typical pixel size of contemporary DSLRs around 0.006mm that
star trail will show up as a small line about 8 pixels long,
which you will readily be able to see in tight crops/large
magnifications of your image. This may or may not annoy you; at
least you now know what you are up against.
So, you want to keep exposure times - and thus
the length of star trails - reasonably short while still
capturing more than but the very brightest stars? Then one
should start - in particular the beginning astrophotographer -
with an ISO of around 1600 and try out the recording capability
of the camera + lens combinations that one intends to use at
varying exposure times. Say, from about 30 seconds and down to a
couple of seconds.
5. First shots
Well, thus prepared I screw on my excellent SMC Takumar 55 f/1.8
, set the ISO value to 1600 and put my camera on a tripod out on
the balcony. I'll take a few shots with 15 seconds exposure time
at full aperture and get ... ...
.... this.
That, I could have said to myself. When I shoot in the
heart of the city I capture not only stellar light but also the
light reflected from atmospheric dust and haze and coming from
street lights, shop windows, cars, fluorescent tubes and sodium
lamps - all providing a hideous mixture of spectral lines in
harsh colours, which may hardly be filtered out. But
fortunately, all hope is not out - this hopeless picture stands
to save. It requires only a simple raster program (I use
PhotoImpact, which cost me about. 1 / 15 the price of PhtoShop
when I purchased it some years ago). The steps in the
post-processing can be as follows:
a) Darken the background and compensate for the attenuation of
the stars in CURVES, like this:
The resulting image (right) is not entirely satisfactory yet,
but
b) Remove the remaining
colour noise by setting the black point with Eyedropper in the
muddy part of the sky in LEVELS:
If needed, repeat this process until you are satisfied and get
the following picture:
Same image as above after just two simple steps in
post-processing.
Now if you have a
sensor that is noisy and thus, have to struggle with
sensor noise and light pollution, the standard technique is
that you take a so-called Dark Frame (DF), i.e.: an exposure
with the lens cap on at the same ISO, exposure time and
temperature. This DF is then subtracted from the sky image
whereby you get compensated for the pixel noise (hot pixels) and
smoothed out colour and luminance noise. After that the picture
may be treated as described above.
But there is a possible shortcut, where you can kill two birds
with one stone and compensate simultaneously for both light
pollution and noise. This shortcut says: Do not shoot a DARK-
but rather a LIGHT Frame through a piece of matt plastic folder!
This smears the star light, but retains the overall distribution
of light pollution the night sky like this:
"LPCN" Frame
Subtracting such a "LPCN" (LightPollutionCameraNoise) frame from
the image of the sky leads us once again to a situation where
the finishing touch can be easily made in CURVES and/or LEVELS:
Original minus "LPCN"
The picture above adjusted in CURVES and LEVELS.
Honoured be the one that
deserves the honour. This ingenious trick I've found here:
http://www.dur.ac.uk/nigel.metcalfe/astro/afocal.html
In a following Tutoral(II) I shall show how to make images
as the one shown at the beginning. You will find the sequel
here.
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