Telescope Optical Parameters
Calculator v.3.1a

Figure 1. Screen shot ofTelescop.exe free for download.

For anyone desiring a simple utility to calculate various telescope characteristics, the Telescop program is offered free for download , and is hereby released as-is to the Public Domain, without any warranty, guarantee or claim of suitability for any specific purpose. The only document of instruction is this webpage which may be freely copied or saved for future reference.

New Features

Comma Decimal Separator Support - While the period decimal point is the default setting, a checkbox on the form will change the numeric decimal separator back and forth between comma and period. Editable Eyepiece Chart - Focal Lengths and Apparent Field of View may be modified by user. Image Scale - photographic guideline. Given in Degrees per Inch. Minor Improvements - cursor roll-over hints, dynamic sizing of Performance and Eyepiece grids for various screen and window font sizes.

Instructions for Use

(New) If your country's standard decimal separator is a comma (,), check the box labeled Decimal Separator: Use COMMA. Otherwise the default decimal separator will remain a period (.). Enter the diameter of the telescope's objective (in inches) into the box labeled Diameter of Primary (in). Then, in the box labeled Focal Ratio enter the telescope'sf/ -number (the quotient of the objective's focal length, divided by its diameter.) (New) You may edit theEyepiece Chart's settings for Focal Length (F.L.)(in millimeters) and Apparent Field (A.F.)(in degrees) to match specific eyepieces. Click on the Calculate button, . . . the results will appear in the appropriate fields below.

Theoretical Performance

# eyes - (Light Grasp, in terms of equivalent Human Eyes) surface area of objective divided by area of 5mm pupil (iris opening) of a typical human eye. Magnitude - (Faintest Star) limiting magnitude of dark adapted eye under very dark, clear skies, using an unobstructed objective of the given size. Assumes unaided eye can perceive Mag 6.2 stars. Res (arcseconds) - (Resoving Power, Angular) Dawes Limit theoretical resolving power of diffraction limited, unobstructed objective. Low Useful X - (Lowest Useful Magnification) Determined by power at which the exiting light cone will just fit in a fully dialated 7mm pupil. As we get older, pupils cannot open as wide as when we are young. By age fifty or so, the average pupil diameter is about 5mm. Light falling outside the pupil is not seen, and is thus essentially wasted. Low Pract. X - (Lowest Practical> Magnification) Determined by power at which the exiting light cone will just fit in a normally dialated 5mm pupil. Hi Pract. X - (Highest Practical Magnification) Magnification at which unavoidable diffraction effects begin to be apparent, limiting further resolution. Roughly 35X per inch of objective aperature. Hi Useful X - (Highest Practical Magnification) Magnification at which Airy disc is obvious. Further magnification produces no additional information or benefit. Useful only for very close double stars. 60X per inch of objective aperature. Image Scale - The angular-to-linear conversion factor for the image at the (Effective) Prime Focus. Useful to astro-photographers, multiply this factor by the linear dimension of the imaging device (CCD, film-frame, etc.) to calculate how much sky (angle) is covered. (This is a fairly good approximation in most cases.)

Most of these values should be understood to apply only to a seasoned observer, using very good optics, observing in ideal conditions. In the real world, observing is limited by atmospheric instability, thermal differentials in and near the optics, observer experience, light pollution, etc. Please refer to telescope usage books (or similar reference material) if you are unsure what these terms mean or how they are used.

Eyepiece Chart

This section provides typical data for an array of common eyepieces. Note that now provision is made for modifying the list of eyepieces, their focal lengths and apparent fields of view. F.L.(mm) - (editable) focal length of eyepiece, in millimeters. A.F.(deg) - (editable) Apparent Field of view, measured in degrees. The angle of view the eye sees when presented with the magnified True Field. Mag (X) - Magnification power. Calculated by: Objective Focal Length divided by Eyepiece Focal Length. Usually a very good approximation of angular magnification. T.F.(deg) - True Field. The amount of actual sky which can be seen at any one time, though the eyepiece. Much smaller than Apparent Field.(For example, to be seen in its entirety, the Full Moon requires a true field of just over one-half degree.) Brightness - Relative Image Brightness of the magnified view. While the image is made brighter by the increased light gathering power of the objective, magnification makes it dimmer by the spreading the light across a larger area. The values stated are relative to the light-grasp of an unaided human eye with a 5mm pupil opening. Any value greater than 1.00 (the maximum perceivable relative brightness) is meaningful only for photographic applications. Exit Pupil - the diameter of the light-cone exiting the eyepiece. Measured in millimeters.

Download & Setup

Download - Click Here! Save the downloaded program to the desired destination folder for execution. No Installer is required. The program is stand-alone, and does not need VBRUNxxx.dll or VCL*.* support files. If the program is helpful, you may wish to create shortcuts to in for your desktop or Start Program menu. Filesize = 463,360 bytes. File Date: Saturday, April 02, 2005, 9:49:18 PM Requirements: Windows 95 or better, 16 MB RAM ?, VGA Monitor Instructions / Help File: This webpage!

Notes

Print- button has been replace by the Decimal Separator: Use COMMA checkbox. This Program was written in, and revised using, Borland C++Builder (v5.0). No further updates are planned at this time.

This site was last updated on Saturday, January 21, 2006.

Copyright 2006, Larry Gilstrap
 

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