Star Tracker UPD

A star tracker is an optical device that measures the positions of stars using photocells or a camera.[1]As the positions of many stars have been measured by astronomers to a high degree of accuracy, a star tracker on a satellite or spacecraft may be used to determine the orientation (or attitude) of the spacecraft with respect to the stars. In order to do this, the star tracker must obtain an image of the stars, measure their apparent position in the reference frame of the spacecraft, and identify the stars so their position can be compared with their known absolute position from a star catalog. A star tracker may include a processor to identify stars by comparing the pattern of observed stars with the known pattern of stars in the sky.

In the 1950s and early 1960s, star trackers were an important part of early long-range ballistic missiles and cruise missiles, in the era when inertial navigation systems (INS) were not sufficiently accurate for intercontinental ranges.[2]

Star Tracker

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Consider a Cold War missile flying towards its target; it initially starts by flying northward, passes over the arctic, and then begins flying southward again. From the missile's perspective, stars behind it appear to move closer to the southern horizon while those in front are rising. Before flight, one can calculate the relative angle of a star based on where the missile should be at that instant if it is in the correct location. That can then be compared to the measured location to produce an "error off" signal that can be used to bring the missile back onto its correct trajectory.[2]

Due to the Earth's rotation, stars that are in a usable location change over the course of a day and the location of the target. Generally, a selection of several bright stars would be used and one would be selected at launch time. For guidance systems based solely on star tracking, some sort of recording mechanism, typically a magnetic tape, was pre-recorded with a signal that represented the angle of the star over the period of a day. At launch, the tape was forwarded to the appropriate time.[2] During the flight, the signal on the tape was used to roughly position a telescope so it would point at the expected position of the star. At the telescope's focus was a photocell and some sort of signal-generator, typically a spinning disk known as a chopper. The chopper causes the image of the star to repeatedly appear and disappear on the photocell, producing a signal that was then smoothed to produce an alternating current output. The phase of that signal was compared to the one on the tape to produce a guidance signal.[2]

Star trackers were often combined with an INS. INS systems measure accelerations and integrate those over time to determine a velocity and, optionally, double-integrate to produce a location relative to its launch location. Even tiny measurement errors, when integrated, add up to an appreciable error known as "drift". For instance, the N-1 navigation system developed for the SM-64 Navaho cruise missile drifted at a rate of 1 nautical mile per hour, meaning that after a two-hour flight the INS would be indicating a position 2 nautical miles (3.7 km; 2.3 mi) away from its actual location. This was outside the desired accuracy of about half a mile.

In the case of an INS, the magnetic tape can be removed and those signals instead provided by the INS. The rest of the system works as before; the signal from the INS roughly positions the star tracker, which then measures the actual location of the star and produces an error signal. This signal is then used to correct the position being generated from the INS, reducing the accumulated drift back to the limit of the accuracy of the tracker.[2] These "stellar inertial" systems were especially common from the 1950s through the 1980s, although some systems use it to this day.[3][4]

Many models[5][6][7][8][9] are currently available. There also exist open projects designed to be used for the global CubeSat researchers and developers community.[10][11]Star trackers, which require high sensitivity, may become confused by sunlight reflected from the spacecraft, or by exhaust gas plumes from the spacecraft thrusters (either sunlight reflection or contamination of the star tracker window). Star trackers are also susceptible to a variety of errors (low spatial frequency, high spatial frequency, temporal, ...) in addition to a variety of optical sources of error (spherical aberration, chromatic aberration, etc.). There are also many potential sources of confusion for the star identification algorithm (planets, comets, supernovae, the bimodal character of the point spread function for adjacent stars, other nearby satellites, point-source light pollution from large cities on Earth, ...). There are roughly 57 bright navigational stars in common use. However, for more complex missions, entire star field databases are used to determine spacecraft orientation. A typical star catalogue for high-fidelity attitude determination is originated from a standard base catalog (for example from the United States Naval Observatory) and then filtered to remove problematic stars, for example due to apparent magnitude variability, color index uncertainty, or a location within the Hertzsprung-Russell diagram implying unreliability. These types of star catalogs can have thousands of stars stored in memory on board the spacecraft, or else processed using tools at the ground station and then uploaded.[citation needed]

I am a full-time astrophotographer who has used many different star trackers from several brands for many years. I have created this resource for those looking to finally dive into astrophotography the right way, with a star tracker.

