Filter Research

Using Planetary (Colour) Filters

With the response curves and transmissivity values I have calculated, I now have a better understanding of what exactly all these different filters are doing.  The blue filters remove greens and reds, the green filters remove blue and red, etc.  One obvious question, and one that luckily many other amateur astronomers have already figured out is:  what are all these colour filters good for?  The simple answer is to increase the contrast between different features when viewing planets through your telescope.  I have come across a number of tables summarizing the application of different colour filters for different planetary features.  The most comprehensive I've found is from the Filter page on the Lumicon website (www.lumicon.com).  I have repeated the tables below; one organised by filter colour, and the other organised by planetary feature.  From my own experience I know it takes a lot of time and patience to assess whether a filter does anything to improve the view of a planet, so my deepest thanks go to the people who contributed to this list!

Table 2   Application of Planetary Filters - Organised by Colour

Table 3   Application of Planetary Filters - Organised by Viewed Object

One aspect of using colour filters that is not often discussed is the fact that they attenuate the amount of light getting to your eye.  This is clearly evident by the % transmissivity values in my Table 1.  People don't tend to be too concerned about the level of attenuation colour filters impose since the objects they are looking at are bright, namely planets.  However the effectiveness of a colour filter at increasing some particular detail on a planet will be affected by the aperture of your telescope.  A smaller aperture will not reveal the same enchancement that a large aperture would.  Figure 9 summarises the recommended minimum aperture size when using colour filters for planetary work, based on scotopic % transmissivity.  You can also calculate the minimum aperture using the simple relation:  Aperture = 0.02*(%Scotopic - 100)^2.

Figure 9   Minimum Recommended Aperture Size for Colour Filters

In addition to improving the view of planets, colour filters can improve the view of other objects as well.  The moon is a good candidate for contrast enhancement through the use of colour filters.  In my experience though, the improvements observed are more due to the attenuation of the moon's brightness than actual filtering out of different colours.  I talk a bit more on the moon below.  Another use of colour filters is as a poor man's (or woman's) violet fringe killer.  As can be seen in my plots from the previous page, a yellow filter is not that different from a minus-violet filter that one would use to cut the violet-blue fringe that is seen around bright objects on achromatic refractors.  Hey, the spectral response curves don't lie, try it some time.  Another reasonably common use of colour filters is for viewing the sun.  In particular an interesting band to look at the sun in is Hydrogen Alpha (Hα), which can be achieved cheaply using a dark red (#29) filter on the eyepiece end in conjunction with a solar filter on the objective end.  You can do the same trick to isolate Hα emissions from nebulae (without the solar filter of course!), but if you recall the graph of eye spectral sensitivity, you can't actually see these emissions.  They are useful for imaging only.  If anyone else has a non-planetary use for colour filters, I'd be happy to add it to the list!

There are some other filters out there that are not technically colour filters, but that are intended for enhancing the views of planets.  Some examples include the Baader Planetarium Moon & Skyglow filter, the Orion Mars filter and the Televue Bandmate Mars-A filter.  These special filters use the fancy coating technology developed for deep-sky filters to make filters that improve contrast when viewing the moon and planets.  I present more data on these filters in my Deep-Sky Filter chapters, under the category "Special".

Figure 10   Some Common Photography Filters Not In Your Accessory Case

While I was researching the various filters sold for astronomy, I wondered why there were some filter colours missing.  The list of Wratten filters is very long, plus there is a whole other family of commonly used photography filters called "Colour Compensating" that introduce colours like cyan and magenta.  Is there a good reason why these filters are not used for astronomy?  Have I uncovered the biggest conspiracy in amateur astronomy history?  I needed more data!  That is why I purchased some common photography filters on top of the usual astronomy ones.  I assessed their spectral response and transmissivity, and did my own visual comparison tests on the objects that were readily available this past April/May:  Moon, Venus, Mars, Saturn, and Jupiter.  The viewing of Venus was very poor due to its location low on the evening horizon, views of Mars were frustratingly bad-good-bad-good, and of course views of Jupiter (morning) and Saturn (evening) were nice.  My observations are summarized below in Table 4.  I evaluated my new Orion Mars filter at the same time for comparison, the results being in Table 4 as well.  In the table I have listed what the commonly observed effect of each filter is as well as what I have been able to observe through my 8" SCT.  Entries with a "n/o" mean that I have not yet had an opportunity to view that object with the filter in question.

Table 4   Summary of My Observations With Colour Filters

Well, needless to say I am very pleased with myself!  I have discovered some potentially useful new filter colours for use in astronomy.  But are they really new?  There are many web and book references to using a Wratten #30 or #34 for Mars observation, but I was not able to find anyone who sells these magenta coloured filters.  Some people also like the Orion Mars filter (included on the "violets" plot) or even broadband light pollution filters (you'll see them later) for viewing Mars, both of which are a dark magenta colour.  So if magenta filters are so great for viewing Mars, and I have personally confirmed that the reports are true, why then is a Wratten #30 not a standard filter in everyone's collection?  Why can I not buy it anywhere, even if I wanted to?...hmmmmm.  There is also the Televue Bandmate Mars-A filter which boasts letting green and red through but not other wavelengths.  The images of this filter look an aweful lot like a #85 to me, and is probably something like the Denkmeier Optical Planetary filter shown with the orange curves.  The Wratten #85 is readily available for photographic work, and I even found a 48mm (2") one pretty easily on Ebay.  So why is there no 28mm (1.25") version of this filter or the #81 which I found worked well with all the planets?...double hmmmmm. 

In my using of colour filters I found that I prefer filters that improve contrast but do not darken the view too much.  That is why I've only included the lighter of the standard colour filters in my Table 4.  I also prefer filters that leave the least amount of colour-cast to the particular viewing object.  This is obviously a personal choice, and perhaps with a different telescope or different viewing conditions, I may change this position.  For now though I have decided to add a magenta (CC30M), light brown (#81b) and light orange (#85) filter to my active list of filters.  I've also added light yellow (CC20Y), light red (CC30R), a real light blue (#82a), and a real yellow-green (#11) for use in future observational tests.  To make this happen I used a diamond Dremel cutting disk to trim the camera sized filter glasses down to fit 1.25" filter housings.  I purchased empty filter housings off Ebay from the seller bjomejag.  The end result can be seen below in Figure 11.

Figure 11   My New Filter Colours Join The Family

Okay, so we've got planets taken care of, but what about DEEEEEEP SPAAAAAACE?...  <next>

Introduction  -  The Human Eye  -  Planetary Filter Research  -  Using Planetary Filters  -  Deep-Sky Filter Research  -  Using Deep-Sky Filters  -  References

Copyrights to all content from the webpages hosted here belongs to Jim Thompson. Nov. 2009.