Observing
The Moon is bright enough to ignore light pollution, large enough to find without a chart, and detailed enough to reward any aperture. It is also the object most often looked at on the wrong night, which is the one it is fullest.
The line between lit and unlit surface throws long shadows, and shadow is what makes a crater look like a crater. It moves across the disc through the month, so the same feature is dramatic one night and invisible a week later.
A thin crescent low in the evening sky shows the same detail as a half Moon, in a smaller strip. Full Moon is the one phase where the shadows are gone and the view flattens.
In a 200 mm telescope the Moon is uncomfortably bright, and looking at it costs the dark adaptation built for everything else. A filter, a smaller aperture or an earlier phase all solve it.
The Moon rewards high power more reliably than anything else, and the ceiling is the atmosphere rather than the telescope. A Barlow is the cheapest way to reach that ceiling with eyepieces already owned.
Variable 1–40 % transmission · 1.25 in · stackable
A two-element polarising filter for 1.25 in eyepieces with transmission adjustable from 1 to 40 per cent by rotating its lower ring.
Best for: The Moon across its whole cycle, where a fixed filter is right at only one phase
≈ $43 / £42 / €38
1.25 in thread · plane-parallel glass · no transmission published
A plane-parallel Moon filter that screws into 1.25 in eyepieces, sold on the quality of its substrate rather than on a stated transmission.
Best for: A 1.25 in eyepiece case where substrate quality matters more than a published figure
≈ $54 / £42 / €45
8 mm focal length · apparent field stated two ways · four elements
A four-element fully multi-coated Plössl of 8 mm focal length in a metal 1.25 in barrel, with a fold-down rubber eyecup and a filter thread.
Best for: A step up from a supplied short eyepiece, on a case-based collection
≈ $17
0.5x focal reducer · 1.25 in · sold under the name “Barlow”
A 0.5x focal reducer in an anodised aluminium 1.25 in barrel, sold under the name “Barlow lens” while its own description correctly calls it a reducer.
Best for: Widening the field on a long-focal-length telescope, or shortening exposures on a camera
≈ €13
80 mm Maksutov · T-2 thread and dovetail
A compact 80 mm Maksutov with a coated front lens, a T-2 thread with buffer ring, a finder shoe and a GP dovetail that fits photo tripods and standard mounts.
Best for: A travel optic that doubles as a long lens on a photo tripod
≈ €166
Because the Sun is behind the observer and nothing casts a shadow. Craters that stand out sharply near first quarter become flat bright patches. The interesting nights are the ones either side of half.
A bright Moon in a large telescope is uncomfortable and destroys dark adaptation. A filter dims it; so does a smaller aperture, and so does waiting for a thinner phase.
Whatever the air will support. On a steady night a telescope will take its full useful magnification on the Moon; on an unsteady one the image boils above a fraction of that.
Any figure written into the body of this page was true on the day it was written. The amount shown next to each instrument is requested from the retailer as the page loads, and it is that amount, not the text, that stands.
Telescopes and mounts are discounted over and over through the year — hardest of all in the weeks before Christmas, which is exactly when most of them are bought — and a struck-through figure on its own says nothing whatever about what the same model was charged at four weeks ago. UniverTrack keeps a week-by-week record for each retailer, so a discount is measured against a figure somebody genuinely paid instead of one invented for the sale — and it says when the moment to buy has genuinely arrived.
