Sharing tips and techniques to help amateur astronomers get the most out of light polluted skies.

Some Thoughts About Filters

All kinds of filters

There are basically two kinds of astronomical filters: color filters primarily intended for use on planets (although they have some other uses), and used primarily to enhance contrast on certain kinds of nebulas. Nebula filters can help in seeing through light pollution. Color filters do not.

But it isn't quite as simple as that....


Edmund 4.25" Palomar Jr. Newtonian

Image of Edmund Palomar Jr focuser and finderMy oldest scope is an Edmund Palomar Jr (Pal Jr). It’s a 4.25 inch (108mm) Newtonian reflector on a German equatorial mount. It’s been in regular use ever since I got it in 1973. These days I use it primarily for solar observing using a Baader AstroSolar Safety Film aperture filter. I also use it to compare views with my Zhumell 10 inch (250mm) Dobsonian (Z-10) and to verify what a smaller aperture can show under washed-out skies.

The optics on the Pal Jr are surprisingly good, consistently showing picture-perfect diffraction patterns. It gives great views of the planets and splits η Orionis whenever the seeing is decent. In dark skies, it’s shown me most of the Messier objects and plenty of NGCs. In washed-out skies it still gives good views of double stars, open clusters, the brighter globular clusters (without any hints of resolution), and the brighter nebulas (M27, M42, M57), but apart from M31 it is exceptionally hard to see any galaxies with it. I’ve made a point of including observations with the Pal Jr throughout Washed-out Astronomy to balance the Z-10 view with a smaller aperture view.

The most interesting thing about the Pal Jr is that it was hit by lightening some years ago….



Pluto reaches somewhere between magnitude 13.8 and 14.1 at opposition making it a rather challenging object to see from washed-out skies. But Pluto’s tiny size helps us pull it out of the light pollution: because it is star-like, we can use high magnification to improve the contrast. Even from my light polluted front yard, I’ve seen stars of this magnitude with my 10 inch Dob using moderate to high magnification (150x to 250x). So in theory, Pluto is within reach.

Unfortunately, Pluto is currently located in Sagittarius. This is not good. That puts Pluto low on the horizon (and I have a particularly cluttered southern horizon). It also brings Pluto into view during the hazy months of summer, when transparency in the Washington DC area is consistently poor. Even worse, it puts Pluto right in the middle of the Milky Way, with lots of other stars of similar brightness. if you think finding a needle in a haystack is hard, try hunting for a particular 14th magnitude star in the middle of the Milky Way....


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