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Untracked Astrophotography

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


Setting up your camera for untracked Astrophotography

Finding the right settings for your camera is crucial to making great astrofotos. This section will give you guidelines for setting up your camera properly.

Exposure Time and the 600 Rule

The main goal of limiting the exposure time is avoiding or at least reducing star trails. As a common rule the maximum possible exposure time should be limited to a single digit second number. The exact value depends on the focal length of the objective and the distance of the motive from the nothern star. A commonly cited rule for supressing star trails is the so called 600 rule (Sometimes also 500 rule): $$t_{min} = \frac {600} {c*f}$$ with

This rule states that the maximum exposure time of a camera with full frame sensor should not be greater than 600 divided by the focal length of the lens. The rule can easily be extended to non full frame cameras by taking the so called crop factor into account.

Focal Length
in mm
Recommended exposure time in seconds
APS-C (Crop=1.6)
(z.B. EOS 50D, EOS 60D, EOS 7D)
Full Frame
(z.B. EOS 5D, EOS 6D)
24 15.6 25.0
35 10.7 17.1
50 7.5 12.0
100 3.8 6.0
135 2.8 4.4
200 1.9 3
280 1.3 2.1
300 1.25 2
500 0.75 1.2
600 0.62 1
Table 1: Exposure times according to the Rule of 600 for cameras with full frame and APS-C sensors.

The exposure times listed in the table above aim at completely eliminating star trails. In reality short star trails can easily be removed so that slightly longer exposure times than listed here are still acceptable. Unfortunately some stacking programs (Deep Sky Stacker i'm looking at you!) have problems working with high noise images that contain star trails. If you experience such problems try stacking with another program (i.e. Fitswork) which might allow manual correction of the stacking result.

Focal Length

The angle of view is related to the sensor size and the focal length of the objective. Telephoto lenses have large focal lengths (200 mm - 400 mm) allowing a good magnification whilst wide angle lenses have a small magnification but a wide angle of view. The following table gives an overview over the image angle of commonly used focal lengths:

Focal Length
in mm
Angle of View Motivs
APS-C
(z.B. EOS 50D, EOS 60D, EOS 7D)
Full Frame
(z.B. EOS 5D, EOS 6D)
24 58.1° 84.1° Übersichtsaufnahmen
(z.B. Milky way)
35 41.7° 63.4° Constellations
(z.B. Orion nubula)
50 29.9° 46.8°
100 15.2° 24.4° Constellations, Large nebulae
(i.e. North Amerika Nenula)
135 11.3° 18.2°
200 7.6° 12.3° Galaxies, Planetary nebulae, Globular clusters
(z.B. Andromeda galaxy, Orion nenbula)
280 5.5° 8.8°
300 5.1° 8.2°
500 3.1° 5.0°
600 2.5° 4.1°
Table 2: Angle of view of common focal lengths for sensor sizes APS-C and Full Frame that are typical for DLSR cameras.

Aperture

Since most astronomical objects are very dark you need a wide opening to gather as much light as possible. Usually using an aperture of f/2.8 is recommended. Unfortunately most lenses show their weakness when being operated fully open. For this reason it is often betten to stop the lens down a f-step. Using lenses darker than f/4.5 is usually not recommended for untracked astrophotography.

ISO Value

Due to earths rotation the stars seemingly move across the sky. As mentioned above short exposure times are a mean for supressing star trails. The downside of short exposure times are underexposed images with a reduced dynamic range. Increasing the ISO settings will help to avoid underexposure and thus the related loss of image dynamic range.

As a general rule the ISO settings should be set as high as possible (> ISO 1600). The only rule is that the motive should not be overexposed. If you take orion for example make sure the stars inside the nebula are still visible and not washed out due to overexposure. it is important to know that high ISO values will also amplify the image noise. The resulting frame is usually extremely noisy but it is important to understand that noise can be dealt with by stacking more images. Missing image information due to a reduced dynamic in the image cannot be fixed later on.

Activate manual mode

A camera suitable for untracked astrophotography should be capable of shooting in manual mode. Usually the manual mode is selected by turning the camera mode selector wheel to the position labelled with the letter "M". In manual mode the operator has full control over exposure time and aperture settings.

Activate RAW mode

Untracked astrophotography requires the use of the so called RAW mode for saving image files. Jpeg compressed images are not suited due to the lack of color resolution. This is caused by the formats limitation to using 8 bits per color channel as well as the introduction of artifical image errors due to the compression algorithm. In Raw mode 12-14 bits are used for storing image data per color channel and no compression takes place.

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