From the Director

by Rex Parker, Phd

Next meeting by Zoom on April 13 at 7:30pm.  While we won’t get together in person at Princeton’s Peyton Hall for meetings this spring, recent vaccination progress gives hope that it may be possible next fall.  For now AAAP will continue to meet virtually via Zoom.  We’re happy to receive the support and continuing interest from returning members as well as newer members.  The current roster (kept by club secretary John Miller) shows 125 total members, about one third of whom have joined AAAP in the past half-year. However, note that the last 4 Zoom meetings had an average attendance of 52 – this means that more than half of members are not showing up.  Please participate in the monthly meetings – we really want you on board.

With the professional speaker presentations that program chair Victor Davis and Bill Thomas have lined up (see the April guest speaker information below), I am confident that this important part of AAAP’s offerings to our members and the public are in great shape.  However, direct hands-on astronomy is another matter. With potential for winding down the state COVID guidelines as summer comes on, an important question is how to handle observatory access by members and future access by public this spring and summer.  A plan will be announced at the April meeting. 

The Gene Ramsey Memorial Reconstruction Fund is over halfway to its goal of $9,000.  Contributions from members are a great way to honor Gene, who did so much for many years for AAAP and the Observatory.  We are seeking ideas on ways to honor Gene with an appropriate gesture, for example a mounted plaque on the new columns when the reconstruction is completed.  Please send me an e-mail with your idea or comment during the upcoming meeting.  Donations can be made directly and securely on the AAAP website (the yellow “Donate” button on the right side of the home page), or checks can be mailed to:  Treasurer, Amateur Astronomers Assoc. of Princeton, Inc., PO Box 2017, Princeton, NJ 08543.  For larger amounts a check is helpful because the PayPal fee is significant.  Contributions are tax-deductible.  Please consider corporate matching if it’s an option for you.

You can help reduce light pollution – here’s some ideas.  I have the honor to give a presentation on lighting issues in astronomy and wildlife in observation of Earth Day during Hopewell Valley Green Week this month, through the Friends of Hopewell Valley Open Space (FoHVOS).  It would be great to have a few AAAP members on this Zoom April 21 at 8:30pm;  register for it here:  I also wrote an article published in MercerMe on the topic. 

Another way to help is to ensure that minimal lighting is part of the emerging park planning for the Moores Station Quarry property in Hopewell Township, adjacent to Baldpate Mountain Park.  The Mercer County Park Commission is undertaking a public planning effort to develop a park master plan for the site.  You can help avoid a future prospect of more lighting interfering with our Observatory at Washington Crossing only a mile away from the Quarry.  To help, review the county plans and take the survey on their website and add your comment to request minimal outdoor lighting in the plan.!/about/moores-station-quarry-park-plan

Revolution in imaging technology – CMOS sensors and amateur astronomy.  The past decade has seen remarkable advances in semiconductor technology which are impacting astronomy in big ways.  The type of sensor used for astrovideo and long exposure astrophotography is shifting rapidly throughout the world of amateur astronomy.  The major companies making sensors, such as Sony and ON Semiconductor, have announced plans to discontinue production of CCD sensors entirely in favor of CMOS technology.  CMOS stands for complementary metal oxide semiconductor, not a very helpful name for the end user.  Of course, amateur astronomers have used DSLRs (with CMOS) for years, but the big push now is development of cooled and optimized astronomy-dedicated cameras using these sensors. The CMOS sensor converts photons to electrons for digital processing, where each pixel amplifies its own signal making the readout lighting quick compared to the slower readout from a CCD sensor.  This enables the near-real time performance of CMOS in the setting of EAA, or what I like to call astrovideo sessions.  Moreover, the read noise (electronic noise from the transfer of signal out of the sensor) is markedly lower than similar sized CCD sensors, critical to revealing faint detail in astronomical targets.  Importantly, cost is substantially lower for CMOS devices relative to CCDs.  With the advent of back-side illuminated (BSI) CMOS chips, quantum efficiency (QE) in some of the newer sensors now exceeds 90%.  For example the club’s ZWO ASI-294MC Pro color camera (used with the Celstron-14 in recent astrovideo live sessions) has a BSI chip with QE ~75%, very high for a color sensor.  Recently, ZWO and QSI astro-camera companies described new astro-cameras based on APS-C sized and full frame monochrome and color sensors using the newest generation Sony BSI CMOS chips.  These developments are game-changers in the field for amateur astronomy, as few sensors of this size were available in the CCD world (and cost a fortune).  Similarly, some of the new cameras with smaller chips along with very small pixel sizes are perfectly suited for shorter focal length refractor telescopes which many amateur astronomers own.  It’s fair to say that there is a revolution in imaging technologies underway, and amateur astronomy is right on top of the wave.

