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Home > Filters > Altair Moon Neutral Density Filter 1.25" (25% light reduction)

Altair Moon Neutral Density Filter 1.25" (25% light reduction)

Altair Moon Neutral Density Filter 1.25" (25% light reduction)
Price: £15.00
Incl. VAT 20% ( £2.50 )
Product code: ND25-125
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The premium neutral density moon filter is a standard accessory for lunar observing, which blocks 25% of the light passing through it, allowing 75% of the light to pass through to the eyepiece.
This is a high quality unit with plane polished optical glass surfaces, and a very even, neutral light transmission with no influence on colour of the image. It should not be confused with cheaper green filters.
Premium Moon Filter Specifications:
  • 75% transmission (cuts out 25%), ideal for viewing the moon with refracting telescopes of  100mm 4" aperture or smaller, and reflecting telescopes of 5" 127mm or smaller.
  • Reduces glare to improve contrast and bring out more fine details, such as smaller craters, ray patterns and "Lunar Seas" or "Mare".
  • Incorporates quality optical glass with flat polished parallel surfaces, accurately installed in a CNC machined cell.
  • Neutral grey - does not affect colour like cheaper green filters.
  • Threaded to fit standard 1.25" astro eyepieces (M28x0.5mm thread). Includes plastic case.
The machining is very precise, and the filter is held nicely orthogonal to it's housing, which is threaded to fit standard 1.25" eyepieces, Barlow lenses and 1.25" diagonal mirror barrels. Most astronomical accessories have this thread on the base, including webcam adapters. 
Why use a moon filter? Telescopes (even the smallest ones) gather so much light, that the moon is sometimes just too bright for comfortable observing. Fine details like lunar seas get washed out in the glare. This premium quality filter removes excess glare from lunar images, bringing out finer detail. It's threaded to fit into the base of standard 1.25" barrel astronomy eyepieces. (M28x0.5 thread). 
What type of filter is suitable for my telescope? Many experienced lunar observers keep both types of filter, swapping them depending on the lunar phase and brightness as well as the magnification being used. As the objective lens or mirror aperture increases in size, so does the brightness of the image. As a general rule, the 25% light reduction filter (code: ND25-125) is preferred for refracting (lens based) telescopes of 4"/100mm or smaller, and reflectors of 5"/127mm or smaller. For refracting telescopes (lens based) larger than 4"/100mm and reflectors of 6"/150mm or larger, the stronger 50% transmission filter (code ND50-125) is preferred.

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