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Minimum Aperture to detect color from DSO

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#26 David Knisely

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Posted 07 March 2013 - 03:04 AM

Seeing color in deep-sky objects is mostly a function of the sensitivity of the eye of the observer rather than just being a function of aperture. While some color can be observed in some of the brighter emission nebulae, most show little color because the eye just isn't sensitive enough to pick it up. In the case of the Orion Nebula, at lower powers, I can see a pale bluish or bluish-green coloration in the brightest portions of the nebula when using eight to 10 inch apertures. However, the color tends to be rather weak. In the case of some of the smaller brighter planetary nebulae, the bluish-green color is a little more intense. However, in most other deep-sky objects, people usually see very little in the way of color at all. I can see some faint reds in M42 using my DGM Optics NPB filter in a 14 inch telescope at fairly low powers (59x to 135x), but they are pretty weak compared to the bluish-green of the core region. However, some people won't see any color at all regardless of how big their telescope is simply because their eyes aren't sensitive to color at the low light levels we have to deal with. Some people see the color while others do not. Try using fairly low powers in your telescope and stare at the brighter regions to see if you can glimpse a little of the bluish-green color M42 has. Clear skies to you.


If it is not too time consuming for you, could you illustrate where in the nebula you detected red tinges? I have always been able to easily see the green and bluish coloration of M42 in scopes as small 8" (indeed, I can see the greenish parts in much smaller apertures), but pink or red hues have eluded me. It could be I have not observed carefully enough in the right areas.

Any help would be greatly appreciated. I will heed Jon, Glen's, and Golgo's comments on dark adaptation being a possible hindrance as well.

Thanks!


Most of the pinkish-red I see is in three distinct areas: first, in the area immediately around the bright "Huygenian" region (the part that contains the bluish-green nebulosity surrounding the Trapezium), and second, in narrow portions of the arc-like "wings" of M42 to the northwest and southeast of the Huygenian area. In particular, the northwest wing has a band like formation off its end that abruptly runs to the southwest that appears distinctly reddish or at least somewhat pink. There are occasionally faint pinkish or tan patches elsewhere, but generally, there is little color beyond the brighter regions of the nebula. One night in the 14 inch, I even saw some red in the brighter portions of M43 when using the NPB filter. In fact, once I got home, was startled to see how much I had seen in my 14 inch that night resembled the large color print of the sword of Orion done by David Malin that I have framed in my living room. The colors visually were nowhere near as saturated as in that color image, but the reds were in roughly the same places as were shown in the image. Clear skies to you.

#27 BluewaterObserva

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Posted 07 March 2013 - 08:45 AM

Not a bright red, but in my 30" in the darkest of sky. M42 is a ruddy red. Really quite striking.

Could be all the red dust in the air around here I suppose though.

#28 bremms

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Posted 07 March 2013 - 09:08 AM

You need high surface brightness. Best bet is a large aperture scope working at max exit pupil on a bright nebula.
M42 will only put so many photons per cell. It's a surface brightness thing. Some people are very sensitive to color. I'm not. My older sister could see color in M42 with my 10" newt many years ago. I couldn't. She said it's redish in the center and maybe some green blue out farther... I was a little jealous.

#29 Astrodj

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Posted 07 March 2013 - 12:31 PM

Glenn,

As I was reading your post I was wondering if filtering out the blue/green would help or not, then you discussed that very topic. I don't believe I own the right filter to try it but would be interested to hear of others who have.

Thanks for the explanations concerning eye sensitivity, very helpful.

#30 Astrodj

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Posted 07 March 2013 - 12:42 PM

Thanks David, I will take my time and search those areas more carefully in the future. Especially the NW wing and Huygenian region.

How does a NPB/UHC filter help to see reddish or tan tints? I was under the impression these wavelengths are filtered out with this type of filter...I am missing something, obviously.

I have a good Lumicon UHC to aid in my search but haven't ever tried it with this purpose in mind. I thought unfiltered would be the best method.

#31 star drop

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Posted 07 March 2013 - 12:46 PM

Thanks David, I will take my time and search those areas more carefully in the future. Especially the NW wing and Huygenian region.

How does a NPB/UHC filter help to see reddish or tan tints? I was under the impression these wavelengths are filtered out with this type of filter...I am missing something, obviously.

I have a good Lumicon UHC to aid in my search but haven't ever tried it with this purpose in mind. I thought unfiltered would be the best method.

Unfiltered works the best for me. Exceptional sky transparency, rare in my area, is essential.

#32 GlennLeDrew

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Posted 07 March 2013 - 01:13 PM

In a spectrogram I've seen published for M42, the red H-alpha and bluish-green O-III emissions are about equal in intensity (as is normal, the bluish-green H-beta line is perhaps 20-25% as strong as the H-alpha, which nonetheless adds to the same-colored O-III emission.)

