Agena
Variable Polarizing Moon Filter 1.25-inch
05The numbers
What These Numbers Mean
TransmissionVariable
Adjustable, roughly 1 to 40 percent
Two polarising elements rotate against each other, so you dial in exactly as much dimming as the phase and your aperture need. A thin crescent and a full Moon want very different amounts of light held back.
What changesComfort
Glare, not resolution
It will not show you more detail than the telescope can resolve. It stops the glare that makes your eye close down, which is what lets subtle contrast in the maria and along the terminator become visible.
Also useful onPlanetary
Venus and Jupiter
Venus is bright enough to be uncomfortable in any decent telescope, and taking the edge off makes phase detail easier to see. It helps on Jupiter in larger apertures for the same reason.
Barrel sizeStandard
1.25 inch
Threads onto the standard eyepiece barrel, so it works on essentially any telescope. Note that adjusting it means rotating the eyepiece or the filter, which takes a moment at the eyepiece.
Trade-offOptical
Two glass elements
More glass in the light path than a single neutral density filter, so on very demanding high-power planetary work a fixed neutral filter is marginally cleaner. For lunar use the flexibility is worth more.