The open star cluster, Messier 67 in the constellation Cancer.  Image taken with a ZWO ASI-071MC color CMOS camera and 12.5” reflector telescope on March 29 from Titusville NJ.  At 3.2 billion years, M67 is thought to be the oldest of all the Messier clusters.  Note the reddish stars – the cluster is so old that its originally bluer stars have passed beyond that stage of development

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From the Program Director

by Victor Davis

The April 2021 meeting of the AAAP will take place (virtually) on Tuesday, April 13th at 7:30 PM. (See How to Join the April Meeting below for details). This meeting is open to AAAP members and the general public. Due to the number of possible attendees, we will use the Waiting Room. This means when you login into Zoom you will not be taken directly to the meeting. The waiting room will be opened at 7:00 PM. Prior to the meeting start time (7:30 PM) you may socialize with others in the waiting room. The meeting room has a capacity of 100 people.

For the Q&A session, you may ask your question using chat or may unmute yourself and ask your question directly to the speaker. To address background noise issues, we are going to follow the rules in the table below regarding audio. If you are not speaking, please remember to mute yourself. You are encouraged, but not required to turn your video on.

Meeting EventParticipant Can Speak?Participant Can Self-Unmute?
Director Rex’s General RemarksYesYes
Program Chair Victor’s  Speaker IntroductionYesYes
Speaker PresentationNoNo
Q&A SessionStart All on MuteYes                                    
5-minute bio break YesYes
Journal Club presentationStart All on MuteNo
Business MeetingStart All on MuteYes
Director’s closing remarksNoNo
Only the Business part of the meeting will be locked.

Featured Speaker: Dr. Alexandra Krull Davatzes is an Associate Professor in the Department of Earth and Environmental Science at Temple University. Her presentation is entitled,  “Precambrian Meteor Impacts and Implications for Early Earth.

About 65 million years ago, at the end of the Cretaceous period, an asteroid impact initiated a mass extinction that, among other things, is famously thought to have ended the reign of the dinosaurs. Less well known are even larger impact events that took place between 3.5 to 2.3 billion years ago. Prof. Davatzes will talk about 18 spherule beds identified in rocks in western Australia and South Africa that represent 13 separate impact events that would have local, regional, and global effects lasting seconds to decades after the impacts. Particularly large impacts, such as those in the Precambrian, may have resulted in: (1) earthquakes, large-scale tsunamis, thermal effects, and distal ejecta reentry; (2) ejection of rock vapor, dust, soot, carbon dioxide, water vapor, methane, and sulfur oxides into the atmosphere, resulting in atmospheric heating and acid rain formation; (3) evaporation in the oceans, leading to destruction of shallow water ecosystems and to hot salty surface waters; and (4) delivery of elements essential to life or transient high oxygen concentration that might have led to biodiversification. In addition, discovery and confirmation of these impact layers is critical for our understanding of the rate of impacts to the early Earth and solar system. This is gaining importance as researchers revisit and redefine the concept of a Late Heavy Bombardment.

Prof. Davatzes’ interests are planetary geology, sedimentology, early Earth processes, and geoscience education. She earned her BA from Pomona College and her PhD from Stanford University. Before coming to Temple, Dr. Davatzes was a postdoctoral fellow at the NASA Ames Research Center, working with the Mars Reconnaissance Orbiter team. She received an NSF CAREER award to study meteor impacts and their effects on the early Earth and the evolution of life. Her field work takes her to the deserts of South Africa, western Australia, and southern California.

