Terminology You Must Know For Astrophotography

TERMINOLOGY YOU MUST KNOW FOR ASTROPHOTOGRAPHY.

I have to admit that when I first started astronomy I found the multiple references to time and coordinate systems extremely confusing. It took some time, helped by the research for this article, to fully appreciate and understand these terms. As the quote above says, “time is an illusion” and as it happens, so too are coordinate systems.

Consider the lonely astronomer, sitting on his planet observing billions of stars and galaxies floating around in space, all in constant motion with respect to each other and his own planet, which is spinning and rotating around its solar system in turn rotating around its host galaxy. One can start to appreciate the dilemma that faces anyone who wants to make a definitive time and coordinate based system. The solution is to agree a suitable space and time as a reference.

Even something as simple as the length of an Earth day is complicated by the fact that although our Earth spins on its axis at a particular rate, since we are simultaneously moving around Sun, the length of a day, as measured by the Sun’s position, is different by about 4 minutes.

An Earth-based coordinate system for measuring a star’s position is flawed since the Earth is spinning, oscillating and orbiting its solar system, galaxy and so on. In fact, one has to make first-order assumptions and make corrections for second-order effects. Our Earth’s daily rotation is almost constant and the tilt of the axis about which it rotates varies very slowly over 26,000 years (over an angular radius of 23°). Incredibly, this slow shift was detected and measured by Hipparchus in 125 BC. The name given to the change in the orientation of the Earth’s axis is “precession” and the position of the North Celestial Pole (NCP) moves against the background of stars.

Currently Polaris is a good approximation (about 45 arc minutes away) but in 3,200 years, Gamma Cephei will be closer to the NCP. The upshot of all this is that there are several coordinate and time systems, each optimized for a purpose. The accuracy requirements will be different for science-based study, versus more humble, down-to-earth systems employed by amateur astronomers.

Even so, we are impacted by the small changes in our reference systems, for instance a polar scope, designed to align a telescope to the NCP has a reticle engraved to show the position of Polaris. Ideally, a polar reticle requires an update every 10 years to accommodate the Earth’s precession and indicate the revised position of Polaris with respect to the NCP.

Time systems.

Local Time (LT)

This is the time on our watch, designed for convenience. Most countries make an hour correction twice a year (daylight saving) to make the daylight hours fit in with sunrise and sunset. As one travels around the Earth, the local time in each country is designed to ensure that the daylight hours and the Sun’s position are aligned.

Universal Time (UT)

Perhaps the most common time system used by amateur astronomers is Universal Time. This is the local time on the north-south Meridian, which passes through Greenwich, London. It has a number of different names, including Greenwich Mean Time (GMT), Zulu Time and Coordinated Universal Time (UTC). It is synchronized with the Earth’s rotation and orbit and is accurate enough for practical purposes. Each night at a given time, however, a star’s position will change. This is attributable to the 4-minute time difference between a 24-hour day and a sidereal day.

Atomic Time.

Time systems based on astronomical events are ultimately flawed. The most stable time systems are those based on atomic clocks; over the course of a decade, small changes in the Earth’s rotational speed add up. Atomic clocks use the ultra-stable property of Cesium or Rubidium electronic transitions. If one uses Global Positioning Satellite (GPS) signals to locate and set your time, one is also benefitting from the stability of atomic clocks.

Barycentric or Heliocentric systems.

Rather than use the Earth as a reference, this time system uses the Sun as the reference point for observation. This removes the sub-second errors incurred by the change in Earth’s orbit between measurements. One use of this system is for the timing of eclipsing binary stars.

Local Sidereal Time.

Local sidereal time is a system designed for use by astronomers. It is based on the Earth’s rotation and does not account for its orbit around the Sun. Its “day” is 23 hours, 56 minutes and 4.1 seconds and allows one to form an accurate star clock. If you look at the night sky at a given LST each night, the stars appear in the same position. It is the basis of the Equatorial Coordinate system described later on.

Other Time References.

Julian Dates (JD)

Julian dates are a day-number system that allows users to calculate the elapsed time between two dates. The formula converts dates into an integer that allows one to quickly work out the interval. For example, the 22nd January 2013 is JD 2456315.

Epoch.

An epoch is a moment in time used as a reference point for a time-changing attribute, for instance, the coordinate of a star. Astrometric data often references the epoch of the measurement or coordinate system. One common instance, often as a check-box in planetarium and telescope control software, is the choice between J2000 and JNow, that is the coordinate system as defined in 2000 AD and today. As the years progress, the difference and selection will become more significant. In many cases, the underlying software translates coordinates between epochs and is transparent to the practical user.

Coordinate Systems.

Horizontal Coordinates.