The eye's sensitivity to H-alpha is at best 10% that of the O-III/H-beta group. And so if we have an admixture of intrinsically equal brightness bluish-green and deep red, but to the eye the red is only 1/10 as bright as the bluish-green, the latter color will greatly dominate, to the point of rendering red as more a subtle modifier, probably biasing the bluish-green to 'green-green.' :grin:

Now, if there are 'gaps' in the O-III intensity distribution which allow H-alpha to be the dominant source, and if that H-alpha emission is of sufficient surface brightness by itself, such patches could very well be seen as red. I concede this as a very real possibility in M42.

There are two (?) planetaries (one being more a proto-planetary, I seem to recall) in which red is definitely seen. But in no others I know of can red be perceived. In these objects O-III is intrinsically of rather stronger intensity than H-alpha (and nitrogen) emission, making the bluish-green visually nearer to 20 times brighter than red. And if the overall surface brightness is such that the bluish-green is hardly perceptible, the red is most definitely below the detection threshold. This includes M57 and M27, I posit, in spite of reports of red being seen; I attribute this to one or both of the color illusion (in which fainter but otherwise colorless bits take on a reddish hue) and bias induced by long familiarity with color images.

#33 Astrodj

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Posted 07 March 2013 - 05:41 PM

Ted,

Thanks. I get decent transparency way more often than good seeing. I'll try without filters too.

#34 Astrodj

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Posted 07 March 2013 - 06:01 PM

Glenn,

Great information. I will stay mindful of this when observing. I have my doubts about my being able to see reds, pinks, or tans, but I will certainly keep trying.

David,

After re-reading your review of the NPB filter I understand the h alpha capability of the filter. Maybe my 90's Lumicon UHC will help some after all.

#35 SKYGZR

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Posted 07 March 2013 - 07:43 PM

Eye Concur..some filters will perhaps change an objects "perspective" and allow hints of coloration to be evident, yet it's all gray scale to the human eye.

#36 MattT

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Posted 07 March 2013 - 11:18 PM

My ability to detect faint fuzzies is pretty good, but my color vision is weak. I have only ever experienced a hint of color on the brightest extended objects, like M42, in ~18" scopes and only seen color clearly the few times I've looked through something like a 30 incher.

#37 David Knisely

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Posted 08 March 2013 - 02:55 AM

Thanks David, I will take my time and search those areas more carefully in the future. Especially the NW wing and Huygenian region.

How does a NPB/UHC filter help to see reddish or tan tints? I was under the impression these wavelengths are filtered out with this type of filter...I am missing something, obviously.

I have a good Lumicon UHC to aid in my search but haven't ever tried it with this purpose in mind. I thought unfiltered would be the best method.


The NPB filter has a rather booming red secondary passband while the newer narrow-band nebula filters often have little or no red transmission at all. To check whether I was being "fooled", I stuck in my Orion Ultrablock narrow-band filter which has no red passband. The reds I saw in M42 vanished. With the NPB replaced, they immediately reappeared. This convinced me quite well that what I was seeing was indeed true red color and not some kind of "grey" illusion. With the width of the red secondary passband of the NPB (a red laser pointer comes through undimmed), some of that red color may be merely scattered "continuum" red reflection nebular light from stars in the nebula with the rest of it being from H-alpha emission. Of course, not everyone can see the red colors in M42, but I have done the same trick on other people and those who can see dim red colors also report reds in M42 using the NPB filter. Clear skies to you.

#38 David Knisely

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Posted 08 March 2013 - 03:02 AM

GlennLeDrew wrote:

There are two (?) planetaries (one being more a proto-planetary, I seem to recall) in which red is definitely seen.


The first one is reds on the outer edges of "the Raspberry Nebula", IC 418 in Lepus. The other is "Campbell's Hydrogen Star", PNG 64.7+5.0 in Cygnus. Of the two, Campbell's Hydrogen Star is the more vivid, although IC 418 is no slouch either. As for M42, the reds are most vivid there using the DGM Optics NPB filter, but again, some of that red coloration may be a little filtered red continuum (reflection nebulosity) light in the nebula as well as some contribution from H-alpha itself. The red secondary passband of the NPB is somewhat broad and bumpy, so H-alpha may not be the only contribution here. Clear skies to you.

#39 GlennLeDrew

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Posted 08 March 2013 - 05:26 AM

David,
An examination of a spectrogram of M42 instantly reveals that virtually all the light in the bright central region is from emission only, with an *exceedingly* dim continuum; the latter is practically nothing, with the emission peaks standing up like tall needles from a less than 1% continuum floor.