Dr. Davatzes collaborates with cognitive scientists to study spacial reasoning in the geosciences and machine learning in aerial drones. Her scholarly work has been published in a variety of journals, including Geology, Science, and Astrobiology. Dr. Davatzes is an associate editor for the Journal of Geoscience Education. In 2020, she received the Lindback Award for Distinguished Teaching at Temple University.

AAAP webcast:  This month’s AAAP meeting, beginning with Rex’s opening remarks and ending at the break before the business meeting, will be webcast live on YouTube and recorded for subsequent public access on AAAP’s YouTube channel. Be aware that your interactions during this segment, including questions to our guest speaker, may be recorded for posterity. Here is YouTube live link

Using Zoom: While we are, social distancing the AAAP Board has chosen to use Zoom for our meetings, based our belief that many members have already have used Zoom and its ease of learning. One of its great features is you can choose whether you want to install the software on your computer or use it within your browser.

How to Join the April Meeting: For the meeting, we are going to follow a simple two-step process:

  1. Please make sure you have Zoom installed on your computer. You do not need a Zoom account or need to create one to join the meeting. Nor are you required to use a webcam.
  2. Please visit our website for the link to the meeting
  3. This session will be recorded and saved on YouTube. Send me an email at if you have any concerns.

NOTE: We plan to open the meeting site 30 minutes to the 7:30 start time. This way you won’t have to rush to join the meeting. A maximum of 100 attendees can join the meeting.

More Information: The Zoom site has many training videos most are for people who are hosting a meeting. If you’re unsure how Zoom works you might want to view the videos on how to join a meeting or how to check your computer’s audio and video before the meeting.

We hope to make these short presentations a regular feature of our monthly meetings. If you are interested in presenting a topic of interest, please contact either or We’d like to keep our momentum going!

Upcoming Programs: Here’s a look ahead at upcoming guest speakers. We’re expecting to conduct virtual meetings for the remainder of this academic year. In an effort to turn necessity into a virtue, we’re casting our recruiting net a bit wider than usual, inviting speakers for whom a visit to Princeton would be impractical or inconvenient. Suggestions for guest speakers for September, 2021 and beyond are welcome.

May 11 – Alex Hayes: Prof. Hayes is an Associate Professor of Astronomy at Cornell University and Director of its Spacecraft Planetary Image Facility. He will speak on Ocean Worlds of the Outer Solar System, plus he will give a brief report on the Mars 2020 mission.

June 8 – Anna Schauer: Dr. Schauer, a new mother, is the NASA Hubble Fellow at the University of Texas at Austin. She leads the team researching what she’s nicknamed the Ultimately Large Telescope, a lunar liquid-mirror telescope that will aim at investigating First Star Formation.

WANTED: Members with interesting stories to tell.  As of this writing, no member has volunteered to offer up a brief story or presentation for Journal Club this month. During the past months, we’ve enjoyed interesting and informative talks from AAAP members, and we’d like to keep the momentum going! We hope to make these short presentations a regular feature of our monthly meetings. We’d like to know what members are doing or what members are thinking about in the broad range of topics encompassed by astronomy. A brief ten-minute (or so) presentation is a good way to introduce yourself and the topics you care about to the club membership. If you are interested in presenting a topic of interest, please contact either or

Looking forward to you joining us on Zoom or YouTube Live webcast at the April meeting!

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Minutes from the March 9, 2021 Members General Meeting (Online)

by John Miller, Secretary

●  The meeting convened at 7:30 PM via Zoom and Yahoo (online).  There were initially about 52 Zoom attendees.

●  Rex Parker briefly reviewed “Galaxy Season” as seen from our latitude. He included a few Parker astrophotos produced at his observatory.

●  Program Chair Victor Davis introduced the evening’s guest speaker: Keivan Stassum, professor of astrophysics, Vanderbilt University.  Professor Stassum’s topic was: “The Royal Road: Eclipses and Transits in the Era of Gaia and TESS.”

●  Member and Sidereal Times Co-Editor, Surabhi Agarwal presented a review of the historic Jantar Mantar Observatory in  Jaipur, Rajasthan, India.

