Dictionary of Astronomy

Dictionary of Astronomy

If you have a word you are unsure of and want it added to this dictionary please ask.



Defect in an image formed by a lens or mirror, e.g. coma

Aberration of light

Shift in the apparent position of stars due to the finite speed of light

Absolute Magnitude

A measure of the true brightness of an object assuming a distance from the observer of 10 parsecs, also see Magnitude


A refracting telescope with a lens (usually a doublet) that tries to correct for chromatic aberrations.


A refracting telescope with a lens (usually a triplet) that tries to correct for chromatic aberrations, corrects better than an Achromat, also known as an ‘APO’.

Apparent Magnitude

A measure of the brightness of an object as seen from the Earth, also see Magnitude


Abbreviation for Altitude


A coordinate used alongside azimuth in the ALT/AZ coordinate system, to mark the position of an object in the sky, the Altitude or ALT as it is also known, is an angular measure of how high an object is in the sky at your location. The ALT/Az system is a local coordinate system and the positions of objects in the sky change with time and location.


The diameter of the primary optical element in a telescope


Abbreviation for Azimuth


A coordinate used alongside altitude in the ALT/AZ coordinate system, to mark the position of an object in the sky, the Azimuth or AZ as it is also known, is an angular measure from North (or sometimes south) eastwards to an object in the sky at your location. The ALT/Az system is a local coordinate system and the positions of objects in the sky change with time and location.


Baade’s Window

A gap between “curtains” of dust clouds which lets us see close to the galactiic center of the Milkyway and see stars on the far side of galactic bulge.

Blazhko Effect

The periodic variation of RR Lyrae type stars isn’t as stable as you might think, the period and amplitude varies over a cycle of about 40 ish days around the extablished period for the star in question, the cause of the effect is not known but there are two popular hypothesis one to do with the resonance of the stars interior and the other to do with the magnetic field of the star when the axis of the magnetic field is out of line with it’s rotational axis. The effect is named after Sergey Blazhko who discovered it.

Blue Shift

Light from an object traveling towards you is ‘compressed’ this pushes the spectral lines towards the blue end of the spectrum.



A telescope that uses mirrors and lenses to focus the light

Chromatic Aberration

This is an aberration in an optical system that results in light from different wavelengths being focused at slightly different points, so it is impossible for all the light to be in focus at the same time, typically this results in a blue or purple haze around bright objects and a loss of contrast.


The lining up of the optical elements in a telescope along with the tube and focuser, essential to get the best out of any telescope, refractors are usually collimated in the factory and don’t need nightly adjustment


An optical aberration that makes stars have little tails directed away from the centre of the field of view


A line of sight phenomena when two or more solar system bodies are visible in the same area of sky.



Abbreviation for Declination


A coordinate used alongside Right Ascension in the equatorial coordinate system, to mark the position of an object in the sky, the declination or DEC as it is usually known is an angular measure similar to that of Latitude, however whereas latitude starts at the Earth’ equator, DEC starts at the celestial Equator.


Secondary mirror used to divert the light from the primary out to the side of a Newtonian telescope


The exact point when the Moon (or Mercury or Venus) are at quarter phase (50% illuminated as seen from the Earth)

Drake Equation

An equation that predicts the number of observable alien civilisations in the Milkyway.

Doublet Lens

A lens made from two separate lenses, usually to combat chromatic aberration.



The orbital plane of the Earth around the Sun, or you can think of it as the apparent path of the Sun in the sky over a year.

Extrinsic Variables

Stars that vary due to external influences, for example binary stars eclipsing each other.



The bright regions on the Sun’s photosphere observed in white light

Fermi Paradox

The contradiction between the estimates of the probability of the existence of alien civilizations see Drake equation – and the lack of evidence for these.

Field of View

The area of sky can see, to see the full Moon you need a Field of View of half a degree


Dark areas on the solar disk when viewed in hydrogen alpha, occurring in the upper chromosphere, if they extend beyond the solar limb they become bright areas called solar prominences.


