Glossary and Useful Formulae
Magnification (Power) of telescope = Focal length of telescope / Focal length of eyepiece
Focal Ratio (f/stop) = Focal length of telescope / Aperture of telescope mirror or lens
A “fast” speed (f/11, f/12 – f/15) is good for high power observing (moon, planets, splitting doubles)
A “slow” speed (f/4, f/5) is good for low power, wide-field observing (star fields, Andromeda nebula)
A “medium” speed (f/6 – f/10) is good for both types of observing
Actual or True Field of View – How much of the sky the eyepiece can see when in the telescope = Apparent Field of View / Magnification of Eyepiece
Apparent Field of View – How much of the sky the eyepiece can see before it is fitted to the telescope
Exit Pupil (the narrowest point of the cone of light leaving your telescope) = Diameter of Primary Mirror / Magnification
The exit pupil should match or be less than the size of your pupil at night. For a 30-year old this is typically 7mm, reducing by 1mm per decade, so for a 60-year old, it will be 4mm. All figures are averages
Focal Length of Eyepiece – The distance between the lens and point of focus (focal plane).
Eye Relief – How far your eye should be from the eyepiece to see all the field of view
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- Stellarium and SkEye are VERY impressive but I still love my 1971 Norton Atlas
- An Avon toiletries bag is a perfect place to hold eyepieces, lens caps and tools
- Feet are colder, backs are stiffer and bending over is harder in 2020 than in 1972
- Yoga teachers’ recommend a viewfinder without a right angle eye-piece
- A height adjustable chair is an essential accessory.
- Put thick walking socks on over a thinner pair at the start of observation periods
- Walking boots are better insulators than trainers and both transfer mud outdoors<>indoors very efficiently
- Just because you can’t see mud in the dark does not mean that it is not there when you go indoors
- Your partner may not wish to see Venus at 6.45am
- Just because SkEye tells you that the Spindle Galaxy is visible from your location does not mean that you can find it on your first date with Dobo.
- Just because you can find Alkaid, does not mean you can find the Whirlpool Galaxy
- Up means down and left means right makes pointing difficult.
- Sometimes just looking though the eyepiece at a load of dim stars is wonderful especially when you think what those little dots actually are, how old they are and how far away they are!
- Patience is a good thing – M51 will still be there tomorrow (weather permitting)
- Plan what you want to see at least the night before
- Just because you can see Andromeda my moving your telescope in a different part of the garden, does not mean that moving your telescope is a good thing to do.
- In terms of easily finding things, my binoculars are better than my telescope
- I need a finder scope on my finder scope!
- FLO are a nice company to deal with.
- You can find the ISS by “accident” without planning a view
- The amount of traffic on roads today compared to 45 years ago equates to the number of man-made objects that pass across your eyepiece
- The number of man-made objects that pass your eyepiece is alluring and frightening at the same time
- You do not need a hearing aid to Skywatch
- Learning star patterns (asterisms) helps you to find “difficult” objects
- Starting with M1 may not be the best / easiest way to start a Messier tick list
- 8” is the Dobsonian sweet spot
- Doctors should prescribe star gazing to insomniacs
- Holding a phone to an eyepiece to capture the moon is like balancing a pencil on its flat end
- Holding a phone to an eyepiece to capture a planet is like balancing a pencil on its pointy end