Filter Research

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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!

#82A Light Blue
Moon: Low-Contrast Features
Mars: Low-Contrast Features
Jupiter: Low-Contrast Features
Saturn: Low-Contrast Features
#80A Blue
Moon: Feature Contrast
Jupiter: Belts
Jupiter: Rilles
Jupiter: Festoons
Jupiter: Great Red Spot
Saturn: Belts
Saturn: Polar Regions
#38A Dark Blue
Venus: Clouds
Mars: Dust Storms
Jupiter: Belts
Jupiter: Great Red Spot
Jupiter: Disc
Saturn: Belts
#47 Violet
Venus: Clouds
Mars: Polar Caps
Saturn: Rings
#56 Light Green
Moon: Detail
Mars: Dust Storms
Mars: Polar Caps
Jupiter: Belts
Jupiter: Atmosphere
Jupiter: Red/Blue/Light Contrast
#58 Green
Venus: Clouds
Mars: Polar Caps
Jupiter: Red/Blue/Light Contrast
Saturn: Belts
Saturn: Polar Regions
#11 Yellow-Green
Mars: Maria
Jupiter: Clouds
Jupiter: Red/Blue Contrast
Saturn: Clouds
Saturn: Cassini Division
Saturn: Red/Blue Contrast

#8 Light Yellow
Moon: Feature Contrast
Mars: Maria
Jupiter: Belts
Jupiter: Orange-Red Zonal
Uranus: Dusky Detail
Neptune: Dusky Detail

#12 Yellow
Moon: Feature Contrast
Mars: Blue-Green Areas
Jupiter: Red-Orange Features
Saturn: Clouds
Saturn: Red-Orange Features
#15 Dark Yellow
Moon: Feature Contrast
Mars: Clouds
Mars: Polar Caps
Jupiter: Belts
Saturn: Belts
Uranus: Dusky Detail
Neptune: Dusky Detail
#21 Orange
Mars: Maria
Jupiter: Belts
Jupiter: Polar Regions
Saturn: Belts
Saturn: Polar Regions
#23A Light Red
Mercury: Planet/Sky Contrast
Mars: Maria
Mars: Blue-Green Areas
Jupiter: Belts
Jupiter: Polar Regions
Saturn: Belts
Saturn: Polar Regions
 
#25 Red
Mercury: Features
Venus: Planet/Sky Contrast
Venus: Terminator
Mars: Maria
Mars: Polar Caps
Jupiter: Belts
Jupiter: Galilean Moon Transits
Saturn: Clouds
#29 Dark Red
Mercury: Features
Venus: Planet/Sky Contrast
Venus: Terminator
Mars: Maria
Mars: Polar Caps
Jupiter: Belts
Jupiter: Galilean Moon Transits
Saturn: Clouds
Rotating Polarizing Filter
Moon: Glare Reduction or Variable Transmission

Table 2   Application of Planetary Filters - Organised by Colour

Object

Features

Recommended Filter

Mercury

 

Planet/Sky Contrast #23A Light Red
Features #25 Red
#29 Deep Red
Venus

 

 

Clouds #38A Deep Blue
#47 Violet
#58 Green
Planet/Sky Contrast #25 Red
#29 Deep Red
Terminator #25 Red
#29 Deep Red
Moon

 

 

 

Detail #56 Light Green
Feature Contrast #8 Light Yellow
#12 Yellow
#15 Deep Yellow
#80A Blue
Low Contrast Features #82A Light Blue
Glare Reduction ND13 Neutral Density
Mars

 

 

 

 

 

Clouds #15 Deep Yellow
Maria #8 Light Yellow
#15 Deep Yellow
#11 Yellow-Green
#21 Orange
#23A Light Red
#25 Red
#29 Deep Red
Blue-Green Areas #12 Yellow
#23A Light Red
Dust Storms #38A Deep Blue
#56 Light Green
Polar Caps #15 Deep Yellow
#25 Red
#29 Deep Red
#47 Violet
#56 Light Green
#58 Green
Deep Sky Filter
Low Contrast Features #82A Light Blue
Jupiter

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Clouds #11 Yellow-Green
Belts #8 Light Yellow
#15 Deep Yellow
#21 Orange
#23A Light Red
#25 Red
#29 Deep Red
#38A Deep Blue
#56 Light Green
#80A Blue
Rilles #80A Blue
Festoons #80A Blue
Atmosphere #56 Light Green
Red-Orange Features #12 Yellow
Orange-Red Zonal #8 Light Yellow
Red/Blue Contrast #11 Yellow-Green
Blue/Light Contrast #25 Red
Great Red Spot #38A Deep Blue
#80A Blue
Galilean Moon Transits #25 Red
#29 Deep Red
Red/Blue/Light Contrast #56 Light Green
#58 Green
Polar Regions #21 Orange
#23A Light Red
Disc #38A Deep Blue
Low Contrast Features #82A Light Blue
Saturn

 