●  Rex Parker mentioned that the State has given permission to proceed with the Washington Crossing Observatory support post repairs.

●  There were a number of comments and brief discussions regarding the pending Perseverance Mars Landing Mission.

# # #

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John Herschel and his Lens Designs

by John Church

Charles Hastings once wrote that the lens which is now on our Hastings-Byrne refractor was similar in design to the types described by John Herschel (1792-1871). He also mentioned that such lenses do not greatly differ from a Fraunhofer form.  I decided to look more deeply into this with the help of Herschel’s own publications.  

Herschel’s first and highly technical paper about lenses was on pages 222-267 of Volume 111 of the Philosophical Transactions of the Royal Society of London for 1821 and was entitled “On the Aberrations of Compound Lenses and Object-Glasses.”  His second and much more useful one for designers was on pages 361-70 of Volume 6, Number 12 of the Edinburgh Philosophical Journal for 1822. It was entitled “Practical Rules for the Determination of the Radii of a Double Achromatic Object Glass.”  Here he writes:

“In the construction proposed in my paper, the destruction of the spherical aberration is insured, not only for parallel rays [i.e. those from objects at infinity], but also for those that diverge from objects placed at any moderate finite distance, so as to produce a telescope equally perfect for terrestrial and astronomical purposes.”  He goes on to say that another advantage would be that more moderate surface curvatures could be used with his construction than with others.

Herschel set up tables for a focal length of 10 in arbitrary units, with the crown element in front.  Two standard glasses were used as a starting point.  A designer would supply the mean refractive indices of the two glasses that were to be actually used, as well as the ratio of the V-number of the flint glass to that of the crown glass.  (A V-number is a glass’s mean refractive index minus 1, then divided by the difference between the indices for blue and red light.) Adjustments from Herschel’s basic coefficients for the standard glasses were then applied. After all the necessary manipulations, the four final radii for any desired focal length were found by multiplying the radii for focal length 10 by a simple scale factor.  

Due to the complex nature of spherical aberration, Herschel must have put an enormous amount of labor into developing this method, testing it, and writing it up for non-mathematicians. After adapting it for a spreadsheet, I’ve been able to verify Herschel’s own worked-out example as well as several other published descriptions of Herschel-type lens. 

What would Herschel’s “prescription” be for our own lens?  In order from the first surface of the crown to the last surface of the flint, the radii in millimeters would be  + 1557, – 570, – 585, and – 2478.   Our actual measured radii are + 1336, – 606, – 625, and – 3435.  These radii are different enough that it appears that we don’t have a genuine Herschel lens.

I still plan to make some empirical tests on nearby daytime objects with our refractor to see if sharp focus can be achieved there as well as at infinity.  Fraunhofer is said to have done similar tests as well, as this was much more convenient than fully mounting lenses for nighttime observations (A.E. Conrady, “Applied Optics and Optical Design,” Part I).  Fraunhofer may well have been doing what Herschel had suggested, although this is probably unknowable now as Fraunhofer left very few records. 

How do these lenses perform?  For celestial objects, our lens is corrected to about 1/40th wave for spherical aberration in brightest light and to about 1/10th of Conrady’s maximum tolerance for coma.  The Herschel design tops out at about 1/10th wave of spherical aberration (Rayleigh’s tolerance is 1/4th wave) and 1/3rd of Conrady’s coma tolerance.  This isn’t bad at all, but Hastings clearly beats Herschel for this particular lens. The spot diagrams that I presented in my January talk confirm the fine performance of our lens, backed up by what we can see in an eyepiece.  

Even better lenses, such as those designed by the Clairaut-d’Alembert-Moser formulas that I published in 1984, are about midway in form between ours and a Herschel.  For one of these with our glasses, the radii would be + 1417, – 592, – 610, and – 3002 mm.  At middle wavelengths, spherical aberration and coma would be negligible with such a lens.

Technical notes: In today’s usage, a “plus” sign indicates that a lens surface is convex to the sky; a “minus” sign indicates concave to the sky even though a surface may be physically convex.  In Herschel’s time, all convex surfaces were considered positive and all concave ones negative no matter which way they faced.  Like all “thin-lens” design methods, Herschel’s omits the effects of element thicknesses and spacings.  The predicted performance of any real objective is always checked by trigonometic ray-tracing before manufacture.  At f/10 and higher, thin-lens designs usually work fine with little or no tweaking. 

