Purpose:

This calculator estimates the true magnitude of a star at which the theoretical reduction in visual acuity between two unequal doubles will begin to be seen. This calculator intended for experimental use by amateurs to probe and define the scotopic visual acuity zone boundary by providing a theoretical estimate of where the boundary is for a particular observing session. Plot the apparent magnitude of the scotopic vsion boundary star and the apparent sky brigthness in the eyepiece on the visual acuity surface.

Explanatory figures:

Explanation of Parameters

Telescope Aperture

The diameter of the objective lens or mirror in inches. (This is the most important determining parameter)

Power

The magnifying power of the telescope. This changes with the focal length of the eyepiece you use.

Power = (focal length of the objective)/(focal length of the eyepiece)

Visual Limiting Magnitude

This is the magnitude of the dimmest star you can see near the zenith.

Use the link to vislimit.html page to calculate this for you OR:
See the section called Sky Transparency in "The Observer's Handbook" published by the Royal Astronomical Society of Canada for a guide to determining this using the stars around Polaris.

Type of telescope

Reflector has a mirror as its objective 

Refractor has a lens as its objective
The Schmidt-Cassegrain has a lens and a mirror

Seeing Disk in arc seconds.

The diameter of the image of a star. This varies on the turbulence of the atmosphere and can vary from 0.5 to several seconds of arc. You have to judge this at the telescope.

MPSAS - Magnitudes per Square Arc Second

An alternative measurement of sky brightness. Magnitude of a stellar point spread over one square arcsecond.

Schaefer, B.E. Feb. 1990. Telescopic Limiting Magnitude. PASP 102:212-229 http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1990PASP..102..212S

Prepared K. Fisher fisherka@csolutions.net rev. 3/17/2008B