When it comes to astrophotography, quite simply, a star tracker allows you to take better images. Your exposure lengths are no longer limited to 30 seconds or less due to a moving sky, and you can dial back camera settings like ISO and F-stop.

Over the years, I have had many opportunities to review star trackers for astrophotography. I have learned that portable tracking camera mounts can vary depending on the brand you choose, from the mechanical design to the polar alignment procedure.

To be effective, a star tracker needs to be capable of accurately tracking the night sky for long-exposure night sky photography. Each model available has its strengths and weaknesses, from the portability factor to maximum payload capacity.

Before I go any further, I want to remind you of the kinds of images possible on a star tracker. There are plenty of night sky photographs shared on Instagram and other social media platforms from adventurous locations, and some of them are downright phenomenal.

However, many of them are ultra-wide field shots of the sky, lacking the reach needed to reveal wonderous deep-sky objects such as nebulae and galaxies. Being a deep-sky astrophotographer means that I aim to expose the incredible objects seemingly hidden in our night sky, and a star tracker helps me accomplish this.

To polar align an equatorial mount for astrophotography (including a small camera tracker), you need to adjust the altitude and azimuth of the base so that the polar axis of the mount is aligned with the celestial pole. In the northern hemisphere, we have the advantage of using the north star, Polaris, to aid in this process.

Without using a star tracker, the stars in the night sky will begin to trail after about 15-30 seconds, depending on the focal length of the lens used. This is because the Earth is spinning on its axis, while the night sky is fixed. Amateur photographers using a stationary tripod can use the 500 rule as a guide for choosing the ideal shutter speed, but a star tracker removes this limitation altogether.

When a tracking camera mount is used, a small motor slowly rotates your camera in right ascension, effectively matching the apparent movement of the night sky, and freezing it in its tracks. The image below shows you what a 3-minute exposure using a 400mm lens would be like without using a star tracker.

A star tracker will usually include a polar scope, which is used to help find the north celestial pole and adjust the mount accordingly. A star tracker that has been properly polar aligned will match the exact apparent rotation of the night sky to freeze deep sky objects in place.

A lot of the star trackers available online today come in a potentially confusing number of packages and bundles. My advice is that you invest in a system that can not only handle your intended payload (and then some) but also provide you with features that make imaging in the field easier and more enjoyable.

You can also photograph planets at short focal lengths on a star tracker, as seen in the image of the planet Venus and the moon below. As long as the mount has been polar aligned and balanced, longer focal lengths (up to about 500mm are practical)

The following tracking camera mounts share many similarities, including the ability to track the night sky at different speeds. Before investing in a dedicated star tracker for landscape or deep-sky astrophotography, make sure the bundle you order includes everything you need to mount your camera and lens.

The portable nature of a star tracker often leads to some of the most memorable deep sky astrophotography sessions in the field, as they offer you the freedom to travel to a dark sky location without a trunk full of gear. One of the most incredible astrophotography adventures of my life was setting up an iOptron SkyGuider Pro and William Optics RedCat 51 telescope to capture the Carina Nebula from Costa Rica.

A star tracker should allow you to quickly get up and running under a clear night sky. It should be a pain-free experience that provides the freedom and flexibility to take amazing astrophotography images wherever, and whenever you want.

I often see comments from beginners about being limited to a maximum exposure time using a particular mount before the stars begin to trail. I honestly believe these situations are almost always due to user error in the polar alignment and balancing procedure. 2351a5e196