AstroNotes 1967 April Vol: 6 issue 04

EDITORIAL . Solar . The Queen Elizabeth II Telescope - A Progress Report . Four Phenomena of Planetary Occultations . The Elongation of Mercury . Observers Group March Meeting

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AstroNotes

The Newsletter of the Ottawa Centre, RASC

Volume 6 April 1967 Issue 4

Editors Dan Brunton 2565 Elmhurst Street, Ottawa 14, Ont. 828-1473
Circulations Joe Dafoe 2366 Malone Cresc., Ottawa 5, Ont. 828-7681

EDITORIAL

You will have noticed that we have a new circulation manager, Joe Dafoe. He has taken over from Bill Dey who has toiled for three years in this time-consuming task. We all appreciate the great amount of time and effort that Bill Dey has given to Astronotes. Selfless efforts such as his make this publication possible and so our sincerest thanks go out to our former circulation manager. It would be fitting at this time to also thank Joe for taking over this job and wish him all the best with it. I received a note from Isabel K. Williamson who is editor of the Montreal Centre's publication, "Skyward". She wrote to say that the Montreal group is enjoying Astronotes and "to congratulate the Ottawa Centre on its activities". It appears that we have established a reputation of sorts, so we had better keep it up!

Thank you, Miss Williamson and we certainly do enjoy reading about the Montreal group through your newsletter.

I received a suggestion from Col.John Stairs a while ago, that we try to include a resume of the main centre's talk each month for the benefit of those that miss some of these meetings. This is an e xcellent suggestion and I am happy to report that it is carried out in this issue. Dr. Hodgson, Director of the Observatories Branch, has given us a very good account of his talk to the centre at our February meeting. I hope this will become a regular feature. Please keep the suggestions coming in; after all, this is your publication.

Next month's issue will contain the first part of a complete list of our library books. Stan Mott, our librarian, informs me that we now have over 100 books available! The list is so long that we can't fit it all into one issue!

Don't forget about the General Assembly being held in Montreal next month. This should be a really good affair and the Ottawa Centre should be very well represented. There will be a more detailed discussion of this event in the May issue of Astronotes.

Have a happy "Centennial" April and good seeing.


Solar

Rick Lavery

to Friday, February 24th, at 8.45 a.m., I began an observation of the sun and certainly wasn't prepared for what I saw!

I started solar observing in 1964 during the "Year of the Quiet sun", and since then any large spot was a wonderful experience for me. THIS spot, however, was 1/7 - 1/8 the diameter of the sun or approximately 100*000 miles across; it contained over 50 individual spots.

THE spot appeared,from its position, to have made its debut one or two days earlier during one of our February blizzards. We had the day off as there was a teachers convention, so I called Rick Salmon and Les MacDonald who both began programs to observe the spot.

By Sunday* Joe Dafoe, Les and I had constructed a temporary solar observatory in the MacDonald's 5 ft. snowdrift. Les obtained excellent results using indirect photography of the sun in our "solar snowdrift". Many other observers have been notified of THE spot's existance and so more will be said in future issues of Astronotes about the various methods employed to observe it. By the time of the March Observers Group meeting, the spot had moved onto the limb of the sun, but it will be exciting to wait for its re-appearance approximately 10 to 14 days after its disappearance.


The Queen Elizabeth II Telescope - A Progress Report

by Dr. J.H.Hodgson, Director

When the Government agreed to the construction of the 150-inch telescope in 1964. it stipulated that the first stage in the development would be the preparation of a "feasibility report". This report would fix the design concept of the telescope, investigate costs and the problems of procurement and recommend on the organization of the associated observatory. This feasibility report is now in its final stages of preparation.

The firm of A.B.Sanderson and Company of Victoria was employed as the prime consultant for the preparation of this report, and they retained the firm of Dilworth, Secord and Meagher, Ltd., of Toronto, to undertake the telescope design. The Observatories Branch of the Department of Energy, Mines and Resources was represented on the design team by a group of astronomers working under the direction of Dr.R.M.Petrie. With his sudden death in the spring of 1966, the Observatory group was led by Dr.Graham Odgers.

Any telescope has to be a compromise between several needs as expressed by the excellence at the several foci. Dr. Petrie decided that the order of importance for our telescope would be the following:
Cassegrain focus - f 8,
Coude focus - f 30,
Prime focus - f 2.8.
The optics will be the Ritchey-Cretien system which will provide 45 minutes of arc of essentially undistorted field at the Cassegrain focus. This system results in a poor field at the prime focus, but this will be overcome by the use of correcting plates.

