RASC Calgary Centre - Imaging the Sky 2
Imaging the Sky 2
by Jason Nishyama
Page last updated November 5, 2018
Imaging the sky - part the second
In this instalment of imaging the sky we'll look at taking the image and what mounting methods are
available and their advantages and limitations.
Regardless of the mounting method, the actual exposure with a DSLR is done in the standard way for
taking terrestrial photographs, that is set your exposure and open the shutter. Generally you will
have to set your DSLR on its manual setting, generally labelled "M" in order to set the shutter
speed. If you have a way of opening the shutter remotely (cable release or remote control) it is
better as there will be less vibration. The length of exposure possible using the camera settings
is limited by the camera, some cameras will allow you to set a time greater than one second,
others won't. If the exposure you want isn't one of the settings you will need to use the "B" or
bulb setting for the exposure. This setting keeps the shutter open as long as the shutter release
is pressed (on some cameras press once to open the shutter, press again to close it, see your
manual for details). This will allow you to hold the shutter open as long as you want and the
camera battery will permit. Purpose built astro-imagers have their exposure set in the computer
software that controls the camera.
The actual shutter speed needed will depend on your mounting method and how dim the object you are
imaging is. The nice thing about DSLR cameras and CCD imagers is that you can check right away if
your exposure is working for you and can then adjust it as needed.
If you are using a DSLR camera you may also be able to adjust the ISO/ASA sensitivity. Generally
faster ISO ratings (larger numbers) provide more sensitivity to light but also provide more noise
in the final image. For starting out just crank the ISO to the maximum and then as you gain
experience adjust it downwards to reduce the noise.
There are four basic ways of mounting a camera, unguided tripod, afocal, piggyback, and prime
focus. In this article I will generally focus on using a DSLR for the imaging. Those of you with
purpose built astro-imagers can skip down to the prime focus section since they can't be used for
the other methods.
Unguided Tripod
The first and easiest method of mounting a camera for astrophotography is to simply place it on a
sturdy photographic tripod, point it to the sky and open the shutter. This is a great way to start
out as there is little in the way of set-up time and all you need is your DSLR, a lens and a
tripod. Since you can use regular photographic lenses, photographs of large areas of sky can be
accomplished making this method ideal for photographing constellations, the moon, and
conjunctions. The primary limitation is that since the camera isn't tracked across the night sky,
stars will trail if the exposure is too long. This limits the technique to relatively bright
objects or for doing photographs of star trails.
It is possible to work out the maximum exposure you can use for a particular declination and still
get relatively round star images by using the formula (for DSLR's and 35mm film cameras):
t = 1000 / (F cos d)
where F is the focal length of your lens in mm, d is the declination of the object in degrees, and
t is the exposure time in seconds. It is important to note at this point that if your DSLR doesn't
have a full size sensor (such as the Nikon DX line) there will be a multiplication factor (1.5 for
Nikons other than the D3 and D700 for example). So a 50mm lens acts as a 75mm lens for a DX Nikon.
Your camera manual should have this factor.
If you want to guarantee round stars you'll need to use this formula:
t = 345 / (F cos d)
Which will make the maximum trail smaller than the resolution of most films or electronic
imagers.
So if you want to image the constellation Orion (at roughly 0 degrees dec) with a 50mm lens with a
Nikon D300 (for example) you can expose for up to:
1000/ ( (50mm*1.5) cos (0)) = 13.3 seconds for a reasonably good shot or:
345 / ((50mm*1.5) cos(0)) = 4.6 seconds for circular stars.
Shorter focal length lenses will have longer maximum exposures as will photographing something at
higher declinations (at the celestial equator is a worst case scenario). Even so 13 seconds is a
good amount of exposure for picking up a bright constellation or planet and would cause the moon
to be very over exposed.
Afocal
This technique is trickier than most but can provide excellent results even with inexpensive
point-and-click type cameras. With the afocal technique you need your camera and lens and a
telescope with an eyepiece. Quite literally you line up the camera and lens so it takes a picture
of what is in the eyepiece. This can provide excellent shots of the Moon and planets. The
disadvantage is getting things lined up properly as well as the difficulty in tracking the object
without somehow attaching the camera to the telescope.
