INTRODUCTION
Long time interested by the visual observation of the double stars, I
came naturally to try to measure them. From the chronometric method to
various reticulated systems, then from the Hertzsprung-Duruy's grid
to the filar micrometer, the experience was rich of teachings. While the
grid comes quickly to its limits with amateur's instrument, the micrometer
remained the absolute weapon. The results are worthy of the pain and energy
but it's not so easy ! Tiredness due to the attention, permanent uncertainty
of the moment you can read the indexes, quasi-necessity of a permanent
installation of the telescope are as many elements that don't encourage
the amateur to practice measurements. Remains the electronic imagery.
By the end of 2000, a CCD camera fixed on a
professional instrument is the privilege of some, and those who can make
measurements with their own instrument are just as rare. Since several
months I followed by curiosity the incredible progress of the webcams in planetary imagery. Their
reputation of little sensitive instruments braked their use in the stellar domain. Owning an old computer,
I however decided to acquire one of these cheap camera. Finally,
in January 2001 I installed a webcam on the telescope telling me that
in the absence of results on the double stars I could come back to the
micrometer.
INSTRUMENTATION
a) Telescope and Optical Settings
All the evaluation course was led on the same instrument. A homemade
8" Newtonian telescope. Numerous combinations have been tested, from the primary focus to diverse
optical trains involving one or two barlows lens. The longuest focal was
about 6 meters.
b) Webcams
The webcams evaluated were Logitech Quickcam VC, Philips Vesta Pro et
Toucam in commercial versions. Apart their intrinsic performances, all models were
very near in terms of quality of measurements so the conclusion applies
to all. The Vesta Pro was later used with modified electronics giving long exposure
times (
mod by Steve Chambers) and finally suited with a black and
white sensor. This last configuration permits high performance on low
brightness double stars.
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