Syke wrote:
Lidl have binoculars from Monday next at £14.99.

These are zoom binoculars. There is no such thing as a good zoom
binocular for astronomy (or anything else, for that matter). There is a
very good reason that you don't find zoom binoculars with the names
Zeiss, Leica, or Swarovski on them.

Not only must the eyepieces zoom at as-near-as-dammit exactly the same
rate (which means absolutely no perceptible rocking of the bridge), but
you are trying to make a system with moveable optical elements that must
hold collimation, ideally to about an arcminute where step (aka
dipvergence, aka supravergence) is concerned if one is approaching ×24;
for the ×125 that I have seen advertised for some zoom binoculars, this
translates to better than 15 arcsec! Now, consider how many good quality
centre-focus 24× binoculars you know of -- I don't know of any, and I am
sure that part of the reason must be that it would be a feat of
technological brilliance (not to say expense!) to bridge two eyepieces
in such a way that they maintain collimation to within the tolerances
that are required. (And remember that it is unlikely that they will have
a "base tolerance" of zero error.)

Does this help?

Best,
Stephen

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