"Well I’m standing on a corner in Winslow Arizona,
Such a fine sight to see...
It’s a girl - my lord! - in a flatbed Ford,
Slowing down to take a look at me..."
The Eagles - "Take it Easy"
I’m sure if Don Henley had been an astronomer he would have had just as much fun by heading west out of Winslow on Route 66, across the Painted Desert to Flagstaff. En route he could drop in on the Barringer meteor crater, but as I’ve already told you about that remarkable site we’ll stay in the fast lane and cruise into Flagstaff.
The MoneyWise Guide to North America defines Flagstaff as "The most boring place on Earth to be stuck in waiting for a lift". But what do they know? Flagstaff has arid deserts and winter skiing, extinct volcanoes and meteor craters, Indian ruins perched precariously on cliffs, and the Grand Canyon barely an hour’s drive away. For the astronomer, however, Flagstaff’s greatest glory is that it is one of the few places on Earth where a new planet has been discovered. (Name the others!).
In Flagstaff’s case, the planet was Pluto, discovered by Clyde Tombaugh in 1930. Tombaugh was a researcher at the Percival Lowell Institute, which to this day is a working astronomical observatory, situated on Mars hill, overlooking Flagstaff city (there is a separate deep sky observatory in the desert to the south of town). The choice of Mars for the name of the site highlights the other great obsession of the Institute’s founder.
Percival Lowell, "the man with the tessellated eyeballs" is one of the great historical characters of astronomy. An amateur observer of wealthy means, he determined to build an independent observatory which would benefit from the high altitude and clear skies of Arizona. Due to a mistranslation of the Italian Schiaparelli’s observation of "canali" (channels), he became convinced that the surface of Mars was criss-crossed by canals, built by martian civilisations to irrigate their arid planet (just like Phoenix, the state capital, actually). Lowell despatched an assistant, one Andrew Douglas, to investigate possible sites. The result was a virtual dead heat between Flagstaff and what later became Kitt Peak observatory, to the south of Phoenix. Flagstaff won because of it’s proximity to the Santa Fe railroad, and other research institutes (including the U.S. naval observatory and Arizona State University) have followed Lowell’s.
Lowell proceeded to observe canals on Mars, a fiendishly tricky planet to observe, despite criticisms of his work from those who failed to agree with his theories. Doubters were not tolerated at the Lowell observatory, however, and in particular Andrew Douglas was dismissed for daring to doubt his employer’s opinions. Douglas, a versatile chap and something of a hero around Flagstaff, went on to develop the new theory of "dendro-chronology" or dating of wooden artifacts by their tree-rings. There are a lot of ancient indian sites around Flagstaff, and they all seem to have been dated by Douglas’s dendro-chronology.
Back to Lowell. He became enthused by the success of Newton’s gravitational theories in predicting the existence of new planets: namely Neptune, discovered by Leverrier and Adams in 1846 (in Berlin, I think), and Vulcan, supposedly within the orbit of Mercury, which was eventually explained away by Einstein's theory of general relativity. Lowell did his own laborious calculations and predicted a massive "Planet X", perturbing the orbit of Neptune from the outer reaches of the solar system. He spent much of the latter part of his career searching for the elusive planet X, but on his death in 1916 the project began to slip into obscurity.
It was thirteen years later before an enthusiastic youngster named Clyde Tombaugh arrived in Flagstaff and began a diligent and laborious search for Planet X. The magnitude of his task should not be under-estimated: during his eighteen month search he discovered several comets and dozens of asteroids. His method of checking photographic plates was to use a "blink comparator", which flipped rapidly between two photographs of the same portion of the sky taken on different nights. The area in the sky to be searched at a given time in the year was such that asteroids would show as large a change in position as possible, to make them easier to identify.
Eventually, Tombaugh spotted a tiny image moving slowly through the star fields - far too slowly for an asteroid. He had discovered Planet X! Pluto, god of the Underworld, was an appropriate name in more ways than one: the first two letters commemorated Percival Lowell, whose vision - flawed in many other ways - had inspired the search.
Pluto was a lot smaller than Lowell had expected, and consequently much fainter. It’s orbit was also highly eccentric and inclined to the ecliptic - indeed, Pluto is currently closer to the Sun than Neptune. Pluto’s unusual orbit has prompted speculation that the planet is actually an escaped satellite of Neptune, particularly since Nereid, the outermost large Neptunian satellite, has an equally unusual retrograde orbit. However Voyager in 1989 found no definitive evidence of a Neptunian catastrophe.
In some respects Tombaugh was lucky to discover Pluto. The planet’s mass is far too low to cause the perturbations to Neptune’s orbit which Lowell had worked from -» indeed, with a longer and more accurate set of measurements Neptune’s orbit now seems well accounted for by the current roster of planets. Additionally, other observers had narrowly missed Pluto in other photographic searches (ten years earlier, Humason at Mt Wilson had photographed the planet but the image lay on a flaw in the plate). However, it would be unfair not to recognise Tombaugh’s tenacious and painstaking observations, and at the Flagstaff observatory the staff are clearly proud of their predecessor’s famous discovery.
The dome in which Tombaugh made his discovery is still in use, so the public aren’t allowed inside, but you can view the exterior of the dome. From the visitor’s centre in the old observatory library the you make your way through the trees along the "solar system walk". Along the path, at intervals corresponding to their mean distance from the sun lie plaques announcing each planet: Merctuy, Venus, Earth and Mars are almost on top of each other, then a big gap to Jupiter, Saturn and Uranus. Almost at the end of the path is Neptune, and then one final plaque, for Pluto, overlooking the dome.
