Space Photography - How Is It Done?

Indulge me if you will. You just have to read this, I have to do this in the middle of nowhere, in cold conditions, in total darkness. 

 


Whilst most can appreciate the splendor of celestial objects,
few can appreciate the technical challenges astrophotography presents.

Meade LX50 with Small Guidescope

Here we see one of my Telescopes with a smaller
telescope piggybacked on top (the guide scope).

Long exposures of 30-120 minutes are required for faint deep space objects.

A long exposure allows the faint light of these dim objects
to trickle in onto the film,
leaving us with bright colourful vivid images.

It is not just a case of attaching a camera to the telescope
and snapping away.

Tracking

The Earth's rotation makes long exposures quite tricky.
At high magnifications, an object being photographed moves quite quickly through the field of view.
So telescopes with tracking motors that slowly move in the opposite
direction of the Earth's rotation are required.


.


To keep the telescope tracking deep space objects accurately,
a guide scope is required (see below). The guidescope sits on top of the main imaging scope, as pictured above.

Telescope Maksutov Cassegrain

Once an exposure starts, the
photographer looks through the guide scope for the entire time.
An eyepiece with an illuminated laser reticule is used to make sure
that the guide star (that has been previously selected) is centered
on the laser reticule throughout the exposure.

If the object being photographed begins to drift, the photographer
makes tiny adjustments to the telescopes tracking motors to correct
that drift.
This technique can be extremely demanding.
 


Cameras

Many people think that these types of images can only
be caught with the latest and greatest camera technology.
Modern cameras with built in light meters and focusing
functions have no place in Astrophotography,
as their light meters and focusing functions are useless
in dark conditions. Cold nights and dampness wreak
havoc on an automatic camera's electronic system too.

It is dangerous to rely on batteries in astrophotography.
The last thing an astrophotographer wants, is to be in the middle
of a 45 minute exposure, only to find the batteries have died.

This is why old fully manual cameras are used.


Cameras Olympus OM1 and Nikon F2

"Fully manual", means full control.

But it is not just a case of attaching the camera to
the telescope either.

Focusing

Before an exposure can even start, the camera that
is attached to the telescope needs to be focused.

Though some deep space objects are
so faint, that they are invisible through
my cameras focusing screen.

So how can an astrophotographer
focus if he/she cannot see the object?

A range of techniques and calculations are required
for this not so easy task (but this is
beyond the scope of this brief description).

On the odd occasion the object being imaged, can
actually be seen through the camera's view finder, it is
still not practical to use the human eye to focus. This is because
your eye has a lens, and can make an 'out of focus'
image, appear 'in focus', so special focusing tools have to
be used.

Also, if an image is so faint that it is invisible through
the cameras focusing screen, how does the astrophotographer
even know if the image is framed properly?

Framing

There are different ways to achieve this. I match star patterns

from the sky with a star atlas. Sometimes this does not
work as the stars are way too faint through a camera's
focusing screen. Good knowledge of astronomy and celestial
co-ordinate systems are a must.

Takahashi Epsilon with Refracting Guidescope

 


Telescope Idiosyncrasies

Optics

Here we can see into the 'Optical Engine' of the telescope.
This type of telescope uses mirrors to bounce light up and down.
This achieves a longer focal length, which results in higher magnifications, in a process called "folded optics".

Telescope with Refracting Guidescope

Collimation

But before light can bounce up and down between the
two mirrors to increase the focal length of the scope,
both mirrors need to be precisely aligned in a process
called "collimation". Aligning the two mirrors
may sound easy, but to a beginner, it can become
extremely frustrating. Every time the telescope is moved,
perfect collimation may be required.

Polar Alignment

Once the astrophotographer is happy with the collimation,
he then needs to "polar align" the telescope. Polar Aligning
is a complicated process for the beginner, where the telescope's tripod and mount need to be adjusted perfectly to the Celestial Poles and also to the exact latitude of the viewing location.
Without precise Polar Alignment, a telescope cannot track
efficiently for astrophotography. This is due to the Earth's tilt
and rotation.

Drift Aligning

I prefer to use a technique called drift aligning, where I
watch what direction the stars slowly drift through my eyepiece.
I then slowly make adjustments to my mount until finally,
the star will not drift for at
least five minutes.

Vibrations

The telescope's size, makes it susceptible to catching wind,
thus causing vibrations. One light breeze is enough to make a
photo appear blurry. Footsteps near the scope too, can also ruin
a perfect photograph. So an astrophotographer needs
to keep extremely still throughout the entire exposure, which
may last for 2 hours in some cases.
Anti vibration pads are used to shield the
telescope from people that may be standing nearby.

Mirror Flop

The primary mirror in some telescopes can be
subject to a phenomena called 'Mirror Flop'. This is caused
when the primary mirror shifts ever so slightly inside the tube
during an exposure. This would obviously ruin a photo. Some
telescopes are now able to lock down a mirror. If you
cannot lock your mirror down, there are
still numerous techniques to fix this annoying problem.

