BMAA is the featured astronomy club again this month at a WHYY radio (90.9FM) SkyTour on Wednesday, March 28 at 8:00p, to be held at McDonald Elementary School in Warminster PA. Two months ago, we were also featured at a SkyTour at North Penn High School in Lansdale PA. Don Knapp, Planetarium Coordinator at McDonald, sent the following e-mail to the Club:

"We are all looking forward to the observing session we will be hosting with WHYY radio at the end of this month. Since we are expecting a very large crowd, we will be having visitors park at William Tennent High School and use school buses to shuttle them to McDonald for the observing and planetarium sessions. Volunteers bringing telescopes will be able to park in the lot behind McDonald School, but we will be barricading the roads leading into the lot by 6:30p. Although the radio broadcast won't begin until 8:00p, we will begin the observing session at 7:00p. I hope we are able to get a good turnout of telescopes for that evening."

Don can be reached at:

There will be astronomical visual presentations ongoing in the cafeteria and continuous programs in the planetarium all evening. The SkyTour and outside observing will fill the playground and rear parking lot. It would be great to see a large BMAA turnout for this.

Don Knapp and McDonald Elementary have been involved with BMAA for some years now, ever since this editor introduced the Club and Don to each other. We started with annual planetarium presentations geared to Club members and recently have been assisting him with StarWatches at the school. The most recent was on February 28 this year, after which BMAA vice president Antoine Pharamond reported: "The StarWatch went well . . . There were five of us [club members]. Skies were pretty steady, though not very transparent (humid). And then it clouded up around 9:00p, although it was clear again by the time I got home."

- Scott Petersen, editor

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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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The equatorial is the mount of choice for most of us, to enable a driving clock or slow motions to ease following the target object. These mounts are usually part of the package as purchased and are normally adequate for viewing. In most cases they are adequate for photography, at least if properly aligned and balanced.

To make life easier when finding an object, the mounts are frequently equipped with either setting circles or a digital off shoot of some type. Although not strictly necessary they are a great aid in locating those targets at the limit of visibility and minimizing the searching time. Especially with the new digital circles, searching is limited to punching in the numbers and zeroing to the target.

There are some items to consider in setting up a mount; most important, to me, is stability. Along with stability one can list smoothness, accurate squaring, limited flexure, adjustable drag and ease of portability. A small, well designed mount can easily be more stable than a battleship mount with poor design.

Before going any further, the primary cause of instability is balance. We'll cover that first.

If the scope balances when aimed at, say, 45 degrees up facing south, it may be fine. Aiming at the east causes the scope to tend to want to drift. This may show up as a backlash after setting, or as a tendency to vibrate more than normal after focusing, or as stickiness in motion. In extreme cases the driving clock may not function correctly, or the drives clutch will slip, or the slow motions will slip or even jump teeth. What's wrong?

So you shift a few things around and it's better but still won't hold in every setting position. Obviously, something is out of balance.

So, being picky, I'm going to start at the very beginning, with a step many will consider unnecessary. That's fine. Go ahead. Ignore this step because it's a pain in the nether regions. Make my day.

In any mount, the first consideration is 'radial balance' of the tube assembly. What's that? Well, the tube assembly's weight, along it's length, should be in dynamic balance. Most likely you've never even considered this. In most cases the mount has adequate drag and mass to not be bothered too much, but in marginally adequate ones it can be a real thorn. How do you check this? Ideally, a fixture with pivots can be made up, on which the scope is set. The tube is removed from the mount, placed in the fixture and rotated. If it tends to drag more one way or the other, the tube should be weighted to offset whatever is causing the imbalance.

Usually, this is the focuser, eyepiece and finder all being on one side of the tube. Weight should be added to offset this. Since it is not really practical to do this with SCT scopes, at least try a bit of empirical weighting, adding some sort of weight, by judgment, opposite the focuser, and at the far end of the tube.

This is important in alt-az mounts also, especially if the tube is rotatable.

So, that is the difficult part, and may not be really necessary, but we're covering all the bases.

