A dynameter is a measuring device that is used to measure the magnification power of a telescope where the eyepiece is attached, which varies somewhat from the actual magnification level of the lens itself. It is usually a handheld or homemade device that resembles a small tubular eyepiece, and is frequently used to determine the power of an old telescope where no manual or model number exists or if the inscribed focal length is not present on the body of the instrument. The eyepiece focal length (EFL), or exit pupil (EP) focal length, usually only affects magnification values by a factor of 2% to 3% from the actual magnification level of the telescope lens, but estimates on older telescopes can vary up to 20% from what the true focal length actually is. This variation can be significant when observing and reporting astronomical sightings using such instruments.
Strictly speaking, a dynameter is one of a wide variety of telescope instruments that has very narrow uses. It is designed to measure three different parameters in tandem. These are the diameter of the exit pupil, or EP; the distance between the physical eyepiece on the instrument and the EP; and the magnification level of the instrument in general. The dynameter does this by its design being built upon the principle of the double-image micrometer, which essentially makes it a miniature compound microscope that compares the image formed on the eyepiece with the size of the image produced by the lens.
When optical devices like telescopes, microscopes, or cameras receive a light image that passes through a lens, this image must then be focused on an eyepiece or recording medium like film that is some distance from the lens itself. This is known as the focal length of the lens, and the lens itself is often referred to as the object glass, object lens, or objective lens. Measuring instruments like dynameters take these two quantities of the produced lens image and the appearance of the image on the eyepiece, and use them to determine focal length and the magnification power of the instrument.
In past telescope design, focal length values were written on the eyepiece based on values derived from similarly-manufactured eyepieces and not actual measurement observations using a dynameter. This could result in the magnification power of the instrument being overstated, such as being given a magnification power of 294 when the actual magnification was 250. While this is not a major problem for casual observations of stellar phenomenon, when recording events for scientific study, it can lead to a significant error in data analysis.
A dynameter can perform a simple magnification calculation by dividing the value of the entrance pupil (EN) by that of the EP. The entrance pupil is defined as the image that enters the aperture of the instrument and impacts on the objective lens. The formula used in a dynameter, therefore, is M = EN/EP and an example magnification (M) would be 200 millimeters divided by 5 millimeters for a M value of 40x.