Conceptually this technique is quite simple. You first aim the telescope at what you want to image
using your eye and eyepiece. Make sure the object is in focus in the eyepiece. Next aim the camera
into the eyepiece and focus the camera. Take the picture. For unguided shots the amount of time
you can expose for will be quite short so bright objects are best.
For an unguided shot it is best to have the camera on a separate tripod and aim it into the
telescope eyepiece. For a guided shot you will need some kind of adapter to hold the camera on the
telescope for the duration of the exposure. Adapters for some point-and-click cameras are
available commercially.
Piggyback
For exposures longer than a few seconds you will need a way to have the camera track the sky. This
can be easily done using the piggyback method. In this case the camera is usually attached on top
of or "piggybacked" to an equatorially driven telescope and then using the camera's lens a long
exposure is taken. Since the telescope is tracking the sky, longer exposures are now possible.
This allows for "deeper" images which show fainter and fainter objects. The wide field of the
camera lens allows for whole constellations to be shot with faint deep sky objects often being
seen in the image. Of course now you need to have access to an equatorially driven telescope and
an appropriate piggyback adaptor to attach the camera. Further the telescope must be polar aligned
to properly track the sky.
It is also possible to do this technique inexpensively without the telescope. Camera tracking
platforms are available commercially or can be built easily by the amateur which can be then used
to have the camera track the night sky, lengthening exposures. If you are interested in making one
of these, type "barn door tracker" into your favourite search engine. Regardless of the tracker
used, they also must be polar aligned to be used.
Prime focus
This is the ultimate in the astrophotography progression. In this case you will use the telescope
as a lens for the camera. In the case of a DSLR, an adaptor (usually a T adapter) will be
required. These are inexpensive and available at professional photography stores or from telescope
vendors. This will allow you to attach the camera to the telescope. Purpose built astro-imagers
usually come with an adaptor that allows the imager to be connected to the telescope.
For this you will need a very accurately polar aligned equatorially mounted telescope to ensure
that the telescope tracks the sky. Also, since most mounts have some form of error in them,
guiding the telescope either manually or with an autoguide system will be required for exposures
over a couple of minutes. To see why this is go back to the section on tripods and put the focal
length of your telescope in for the lens and see how short the times get, there's not a lot of
margin for error at telescope focal lengths.
It gets trickier from here. Focusing a camera on a telescope can be an exercise in frustration.
The viewfinder in most DSLRs will be too dark to see anything but the brightest stars in. Some
have a "live view" function which allows a real-time image to be viewed on the camera display
which can be used to focus the telescope. If this feature isn't available, or if you have a
purpose built astro-imager, you will have to use trial and error to focus by taking an image,
checking the focus, adjusting the telescope focus, taking another image and so on. A focusing mask
can help with this process (look up Hartmann or Bahtinov focusing masks in your search
engine).
Once you have everything lined up and focused, you can then take your image. Those of you with
monochrome imagers of course will have to take at least three images through different filters if
you want colour (see last month's article).
So why go to all this trouble to take a prime focus shot? You get to use the aperture of your
telescope to image quite faint objects in a way that a rather small aperture photographic lens
can't. Further telescopes usually have quite long focal lengths, providing magnification to see
small distant objects (at the expense of field of view). This makes prime focus photography the
method of choice for most deep sky objects.
Which one to use?
Which of the methods do you use? It all depends on what you want to take images of. Bright objects
that occupy wide areas of sky are best imaged with the tripod or piggyback methods. Planets and
the Moon with afocal and prime focus (afocal can provide even more magnification than prime focus
does). Deep sky can be done with piggyback for objects that are visually large (arc minutes or
degrees across) and prime focus can collect a lot of light to see faint objects. So figure out
what you're looking for and pick the method that works for you!
Imaging 3 - processing those images or what the heck are bias, dark and flat frames?
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