Replace the OTA in the cradle, or bolt it on or whatever. Now aim the scope at the meridian, so that the declination axis is on its side, parallel to the ground; aim the tube at the horizon. Shift the OTA up and down to achieve a balance. Do this with all drag removed. A light pressure at either end of the tube should move the scope, and if it feels even it is close enough for now.

Leave the dec axis aimed south. Aim the tube high towards the zenith. If the same little push is still about equal the tube is now balanced in the cradle within reason. Now adjust the counterweights to balance the scope on the polar axis. Now flip the scope to the other side, around the polar axis, 180 degrees. The same conditions should apply if the scope is balanced. If it is not, go back and do the settings again, right this time.

Aim the scope all over the place. It should remain where aimed, without adding drag to keep it from drifting.

It's really not that difficult once done a few times. When changing to a heavier eyepiece all the balances will be upset, as also adding a camera, finder or whatever. If the scope was balanced correctly an eyepiece change will not overly affect things, but the heavier items will require allowance.

A mount that has sticky or jerky motion is a trial to ones patience. Mounts having this problem should be disassembled and cleaned. If the bearings or bushings are gummy, use WD40 to clean them, drying carefully afterward, then lubing as required.. Don't just load up with lubricant in an effort to avoid cleaning the bearings. This is just making a perfect collection point for dirt, dust and whatever.

If the bearings themselves are rough, you may be able to smooth them, but this is not easy to do without damage, especially to ball bearings. If they are really rough after cleaning, consider replacing them.

Loose fitting bearings will cause the shakes. Some scopes allow adjustments for tension and endplay. Follow the book on this if available. If not try some things that look like common sense. If there is a collar, adjust it to stop endplay. If the bearing has a setscrew to clamp it tighter, try that. Get someone with experience to help. Adding shims of thin Teflon does wonders for bushing type mounts, in smoothing motion and damping vibration.

To find which is the most likely member causing flexure, use a trick taught me by Bob Post. Attach a long thin rod to the suspected member and give a rap. Watch the end of the rod. It will follow the motions set up in the scope, and its motion will magnify. Figure out the remedy or at least the cause and address it. Perhaps it will require replacing some component, but usually a method can be found to eliminate or minimize the flex.

A prime source of wiggles is dinky, inadequate legs for the tripod, or a narrow attachment point for them. Wooden legs are good, as they are naturally vibration dampers, provided they are rigid enough and strong enough to support the scope's weight.

A little bit of added weight to set up is a small price to pay for ease of use. Do this now, while the weather is poor, so you'll be ready for the season.

There is much more to this. If you suspect your mount can be improved without a killer amount of money, talk to someone who has made one. The experience goes a long way in providing simple cures for the shakes.

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ABERRATION: 5) OPTICS a) the failure of light rays from one point to converge at a single focus; b) a fault in a lens or mirror causing such failure. (Webster's New World College Dictionary).

Aberrations can be classified according to their appearance, and analyzed by reference to a standard coordinate frame. This is the first in a series of articles designed to explore the six aberrations in optical systems. Each aberration will be examined in detail in subsequent articles. An understanding of the coordinate frame is a prerequisite to further discussion.

Light is treated as a bundle of parallel rays arriving at the objective parallel to the optical axis (generally the geometric center of a lens or mirror). The rays are caused. by reflection and/or refraction to converge to form an image on the optical axis at the focal plane. In a perfect optical system this image, from a point source (star), has a measurable dimension, the Airy disc, and is surrounded by diffraction rings, also of known dimensions. Any departure from perfection results in changes in the dimensions and brightness of the Airy disc and rings. The size of the disc CANNOT be improved upon - it can only be degraded by a) design limitations, b) faults in quality of the lens, mirror, or system, c) lack of collimation of the optical system, d) atmospheric disturbances. Note that a) and b) correspond to Webster's a) and b). For these article we will focus on the first three sources of aberration (having addressed the atmosphere and tube currents in other articles).

The optical axis is referred to as the "Z" axis. The focal plane is everywhere at right angles to the Z-axis. The image formed by an objective only approximates a focal plane. Nevertheless, this is a useful convention. Curvature of field and distortion are related to failure of the image to conform to a plane.

