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Baader Moon and Skyglow Filter

14 September 2001

- 15 April 2003 Update-
Baader has release another similar filter, the Contrast-Booster.  The Contrast-Booster has many of the same characteristics as the Moon and Skyglow filter, but it is a bit more aggressive.  In other words, it provides slightly higher improvement in contrast at the expense of a slight discoloring of the view.  It is available in North America from Celestron dealers or from Alpine Astro ( or in Europe directly from Baader (

The Baader Moon and Skyglow Filter is not a standard Moon filter.  This filter was designed with Neodymium for it's inherent light spectrum characteristics.  This spectrum, shown below, passes most light while blocking light from 565 to 600 nanometers - the wavelength of many street lights, the source of much sky glow.  Additionally the filter blocks most light below 375 nanometers - deep violet in color.  This makes the filter ideal for color correction on short-tube achromatic refractors like the NexStar 80.  {Update - at the end of 2003, Baader introduced a true minus-violet filter - the Fringe Killer - read my review here.}   Additionally it should be noted that this is a precision ground, fully coated optical filter - as are all the Baader filters.

My first look through the Baader Skyglow filter showed that it definitely reduces sky glow in my suburban skies.  But the question was whether it made deep-sky objects too faint to detect.  In my 80mm scope, I could still easily see globular clusters such as M22 and M13, but not with any additional detail.  It did not enable me to see objects that I could not see without the filter - such as the Trifid and Lagoon nebula.   It seems that it decreased the brightness of planetary nebula such as M27 and M57 such that better detail was visible without the filter.  Bottom line, in an 80mm refractor, with it's limited light-gathering capability, the Baader Skyglow filter doesn't seems to help much for deep-sky objects.

Regarding color correction, here the Baader Skyglow filter really shows its stuff.   On bright objects like the planets and the Moon, achromatic refractors are plagued by false color, particularly around the edges of the objects.  This is especially true with short-tube refractors.  The Baader filter reduces color aberrations dramatically and really improves the view of the moon and bright planets.  This is much improved over using yellow or blue filters as the Baader filter adds no noticeable color of its own.  For color correction, I can highly recommend this very economical filter.

At high magnification on the Moon (100-200x), I prefer the Baader filter over traditional Moon filters as it shows noticeably more lunar detail.

The Baader filter also improves contrast and detail on Jupiter and Mars. I did not have the filter in time to test it on Mars, other than the noted decrease in false color.   This filter is by far my favorite choice for pulling out subtle detail on Jupiter - both in my NexStar 11 and NexStar 80.

At such a reasonable cost, in larger aperture scopes (say 120mm or larger), I would think that anyone in urban or suburban skies might like to thread this filter in the front of their diagonal and leave it there except for trips to a dark-sky site.   Particularly if your larger scope is an achromatic refractor.

The Baader Moon and Skyglow filter is available in 1.25" ($40) and 2" ($65) sizes from  Alpine Astro (, Astro-Physics ( or in Europe directly from Baader (

Join the Baader Planetarium discussion group on Yahoo Groups:

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Michael Swanson
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