There are several techniques to discover new double stars for instance speckle interferometry involving large telescopes and special equipment.

Nevertheless, amateur observers with relatively small instruments are able to discover new double stars far beyond the theoretical resolution of their telescopes. How is this posible ? Well you can simply use the moon or other objects in our solar system like asteroids to realize the extra resolution.

The moon moves with a speed of roughly 0.5 arcseconds per second relative to the star background seen from the Earth. Visual observations of occultations of stars by the moon can be timed with an accuracy of 0.1 seconds resulting in a resolution of 0.05 arcseconds. When video records are made of the occultations this resolution can be even 3 times better. This is far better than the largest telescopes can realise in direct measurements of the star, especially when components are quite different in brightness. This means amateur atronomers can make valuable contributions in the field of double stars.

Every now and then during an occultation the light of the star seems to fade or disappear/reappear in clear steps. We call this gradual and step events respectively. In case of a gradual event the observer estimates the duration of the gradual event. In case of a step event the time difference is either measured by stopwatch or estimated and the brightness of the component in relation to the total brightness of the pair can be estimated as well. Do not worry about the accuracy of such estimates. Any estimate is better than no estimate. The estimated values are reported as remarks in the ILOC report form. Such forms can be generated with most prediction programs like WinOccult.

Video observations of occultations can be analysed using LiMovie. This program produces the lightvalues of an object for each frame. This produces graphs like the one included here which was made by D. Gault from Australia. In this case a clear step event is visible. Both the separation and the magnitudes of both components can be measured from the graph. If you need any help with this you can consult me.

[Video contains 25 or 30 frames per second. A frame consists of two interlaced fields, so video contains 50 or 60 fields per second. This rate depends on the type of video recording and is partly determined by the frequency of the electricity network in the country of origin, 50 Hz for PAL and SECAM, or 60 Hz for NTSC.]

Reports of gradual or step events in ILOC format can be sent to me. Each year around August/September a new update is made of the double star files, which are used in prediction programs like WinOccult. These updates are published in Occultation Newsletter, a quarterly of IOTA.

Not only new discoveries can be reported, also observations of already known doubles can be reported in the same way. Every now and then you can observe a known double or a recently discovered double. Even if no fades or steps are observed it is important that you report this to me. If two independent observers observe no double star effects for a known double the star can be declared single. This is important because especially in the past, when only visual observations were made, quite a number of "suspected doubles" were identified. Some of these observations are the result of bad seeing, bright dark limb observations or even bright limb observations. In such cases the star seems to diminish in brightness just before disappearing or seems to fade. To rule out such mistakes from the past negative observations of known doubles are important.

XZDoubles.dat (format: see XZ80Q documentation chapter 21) XZDoubles Discoveries.dat (format: see XZ80Q documentation chapter 26) XZConfirmations.dat contains OCC/XZ star numbers, date of observation, observer that confirmed duplicity and the ON reference of publication

In XZDoubles/XZDoubles Discoveries each newly discovered double star gets an OCC number. In XZConfirmations the first 4 observers that confirm the duplicity of a star with OCC number are mentioned. All three files are used in the WinOccult prediction program. If you download them copy them to the StarCats directory of WinOccult and use option 6 in the StarCats menu to activate them.

Henk J.J. Bulder (hjjbulder@scarlet.nl)