What is the shortest distance in front of a telescope that an object can be brought into focus? To what extent does the answer to this depend on:
- telescope focal-length
- telescope focal-ratio
- eyepiece focal-length ?
Assume that sufficient aperture to satisfactorily resolve the object is not an issue.
This question came up last weekend when I was exhibiting two of my telescopes in the North Carolina Science Museum's Astronomy Days event, my 90mm f/6.3 WO Megrez 90 refractor, and my 12" f/4.9 reflector. The event was indoors during daytime, so the only object available to view through the telescopes was an image of the moon hung on a balcony about 60 feet diagonally above our exhibit site. I could get the moon image to come to focus in the 90mm f/6.3 refractor using a 13T6 Nagler, but even with my lowest-power available eyepiece (31T5 Nagler) I could not come anywhere near close to bringing the 12" f/4.9 reflector into focus - I could only do so by holding the 31T5 in hand several inches outward beyond the farthest available out-focus in the focuser. In principle, the same problem would have occurred with my refractor, had the moon image been at a sufficiently close distance.
I ask this in the refractor forum because refractors are sometimes used for terrestrial viewing, where too-close proximity could potentially be a problem (at least without a sufficiently long focal-length eyepiece handy) e.g. bird-watching, whereas 12" reflectors are almost exclusively used for astronomical observing, where objects being too close to come into focus in any available eyepiece is obviously never any problem. And so, it's more likely someone here in refractors may have studied this issue.