On the evening of July 23, 2001, the owners of Camp Onas near Ottsville Pennsylvania were startled when they heard their windows rattle. Soon afterward, Ollie the camp dog began barking at something outside. Smoke was later seen rising from the south end of the field, apparently coming from a large black circular area. Upon further investigation it was found that in the center of the crater lay a strange charred oval object. Weird clanking noises were heard coming from inside the object, and as the top began to slowly unscrew..

Okay, so all kidding aside... It's that time of year again fellow stargazers, when the weather gets cool and crisp, the leaves on the trees begin to show their beautiful colors, and of course, time for our annual Astronomy Campout/Convention Star-party at Camp Onas. Now I can't tell you exactly what last year's event was like because even though I was involved with the planning, I came down with a bad case of the flu and couldn't attend. But just ask anyone who attended; I think it's a given that it was a very enjoyable time.

As always, running an event like this requires the involvement of as many BMAA members as possible. IF I'VE SAID IT ONCE, I'VE SAID IT A BILLION TIMES: THE MORE PEOPLE THAT HELP OUT, THE LESS THAT EACH PERSON HAS TO DO!

Just a little bit of your time and effort will insure that we have as great a time this year as we did last year.

I will begin filling slots for the various positions at the October meeting, and also calling the rest by phone, but if you're really smart you'll pick your favorite position from the list below as soon as possible rather than being asked to fill one later. Besides, SDV attendees usually rather enjoy pitching in when they see everyone else helping out, so come and join the team! Hope to see you all there.

Think Clear Skies!

Alan Pasicznyk

On-Site Coordinator, Stella Della Valley XV

POSITIONS AVAILABLE:

FRIDAY: Assist in Guiding/Parking Cars (A 1.5 hour shift, and you're FREE from volunteer work the rest of the weekend!)

Post and later remove signs to SDV.

SATURDAY: 8:00 Setup for Breakfast

Setup for Flea Market

10:00 Texas Bldg. Chair Setup

BMAA Table (2 Hr. Shift)

4:00 Setup for Dinner

SUNDAY: 8:00 Setup for Breakfast

Pre-Departure Cleanup (check for small litter, tidy up dining hall, generally restore the site to original condition).

AND NOW.... From our home office in Danboro, Pennsylvania...

ALAN'S TOP TEN LIST FOR ATTENDING SDV XV:

10 The crisp, beautiful Fall weather

9 You get to see Bernie Kosher go nuts buying flea market stuff

8 Clear skies, Clear skies, Clear skies !!!

7 The return of the "all you can eat" pizza banquet

6 The tastykakes after the banquet

5 Meeting with old friends you saw last year

4 Meeting with young friends you saw last year

3 Prizes, prizes, and even more prizes!

2 The guest speaker with his dancing binary stars

1 You get to see Alan Pasicznyk's stockpile of Vitamin C!

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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.

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Groeten (greetings):

I am very pleased to be sharing the byline for this article with my Dutch friend Cor Berrevoets. Rightly, his name should be on other parts of this series as his work has added greatly to my own appreciation of the subject and made many of the illustrations possible.

Timing was close! As I write these words the chromatic aberration capabilities of ABERRATOR (Cor's "machine" for analysis of lens aberrations) nears completion. I had been planning to explore chromatic aberration at the end of the series, unsure of how to demonstrate the phenomenon, let alone explore it well, other than at the eyepiece. This is the main power of ABERRATOR - it is a tool for exploring nature because it is based on the rules of nature. What is learned with ABERRATOR is carried through to sessions at the eyepiece (and the other way around as well!). - JCD

Chromatic Aberration: Occurs as a result of the inability of a lens to bring assorted wavelengths (colors) of light to a common focus. Longitudinal Chromatic Aberration occurs when a simple lens refracts each color to a VARIABLE degree, resulting in a separate "best focus" for each color (the others being out of focus when any one is in focus.). Lateral Color occurs when the assorted colors fail to reach a common focus on an off-axis star (each color has a different scale, resulting in a spectrum spread out with an axis directed toward the center. When a lens is corrected for spherical aberration in green light the lens is under corrected in red light and over corrected in blue light, resulting in Spherochromatism.