While doing this work, I learned more about Herschel himself. He was the only child of his famous father, Sir William. Mathematics was Herschel’s strong point; he was “senior wrangler” at Cambridge in 1813.  He was awarded the prestigious Copley Medal of the Royal Society in 1821 for his papers in the Philosophical Transactions.  In 1824 he visited Fraunhofer in Bavaria, but was disappointed when Fraunhofer declined to discuss his own design methods or even show him his workshop.  

Fraunhofer died in 1826.  He had transferred his trade secrets to his successors, who continued with the glass and telescope business that Fraunhofer had built up. 

Knighted and titled in 1831, Sir John Herschel went on to many other nonmathematical accomplishments including greatly extending his father’s work on nebulas and double stars.  He coined the words “photograph” and “photography” as well as “positive,” “negative,”  and “snapshot” in this context.  (Imagine the copyright possibilities.)  He discovered how to fix silver images with thiosulfate (“hypo”) solutions, still occasionally used to this day.  He invented the calotype (later named the blueprint), named the seven known satellites of Saturn and the four of Uranus, and initiated the use of the Julian day system in astronomy.  He lies in Westminster Abbey next to his good friend Charles Darwin (1809-1882).

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compiled by Arlene & David Kaplan


The Most Intimate Portrait Yet of a Black Hole
The Event Horizon Telescope collaboration, an international team of radio astronomers that has been staring down the throat of a giant black hole for years, on Wednesday published what it called the most intimate portrait yet of the forces that give rise to quasars, the luminous fountains of energy that...more

-David Trood

Space is the place for impossible molecules Molecules containing noble gases shouldn’t exist. By definition, these chemical elements — helium, neon, argon, krypton, xenon and radon — are the party poopers of the periodic table, huddling in the rightmost column and refusing to make molecules. Indeed, no one has ever seen any naturally occurring noble gas molecules on Earth. Earlier this decade, though, astronomers accidentally discovered one of these aloof elements in molecules in space….more


Interstellar Comet 2I/Borisov is One of Most Pristine Comets Known 2I/Borisov, also known as C/2019 Q4, was discovered on August 30, 2019 by Gennady Borisov, an amateur astronomer from Crimea, Ukraine. Its orbital eccentricity shows that the object is not gravitationally bound to the Solar System, making it the first unambiguous case of a comet arriving from interstellar space…more


These giant mirrors will help astronomers see to the edges of the universe When completed, the Giant Magellan Telescope being built in Chile’s Atacama Desert will gather images of the universe that are 10 times sharper than those produced by the Hubble Space Telescope. It will snap photos of distant planets and search them for signs of life, reveal the masses and compositions of infant galaxies and analyze how stars are born and die….more


Hubble Team Releases Reprocessed Image of Veil Nebula A small portion of the Veil Nebula, which is part of a supernova remnant called the Cygnus Loop, was featured in previous Hubble photos, but now new processing techniques have been applied, bringing out fine details of the Veil Nebula’s delicate threads and filaments of ionized gas. The Cygnus Loop is a large donut-shaped nebula….more

-Boatwright & Head

New Type of Paleolake Spotted on Mars A 54-km- (33.5-mile) diameter Noachian-aged crater with neither inlet nor outlet channels is distinct from previously documented crater basin lakes on the planet, according to new research from Brown University. The newly-discovered ‘closed-source drainage basin’ crater is located in the southern highlands of Mars. It contains unusually well-preserved stream beds…more


 Subsurface Ocean of Enceladus Has Currents, New Theory Suggests A novel theory proposed by planetary scientists from Caltech and NASA’s Jet Propulsion Laboratory challenges the current thinking that the saltwater global ocean of Enceladus, the sixth largest moon of Saturn, is homogenous. In 2014, NASA’s Cassini spacecraft discovered evidence of a large subsurface ocean on Enceladus and sampled water from geyser-like eruptions…more

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