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Loren Toole
super member
Reged: 03/23/04
Posts: 124
Loc: New Mexico USA
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A few months ago, in a thread on lunar filters, I added comments
about a comparative color filter test I ran on rays, near full
moon. I was asked to verify violet as better than red to enhance
ray detail. This night (Halloween) I had a chance to go over this
again. For the record I used a carded film set of eight color
filters plus a variable density filter. My 4" f5 refractor was
used at 50X for all tests with a 10mm Radian.
I focused on Tycho's ray system, and repeatedly flipped between
no filter, one color then another.
So, the basic result was: colors toward blue are definately less
effective than colors toward red. I suspected filter density might
play some part in this comparison since my red is generally more
dense than the other colors. So, I also compared to the variable
density filter adjusted for approximately same throughput. I saw
again (as in the earlier test) that red tended to darken maria
more than the rays, which increases contrast. Blue or violet does
not really darken maria, which results in poor contrast with rays.
And filter density did not play a significant part in the result,
because I allowed my eyes to adjust to the dimmer view before
making the comparison. I suspected that medium density orange
might be more effective on rays than dark red, but this was not a
definite conclusion. My eyes can't distinguish small differences
such as dark or medium density easily.
One curious effect I did notice this time was the degree of maria
darkening with filters is not uniform across the disk. Mare Imbrium
generally shows less darkening, the eastern maria such as Mare
Tranquillity are much darker with red. At first I thought this is
a phase effect but looking at the geometry, there is clearly more
going on than just phase. Does anyone have an idea about this?
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My binos: 6x30, 7x35, 10x70, 15x60, 20x80
My scopes: 5"f5 newt, 4"f5 TV Genesis, 4"f5 TV 101, 3"f9, 3"f5
ARCO OBSERVATORY 7200'
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Sarkikos
Pooh-Bah
Reged: 12/18/07
Posts: 1075
Loc: Suburban Maryland
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Loren,
Good test and interesting results! I may have been in that thread you're talking about, because I usually support the use of #47 Violet on the Moon, contrary to the majority opinion favoring Red or a polarizing filter. But I've yet to make lunar observations specifically on the rays near Full Moon. That's something I intend to do ... if the weather ever clears. I usually concentrate on the terminator instead. For that, I prefer the #47 Violet. I had read in various places that a red filter is best for observing the rays, so I am not really surprised by your results. I've also read that it is good to alternate between red and blue filters to catch differences caused by the subtle color variations on the lunar surface. Maybe that is something else to consider when doing your filter comparisons on different areas of the Moon. I usually don't look forward to any type of astronomical observations, lunar or otherwise, during Full Moon, but your filter tests have caught my interest. Now if the weather would just cooperate! 
Mike
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Celestron 10" f5 Newt on 1stBase (DSO)
Zhumell 8" f6 Newt, Bosma 6" f12 MCT (NSO)
6" f5 Newt, 130ST, 4.5" f4.4 Ball w/GLP, ST80 w/Crayford (RFTs)
C4-R (NSO/DS)
90mm f13 MCT (Luna/DS)
SkyMaster 25x100, 15x70
Zhumell 20x80
Barska XWA 10x50, 8x40
OptiView LPR 10x50
Const View 2.3x40
BV-125C
CG5, 2 drv / CG4, 2 drv, wood legs, ScopeStuff saddle / CG3, 1 drv
SV AZ / 501HDV on Bogen 055XB / P+ on Oberwerk
QuikFinder, Telrad
Orion Dynamo Pro 12, Dew-Not
Have GLP and not afraid to use it!
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JimK
sage
Reged: 09/18/05
Posts: 231
Loc: Albuquerque, NM USA
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Tonight had a full Luna, so I decided to look at its bright rays using several different filters in my 90mm Mak from about 80x to 130x. A couple months ago I tried some of this with a 100mm f/10 achro and although I was clouded out early in my tests, the results were consistent.
My results/opinions, for various Wratten numbered filters, are:
#23 (light red) & #21 (orange) = I don't like red/orange filters on Luna
I saw good detail in the valley/lava tube/ridges near crater Aristarchus, but the rays of Tycho were unimpressive and the different shadings in the crater floor of Schickard did not stand out. Maybe if I had tried a #25 (red) or #29 (dark red) my opinion would change.
#56/58 (green) = I like green filters for bright lunar detail
Green filters provided the same effects as the red/orange ones, but for me the effect is more enjoyable. If I wish to study bright details near a fully illuminated lunar disk, such as the many central mountains and hills of Gassendi when fully illuminated, then I will use a green filter.
#11 (yellow/green) & #80 (blue) = I see no benefit in their use on Luna
Everything looks somewhat washed out, as if no filter was used.
#38 (dark blue) = I like this filter to locate bright and dark lunar areas
With some effort I could make out detail in the valley/lava tube/ridges near crater Aristarchus, but bright and dark lunar areas were readily discerned. It was generally easy to see bright rays of Tycho, different shadings in the crater floors like Schickard, as well as the locations of bright craters like Dionysius. This seems consistent with my experience with using #80 (blue) on Mars to highlight the polar ice caps. However, for study of bright details I would use the green filters.
#47 (violet) = I am not able to easily use this filter on Luna
Everything looked dim yet washed out to me using this filter, and I had a very, very hard time seeing/focusing any detail in the valley/lava tube/ridges near crater Aristarchus.
I did NOT use a neutral density nor a polarizer in these observations because I wanted to concentrate on looking at the bright/dark area differences instead of an overall perspective of Luna at/near full.
I suspect that larger aperture telescopes (and different eyeball sensitivities) may favor other filters (such as #25-red or #47-violet), but I have provided the above as another opinion for smaller aperture telescopes.