Illustration 1) treats a simple lens as two prisms joined at the base. In fact, calculations employed in lens design treat the lens as an infinite series of such prisms. The light is bent (refracted) to a focus, but each color is bent to a different degree, resulting in a different focal length for each color. Early builders found that by employing long-focal lengths (high f/values) the effect was reduced. The problem is that the aberration does not scale so that very long focal lengths are required for each modest increase in aperture, and the telescope quickly takes on gigantic proportions. Both longitudinal and lateral color are present.

Chester Moor Hall built the first achromatic objective, while John Dolland (and son Peter) greatly improved the method. The idea is this: if one lens bends the light inward, each color to a variable amount, then a second lens, bending the light outward, could be employed to bring the colors to a common focus. This is the idea behind the Achromat. The word means "without" color, but some color aberration always remains. Illustration 2).

Each glass available to designers has an associated refractive index which tells the designer how much the glass bends light. In addition the Abbe numbers inform the designer how the glass will disperse the colors of white light. By prudent use of this information, combined with selection of lens radii and spacing between the elements, chromatic aberration can be brought under control. The focal lengths of the two lenses in achromatic doublets are inversely proportional to their dispersion numbers.

The refractor objective must be corrected for all of the Seidel aberrations plus chromatic aberration. Fortunately, the great number of design variables inherent in the refractor objective allows the opportunity to correct for these aberrations. In the process of correcting for spherical aberration some chromatic aberration is introduced into the design. This is termed spherochromatism. Illustration 3) shows the remaining difference in focal length for "red-blue" and green when a doublet is employed to bring red- and blue light to a common focus, while green remains short. The difference between red-blue and green focus is the secondary spectrum. Variations in focal length with color remain, as Illustration 4) shows.

Illustration 5) shows traditional solutions for dealing with chromatic aberration. The Fraunhofer places the positive lens in front of the negative lens, with the negative lens having the higher refractive index. The Steinheil places the negative lens in front. There is loss of variability in design with the cemented doublet in exchange for ease of manufacture (the cemented surfaces have the same radii and are often less well polished- saving time and money). Objectives employing two elements are corrected for two different colors of light, and achieve this goal to varying degrees. New (and expensive) rare glasses provide room for greater correction, as little as 1/4 the chromatic spread in traditional glasses.

In a conventional doublet the spot diagram for red and blue light should not be greater than three times the Airy disc diameter! For apertures of 100 mm good control of chromatic aberration is possible at F/12 while with a 200 mm objective comparable control is had by moving to F/24! Greatest aperture for fully achromatic refractor is 160 mm. In order to achieve good color and reasonable instrument length, or to control color to a greater degree, designers found it necessary to build objectives with three elements, and the apochromat was born. These can be cemented, the rear element "broken", both broken or the front element broken (this last form is NOT shown in the illustration). With so many design options the objective can be corrected for three or even four colors of light, and the same instrument can perform well both visually and photographically.

Chromatic aberration is non-existent in the reflector (if you see it try different eyepieces), and insignificant in catadioptric designs. If you own a reflector and wish to observe chromatic aberration take a low power eyepiece apart (Kellner's work fine) and leave only the plano-convex lens in place. Your stars will be surrounded by color! The image of say Polaris (because of lack of motion and color balance) is a good test star. Star images should be observed without filters when looking for chromatic aberration (in contrast to other star testing- see CONSTELLATION February 2001).

The refractor will show the star typically "yellow" relative to the reflector because of the loss of blue light in the central disc. A faint purple halo will be seen around the star. Shifting the focus outside best focus by a small amount will show the star ringed in red, while distances still further outside focus will show green ringing a star with a blue cast. These and other effects can be explored in performing the star test on your refractor. Let's turn to ABERRATOR to explore the issue further.

With careful mixture of three primary colors a complete color scheme can be generated. This three-color method is employed in most technological systems that generate color images. Indeed, the original color system, the retina, employs a primary color system to enable us to enjoy the entire spectrum. ABERRATOR makes use of the three-color system to generate full color displays of the Airy disc and rings. Analysis of a three-color system forms the basis of objective design. If you have used ABERRATOR in the past you are familiar with looking at the Airy display and axial slice of the focal region in green light. ABERRATOR employs this idea by considering what happens in each of the primary colors and summing the effects into a composite color image.

