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 On… |
My Observed Affect On… |
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>
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