One post-script to the story of Pluto. In 1978, James Christie observed Pluto very accurately to see if it was due to occult any stars. To his complete surprise the planet’s image was elongated - Pluto had a moon! Charon, the moon, is a substantial proportion of the mass of Pluto, making Pluto/Charon the best candidate for a double planet in the Solar System.
Where do you suppose Charon was discovered? At the U.S. Naval Observatory in Flagstaff, of course - so the city still claims Pluto as it’s own!
FORMULAE AND TIPS
by JM Townrow
1. Angular resolution (arc sees.) = 4.5 where A = Aperture
A in inches
or = 115 with 'A' in millimetres
The above should be regarded as the maximum possible.
2. Resolution in the focal plane (in microns)
= focal ratio (i.e. f no.)
(actually more like .55f no.)
The above should be regarded as the maximum possible.
3. Maximum angular resolution of the human eye = 1 arc minute
For practical purposes allow a margin and take this as 2 arc. minutes - about 1/7 mm. viewed from 25cm. (10").
To achieve maximum resolution visually a telescope needs only sufficient magnification to make its finest resolution subtend 2 arc minutes, i.e. 120A (A in inches)
This approximates to the aperture in millimetres.
Greater magnification serves only to reveal diffraction phenomena.
4. Limiting magnitude (visual). Apertures in inches.
Aperture Mag Aperture Mag
2 12.1 8 15.1
3 13.0 10 15.6
4 13.6 12 16.0
5 14.1 15 16.5
The above should be taken as maxima.
5. Telescope magnification = focal length objective
focal length eyepiece
6. Scale of 1 degree = focal length of objective
A degree is one part in 57.3, i.e. 1" viewed from 57.3" subtends 1 degree.
From this simple rule one can work out how much sky is subtended by various parts of one's hand.
7. Combining resolutions;
r1 + r2 + r3 + . . . . = r (in distance units - typically microns)
or 1 + 1 + 1 + . . . . = 1 (in lines per mm. - e.g.)
Rl R2 R3 R
It follows from the above that where maximum film resolution is equal to maximum image resolution the maximum resolution of the combination is only half as good as either.
8. Film factor. This is intended to express the usefulness of a particular film for high resolution astronomical photography.
The higher the figure the better.
Factor = film speed
Where r = resolution e.g. in microns. For ordinary films this is typically between 5 and 20 microns but for Kodak Technical Pan is claimed to be 2.5 microns.
The factor assumes that blur is directly proportional to exposure time. This tends not to be the case with very short or long exposures. Don't forget about reciprocity failure, which reduces
the effective film speed and differs from film type to film type.
9. High resolution photography with an UNDRIVEN TELESCOPE. If image blur is to equal maximum possible resolution then the maximum permissible exposure (in seconds) is given by :—
4.5 (for A in ") or 115 (for A in mm.)
10. Photography with an UNDRIVEN CAMERA. The criterion here is that the blurred image of a point object should have a diameter not exceeding 25 microns (or .001"). Exposure in seconds.
maximum exposure = 350
focal length (in mm.)
11. Simple formula for lens imaging;
1 + 1 = 1
— — —
u v f
Where u = object distance from lens
v = image distance from lens
f = focal length of lens
12. Make extensive use of similar triangles for simple optical calculations. Projection distances, magnifications etc, can be computed easily.
13. How to place your meridian to a fraction of a degree.
1. Suspend a line between two points, (i.e. as a clothes line)
2 Suspend from this line a plumb line.
3 Work out when your local noon is from the BAA Handbook using
your exact longitude (1 degree = 4 mins. time) and the Solar Equation
4 Mark the shadow of the plumb line at local noon.
14. How to set up an equatorial mounting fairly accurately.
1 Construct an angle equal to your latitude in some stiff sheet material.
Offer this up to your polar axis (to be precise "axle") and using a spirit
level set the axis.
2 Use an engineer's "V block" plus a spirit level to make the declination
3 Use the shadow of the telescope tube (or projected solar image) together
with knowledge of the local noon to rotate the whole telescope to the correct
Remember a degree is 1 part in 57.3 and represents 4 mins. of time.
15. Construct rectangles on tracing paper to lay over the maps in Norton's Star Atlas indicating the fields of view of your various camera/lens combinations.
16. Newtonian diagonals. The formula (straight out of the text book but saves a lot of trouble) for computing the minor axis of an elliptical diagonal for a Newtonian reflector is:—
w = d (D - a) + a
where w = minor axis of diagonal
D = diameter of main mirror
f = focal length of main mirror
a = width of focal plane (i.e. field to be fully illuminated)
d = distance from focal plane to diagonal
17. Try to be familiar with both imperial and metric units, particularly those of distance. Metric units are fine for passing examinations but in the real world you need to know both.
Learn to convert in your head - very easy.
1m = 39.4" (approximate to 40")
1" = 25.4mm. or 2.54cm. (approximate to 25 & 2.5)
.001" (a 'thou') = 25.4 microns (approximate to 25)
1mm = 39.4 thou (approximate to 40)
The approximations suit mental arithmetic admirably»
If any members have any other Hints and Tips, this is the place to pass them on. Not only will they help other members if printed in MIRA, but they will be a useful source of reference in the future. All kinds of ideas for observing, use of telescopes, things to make, best books to read on cloudy nights...... you know the sort of thing. Just send them to the Editor and I'll do the rest.
By Vaughan Cooper