J. Brit. Astron. Assoc., 109, 2, 1999, p.xx-xx
J. Brit. Astron. Assoc., 109, 2, 1999, p.xx-xx
(Note: The Association is not responsible for individual opinions expressed in articles, reviews, letters or reports of any kind.)
From Mr David Frydman
The formula 2 + 5logD for limiting magnitudes given in the Handbook matches quite closely the figures stated by telescope manufacturers for their products. If this is changed without explanation confusion will result. ( JBAA, 1999 February p.42).
These existing formulae seem to stem from the idea that a telescope of, say, 70mm aperture is one hundred times the area of a 7mm naked eye dark-adapted iris. The telescope limiting magnitude is then five magnitudes fainter than the naked eye. The implication is that all 7mm eyes have a fixed limit of about magnitude 6.5, and that the faintest telescopic star is seen with an exit pupil of 7mm. Both of these assumptions are incorrect.
However, the existing formulae do give an indication of the faintest stars that a newcomer to astronomy can expect to see using direct vision from a suburban site. It is true that experienced observers using averted vision often see fainter, and the formula 3.7 + 5logD should be given in addition. One could argue that there is no such thing as a limiting magnitude. In my opinion, though, the formulae are valuable. A 3mm maximum iris should not be of concern when using a telescope (Gordon Taylor's letter), as in practice the faintest magnitudes are achieved with an exit pupil of about 1mm. Only extended faint objects sometimes benefit from large exit pupils. 20×60 binoculars have a 3mm exit pupil, and provide good views. 12×40s are also popular. Canon image-stabilised binoculars have a 3mm exit pupil.
A solution to the problem may be to state: 'The two existing traditional formulae apply to a telescope exit pupil of 7mm. In practice, with a 1mm exit pupil, an experienced observer using averted vision in good conditions may reach a limiting magnitude described by 3.7 + 5logD.'
David Frydman
London NW11 8BG
From Mr Gerald North
In the February 1999 Journal Messrs Newman, Neville, Taylor, and Mitton discuss the possibility of revising the formulae given in the Handbook for telescope limiting magnitudes. Mr Taylor invites further discussion. Could I refer interested readers to a short paper of mine in the April 1997 Journal,[1] where I proposed a formula based on the results of a survey conducted by Bradley Schaefer.
The formula I suggest in my paper fits Schaefer's results rather better for a wide range of apertures than do the alternatives discussed by the aforementioned correspondents. I further propose that the formula be called 'Schaefer's formula', as it stems from his significant study of what people actually see through their telescopes.
Gerald North
Bexhill-on-Sea, East Sussex TN39 5BE
[1] - North G., 'A better formula for telescopic limiting magnitudes', J. Brit. Astron. Assoc., 107(2), 82 (1997)
From the Director of the Variable Star Section
Mr Newman ( Journal, 1999 February) need not worry about his results being looked upon with scepticism, just because he is hitting faint limits with a small instrument. If his results do not agree with the majority of other observers, then we will contact him to discuss the problem (as we would with any other observer).
I have always regarded formulae for calculating theoretical limiting magnitudes in telescopes with the utmost scepticism. I have known amateurs reach absurdly faint limits with small apertures in very good conditions, making such formulae look ridiculous in comparison. I also believe that such things can bias an observer, to the extent that he or she looks upon their own results with a certain amount of doubt, if those results do not comply with a given formula - and here may be a case in point!
Gary Poyner
Birmingham B44 0QE. [gp@star.sr.bham.ac.uk]
From the Director of the Asteroids & Remote Planets Section
The February 1999 issue of the BAA Journal contained my paper on observation and research related to Pluto.[1] In a section headed 'The status of Pluto' I noted that Pluto was unlike both the terrestrial planets and the gas giants. I remarked that the nearest equivalent body to it known at present was Triton, and that there appeared to be significant differences between it and other bodies in the Edgeworth-Kuiper Belt. After reporting that there were those who wished to demote Pluto to the status of a super-asteroid or comet, I confirmed my view that it should retain its historical status as a major planet although it could eventually become the first of a hierarachy of remote solar system objects.
This was written in December 1997. Since December 1998 there has been a significant debate about the status of Pluto. In part this has been occasioned by the imminent occurrence of asteroid numbering reaching 10,000 (in the February 1999 batch of Minor Planet Circulars) and a wish for this asteroid to be significant. In addition a number of the Edgeworth-Kuiper objects have known orbits where the precision is sufficient for them to be numbered, and it would be satisfying if the first object to be found in this region were to be the first numbered. The thrust of the argument is that Pluto is so small it hardly warrants the importance of a major planet, and additionally we now know there are many others in similar orbits (those EKOs known as Plutinos).
The IAU and MPC started a discussion to sound out those prepared to comment on whether Pluto should be designated an asteroid, and given the number 10,000. Initially this was considered a demotion, but later the suggestion was for Pluto to have a dual status. Whilst there was a slight bias in favour of this the prevailing view was not decisive.
The discussion does however raise several valid points for consideration. Firstly what is a planet or indeed a major planet? How does it differ from asteroids and comets? What are the distinguishing features? This is not an idle question, because in recent years there have been several reported planetary systems identified round nearby stars and they are presumably (if real) only vaguely like the Sun's family, as most of the planets are several times more massive than Jupiter. If we consider the Sun's family we find the gas giants - large independent bodies which, though not undergoing nuclear reactions, emit more radiation than they receive from the Sun. Then there are the terrestrial planets - inert independent rocky bodies - and the main belt asteroids appear similar. Pluto appears to be an icy independent body as do the outer asteroids, the EKOs and indeed most comets.
On this basis one could suggest that there are only four major planets, Jupiter, Saturn, Uranus and Neptune. These are big enough to significantly perturb passing comets and asteroids. We then have many thousand minor planets, the largest of which would be the Earth, with a much lower perturbing influence on passing bodies. Finally the comets - that is the icy bodies which outgas when approaching the Sun, as occurred with Pluto and Chiron near perihelion and all those objects traditionally known as comets. On this approach members of each group are similar to each other and different from the other classes, which is not the case at present. The planetary satellites could be attributed to the correct type based on composition.
All this is idle speculation and perhaps pretty silly - but then so are many of the comments that have been passed on this subject.
Andrew J. Hollis
Marton, Cheshire CW7 2QE. [A.J.Hollis@open.ac.uk]
[1] - Hollis A. J., 'From the darkness emerging: Pluto and the Edgeworth-Kuiper Belt', J. Brit. Astron. Assoc., 109(1), 9 (1999)
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