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Sarkikos
Pooh-Bah
Reged: 12/18/07
Posts: 1075
Loc: Suburban Maryland
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Last night, November 3rd, I observed the Moon through my 130ST (5.1") f5 Newt on a driven CG4. It was pleasant to view the Moon without having to nudge the scope. I had collimated the scope using a Cheshire/sight tube combo and then an autocollimator. I was satisfied that it was very well collimated. According to Clear Sky Chart, the transparency was excellent but the seeing was poor - a better night for DSO if the Moon had'nt been full! I used a Zhumell Zoom, 8mm-24mm, with and without a Smart Astronomy (GSO) 2.5x Barlow. (Unfortunately, this Barlow did not have threads for filters.) Luna looked nice at lower and medium powers, but not at higher powers, for instance 233x, in this scope on this night. (I prefer not to push up the magnification anymore than I need to observe whatever detail the seeing will allow.) I could barely see three craterlets in Plato during moments of clearer seeing.
I'll give my impressions using different filters.
The red filters did make the crater rays stand out. I tried #29 Dark Red, #25A Medium Red, and #23A Light Red. All three showed the rays to good effect, but for my 5.1" reflector, the view was probably best through #23A Light Red. Even the Dark Red, though, was surprisingly good near Full Moon. I had tried all three red filters before during First and Last Quarter, and I did not like them at all during those phases.
I did not like #21 Orange. Too bright for Full Moon, IMHO.
I did not like the Green filters. Too bright, even #58 "Dark" Green. I did not even try the Yellow filters. I knew they would be too bright for Full Moon.
I did not like the 13% Neutral Density filter. Not dark enough.
My Polarizing Filter was merely OK. At it's darkest, it was too dark. If I backed it out about 20% or so, it wasn't too bad, but it's not my preference. In my reflector, I have to take the eyepiece out each time to adjust the setting on the filter: not my idea of user-friendly astronomy. I suppose I could screw one of the polarizers onto a Barlow or extension tube, and the other on the eyepiece, then turn the eyepiece, but what if I can't or don't want to use that setup for an observation? I don't like it.
#47 Violet was too dark for a 5.1" reflector. (I prefer Violet in 8" or 10" reflectors for observing details on the terminator.)
The #38A Dark Blue had a surprise for me. It was really good for seeing the floors of small craters. With a #38A Dark Blue filter on the Zhumell Zoom, the smaller craters really popped out for me: their floors appeared very bright in contrast to the rest of the lunar surface. It looked like the Moon had caught chicken pox! I switched back to a red filter to verify that the effect was not apparent with red, but was with the blue.
I had read that it is good to switch between red and blue filters to see different contrast effects on the Moon. Next time I look at the Full Moon, I might preload my little 3-filter wheel with a #23A Light Red, a #29 Dark Red and a #38A Dark Blue filter so I can see immediately the different contrasts and surface features that these filters emphasize. I'll still use #47 Violet during First or Last Quarter for observing the terminator, at least with my larger Newts.
Note: For what it's worth, my wife and daughter were with me through most of my session last night, and they agreed with my observations.
Mike
P.S.: I knew that Luna was beginning to move through M45 last night, and that was one of the reasons why I went out, but I was not impressed, at least not at the view through my 130ST. Only the brighter stars in the Pleiades were visible near the Moon, so the cluster lost most of its appeal to me. Maybe it would have looked better in binos. Maybe next time.
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Celestron 10" f5 Newt on 1stBase (DSO)
Zhumell 8" f6 Newt, Bosma 6" f12 MCT (NSO)
6" f5 Newt, 130ST, 4.5" f4.4 Ball w/GLP, ST80 w/Crayford (RFTs)
C4-R (NSO/DS)
90mm f13 MCT (Luna/DS)
SkyMaster 25x100, 15x70
Zhumell 20x80
Barska XWA 10x50, 8x40
OptiView LPR 10x50
Const View 2.3x40
BV-125C
CG5, 2 drv / CG4, 2 drv, wood legs, ScopeStuff saddle / CG3, 1 drv
SV AZ / 501HDV on Bogen 055XB / P+ on Oberwerk
QuikFinder, Telrad
Orion Dynamo Pro 12, Dew-Not
Have GLP and not afraid to use it!
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Sarkikos
Pooh-Bah
Reged: 12/18/07
Posts: 1075
Loc: Suburban Maryland
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Reading back over my observations, I see that I said the 13% ND filter was not dark enough, although it is supposed to have less VLT (Visual Light Transmission) than the Light Red (25%) and the Dark Blue (27%), both of which I liked. So it must have been something else I didn't like about the view through the ND. Maybe it was because the ND didn't make any particular feature "pop," as the other two filters did. Or maybe I just like looking at a colorful Moon!
Mike
--------------------
Celestron 10" f5 Newt on 1stBase (DSO)
Zhumell 8" f6 Newt, Bosma 6" f12 MCT (NSO)
6" f5 Newt, 130ST, 4.5" f4.4 Ball w/GLP, ST80 w/Crayford (RFTs)
C4-R (NSO/DS)
90mm f13 MCT (Luna/DS)
SkyMaster 25x100, 15x70
Zhumell 20x80
Barska XWA 10x50, 8x40
OptiView LPR 10x50
Const View 2.3x40
BV-125C
CG5, 2 drv / CG4, 2 drv, wood legs, ScopeStuff saddle / CG3, 1 drv
SV AZ / 501HDV on Bogen 055XB / P+ on Oberwerk
QuikFinder, Telrad
Orion Dynamo Pro 12, Dew-Not
Have GLP and not afraid to use it!
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