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Telescope Formulas and Design Comparator

This page lists various telescope related formulas and it calculates telescope & mirror parameters for you. It is primarily oriented to the Newtonian reflecting telescope, which is what most first time telescope makers build, although many sections of it are applicable to other types of telescopes. Since many telescope makers want to compare two designs in order to decide on the right telescope for them, this calculator computes parameters for two telescopes at once and also calculates the differences between them.

You don't need to use the calculation feature - just scroll down the output table to find the formula you need. All variables used are always defined in rows nearer the top of the table.

If you have measured a Sagitta (depth of Curve) and want to calculate Focal Length, use our Sagitta to Focal Length Calculator.

User Inputs:

Parameter Scope #1 Scope #2 Units Typical Values
Objective Diameter (D): 50 mm to 20+ in
Objective Focal Ratio (f): (none) 4 to 12
Eyepiece Focal Length (Fe): mm 6 mm to 50 mm
Eyepiece Apparent Field (AF): deg 30 to 82 deg
Observer's Fully Dilated Pupil Size (Deye): mm 7 mm average
5 mm if older


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Page last revised on Thursday, 2017-Sep-07 13:31:13 -0400