A coloured glass used to block specific wavelengths from the sensor (or eye) of the telescope.
V (Visual) Filters are used to approximate the visual view when using a CCD and are a common filter for photometry ensuring different observers have comparable observations.


Fast Low-Ionization Emission Regions. Emission regions moving outwards at supersonic speeds. Their formation is not easily explained by any models of stellar evolution.

Focal Length

The distance the focus is away from the primary objective

Focal ratio

Simply the focal length divided by the aperture of a telescope, usual expressed as FN, where N is the number of times larger the focal length is compared to the aperture. When ‘N’ is small this is termed ‘Fast’ when N is large this is termed’ Slow’



German equatorial mount, a type of telescope mount.



Intrinsic Variables

Stars that vary due to the internal nature of the star, for example a star that is pulsating.



The point in an orbit where you become nervous about having possibly made the wrong decision. Well it made me laugh.




Leptogenesis is the term used to describe the (hypothetical) physical processes that resulted in the universe being made up of matter rather than an even mix of matter and anti matter, ‘Lepto’ referring to particles called leptons, most famous of which is the electron.

Light Year

The distance light can travel in one year, equal to about 10 trillion kilometres, the nearest star (after the Sun) is about 4.2 light years



The ‘brightness’ scale used in Astronomy, the lower the figure the brighter, it is a logarithmic scale, so a difference of 1 magnitude is about 2.5 times, 5 magnitudes difference is 100 times thus a difference of 8 magnitudes would be (5+1+1+1) (100*2.5*2.5*2.5= 1560 times brighter/fainter)


An imaginary line in the sky passing north south

Meteor Train

A meteor train appears as a faint nebulous streak of light left behind, along the track of a meteor, but AFTER the meteor itself has extinguished, about a quarter of meteors leave a momentary train and trains lasting over a second are quite rare .

Meteor Trail

The moving streak of light a meteor



A type of reflecting telescope



Axial tilt of a body, for the planets and the Sun in relation to its orbital plane.


The main mirror or lens in an optical system

Observed – Calculated (O-C)

This is generally a plot of an observed measurement minus the calculated measurement, by doing this you plot the residuals and can quickly identify any changes, used commonly for the measurement of eclipsing binary stars.


The passing of one object behind another, for example an occultation of Saturn by the Moon, the object behind is said to be occulted, see also transit.


Optical Tube Assembly, a telescope is made from a mounting system and an optical tube assembly.



A measure of distance, based on the parallax of the object using the Earth as a base line, 1 parsec is equal to 3.2615 light years.


The measurement of an objects intrinsic brightness


Bright area in solar chromosphere seen in Hydrogen alpha


Generally referring to the main ‘light gathering’ optical element in a telescope




The point in the sky where meteor showers appear to originate, this is not to say that a meteor will start at this position, but if you plotted the observed meteors from a specific shower and extended their trail backwards they would all converge onto the radiant

Red Shift

Light from distant galaxies is stretched as space between the galaxy and you expands this results in the spectral lines appearing to be shifted towards the red end of the spectrum


A telescope with a mirror that focuses the light


A telescope with a lens that focuses the light


Measured in arc seconds, this is the finest detail you can see, or image, it is generally limited by aperture and atmospheric conditions.

Right Ascension

A coordinate used alongside declination in the equatorial coordinate system, to mark the position of an object in the sky, the right ascension or RA as it is usually known is an angular measure similar to that of longitude, however whereas longitude starts at Greenwich England, RA starts at the position of the vernal equinox. Rather than using degrees, as longitude does, it uses hours, minutes and seconds, where 1 hour = 15 degrees.


Abbreviation for Right Ascension


A telescope with a lens that focuses the light


Schröter’s Effect

This is the effect where by the theoretical and observed times for Venus to be at dichotomy differ by a day or two.