Because of the northern latitudes of the telescope a fork mounting is necessary to provide a complete operation. In order that this fork should not suffer any flexures two features are incorporated in its design — it is very massive, and the various facilities that must be provided at the upper end of the telescope are removable.

There will be sets of rings — one to carry the prime focus cage, another to carry the Cassegrain system, a third the mirror for the Coude system. In most large telescopes all these parts are carried all the time, flipping out of the way when not needed. In the Queen Elizabeth telescope each system will be on its own removable ring. This results in much less weight at the upper end, and again reduces danger of flexure.

The telescope mirror, which will weigh 17 tons, or 40 tons in its mounting cell, is now nearing completion at the Corning Glass Works. It will be figured and polished in a shop being constructed on the campus of the University of British Columbia.
One of the major design problems has been the support system for this mirror. This system must retain the figure correct to 1 millionth of an inch in any position. The problem has been solved by the use of air cushions in which the pressure is varied
according to the position of the mirror by a system of valves operated by counterweights.

The Canadian design group has been very much helped by their colleagues in the United States. It is a matter of pleasure to them that in the design of the mirror support system they are in the lead and so able to repay, to some extent, the help they have been given.


In Next Month's Issue

Spectrographs - by A.H.Gillieson
Observational Reports
The Planet Mercury - by F.Evraire
Ottawa Centre Library List - by Stan Mott


Four Phenomena of Planetary Occultations

Tom Tothill

Most people are aware that the setting sun is still wholly visible when the true sun is wholly below the horizon; the sun's light is bent by atmospheric refraction. At sea level this averages about 34.5' at an apparent zenith distance of 90°. The figure shows a cross-section through a planet, with an atmosphere exaggerated for clarity. Parallel rays coming from a distant star on the left are unrefracted if they pass clear of the atmosphere, like A and H. Rays D and E graze the surface and have maximum refraction which we will call "base refraction" (the base may be the top of clouds or mountains, of course). B, C, F, and G are examples of an infinite number of intermediate rays suffering intermediate refraction.

For the earth, if D and E graze sea level and there are no clouds, the base refraction is 2 x 34 .5 ' for each (the ray is bent an equal amount going in and coming out of the atmosphere) so the angle DIE where they meet is 4 x 34.5 ' = 2° 18'. As the earth subtends less than 2° at the mean distance of the moon, point I is nearer than the moon.

We thus arrive at the startling realisation that for an Observer on the moon the earth is incapable of occulting a star under the stated conditions. By the same token, any other planet is incapable of occulting the light from a star if its base refraction exceeds the observed angular radius of the planet. As we are now talking about angles ranging downwards from 0.5', there is a strong possibility that most and perhaps all of the planets have a sufficiently refractive atmosphere to prevent occultation.

Consider an observer at J, chosen such that rays C and F pass comfortably clear of clouds or mountains at the limb. He would see the star along C, and again along F, and because of rotational symmetry the star would appear as a thin ring of light all around the planet. Since the observer cannot normally stay at J, but only pass through it on some orbit or other, this phenomenon can only be seen for an instant and we will call it the Flash Ring.

Now consider an observer at K. He sees the star along C close to the limb and again, more brightly farther from the limb along G. He is in the plane of the drawing but not on the occultation axis; therefore light from the star does not reach him around the sides of the planet and he sees the star in two places, above and below the planet.

Remembering that the refraction takes place in a direction normal to the limb, we can plot the successive limb positions of the star by drawing normals to the limb from its true positions. The actual phenomena to be observed depend on the zone through which
the observer is passing at the time. From Zone 1, the true umbral zone, he sees a true occultation. From Zone 2 he sees the star on one side of the planet only. From Zone 3 he sees the star on both sides of the planet (or, at the central moment of a central
occultation, the Flash Ring). And from Zone 4 he sees the star clear of the planet, unrefracted and undimmed, plus a refracted false star at the opposite limb.

In an actual planetary occultation the observer must pass through Zone 2 or Zone 4, depending on his distance off, in order to reach Zone 1 or Zone 3. Or his orbit could miss the latter two zones completely. The phenomena to be seen will therefore change as he passes from one zone into another; the star will disappear as he crosses into Zone 1, a second star will appear as he crosses into Zone 3, and a false star will appear as he enters Zone 4 from the outside.