There are several fundamental coordinate systems, each with a unique frame of reference. Perhaps the most well-known is that which uses the astronomer’s time and position on earth, with a localized horizon and the zenith directly above. The position of an object is measured with a bearing from north (azimuth) and its elevation (altitude) from the horizon. This system is embodied in altazimuth telescope mounts, which are the astronomy equivalent of a pan and tilt tripod head, also abbreviated to “alt-az mounts”. There are pros and cons with all coordinate systems; in the case of horizontal coordinates, it is very easy to judge the position of an object in the night sky but this information is only relevant to a singular location and time. In the image-planning stage, horizontal coordinates, say from a planetarium program, are an easily understood reference for determining the rough position of the subject, if it crosses the north-south divide (meridian) and if it moves too close to the horizon during an imaging session.

Equatorial Coordinates.

Unlike horizontal coordinates, a stars position, as defined by equatorial coordinates, is a constant for any place and time on the Earth’s surface. (Well, as constant as it can be in the context of star’s relative motion and Earth’s motion within its galaxy). For a given epoch, planetarium programs or the handset with a programmable telescope mount will store the equatorial coordinates for many thousands of stars. It is a simple matter with the additional information of local time and location on the Earth for a computer to convert any star’s position into horizontal coordinates or display on a computer screen.

Other Terms.

Galactic Coordinates.

Galactic coordinates are used for scientific purposes and remove the effect of the Earth’s orbit by using a Sun-centered system, with a reference line pointing towards the center of the Milky Way. By removing the effect of Earth’s orbit, this system improves the accuracy of measurements within our galaxy.

Ecliptic, Meridian and Celestial Equator.

There are a couple of other terms that are worth explaining since they come up regularly in astronomy and astrophotography. The ecliptic is the apparent path of the Sun across the sky, essentially the plane of our solar system. The planets follow this path closely too and planetarium programs have a view option to display the ecliptic as an arc across the sky chart. It is a useful aid to locate planets and plan the best time to image them. The meridian is an imaginary north-south divide that passes through the North Celestial Pole, the zenith and the north and south points on the observer’s horizon.

This has a special significance for astrophotographers since with many telescope mounts, as a star passes across the meridian, the telescope mount has to stop tracking and perform a “meridian flip”. (This flips the telescope end-toend and side-to-side on the mount so that it can continue to track the star without the telescope colliding with the mount’s support. At the same time, the image turns upside down and any guiding software has to change its polarity too.)

During the planning stage it is useful to display the meridian on the planetarium chart and check to see if your object is going to cross the meridian during your imaging session so that you can intervene at the right time, perform a meridian flip and reset the exposures and guiding to continue with the exposure sequence.

Degrees, Minutes and Seconds.

Most software accepts and outputs angular measures for longitude and latitude, arc measurements and declination. This may be in decimal degrees (DDD. DDD) or in degrees, minutes and seconds. I have encountered several formats for entering data and it is worthwhile to check the format being assumed.

Common formats might be DDDMMSS, DDDO MM’ SS” or DDD:MM:SS. In each case a minute is 1/60th degree and a second is 1/60th of a minute. In astrophotography the resolution of an image or sensor (the arc subtended by one pixel) is measured in arc seconds per pixel and the tracking error of a telescope may be similarly measured in arc seconds. For instance, a typical tracking error over 10 minutes, without guiding, may be ± 15 arc seconds but a sensor will have a much finer resolution of 1 to 2 arc seconds per pixel.

Distance.

The fourth dimension in this case is distance. Again, several units of measure are commonly in use, with scientific and historical origins. The vastness of space is such that it is cumbersome to work with normal measures in meters or miles. Larger units are required, of which there are several.

Light-Years.

Light-years are a common measure of stellar distances and as the name suggests, is the distance travelled by light in one year, approximately 9 x 10 15 meters. Conversely, when we know the distance of some cosmic event, such as a supernova explosion, we also know how long ago it occurred. Distances in light-years use the symbol “1y”.

Astronomical Unit.

The astronomical unit or AU for short is also used. An AU is the mean Earth-Sun distance at about 150 x 109 meters. It is most useful when used in the context of the measurement of stellar distances in parsecs.

Parsecs.

A distance in parsecs is determined by the change in a star’s angular position from two positions 1 AU apart. It is a convenient practical measure used by astronomers. In practice, a star’s position is measured twice, 6 months apart. A star 1 parsec away would appear to shift by 1 arc second. It has a value of approximately 3.3 light-years. The parsec symbol is “pc”. The further the star’s distance, the smaller the shift in position. The Hipparcos satellite has sufficient resolution to determine stars up to 1,000 pc away. All these measures of large distances require magnitude uplifts; hence kiloparsec, megaparsec, gigaparsec and the same for light-years.

Terminology You Must Know For Astrophotography