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Lunar observing is very popular amongst a large number of amateur astronomers. For some, it has been the Moon that has been the original source of astronomical curiosity and fascination that has led them to further develop their interest of "more things astronomical".
The Moon has a huge number of craters formed from meteorite/asteroid impacts, mountains, valleys, and seas from volcanic eruptions and lava flows long ago. These wide ranging Lunar features can be quite mesmerising to observe or image. However, depending on its phase, the Moon can be quite bright to look at, and therefore observing the Moon, like it is at present, can therefore be uncomfortable and can also mean that fine Lunar details can get "washed out".
Baader Planetarium offer a range of neutral density filters to help cut down the brightness/glare and improve contrast whilst Lunar observing, making observing the Moon a much more pleasurable experience.
Baader offer four different optical densities (transmissions) of their neutral density filter. The higher the ND-number the darker the filter appears, and less of the light in transmitted. The four optical densities available are listed below in order of decreasing transmission.
For Lunar observing, which ND filter to use would depend on the phase of the Moon, telescope aperture and your own personal preference. For example you could use the ND0.6 which has a 25% transmission for around the first/last quarter phases of the Moon, with the ND1.8 or even ND3.0 for viewing the Moon when full.
The glass used in each filter is finely and accurately optically polished so they can be used at high magnification without any loss in sharpness. Each filter also has Baader's Phantom Group coatings applied, which results in no ghosting or reflections.
Although Lunar observing is often done at high power, which usually means the use of 1.25" barrel eyepieces, each of the above four filter optical density options are available in both 1.25" and 2" sizes, so they will fit any of your eyepieces. To use them, you simply screw a filter into the barrel of your eyepiece and then insert the eyepiece into your telescope. Alternatively you can mount them in a nose-piece of a diagonal or in a filterwheel.
Above: Image showing 1.25" and 2" Baader Neutral Density Filters (not to scale)
The filter housings are threaded on both sides. This allows other filters, such as the Baader coloured filters, to be combined (stacked) with the neutral density filter, for planetary observing (more about this in a future post).
In addition to Lunar and planetary observing, the Baader neutral density filters can be used for solar observation too - but *only* when used with an objective safety solar filter such as Baader Astro Solar Film or Baader ready-made Astro Solar Filters (which use the Baader Astro Solar film). The neutral density filters can also be used with the Baader Solar Herschel Wedge too.
Each of the filters comes in a sturdy, strong plastic storage case with an internal filter drawer that slides in/out. The Baader filter cases can be stacked one on top of another, or joined laterally/horizontally, to create a sturdy multiple filter "storage unit".
Above: Baader Storage Filter Box for one of the Neutral Density Filters.
The Baader Neutral Density filters are available from your local or favourite astronomy retailer.