When two or more lenses are employed as a telescope objective of primary consideration is the correction of chromatic aberration. The Fraunhofer doublet is a traditional design for doing just this. Illustration 6 shows a longitudinal slice in each of three primary colors. Immediately we see the green focus is much shorter than the focal length of red or blue. The objective is red-blue corrected as these are at similar distances (notice too how the diameter at focus from blue is narrow, a consequence of wavelength). The Airy diagrams tell the story. At one wavelength shy of focus (-1) the green disc is large, with red and blue still larger. At one wave beyond (one wave of green light that is!) blue is tighter than green and is ringed by red. At focus the light shows a slightly yellowed or green star as commonly visualized with refractors.

A well-corrected doublet, supplied with ABERRATOR, can be examined for chromatic aberration (this design is by Klaas Compaan). Try various distances each side of the green focus and watch the displayed Airy disc and diffraction pattern. At the best focus we see a central well lit spot containing most of the light but absent some of the red and blue that is NOT at focus. This light appears as rings of color around the central disc containing green and yellow light. The images at 0.8 waves inside focus and 0.8 waves outside focus show a similar green pattern. A large, faint outer ring contains the red light (denied the center), while green is removed from the center leaving only the blue (nearly at focus). At 0.8 waves outside the green focus the extreme outer portion contains most of the blue light, with green removed to a smaller diameter leaving red to occupy the center (nearly at focus). At intermediate points other nodes of bright and dark for each color can be explored.

A lens with a single element spreads white light out over a range of distances along the axis from blue (near the lens) to red, with the entire spectrum between in an orderly fashion. Typically the achromat "folds" this rainbow over itself (with green at the fold) to bring red and blue to a common focus. The apochromat seeks to fold the rainbow once again to bring 3 or even four colors to a common focus. Just where one design reaches a limit and the next begins can be hard to say! A well-designed achromat may perform nearly as well as some apochromats. The distinction between the apochromat and the super-apochromat can be even harder to make.

This illustration shows a very nice triplet. Here all three wavelengths overlap at the focus, starting with green and closely followed by blue and red. The total range, relative to green light, over which the colors focus is further reduced relative to the other designs considered. At focus the star is little changed from white, while before focus the star is blue and beyond red light will form the core.

In low light levels we are all more or less color blind. Only at moderately high brightness levels is the color system fully operative. Increased image brightness, as obtained with larger aperture, allows better and better visualization of color aberration. Large refractors require the use of filters to retain contrast.

Start by observing chromatic aberration on the moon. Look for color fringes along the limb. Vary the focus and observe where the color develops. After exploring chromatic aberration "on the moon" shift to a second or third magnitude star that is not strongly colored. You will want to establish a standard- Polaris is a good one, at least for those of us in the Northern Hemisphere. Center on the star and focus carefully. Now hold a deep green filter OVER the eyepiece and re-check the focus. Our eyes are most sensitive in the green or yellow part of the spectrum and so we tend to focus here. Slowly shift the focus, observing the redistribution of light levels and color. Where is the green light? When is red most intense? Blue? If you suspect the eyepiece is introducing color aberration of it's own try a different eyepiece (or use the same on a reflector- if color remains then the eyepiece is at fault). If color seems excessive any number of things may have resulted in tilted, decentered, or incorrectly spaced elements. One component may be turned around (after cleaning?). Perhaps a dented tube has resulted in a tilted cell. Sometimes small spacers are used in an air-spaced objective and these may be misplaced (some objectives are oil spaced). If there is no obvious problem then contact with the maker is called for.

Look for "hot-spots" when some colors manifest themselves to the exclusion of others. Can you find the deep, ruby-red spot discussed by Suiter in Star Testing Astronomical Telescopes? Turn your refractor to the planets, Venus can be a lot of fun, particularly when confounded by a position low in the sky. Turn toward colorful pairs of stars to see what happens with color when the focus is shifted.

This article concludes our survey of image aberrations as they concern astronomers. Continue to explore with ABERRATOR as rain or shine it sits ready to open up the door to study of aberration. It is, additionally, like having many telescopes to study with. And, of course, continue to use your telescope to look inward, to explore the hidden, very tiny, world of optical phenomena. These things are just as much a part of nature as stars and planets. While each aberration was dissected from it's neighbor, and presented here as a separate entity, they do not occur in isolation. Each telescope design, even with "perfection" in manufacturing, exhibits it's own set of

aberrations. On top of aberrations inherent in design others are exaggerated when tolerances are not met in workmanship or materials are less than ideal. And, finally, in your hands is the responsibility of collimation. This we will explore in coming issues of CONSTELLATION.

July 2001

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