 

 

 

 

 

 

Clouds #11 Yellow-Green
#12 Yellow
#25 Red
#29 Deep Red
Belts #15 Deep Yellow
#21 Orange
#23A Light Red
#38A Deep Blue
#58 Green
#80A Blue
Polar regions #21 Orange
#23A Light Red
#58 Green
#80A Blue
Rings #47 Violet
Cassini Division #11 Yellow-Green
Red/Blue Contrast #11 Yellow-Green
Red/Orange Features #12 Yellow
Low Contrast Features #82A Light Blue
Uranus Dusky detail #8 Light Yellow
#15 Deep Yellow
Neptune Dusky detail #8 Light Yellow
#15 Deep Yellow
Double Stars Bright Primary ND13 Neutral Density

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.

 Wratten/ Name  Colour Reported Affect Onů My Observed Affect Onů
Moon Venus Mars Jupiter Saturn Moon Venus Mars Jupiter Saturn
#47H light violet none reported improve view of atmosphere improve view of polar ice caps & clouds none reported improve contrast on rings & surface detail slightly improve  all details, large boost to light feat. like ejecta, violet hue n/o improvement to contrast between light & dark regions improved contrast, fine details in zones, very little colour cast improved contrast of n.hemisphere dark bands
#82B light blue improve contrast slightly improve surface markings improve contrast improve contrast improve contrast similar to #47H but more contrast, neutralise orange colour of moon when low in east n/o view not improved view not improved view not improved
#11H light green improve contrast none reported improve view of polar ice caps & dust storms improve contrast between red & blue features none reported improve contrast of light features n/o view not improved improved contrast dark to light, but darkens view surface detail less visible!
#8 light yellow improve view of features slightly improve low contrast surface features slightly improve contrast between light and dark regions improve view of red & orange features improve view of red & orange features very small improve n/o view not improved view not improved minor improvement in surface detail contrast, almost no change in colour or view
#21 orange none reported none reported improve maria regions & edge details polar regions, belt contrast, red spot polar regions, slightly surface details n/o n/o improved detail around polar cap n/o very dark, no improvement observed
#23A light red none reported daytime contrast with sky improve maria regions & edge details polar regions, belt contrast, red spot polar regions, slightly surface details improve contrast but dark, loose detail in dark areas n/o improved detail around polar cap small increase in contrast view not improved
Variable Polarizer neutral improve contrast, glare reduction none reported improve contrast, glare reduction improve contrast, glare reduction improve contrast, glare reduction reduced glare, improved contrast, wash out dark areas reduced glare, some dark features visible view not improved n/o view not improved
Orion Mars magenta none reported none reported dramatically improve views, improve contrast & detail of all regions at same time some improve in contrast between light & dark belts none reported reduced glare, improved contrast, very dark and very pink (not pleasing) no improvement observed large increase in contrast between all regions: light, dark, and polar caps, but view was dark and very pink, no bluish clouds n/o improved contrast of n.hemisphere dark bands, dark and purple colour
Baader Moon&Sky neutral improve contrast & features none reported improve contrast & features but leave natural colour improve contrast & features improve contrast & features reduced glare, improved contrast mostly to light areas, minor colour cast but practically neutral, still quite bright n/o very dramatic increase in contrast & detail visible, clearly see light & dark regions, polar caps, & bluish clouds near pole, natural colour & nice bright view large increase in contrast & visibility of fine detail & shading, very nice colour all around increase in contrast
#81B light brown not known reduced glare, improved contrast, pleasing tan colour slight reduced glare, brought surface details just into visible threshold improvement to contrast between light & dark regions, and edge of polar cap improved contrast between belts & zones increase contrast of surface features, nice natural colour
#85 light orange not known reduced glare, improved contrast, more orange and darker than 81B similar improve to 81B, but darker view improvement to contrast between light & dark regions, and edge of polar cap, but more than 81b improved contrast between belts & zones increase contrast of surface features more than 81B, but more orangish colour
CC20M light magenta not known reduced glare, improved contrast of light and dark features, slight pink colour not distracting view not improved similar view to Moon&Sky but with a bit more contrast, natural colour, clearly see polar cap & light/dark regions, hard to see bluish clouds surprising improved contrast in belts & zones, like 47H & Moon & Sky, pleasing colour small increase in contrast
CC30M light magenta not known reduced glare, improved contrast of light and dark features, slight pink colour not distracting n/o similar view to Moon&Sky but with a bit more contrast, a bit more pinkish  colour, clearly see polar cap & light/dark regions, hard to see bluish clouds almost same as CC20M small increase in contrast
CC40M magenta not known reduced glare, improved contrast of light and dark features,  pink colour starting to get distracting view not improved view midway between  Moon&Sky and Orion Mars, pinkish colour, clearly see polar cap & light/dark regions, not see bluish clouds like CC30M but darker so harder to see details in zones small increase in contrast

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

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Last updated: 07-Mar-12

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