So on the way home from pizza after the Willingboro meeting, I noticed the sky breaking up, a good sign since there was an aurora alert. So I looked for red patches. Disappointed. No red. But I did notice that due east there was a green tint to the sky. The sky got a bit clearer as I went along I295, heading due north to home. The sky began to show green streaks. Ah ha. By the time I got home the streaks had subsided, but I loaded a roll of film in the camera anyway, and set up the tripod. About 120 AM the sky began looking red to the west and east, and about 10 minutes later there was a red and green streaky aurora in Leo. This is getting better. It became so obvious I woke up my wife (with some trepidation) and for the next 20 minutes we watched red and green streaks ascend from above the horizon to directly overhead, so bright it blocked out the stars in Corona Borealis. The entire sky from east to west and well past overhead was streaks of color apparently converging directly overhead. There was a patch of green about 20 degrees across due south, in Scorpius. The display began to subside, and by 2 AM the sky was clear except for faint, diffuse patches.

I do hope these pictures come out. Tried various exposures but was so overwhelmed I really have no idea how long some were. Some of the patches during the height of the display were far brighter than clouds lit up by the city lights to the east. I am awed. Probably should have called you guys, but it never occurred to me. BK

Hi Ed

5 People showed up. It was a little windy and cold. I got there first, about 6:15. Then Ed Sturm, Wayne, Wayne and Kathy. We had my 10" lx200 setup and used Ed's 2" 32mm lens. We circled the wagons for a windbreak.

We observe about 50 M objects. Wayne Philips was keeping a log, He knows exactly which ones. Ray would of proud of us using the goto. We finally packed it up at 10 pm, well chilled All in all I had a lot of fun. Dave Stitz

Ed we had a great time at the mini-marathon. .Although I love the challenge of finding celestial objects I must admit the "GO TO" was a beautiful thing taking into account the observing conditions. We pulled down 50 messier objects, some of a magnitude of 10, and other objects I have never seen before. .I saw the Sombrero galaxy for the first time and believe I saw some structure to it (I second guess myself because no one else did).Cant wait to get back out,60 more to go (probably the old fashioned way, but if not, I can live with "GO TO" technology). Wayne W Adams

Hi Ed, I showed up for the marathon, there were 5 of us total the sky was just absolutely stunning, great for viewing. However it was very cold and windy. The airport is a great place to skywatch, but it was hard to find, especially in the dark. Hope to see you soon, Kathleen Smith

___________________________________________________________________________________________________________

The CONSTELLATION is the official publication of the Bucks-Mont Astronomical Association, Inc, a 501(c)3 non-profit organization incorporated in the Commonwealth of Pennsylvania and exists for the exchange of ideas, news, information and publicity among the BMAA membership, as well as the amateur astronomy community at large. The views expressed are not necessarily those of BMAA, but of the contributors and are edited to fit within the format and confines of the publication. Unsolicited articles relevant to astronomy are welcomed and may be submitted to the Editor.

Reprints of articles, or complete issues of the CONSTELLATION, are available by contacting the Editor at the address listed below, and portions may be reproduced without permission, provided explicit acknowledgement is made and a copy of that publication is sent to the Editor. The contents of this publication, and its format (published hard copy or electronic) are copyright ©2001 BMAA, Inc.

___________________________________________________________________________________________________________

___________________________________________________________________________________________________________

ASTIGMATISM: is an error of spherical lenses peculiar to the formation of images by oblique pencils. The image of a point when astigmatism is present will consist of two focal lines at right angles to each other and separated by a measurable distance along the axis of the pencils. The error is not eliminated by reduction of aperture, as is spherical aberration. Handbook of Chemistry and Physics, 49th edition.

ANASTIGMAT: Optical system showing little or no astigmatism. An optical system in which astigmatism is suppressed.

Astigmatism occurs when the images are "without stigmatism" (roundness), so that the star images are not round but represented by short lines. Astigmatism causes slightly out-of-focus star images to be imaged as short lines oriented at right angles to each other on either side of focus. Set your scope up and look for it- most systems show some astigmatism (harder to see in the Newtonian- read on). Rock the focus from one side of the best focus to the other while paying special attention to the images of stars near the edge of the field of view. The stars will be imaged as short dashes on either side of focus. On one side of focus the line points toward the center of the field of view, while on the other side of focus the line is at right angles to the radial. Keep in mind you must be very near, but not quite at, the best focus. As you defocus more the short lines becomes an ellipse with less and less eccentricity as you move away from best focus. The best focus is a compromise and is not as discrete as would be seen in the absence of astigmatism. Once you start looking for it you will see plenty of examples of astigmatism. Why should this be so? For one thing, residual astigmatism is present, to a small degree, in many designs. Additionally, astigmatism can be introduced by poor collimation and be mechanical stress in the lens or mirror. The ANASTIGMAT ("not-not round"-wow!) is an optical system designed to suppress astigmatism.

