Orwell Astronomical Society (Ipswich)
How Far Can You See With An Amateur Telescope?
How far can you see with your telescope? Like most amateur astronomers, I've been asked this question many times, and have always found it difficult to give a reply that captures the full extent of the astronomical distance scale. Dr Frank Flynn, in his lecture to OASI in April 1999, gave an approach to the question of distances in astronomy. His approach was to divide the universe into four zones of increasing distance from the Earth, and to use in each a different unit of distance, choosing the units to represent naturally the linear distances involved.
Zone 1 comprises the domain of familiar things on the Earth, containing all the objects that we deal with in everyday life. The Moon is the furthest object in the universe upon which humans have yet been able to set foot, and it therefore defines the natural limit of this zone. The familiar units of metres and kilometres, based historically upon the circumferential dimensions of the Earth itself, are appropriate to measure distances within zone 1. In these units, the centre of the Moon lies at a mean distance of 384,400 km from the centre of the Earth.
Figure 1 is a seminal image from zone 1. It is the first good image of the Earth and Moon taken together from the vicinity of the Moon. Lunar Orbiter 1 obtained the image on 23 August 1966, at 16:36 UT, while approaching the crater Pasteur (centre image)1. The image shows a crescent Earth, some 383,800 km distant, with sunset terminator running through Odessa, Istanbul and slightly west of Capetown.
Zone 2 comprises the Solar System. Within this zone, the appropriate measure of distance is the astronomical unit (AU), defined as the mean distance between the Earth and Sun, approximately 150 million km (the current accepted value is 149,597,870.66 km). Within the Solar System, Venus is the nearest planet to Earth, at a distance of 0.26 AU at closest approach. Mars, the next nearest planet, can approach Earth to within 0.38 AU. Neptune, the most distant planet of the Solar System, has a mean distance from Earth of 30.1 AU and a distance of 28.8 AU at closest approach. Within the scale of zone 2, the Earth-Moon distance (the limit of zone 1) is reduced in comparison to an insignificant dot.
The Hubble Space Telescope (HST) took images of all the above planets at an early stage in its observing programme; they are reproduced in figures 2-4. Figure 2 shows Venus, taken in ultraviolet light on 24 January 1995, when the planet was at a distance of 0.76 AU. In visible light, only very subtle cloud features are occasionally visible but, in ultraviolet light, enhanced by the use of false colour, they become much more distinct. In particular, a horizontal "Y"-shaped cloud feature is visible near the equator; it may indicate atmospheric waves, analogous to high and low pressure atmospheric cells on Earth. Figure 3 shows Mars in May 1999 at a distance of 0.58 AU. The dark "shark’s fin" feature in the centre of the image is Syrtis Major (first seen telescopically by the astronomer Christiaan Huygens in the 17th century). To the south of Syrtis Major is a large circular impact crater called Hellas, filled with surface frost and water ice clouds. Along the right limb, late afternoon clouds have formed around the volcano Elysium. The North polar cap is to the top of the image. Figure 4 is an image of Neptune, taken in close to true colour, on 03 February 1995. it shows a bright cloud feature near the south pole of the planet (bottom) and bands of bright cloud at other latitudes.
Zone 3 is the next zone outwards; it encompasses the local galactic group, comprising our galaxy, the Milky Way, together with some 35 others. In zone 3, the unit of measurement is the distance that light travels in a year, the light year (ly), or approximately 10 million million km (10,000,000,000,000 km).
The closest galaxies in the local group, and also the smallest, are the Magellanic clouds (visible from the southern hemisphere). They are both irregular dwarf galaxies in orbit about the Milky Way. The Large Magellanic Cloud lies at a distance of approximately 160,000 ly and has a diameter of approximately 14,000 ly; the Small Magellanic Cloud lies at a distance of approximately 200,000 ly and has a diameter of approximately 7,000 ly. The largest member of the local group is the Andromeda galaxy, visible to the naked eye from the northern hemisphere as a small fuzzy patch of light: it lies at a distance of 2.2 million ly. The most distant member of the local group is galaxy M33 in Triangulum, visible in small telescopes, lying at a distance of almost 3 million ly. Figures 5-7 illustrate these members of the local group.
Zone 4, the final and largest zone, encompasses all of the universe outside the local galactic group. In this zone, the unit of measurement used by cosmologists is the megaparsec (Mpc). One Mpc is equivalent to approximately 3.26 million ly, or 32,600,000,000,000,000,000 km. The nearer objects in Zone 4 are visible in small telescopes - a good example is the giant elliptical galaxy M87 in Virgo (see figure 8), lying at a distance of 16 Mpc. M87 is a member of the Virgo group of galaxies. It has a mass of some 25 million million times that of the Sun and is notable in that it is surrounded by some 12,000 globular clusters (compared to approximately 200 for the Milky Way) and appears to be expelling a jet of material, of extent 5000 ly, from a super-massive black hole at its centre.
