The acquisition of a telescope is a personal thing. Indeed, though some may deny it, we can become very attached (even fondly) to the instruments that give us such a thrill observing the Universe. It can come very hard, then, for us to question their quality. It might even seem a betrayal of friendship, that we would look into their very bowels to examine their integrity.
But before we get carried away in a wash of sentimentality, let us not lose sight of the fact that we all spend a great deal of money on observing equipment. Being able to challenge the claimed quality of our purchases is essential. Although it might be a brave observer who dares to find out if poor images are the result of optical quality, not the ‘seeing’, that knowledge is essential for assessment of the observations themselves.
A star test — the live assessment of a star’s focused and extra-focal images — is a definitive and exacting exposure of a telescope’s optical quality. But in order for it to reveal the finest details of that quality, the Earth’s turbulent atmosphere has to co-operate by providing steady untroubled air through which the light is passing. (So, little hope there!)
A Ronchi eyepiece can provide information only approaching that of a star test, but is not so dependent on atmospheric stability. The device turns a star’s light into a series of lines that demonstrate the presence of errors of form in the optics encountered in the optical train. This new version employs a chrome-on-glass grating of 10 lines per millimetre, providing a high-contrast display of starlight that enables good analysis. A manual is provided that explains what can be seen and the conclusions that may be drawn. (A photographic version is also available.)
I wonder which telescopes will view the approach of this eyepiece as a chance to shine, or shrink back with the fear of exposure?
Price: £32 (visual) £39 (photo)
For more information: www.gerdneumann.net
