Eyepiece Filters
Published on Dec 31, 2006 at 8:54 pm.
3 Comments.
Filed under amateur astronomy, telescopes.
Once you get comfortable with your telescope, you might want to consider buying filters for your eyepieces. As I said in my eyepiece post a couple of days ago, telescope eyepieces come in standard diameters. For at least half a century, they have also had standard thread in them, allowing threaded filters to be screwed into them. But, why would you want filters?
One of the first filters that most people buy is a Moon filter. What you quickly discover when you look through a telescope at the Moon is that it is bright. In fact, the Moon can be rather uncomfortably bright through a telescope. Often mass market telescopes come with a Moon filter. The simplest Moon filter is a dark filter that dims the Moon. Better quality Moon filters are neutral density filters, meaning that they dim all colors of light equally, keeping the same color ballance in the dimmed image. Specialized Moon filters can be created by using two polarized filters at angles. The bigger the angle, the more light that they block. This is convenient because the different phases of the Moon are different brightnesses. But, personally, I like colored filters for the Moon.
As you can see in the photo above, I have several colored filters. These are good because they can tend to emphasize different features. For example, the Seas on the Moon tend to be slightly bluish. So, a red filter makes them even darker than it does the rest of the Moon. That improves contrast, making the Seas stand out. The other advantage of a red filter is that it makes the annoying bright Moon more managable. Dark yellow filters tend to make the rays on the craters easier to see.
I also like to use yellow filters for Jupiter and Saturn. They tend to bring out details a bit. Blue filters are also good with Jupiter, Saturn, and Mars. Blue filters make the bands on the planets stand out a bit better, and sometimes helps with the Martian ice caps. Blue and yellow filters are also sometimes good with comets. There are two types of tails on comets: dust tails, which are yellowish, and gas tails, which are bluish. Blue and yellow filters tend to make one or the other stand out a bit more. Green filters are often good for dust storms on Mars. Now, this is a general rule of thumb, and sometimes one filter works better one time, and another at another time. Also, sometimes you want a dark blue, and sometimes a light blue. Amateur astronomers often spend quite a lot of money on their filters! You can buy filter starter sets containing the most commonly used filters from any of the places that sell telescopes and eyepieces.
Besides the broad band color filters, other filters that are often used with eyepieces are specialty filters that block certain wavelengths of light, or transmit certain wavelengths. These are more advanced filters for most amateur astronomers. Such filters include filters that specifically block the most common light pollution in cities. These light pollution filters are often advertised as wonder filters that significantly help with the bright sky glow of the city. Personally, I have not been so impressed with them. They to tend to improve contrast with emission nebulae and spiral galaxies. But, they do little to help with elliptical galaxies and star clusters. They also don’t do much with planets, but the planets are so bright that light pollution isn’t such a factor anyway. Other filters specifically transmit the light of ionized oxygen, so they are really good for planetary nebulae. Some filters tend to cut down on the violet end of the spectrum, thus reducing some of the chromatic abberation common with refractors (see my telescope post). But, really, you should hold off buying these rather pricey specialty filters until you get really comfortable with your telescope.
The nice thing about these filters is that they come in standard sizes, so if they fit one 1.25 inch eyepiece, then they’ll fit all of them. You can find pretty much any filter that you want in an inch and a quarter size. But, there are also a lot of filters available for two inch diameter eyepieces. Just a very few years ago, you used to be able to get filters for 0.965 inch eyepieces, but I haven’t seen many good filters in that size sold for several years.
One filter that you should avoid, though, is a solar filter that screws into the eyepiece. There are separate solar filters that go on the front of the telescope, preventing light from entering the telescope in the first place. But I have heard reports that sometimes the eyepiece filters can crack in use, and that is bad! So, avoid eyepiece solar filters.
But once you get used to your new telescope, you might consider buying one of the color filter starter sets for use with night objects. But, you aren’t limited to just night observing. I have found that often a red, yellow, or green filter also helps a lot when you observe the Moon in the daylight.
-Astroprof
Andrew on December 31, 2006 at 10:00 pm: 1
Happy New Year!!
I recently purchased a 2″light pollution filter and was totally amazed.It more than doubled my exposure time,which is good for getting more detail.One downside however,it tends to create unwanted blue halos around stars,which is a pain to get rid of.I agree with you about eye piece solar filters.Not only do they run the risk of cracking and causing eye damage but,it could also damage the scope its self.
Blue Girl, Red State on January 1, 2007 at 7:06 pm: 2
Good piece. A hundred adn fifty years ago - okay 1994 - I was a TA for a class at Wichita State taught by the venerable anthropologist Clay Robarchek titled “Magic, Wichcraft and Religion” I convinced him I was the right person because I had a decade of healthcare and really, didn’t magic, wichcraft and religion really spring out of illness and injury? I made my case wll enough, I guess. I got the gig.
I’m a biochemist, so the physics I took was not astronomy. What I know about the topic all came from that class until I read this post. See? We learn something even on semester break.
Now - Are your syllabi ready for next semester? Mine are…
(aka Global Citizen)
Astroprof on January 2, 2007 at 11:10 pm: 3
Hey there, Global Citizen. I have almost all my syllabi ready. We don’t start for two weeks, though, so I’ve still got some time …