An occultation from Zones 2-1-2 would look somewhat as shown on the right. The star would seem to pause on reaching the atmosphere and deflect away from centre before disappearing into the limb. The duration of the true eclipse would be appreciably
shorter than the geometrical eclipse. Note that the apparent star must lie on the normal to the limb from the position of the true star. These normals do not go through the centre of the planet if it is oblate, except when the star is crossing one of the axes of the ellipse.

The second diagram shows a phenomenon which we will call The Walkaround. It is visible in pure form only from Zone 2. As before, the star will appear to pause on entering the atmosphere, and deflect away from centre. It will then move rather rapidly around the nearer limb, getting dimmer and closer to the planet up to the mid point of the event. The egress will be the same but, of course, in reverse order.

From Zone 3, the walkaround will be accompanied by the false star. Oddly enough, in the case of an oblate planet there can be one, two, or three false stars at different stages of a nearly central occultation, in addition to the walkaround star. At the central moment of a central occultation these four stars will mark the axes of the ellipse and there will be no flash ring.

The third diagram shows the False Star in pure form, visible only from Zone 4. The star, passing clear of the planet's atmosphere, enters Zone 4. At this moment the false star appears on the opposite side of the planet. It grows in brightness and rate of motion up to the mid-point of the event, and is eventually extinguished in symmetrical fashion.

An occultation of the sun by the earth, as seen from the moon during a lunar eclipse, would exhibit these phenomena for every point of the sun's disc. Such eclipses may be divided into "centrecutting" and "non-centre-cutting" the latter being shown in the fourth diagram. There would be an elongated, lozenge-shaped Walkaround Sun sweeping around the near limb, and frequently a False Sun on the opposite limb. In centre-cutting eclipses the Flash Ring could exist for periods up to an hour. These phenomena go far towards explaining the illumination of the moon during an eclipse, and possibly to predict it. Conditions at the earth limb are also important; a mountain range or high cloud could break up the refracted solar images into beads (like Bailly's) or extinguish them altogether.

Has anyone observed these phenomena?


Observers Exchange

Wanted; A 1" eye-piece and any size camera shutter. Both are needed for the lunar camera under construction at the Small Dome.
Contact: Rick Salmon, Co-ordinator of the Small Dome 731-8427


The Elongation of Mercury

Rick Lavery

My first observation of this elongation was on the 6th of February at about 6 p.m., 10 days before its maximum displacement east of the sun. It was very conspicuous in binoculars and by telescope. I contacted Rick Salmon, Les MacDonald and several others, making it very well observed even though it was not listed in the Handbook as a favourable evening elongation.

On February 11, Les MacDonald and I braved below zero temperatures and a 30 mph wind to photograph the configuration of the Moon, Saturn, Venus and Mercury, all in the same field. I obtained fair results on Anscochrome 200 at 6.20 p.m. with a 20 second exposure, but the bitter cold refused to allow my camera to focus properly, slightly spoiling the photography. Under similar conditions on the 12th, this time using HPS, ASA 800, b. and w. film, I made 6 exposures, but again, the slightly out-of-focus lens left the results nconclusive as Mercury's presence on the film was questionable.

The 13th was Dan Brunton's turn to face the deep-freeze and take 4 pictures on Plus X, ASA 125 film. By then Mercury was easily visible to the naked-eye observer, about 5 degrees NE of Venus. Bad weather after the 16th put the ice on any further observations.

For further elongations, I have concluded that with a medium speed film, (ASA 125 - ASA 250), a 20 second exposure at about 1½ to 2 hours after sunset or before sunrise and a well focussed camera at about f 3.5, will provide good to excellent results.


Observers Group March Meeting

The last meeting of the Group was held on March 4 1967. A large variety of topics were dealt with at this meeting.

Unfortunately, we were informed that a camping trip to Algonquin Park is out for a number of reasons which are too involved to go into here. Well, never let it be said that we Astronomers are quitters! The problem of site and time has been put to a two-man committee consisting of Rick Salmon and Tom Tothill. Arrangements should now be finallized by the next meeting.

Rick Lavery and Les MacDonald conducted an informal but well-prepared discussion on recent solar and planetary phenomena. This was accompanied by a series of slides on both topics. ( see pages 1 and 5 of this issue for these observations).

Displays for the General Assembly in Montreal next month were discussed and several groups agreed to construct some form of exhibit. Also, Lunar co-ordinator Tom Tothill ( alias "Chairman, Observers Group") rounded up a crew to observe the grazing lunar occultation on March 16th ( weather permitting). More about that later.

To round off the evening, Gordon Grant showed his excellent slides of the various telescopes that had appeared at Stellafane at the time of his two visits, to the 23 members and 8 visitors that were present.