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Untracked Astrophotography

An introduction to fixed tripod low cost DSLR Deep-Sky Photography


Astrophotography Gear

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Fixed tripod deep sky astrophotography is a relatively new branch of astrophotography made possible by the arrival of digital image acquisition devices in the consumer market segment. Nowadays a standard DSLR or bridge camera is enough to create images of deep-sky objects if you know the proper technique. This page will give a brief overview over camera hardware and acessories that will help you making astrophotos.

Digital Camera (DSLR or Superzoom)

Untracked deep sky photography requires a digital camera. Most DSLR, Superzoom or System cameras should work but camera systems with interchangeable lenses should be preferred since they offer more flexibility.

The camera should at least have an exposure index rating of ISO 1600. (More is better; The images presented here were mostly taken at ISO 12800.) Since optical viewfinders are almost useless when aiming at the night sky a live view feature is needed for focussing. Having an additional digital magnification is a plus since it greatly increases the focus quality. Another feature to look for is the capability to take RAW images. The default image type used by most digital cameras is jpeg but this format is not suitable as input material for further processing due to the limitation of 8 bits per color channel and the introduction of image artifacts.

The camera should also be able to use an external shutter with integrated timer since this will greatly simplify the process of taking image series semiautomatically. Usually these timers allow setting up exposure times, time between sucessive shots as well as the number of images to take. For Canon cameras I can recommend the TC80N3 timer or similar products. And lastly i recommend having a articulating screen. Most objects are high in the sky and if the camera is pointing upwards at a high angle looking at the display is easier if you can tilt and rotate the display.

In a nutshell

Sensor size, Pixel number and Noise Reduction

Modern digital cameras are often advertised as having a high number of pixels. Unfortunately the number of pixels doesn't say anything about the quality of the camera. A high number of pixels means that the size of each individual pixel is very small and often entry level cameras combine high pixel numbers with a small sensor. As a result fewer photons reach the individual sensor cell and the signal to noise is negatively affected.

Image 5: Common Sensor formats. (Based on an Image from: User Chriusha auf Wikimedia Commons; Lizenz CC- BY-SA 3.0)

Since no one likes noise images most camera vendors add software filters to eliminate the noise. Since those filters aren't particlary clever they also eliminate image information. A camera suitable for astrophotography should not have built in noise supression or it should an option that can be disabled. Usually it is suficcient to dave the images in RAW format in order to bypass any internal postprocessing.

Lens Selection

Usually the lens is the determining factor for the quality of the astrophoto. Astronomical images are very sensitive to lens problems due to the pointy nature of stars. A lens that creates good images at daylight may be completely unsuitable for making astrophotos. Colors may be botched or stars appear blurry.

Relative Aperture (f-number)

If you want to start astrophotography with an existing superzoom Camera you don't have a choice lense wise. If you want to purchase a new camera make sure to select a model capable of working with small f-number at large zoom levels. For astrophotography a f-value of 2.8 at all zoom levels is recommended but be warned that only few superzoom cameras can deliver that (i.e. Panasonic Lumix FZ-200)

DSLR cameras and system camera offer a variety of lens options. For deep sky photography the large magnification provided by long focal lengths is needed. In addition the lens should collect as much light as possible which means the relative aperture of f-number should be low (recommended f/2.8 or below). Such lenses are usually expensive so that beginners may also start with f/3.5 to f/4.5. This is enough for bright objects like the Orion nebula and the andromeda galaxy but you will miss sone details. Cheap mirror based lenses though cheap are not suitable for untracked deep-sky photography due to their high f-number.

Sometimes it is better to stop down the lens a f-stop (close the aperture a bit) in order to increase the quality of the image. Usually the lenses do not have the best image quality when operated fully open. This is especially true for cheaper lens models.

Fixed Focal Length vs. Zoom Lens

Fixed focal lenses usually give a better image quality and brightness due to their simpler mechanical layout with fewer internal lenses. Therefore they are usually cheaper. A good zoom lens will work too but the really good ones tend to be more significantly expensive then fixed focal lenses.

The recommended focal lengt depends on the motive. A larger focal length will capture a smaller area of the sky thus giving you a greater magnification. For capturing galaxies the focal length should not be around 200 mm or longer. Shorter focal lengts (i.e. 50 mm) are suitable for imaging larger objects such as wide nebulae or making overview images of constellations like the orion nebulae. Wide angle and fisheye lenses have their use in imaging large sections of the sky at once. A complete list of focal lengths and recommended objects can be found in the section camera settings.

Lens Hood

Using a lens hood at night might be counterintuitive but it has benefits for astrophotography:

Lens hoods can easily be improvised by using cardbord and tape.

Image 6: A lens hood blocks incoming light from car headlights and and delays the dew formation on the lens.

Dew formation is hard to avoid without additional equipment such as a lens hood heating element. Especially in colder temperatures dew formation will set in as soon as the equipment has cooled down to ambient temperature. It is not uncommon to find the lens covered by dew after 2 hours of shooting in th cold.

Lens Clamp

A lens clamp is attached to the lens and provides an alternate mount point to attach the objective to a tripod or spherical head.

Image 7: Left: A camera mounted without a lens clamp. The center of mass further away from the camera body. Right: The center of mass is located close to the lens clamp. This arrangement is less prone to oscillations.

When using bulky objectives a lens clamp helps to distribute the weight of the camera more evenly by providing a camera mount point close to the center of mass of the entire system. Without a lens lamp the long objective will act as a lever with a tendency of pushing the camera down. You might loose aim and in addition the stress on the cameras internal lens mount might cause additional wearout of the mechanical elements holding the lens.

Camera Tripod

To start with astrophotography you can use any tripod but a sturdy tripod that capably of carrying at 4-6 kg of gear is recommended. Make sure the tripod is large enough so that you don't have to crouch behind it when shooting objects high on the horizon. If you plan on advancing to more complicated gear later on you might also want to look at special tripods made for astronomy and astrophotography. Their design is more robust that they can carry higher weights. Their legs might be made of wood or carbon fieber to give them more torsional stiffness.

Spherical Head

Due to the apparent motion of the stars across the night sky the deep-sky objects will slowly wander out of the cameras field of view. Taking images over an extended period of time requires adjusting the camera position manually several times to obtain enough frames for the stacking stacking process. A spherical head is a usefull piece of equipment that allows quick repositioning of the camera when taking multiple image series. Whilst this is also possible with the tripod alone the spherical head makes it much easier.

A spherical head may also come in handy when you dont want to use a tripod at all and simply put the camera on a flat surface like a table. This only works for smaller camera models tough.

Red Dot Finder

A red dot finder helps you to point the camera to the right position in the sky. The first thing you will realise when standing in the darkness trying to find the point in the sky where you want to point the camera is that you don't see much. The viewfinder is not of much use since it only shows bright stars and even then you can't really be sure which star is currently displayed. It literally does not provide the bigger picture. This is where a red dot finder comes into play. It is an acessory that helps you to aim the camera at the proper location in the night sky.

A red dot finder usually gives the user an aim point in the shape of a red dot or a cross. The amazing thing is that you do not have to align the eye exactly behind the red dot finder. The red marker will also appear at the correct position if you look slightly from the side. This is an amazing property that makes aiming the camera very easy (as opposed to aiming with iron sights).

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