Astigmatism can not be controlled in the simplest of telescopes, the Newtonian. Astigmatism can not be controlled in a doublet (refractor with two lens elements making up the objective). Only with the triplet and catadioptric designs are there enough degrees of freedom to fully address the problem.

Astigmatism increases with the square of the distance off-axis and directly with the aperture. Under most circumstances the amount of astigmatism in the telescope is not significant, as the distances off-axis are small. Hence the simple designs perform acceptably well. In larger instruments covering wide fields the problem is of greater concern. Not until the advent of the Schmidt camera in the 1930's do we see the problem controlled in large aperture wide-angle instruments. Of course camera lenses must cover wide fields. Dr. E. von Hoegh produced the first anastigmat in 1893. Thus far we have considered astigmatism from a design standpoint.

There are three sources of astigmatism:

1) by design (residual)

2) workmanship and materials

3) collimation and mounting of the optical components

Illustration 1 shows an idealized example of astigmatism. With the eyepiece racked outside the best focus stars are imaged as short dashes pointing to the center (RIGHT image), while moving the focus inside we see lines at right angles to these (LEFT image). Look for this pattern in your telescope. It is common to refractors, particularly evident in finders.

Illustration 2 is a guide to why this occurs. The heavy line represents the optical axis of a refractor pointed starward. Light from a star well off-axis arrives from below and is represented by two crossed planes. Importantly, the optical axis and the line from the center of the objective to the star are on the same plane, the tangential plane (see March 2001 CONSTELLATION). This starlight is coming to a focus along the dotted line, also in the tangential plane, located above the optical axis. The advancing wavefront "W" tells the story. The wavefront parallel to the tangential plane is more sharply curved than the wavefront at right angles to the plane (the sagittal plane). Consequently the tangential axis of the advancing wavefront comes to a focus at "T" (where the sagittal axis is still large), while the sagittal axis will not reach focus until "S" (at "S" the tangential axis has enlarged). Between "T" and "S" there is a point of good focus, acceptably small in most systems. As stars further and further off-axis are considered the difference between "T" and "S" grows ever larger. Illustration 3 (below) shows the resultant two focal surfaces. There is a separate focal length (and image scale) for each location off-axis. Why should this happen?

If you are a refractor owner set a dinner plate in front of you. Now turn another plate upside-down over the first to construct a large "lens". Seated at a normal height and viewing the plates from a distance it is easy to see the shape of the lens on the vertical axis is much different than the shape of the lens on the horizontal axis. Consequently, starlight in one plane sees a different optical system than starlight on a plane at right angles. If you are a reflector user perform the same exercise with a single plate.

Let's look at some examples. Illustration 4 shows spot diagrams for an apochromat of 100mm diameter of F/8. On the left are two spot diagrams showing how stars appear inside focus at 10mm off-axis and 7mm off-axis. On the right are two spot diagrams as seen outside focus at the same distance off-axis. Spot diagrams are faithful representations of what happens to rays of light as they pass through the optical system. Between the two sets of spot diagrams we see a graph that shows the tangential light coming to a focus before the sagittal axis. Each axis has it's own curved focal surface.

Illustration 5 is particularly interesting. Here we see the results of ray trace for a 6 in Newtonian reflector of F/8. The stars are imaged as small "comets", called coma, an aberration resulting from shaping the mirror surface as a parabaloid. If this shape were NOT employed another aberration, spherical aberration, would do greater harm to the image on-axis. Off course stars with this shape are difficult to measure particularly in regards to position. Where is the astigmatism? Outside focus (on the RIGHT) we see a central tail between the "arms" of the coma. The graph explains the absence of astigmatism inside focus. The sagittal focal distance is the same as the focal distance for a star located right in the center of the field of view! The sagittal axis expands at the same rate inside and outside of focus.

Illustration 6 shows spot diagrams and graphs for focus of a 200mm f/8 Schmidt-Cassegrain. The two curves are the same, and produces circular star images inside and outside focus. This system has simple field curvature, so that if the film plane is curved the star images remain in focus! Moreover, the field curvature is not at all great at modest distances off-axis. This points to the close relationship between field curvature, astigmatism, and distortion.

Astigmatism can occur either side of focus of each of the two planes, resulting in four basic plans. When a camera lens is corrected for astigmatism the surfaces assume complex shapes (Illustration 7).

Residual astigmatism in the design is generally not a serious limitation for most of our applications as amateur astronomers. The story is different, however, if astigmatism becomes manifest from other sources.

Astigmatism can be polished into an optical surface. Consider Illustration 8 where we see two different radii on the same mirror surface. Of course, such an extreme would never work! Nevertheless, mirror and lenses are made with more than one focal length- this results in astigmatism, and can be seen at the center of the field of view. Improperly annealed glass can result in stress that produces the same kind of surface. Short of replacement or refiguring the situation can not be remedied. Astigmatism from this source need NOT point directly toward the optical axis!

* * * * * * *