[astro744]

Try the following all with your arm extended as far as possible (arms length).

Fingers spread as much as possible gives you a span of approx. 20 degrees.

Fist clenched gives a span of approx. 10 degrees.

The width of your fingernail at arms length is approx. 1 degree.

The Moon is 1/2 degree.


To work out what your telescope shows use either one of the following formulae:

True Field of View (TFOV) = Apparent Field of View (AFOV) divided by magnification.

OR. TFOV = Field stop diamater (FSD) of eyepiece x 57.3/focal length of telescope. This is more accurate but you need the field stop diameter available from most manufacturers or measured if you can see the field stop at the bottom of the eyepiece.

Don't worry about the accuracy too much and use the first formula as a guide and simple calculation.

eg. telescope = 114mm f7.9, f.l. = 900mm

A 32mm eyepiece will give a magnification of 900/32 = 28x
A 32mm (1.25") eyepiece generally has a AFOV of 50degree.
Therefore TFOV = 50/28 = 1.8degree.

OR. A 32mm eyepiece generally has a FSD of 27mm. Therefore TFOV = 27 x 57.3/900 = 1.7degree.

Eyepiece distortion can mean the figures are slightly different but either formula will give you a guide to the TFOV you are seeing.

A red dot finder is a good idea and if you get the Telrad you also get 0.5, 2 & 4 degree red circles as you look throough the Telrad which helps in seeing TFOV against the sky.