Diagrams of optical systems are most often made in flat perspective, with the plane of the page as the tangential or meridianal plane. Illustration 1) shows the relationship between the common 2D representation and the 3D nature of the optical geometry. The sagittal plane, viewed on end, contains the x-axis.

Keep in mind the coordinate system is not defined in an absolute sense. That is, the tangential and sagittal plane are defined relative to each other, always at right angles, but not strictly "anchored" to the lens.

Light from an object at infinity arrives at the objective parallel to the optical axis (in most systems) and forms an image on the axis at the focal plane. Light from objects located increasingly off-axis strike the lens or mirror a greater and greater angles to contribute to image formation at greater distances from the optical axis at the focal plane and are referenced to the position on the plane by the X-axis and y-axis. The tangential plane contains the y-axis.

The control of image aberrations is related to the degrees of freedom available to the designer. The simple Newtonian, with one active element, cannot be fully corrected for spherical aberration and coma at the same time. The effects of coma can be greatly reduced by using a suitably long focal length, making the coma less visible, or by leaving the mirror as a sphere and so introducing spherical aberration (also reduced by long focal-lengths). Of course, lenses can be employed to correct coma or spherical aberration as well. Chromatic aberration is automatically absent from systems employing mirrors only, and is only encountered in refractors (or eyepieces with reflectors!). Presence of aberration is a result of design OR less than perfect workmanship in production of the lens or mirror. No design is perfect in all respects. That is, there is a constant give and take, a sort of ebb and flow, between the assorted aberrations as the design parameters are varied as the designer searches for the desired performance. This is always done with a view toward application: is the objective wide or narrow field? Will it be used strictly for film photography? Astrometry? Planetary observation? Portability? The list goes on and on, and includes considerations of cost and ease of manufacture. After the design is worked up with suppression of aberration as far as possible, the optical system is built to very high standards. But no objective is perfect! All, more or less, suffer from aberration above and beyond what is a result of design. Finally, after great care is taken in design and production of the telescope, the optics must be cared for by maintaining collimation. Lack of collimation will introduce aberration in the best of telescopes.

An ideal optical system would produce an image of a plane located at infinity as a plane at the focus. Considering monochromatic light as infinitely small rays Seidel explored the mathematics of image formation, resulting in the five Seidel sums, each of the sums related to each of five image aberrations:

Seidel theory ignores the wave nature of light, and so does not account for diffraction. Issues related to chromatic aberration can be addressed by considering the path of each color through an optical system in turn and considering the composite result. The final image of a point source (star) will show the composite effects of each aberration present, and is called the scattering figure.

Each ray of light passing through an optical system can be raytraced and then compiled as a graph or as a spot diagram. Spot diagrams are easy to relate to and can be considered as representing an image of the star. Graphical representation shows how the rays end-up at the focal plane relative to other rays (paraxial rays- those right done the center on axis). The information can also be presented in tabular form, but this is generally unnecessary in the age of computers. In coming issues of CONSTELLATION each of the five Seidel aberration plus chromatic aberration will be explored. Being familiar with the terminology relating to the three dimensional geometry of image formation is key to understanding the discussions of image aberrations.

under consideration in order to facilitate discussion. The 2D representation is most commonly employed.

Forum

Don Knapp, Planetarium Coordinator at McDonald Elementary School in Warminster, sent the following e-mail to the Club:

Please extend my thanks to everyone who took part in last night's [Feb 28] Community Sky Tour that was held at McDonald Elementary School. Based upon tickets given out for the planetarium programs, about 110 visitors took part. I've gotten many compliments about the program, especially the opportunity to view objects through your telescopes.

Someone (I assume from the club) left a plastic step stool in the observing area last night. I have it in the planetarium, and its owner can contact me at 215-441-6154 to pick it up.

Peter van der Spek mentioned Astronomy 101 at the February General Meeting. Since Peter and Linda presented the informative sessions with Jim and Marian Shearer, and that the Shearer's have moved to upper NY state, they will be cutting back to occasional A101 presentations. They also welcome anyone who could run a session. Any scheduling for Astronomy 101 will be posted in the CONSTELLATION and on our website.