Periodic Error

No matter how well your telescope tracking
motors and worm gears are made, they will always suffer
from Periodic Error. Or the tiny amount of slack in between
the worm gear teeth. Over many minutes, this
can build up and can confuse autoguiding software, thus
ruining that perfect photo. So calculating the amount of
Periodic Error that your mount has is a must before
a photo can start.



Balancing

At the point when the telescope is ready to take photos,
it has much equipment attached to it. For example, camera,
guidescope, CCD Camera for guiding, heaters to remove
unwanted dew, camera adapters and so on. This
change in balance can wreak havoc on a mount's
tracking ability, so counterweights are used to
compensate.


Telescope Equipment 
 


Planning & Preparation!

Timing

Prior to an astrophotographer even setting up
the telescope for deep space images, he/she will need
to plan exactly what deep space objects will be photographed
that night. This is because different deep space objects
appear at different times throughout the night. The stars
overhead completely change their positions in as little
as an hour. This is due to the Earth's rotation. So planning
what objects will be up first, and where they are
is essential for an effective night of Astrophotography.

New Moon

But there is another reason for planning the night
well in advance.
Deep space objects can only really
be photographed on the night of the new moon
(no moon, which is once a month). Moonlight "cancels out"
the dim light from deep space objects. Only getting
one night a month to take these types of
photos, forces the astrophotographer to become a
meticulous planner.

Emulsions

Another factor to consider in astrophotography, is the type
of film used. Different films are sensitive to different colours.
So an astrophotographer needs to adjust the type of
photographic film being used for each individual image. This
knowledge comes from the classic trial and error
routine.

Predicting Weather

Predicting atmospheric turbulence and knowing
the weather is also a must. Obviously learning meteorology
is extremely important for an astrophotographer.

I use the Australian Bereau of Meteorology's
web site for this.

http://www.bom.gov.au/weather/wa/

Scintillation

Scintillation is what you are seeing when the stars
appear to twinkle. Scintillation is bad
for astrophotographer's, because it means
the atmosphere is unstable. An unstable atmosphere
may not look like much of a problem to the naked
eye, but through a high powered telescope, the stars
wobble haphazardly, and this is obviously not good
for imaging. So learning meteorology and keeping
up to date with jet streams, by using satellite
images is a must. I use the following links to monitor
jet stream activity in the upper atmosphere.

http://squall.sfsu.edu/crws/jetstream.html

http://www.wunderground.com/global/Region/AU/JetStream.html


Flight Paths
Also, if you are close to a flight path
checking up on local flights is a must.
A jet with bright lights streaking through a one
hour exposure would not be helpful. See picture below.

737 Ruining A Great Shot

I took my eyes off the sky for five seconds here, and
a 707 decides that it is not camera shy, ruining my 30 minute
exposure.

Not Happy Jack!

Satellite Tracking Software

Also having a satellite streak through
your photo would not be helpful,
so I use commercially available astronomy software
to monitor satellites. Whilst conducting long exposure
photography, I have my laptop sitting near me
running software showing me real time movement
of Earth's artificial satellites. I keep a close eye
on this during an exposure.

The occasional satellite that passes overhead
which is not on my astro-software is usually
a spy satellite or space junk.

Satellites that appear to blink on and off
as they travel through the sky are
classified as detatched satellites which
are decomissioned satellites
that are simply tumbling around
in orbit.

Recapping

So after the astrophotographer has
Checked the weather and jet streams
and waited for the new moon, and also
collimated his scope
polar aligned his scope
calibrated his guidescope
selected the right film
found the object to be imaged
focused the image
then framed the image
the exposure is close to starting. But not quite yet.
When using the cable release to open the camera's
shutter. The movement of the mirror flipping upward,
inside the camera, vibrates the telescope enough to ruin
the photo.So an artificial shutter must be used to avoid
this problem. Which won't be covered here.

Now a lengthy guided exposure can start.

The reason an astrophotographer uses different exposures times
for different objects, is that the object's magnitudes (brightness)
vary from moderately bright, to extremely faint.

Astrophotography is the culmination of two
demanding disciplines: Astronomy and Photography.
It is easily the most technically demanding form of
photography today.

You have to be a fanatic for this type of photography.

Because after lugging
all your heavy equipment out to the country
and spending hours setting it all up, you
end up sitting in total darkness
in freezing conditions for hours on end, uncomfortably
huddled over a guiding eyepiece.
It is not most people's idea of
an exciting night, but the challenge and
difficulty in taming these
objects is what drives me.

Many other techniques and gadgets are used for this form
of photography. This has simply been a brief
overview of the field (phew).

Ray Palmer
Deep Space Photographer

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