So, given the above background, what’s the answer to the original question - how far can you see with your telescope? Zone 4 contains the most distant objects in the universe. Within Zone 4, the furthest objects themselves are the enigmatic quasars2. Radio astronomers discovered quasars in 1960. Early radio maps of the sky were not very accurate, so it was not possible to find the optical counterparts immediately. When, eventually, astronomers did locate the optical counterparts of the first quasars, they appeared as unimpressive faint stars. However, the optical spectra of the first quasars appeared anomalous. In 1963, Maarten Schmidt at Palomar Observatory was able to identify and analyse the spectrum of the quasar designated 3C273 (object 273 in the Third Cambridge Catalogue); it showed a large redshift, implying that the object was extremely remote. The spectra of other quasars could also be interpreted in terms of large redshifts.
In fact, 3C273 exhibits a redshift of 16%, implying a velocity of recession of 15% of the speed of light and a distance of 600 Mpc. In the typical amateur telescope, the object appears as a faint star of magnitude 12.9. In order to explain its apparent brightness, given its vast distance, 3C273 must emit as much light as 300 giant galaxies, or 30 million million suns. The only object capable of generating such prodigious energy is a black hole. Current theories suggest that the nucleus of a quasar consists of a supermassive black hole (with mass equivalent to 1,000,000,000 suns). Matter spiralling into the black hole emits energy over a wide spectrum, which can include radio emissions and visual wavelengths.
At magnitude 12.9, 3C273 is within range of the larger amateur telescope. It lies within Virgo and so is best placed for observation during late spring and early summer. Figure 9 illustrates the position of 3C273 within Virgo. The easiest approach to finding it is to start by locating (with the naked eye) the right-angle triangle formed by Porrima, eta Virgo and 16 Virgo. Then search telescopically the midway point between Porrima and 16 Virgo – the excellent finder map in Burnham’s Celestial Handbook3 provides reference stars down to approximately magnitude 16, and facilitates positive identification of 3C273 itself.
I started searching for 3C273 during April 1998, from my back garden, using a 254 mm Schmidt Cassegrain reflector. High level haze thwarted my efforts during 1998 and, by the time the weather had improved sufficiently, the year was too far advanced and evening twilight persisting too late to make the search worthwhile. I resumed the search in April 1999. On the evening of 09 April, sky conditions were steady, although a little hazy. Venus presented a beautiful spectacle in the western sky, shining prominently after sunset. Around 23:00 UT, once the Moon had set and Virgo was rising in the south-east, I began searching for 3C273. By midnight, after some initial difficulty, using Burnham's finder chart, I was able to locate the object. I found it necessary to use averted vision to discern 3C273 in the first instance, but once I had positively located the object, I was then able to see it with direct vision.
Encouraged by the success with 3C273, I then decided to try for something even more distant. Table 1 lists the most remote quasars visible from northern latitudes with magnitudes down to 15; the data is extracted from the NASA catalogue Quasars and Active Nuclei4. The object PG 1718+481, at visual magnitude 14.6 would likely be the faintest object visible above the general sky-glow spoiling the view from my back garden, and had the virtue of being at a relatively high northern declination (thus suffering less from atmospheric extinction), and being at the colossal distance of 2500 Mpc. I used printouts from the Sloan Digital Sky Survey as finder charts. On 11 October 1999, at around 22:00 UT I was able to glimpse the object using averted vision; it was, indeed, just on the threshold of visibility above the background sky-glow.
Table 1. Remote quasars brighter than mag 15 visible from northern latitudes.
The light grasp of the Tomline Refractor is approximately equal to that of my Schmidt-Cassegrain, and the sky-glow at Orwell Park is broadly similar to that at my home. PG 1718+481 likely represents therefore the most distant object which the Tomline Refractor could glimpse under good seeing conditions (although this has not, to date, been verified). So, in answer to the opening question, the Tomline Refractor should be able to see a distance of 2500 Mpc (or 8*1022 km, 80,000 million million million km). The light that we observe nowadays as PG 1718+481 began its journey to Earth billions of years before the formation of the Solar System!
The high angle of the Sun in figure 1 makes it difficult to discern individual craters.
Quasar is a contraction of quasi-stellar object. Another abbreviation sometimes found is QSO.
Robert Burnham, Jr., Burnham's Celestial Handbook. An Observer's Guide to the Universe beyond the Solar System, Dover Publications, Inc., New York, 1978.
NASA catalogue Quasars and Active Nuclei, 7th edition, by Veron-Cetty et al, 1996, available on the NASA ADC CD-ROM Selected Astronomical Catalogs, volume 3, December 1996, file 7188.