Getting in Gear

With so many large telescopes now in amateur hands, owners of small instruments sometimes feel left out of the action. When it comes to accurate double-star measurement, however, optical quality is far more important than aperture. To illustrate this point, I recently conducted a visual program with a 3.5-inch Questar. My results show that observers with high-quality, solidly mounted telescopes in the 3- to 4-inch range are capable of accurately measuring double stars. Observers with larger instruments can expect even better results.

Apart from a good telescope on a solid motorized mount, you need one other piece of equipment to measure double stars: a micrometer. Although a variety of measuring devices have been used by amateurs, the reticle-type micrometer is the easiest. One of the very best is the Celestron Microguide eyepiece. The eyepiece itself is a 12.5-mm Orthoscopic of the classic Abbe design with a diopter adjustment at the eyelens and a battery-powered LED illuminator. The only modification necessary for double-star work is the addition of a position-angle circle and a pointer. You can also make your own reticle micrometer from an existing eyepiece and a reticle available through Edmund Optics.

A Barlow lens will be needed to change the telescope's magnification. In fact, several Barlows, each with a different amplification factor, will give a wide range of magnifications.

Calibration Matters

Before you can begin to measure double stars with the reticle eyepiece, its linear scale must be calibrated with great accuracy. This is a one-time procedure that need not be repeated so long as you continue to use the same configuration of eyepiece, Barlow lens, and telescope. To accomplish this calibration, four to six reference doubles must be selected from the list of stable pairs on the next page. These are stars that have not changed separation since they were first observed.

To begin the calibration, simply note the number of divisions on the reticle's scale separating the primary and secondary of each calibration star. For example, if the components of the lovely double star Albireo, which are 34.5 arcseconds apart, span 10.2 divisions, then the scale of the reticle is 3.38 arcseconds per division (34.5 divided by 10.2). A minimum of four different calibration pairs should be used and the results averaged. To reduce the effects of differential refraction, these stars should be measured only when they are 30° or higher above the horizon. Don't forget to repeat this procedure for whatever Barlow lens is added to the system.

When I calibrated my micrometer with the Questar, I took a large number of measurements of each star and then averaged them. Remember, work carefully and take your time; your calibration can never be too accurate! The Celestron reticle micrometer fitted to my Questar yields about 100x and gives a calibration value of 16.093 arcseconds per division — useful for very wide pairs. Engaging the Questar's 2x internal Barlow lens produces a magnification of 200x and a calibration value of 8.062 arcseconds per division. For the closest pairs I use the Questar's internal Barlow and a Celestron Shorty 2x Barlow, for a magnification of 487x and a reticle scale of 3.389 arcseconds per division. Although this much magnifying power might seem excessive, I use it for most of my double-star work.