Generally referring to a mirror in a telescope used to change the direction and or point of focus in a telescope


Stands for Sudden Ionospheric Disturbance, a change in the ionosphere due to energetic matter from the Sun interacting with it. see our SID page



The passing of one object in front of another, for example the transit of Venus in front of the Sun, the object in front is said to transit the other object, see also occultation


A lens made of 3 components used to make Achromatic Refractors



Abbreviation for Universal Time

Universal Time

Closely related to Greenwich Mean Time (GMT) UT is measure of time used for all civil time keeping, for us astronomers in Crayford we just need to remember to remove an hour if we are in British summer time



Very Low Frequency, name given to the 3-30kHz range of the electromagnetic spectrum, VLF radio signals are used to indirectly monitor solar activity see more here






Point in the sky directly overhead.

Zenithal Hourly Rate

The expected number of meteors that would be observed if the radiant is at the Observers Zenith.


Abbreviation for Zenithal Hourly Rate

2018 Perseid Meteor Report

Interim Report

This year was the first year of successful operation of the Dick Chambers Observatory Automated Meteor Camera coordinated by Janice McClean. This is an interim report and may be replaced as more information on the Perseid’s captured comes in from the meteor group.



Jim goes up a ladder and comes down a hero


Meteor camera success features in BAA newsletter.


In all seriousness, there are a lot of people to thank for going up and down the ladder and providing input into getting the Meteor Cam working, so many that I daren’t not mention anyone because I’m bound to miss someone – you know who you are – we still have a small bug that Simon is working on to prevent the sorted meteor files from reappearing on the myCloud drive.



2018 Summer BBQ

The summer BBQ was a great hit again. About 25 members attended and we had food for about 100! The day was very relaxed. We had a few glimpses of the Sun through our Dobsonian telescope, Anita using Baader Solar Safety Film, unfortunately there were no Sun spots visible.











Welcome to our new website!

Our new website is live, come back regularly to see new updates.

This post lists the progress of migrating data from the old website to the new one.

Our old site remains operational for a short period and can be found on this link as sections of the old site are migrated more and more of the old site will point to this new site, however please be patient at we have 560 pages and 2.5Gb of images to migrate!


Added Making dew heater controller and dew bands to equipment section


Added Automatic Photometric Telescope post to Papers  and Equipment sections


Added new post for BAA Exhibition meeting in 2010
Added new post for BAA Summer Exhibition Meeting 2012
Added Paper
Added Paper
Added Paper


Added book review posts

Updated Aurora page

Added news posts

Updated Atmospheric Optics Page and added some posts


Added Frankenscope to equipment section

Added Observatory Section


Added post on summer BBQ

Fixed Event section which wasn’t displaying event posts


Updated space Junk section and added post


Added Equipment Section and added to Activities menu

  • CEM post
  • Drift scan post
  • Battery Free electric focuser
  • DIY Finderscope
  • Variable Barlow
  • Large Telescopes

Added Variable star page and 3 posts showing observations


Updated Mars Page


Added Spectroscopy page


Added post for 2015 Lunar Eclipse – all Lunar Eclipse reports from old website are now covered.
Modified the Programme- you can now import our programme into your own Calendar!


Added page on Detecting meteors with Radio
Updated Lunar Eclipse Page added posts for 2001 and 2007 Lunar Eclipses


Complete upload of member solar images
Papers section added and 2 papers published
Meteor Page updated with posts for previous society reports of meteor showers
Radio Section Added, added SID page
Double Stars Section Added


Comet page updated, including all the previous comet reports by the society.
Venus page updated with members images plus a paper on the dichotomy of Venus 
Sun pages updated with members images


Changes to Membership

New Membership Fees: 

Our society is booming, as a rsult our income has become a little high so we have made the decission to reduce membership fees .

