Some Notes on Filters
Essentially a filter is an optical device that blocks undesirable wavelengths of light and therefore only allowing light of desirable wavelengths for a given target, therefore "enhancing" that target. However a filter does not increase available light: it decreases the total light passing through the light path of your telescope and eyepiece configuration.
A filter is not a magic wand.
Often the improvement of a filtered view of a target is very subtle.
Filter selection can be very subjective. I am not trying to write a definitive handbook, merely offering my opinions based on personal experience, and reliable research, hopefully to assist beginners.
I do welcome discussion, and varying opinions or criticisms.
Throughout I am referring to visual application only, also I am deliberately not quoting percentage cuts, or spectral zones in nanometres, I prefer to leave that to those that have a better understanding of the Physics involved than I do.
Probably the first filter that a beginner will want is some type of Lunar Filter. The purists will argue that no filter is necessary for lunar observing but personally I find lunar brightness to be rather annoying.
A common filter used is an ND96 ND being Neutral Density. In effect this is simply a filter of fixed properties designed to reduce glare. Some observers also like to use an ND filter for Venus.
Gaining in popularity (and with good reason) is the Variable Polarising Filter. This is in fact a pair of Linear Polarising filters mounted together so that one rotates against the other, and thereby enabling the glare reduction to be variable in relation to any given part of the Moons phase.
Out of interest, I often find that using one half only of a VP cuts edge glare when observing both Saturn and Jupiter at times when detail tends to otherwise be washed out by excessive brightness.
These are the common “Coloured” filters that were originally introduced by Eastman Kodak some 90 odd years back for photographic use. These days there are around 100 different colours and shades available, but for Astro Observing only about a dozen or so are commonly used, and actually stocked by Astronomy equipment retailers.
Wratten filters are mostly used for planetary observing and are actually quite cheap to buy, so certainly anyone can have a tinker with a few different ones with little outlay.
Here are some common Wrattens:
- #8 Light Yellow: Used to enhance detail in red and orange features in the belts of Jupiter. Is useful for increasing mare contrast on Mars and improving disc resolution of Uranus and Neptune with larger telescopes. Can be used to enhance lunar detail also.
- #11 Yellow-Green: Excellent for bringing out surface details on Jupiter.Darkens the mare regions on Mars.
- #12 Yellow: Enhances red and orange features of Jupiter and Saturn, while blocking blue and green wavelengths. It also lightens red and orange features on Mars, while reducing, the transmission of blue and green areas. This filter increases the contrast between the two. Also enhances the blue clouds in the Martian atmosphere. Can also be used to increase contrast in lunar features with telescopes of 150mm aperture and larger.
- #15 Deep Yellow: Used to bring out Martian surface features, and the polar ice caps. It can also be used to enhance the orange and red features, bands and festoons, on Jupiter and Saturn, and low-contrast cloud details on Venus.
- #21 Orange: Reduces transmission of blue and green wavelengths, thus increasing the contrast between these areas and red or yellow or orange areas. Works very well with Mars. Sharpens the boundaries between these areas on the planet's surface and can sharpen the contrast in the belts of Jupiter also enhancing the Great Red Spot. Has slightly more contrast than the Wratten #15.
- #23A Light Red: A popular filter for use when observing Mars, Jupiter, and Saturn. Due to the lower light transmission of this filter, it is best used with telescopes larger than 130mm aperture. It performs many of the same functions as the #21 and the #15 but with somewhat more contrast. Can be a useful filter for daylight observations of Mercury and Venus. improving contrast between these planets and the pale blue sky.
- #25A Red: The #25A filter strongly blocks the transmission of blue and green wavelengths which results in very sharply defined contrast between the cloud formations and the lighter hue features of Jupiter. Also quite useful for improving definition of the Martian polar ice caps and darker mare surface regions.
- #38A Dark Blue: Very good for use on Jupiter because it strongly rejects red and orange wavelengths in the belts and in the Great Red Spot, thus increasing contrast. It works well on Martian surface phenomena, like dust storms, and increases contrast in the rings of Saturn. Most suitable for Venus given its low light transmission and readily increases contrast of subtle cloud markings.
- #47 Violet: Strongly rejects red, yellow, and green wavelengths, making it suitable to use on the Martian polar ice caps. It is the best filter for observations of Venus due to its low light transmission and ability to enhance upper atmosphere phenomena. Also can be used for enhancing lunar detail. Presents interesting views of Jupiter and its satellites.
- #56 Light Green: Very good for observing Martian polar ice caps and yellow tinted dust storms on the planet's surface. Also increases the contrast of the red and blue regions in Jupiter's atmosphere and cloud belts.
- #58 Green: Strongly rejects red and blue wavelengths increasing their contrast on the lighter regions of Jupiter's cloud tops. Useful for enhancing the cloud belts and polar regions of Saturn. Great for increasing contrast in polar ice caps of Mars and does a reasonable job at improving contrast of atmospheric features on Venus.
- #80A Blue: A magnificent all-round filter. Perhaps the best economical filter for studying planetary detail of Jupiter and Saturn. Enhances contrast of festoons and other disturbances in Jupiter's cloud belts, along with detail of the Great Red Spot. Works great with Saturn revealing terrific detail in its belts and polar features. Can be useful for lunar observations.
- #82A Light Blue: Another outstanding performer like the #80A and works well with Jupiter, Mars, Saturn and the Moon. Its pale blue colour enhances low contrast areas and avoids significant reduction of overall light level at the same time.
Personally I use either an 80a or an 82a more than any other, usually on Saturn and Jupiter
These cross over in many aspects with Light Pollution Reduction (LPR) filters.
The two major Nebula filters are the UHC and OIII. Both are admirable performers on most commonly observed Nebulae, the general rule of thumb being that the UHC is preferred from dark sky sites, and the OIII from light polluted sites.
A third Nebula filter, the H-Beta, is a particularly specialised tool usually only used for the California and Horse Head Nebulae.
I highly recommend Nebula filters. They really work.
Light Pollution Reduction filters
There are an increasing number of specialist LPR filters becoming available. These are designed to reject the light produced by Mercury vapour and Sodium vapour lamps. Use of LPR filters is quite an area for debate. Some claim significant success, others claim no improvement at all. The problem arises as to just how much artificial light is present in the first place, and selection and use of an LPR is a choice that should be made with direct comparison to a user in a situation as close as possible to your own.
Unfortunately, at the end of the day there is no substitute for dark skies.
Chromatic Aberration Filters
A specialist group designed to reject the violet end of the spectrum which constitutes Chromatic Aberration as commonly seen in Achromatic Refractors on bright objects.
Various brands and names are available, commonly called Minus-V or Fringe Killers. Probably the pick of these is the Baader Semi Apo, having less total light cut than most others, but its not a cheap filter.
This article was originally posted on the AstroHolic forum and has been reporduced with the kind permission of Vince Legge.