The new membership structure is as follows…

Full Membership

  • £95 per annum
  • All meetings including lectures

Basic Membership

  • £35 per annum
  • All meetings except lectures (lectures can be attended at £7 per lecture)

Student Membership

Only availble to full time students 

  • £35 per annum
  • All meetings including lectures

John Wall 1932 – 2018:

On Saturday the 27th of January 2018, the Society learnt of the death of Mr John Wall, an inspirational inventor who make telescopes out of household items and inventor of the Crayford Eyepiece Mount (referred to now as the Crayford Focuser), John was a member of the Society for many years and will be remembered for his laugh, inventive genius and great character. An obituary was posted in ‘The Times’ and the ‘BAA Journal’

Below is a copy of the obituary for the BAA Journal 

John Wall Obituary

Obituary as it appeared in the BAA Journal

BAA Summer Meeting 2012

This meeting was hosted by Crayford Manor House Astronomical Society, the weather wasn’t great but we did manage to get the Solar scopes out for a short while. The lectures were great.

Europe goes to Jupiter

Dr Chris Arridge
All about project JUICE an ESA mission to Jupiter


Dr Dan Andrews
All about missions to comets with some insight into the Beagle 2 mission

Application of penetrators for the Exploration of planets & the Moon

Dr Rob Gowen
Title says it all really

Differentiating between dead comets and Asteroids

Prof. Iwan Williams
All about categorisation of asteroids and coments and how/ if they can/should be classified differently

Working on Mars

Dr Craig Leff
All about how to manage robotic missions on Mars.
























BAA Exhibition Meeting 2010

Location: Greenwich, London, UK.

This meeting included a (Free) trip to the Planetarium organised by the SPA, and then the BAA exhibition meeting down the hill. At the meeting our founder, Gordon Taylor received an Award from the BAA.

Discovery of Asteroid 1997 WQ28 by Crayford Member

On the evening of 30th November 1997 we were obtaining CCD frames for the UKNova/ Supernova patrol using a 10″LX200, operating at f3.3 and StarlightXpress. At 21.30UT we imaged a suspect object near ngc765, a type SBb/Sc, 14th mag spiral galaxy in Aries. Estimating the magnitude at brighter than 16 we initially thought we might have discovered a second supernova as this was rather bright for a new asteroid and no known asteroid was in the region of the galaxy. We used the Sky v4 to control the LX200 and always load the 34000 minor planets before we start patrolling so are forewarned of any asteroid in the field. We also checked Megastar and the new service to check supernova suspects for minor planets at the Central Bureau for Astronomical Telegrams Web site:

A second image 25 minutes later clearly showed movement, ruling out a supernova. Ignoring our chagrin we took a final image at 22.28UT, measured the positions using astrometrica, a commercial astrometry software package and sent a report to Martin Mobberley, deputising for Guy Hurst who was away on business.

Asteroid 1997 WQ28 discovery image by Mark Armstrong

Asteroid 1997 WQ28 Follow-up image by Stephen Laurie

The skies remained cloudy until on 3rd December we imaged six overlapping fields twice, approximately one hour apart to try and recover the suspect. At this stage we had to estimate where we thought the suspect should be as we don’t have the software to calculate a rough ephemeris from initial observations. We picked up only one object showing movement and not appearing in the `Real Sky` field but were doubtful that it was the suspect as it appeared at least 0.5 magnitude fainter and CCD images taken near the ecliptic can often reveal faint asteroids. However we measured the new positions and reported to Guy Hurst.

Meanwhile on 2/3 December Stephen Laurie, having seen our original report but unable to respond immediately due to business, imaged an object in the approximate position. Using Computer-Aided Astrometry, another commercial software package, he imputed all the measured positions to date and concluded that they were all the same object!
Guy reported the observations to the Minor Planet Centre on 6th December and the asteroid received the official designation 1997 WQ28. We have now secured 28 observations over a 55 day arc and the object has still not been linked to a previously designated object. Our previous `discovery` 1997 DV was linked to 1990 QN5, discovered at Palomar!

Article originally by Mark Armstrong