Of the reflection component, it's almost purely confined the outer part of the nearly circular 'bubble' being blown out by the Trapezium. In good color images, this is seen as a somewhat bluish color, the most prominent being the long 'wing' wrapping around the E/NE side, with very sharply defined edges.

Rare are the reddish reflection nebulae, none of which remotely approaches the color purity of H-alpha light. The reddish RNs arise from illumination by red stars (like Antares, where the lit up dust is more yellow), and where intervening dust reddens an RN somewhere beyond.

In the latter case, the RN would have to be of quite high surface brightness in order to be reddened significantly and not be too greatly dimmed. The brighter such nebulae are vastly more likely to be illuminated by brilliant blue or white stars, and the selective scattering would result in a nebular B-V certainly no whiter than -0.4. To result in a good reddish color, with B-V of, say, +1.5 to +2.0, the visual dimming would be fully 6 magnitudes (!).

Reddish reflection nebulae are rare enough to begin with, and that most seem to result from reddening by intervening dust, with a concomitant dimming, I doubt any have sufficient surface brightness to exhibit a perceptible red hue.

#40 gk5481

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Posted 13 March 2013 - 11:16 AM

When seeing conditions are good in the suburbs of Los Angeles I can clearly see a green-blue colored background in the Orion Nebula using my 8" reflector at low power. Under the same seeing conditions I have detected the greenish color with both a 4" and an 80mm refractor (but not with a 60mm refractor). Others who have observed with me at the same time have also noticed (or not noticed in the case of the 60mm) the greenish color.


#41 David Knisely

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Posted 13 March 2013 - 06:36 PM

David,
An examination of a spectrogram of M42 instantly reveals that virtually all the light in the bright central region is from emission only, with an *exceedingly* dim continuum; the latter is practically nothing, with the emission peaks standing up like tall needles from a less than 1% continuum floor.

Of the reflection component, it's almost purely confined the outer part of the nearly circular 'bubble' being blown out by the Trapezium. In good color images, this is seen as a somewhat bluish color, the most prominent being the long 'wing' wrapping around the E/NE side, with very sharply defined edges.

Rare are the reddish reflection nebulae, none of which remotely approaches the color purity of H-alpha light. The reddish RNs arise from illumination by red stars (like Antares, where the lit up dust is more yellow), and where intervening dust reddens an RN somewhere beyond.

In the latter case, the RN would have to be of quite high surface brightness in order to be reddened significantly and not be too greatly dimmed. The brighter such nebulae are vastly more likely to be illuminated by brilliant blue or white stars, and the selective scattering would result in a nebular B-V certainly no whiter than -0.4. To result in a good reddish color, with B-V of, say, +1.5 to +2.0, the visual dimming would be fully 6 magnitudes (!).

Reddish reflection nebulae are rare enough to begin with, and that most seem to result from reddening by intervening dust, with a concomitant dimming, I doubt any have sufficient surface brightness to exhibit a perceptible red hue.


In that case, the Oyxgen [I], Nitrogen II, and H-alpha lines should be the prime contributors to the visible pinkish-red coloration in M42. Last night, I could even see some very faint red in the northern wing of the nebula in my 14 inch without using the NPB, although it was a little more prominent with the NPB. Still, the colors seen visually are very weak compared with what is shown in most good color images. Clear skies to you.

#42 RocketScientist

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Posted 13 March 2013 - 10:24 PM

Few or no celestial objects have genuinely vivid colors; probably the ones that come closest are carbon stars. Double stars often have interesting color contrasts, but they're subtle.


Take a look at Mu Cephei (the Garnet Star) and R Leporis (Hind's Crimson Star). The first is circumpolar, the second is well-placed in the evenings right now. How much color do you see?

Now look at the "winter Albirero", h3945 in Canis Major. How obvious is the color contrast of this double?

These objects will help you get a calibration of your ability to see colors in the sky. They are intense or high-contrast objects.

#43 RocketScientist

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Posted 13 March 2013 - 10:27 PM

And in a 30 inch scope planetary nebulas can really start to look cool.


I suspect that in a 30 inch scope, everything looks cool.
:whee:

#44 starbux

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Posted 14 March 2013 - 01:21 PM

Color in M42 IS subtle and varies from person to person. In my 30s I had a 10" dob and a dark sky and regularly saw grey green in M42 and on occasion subtle pink in some places. I'm in my 50s now and probably wouldn't see quite that much.

For color in DSOs, planetaries can actually be quite vivid, relatively speaking. You may find these rewarding in that regard:

NGC 2392 in Gemini. Eskimo Neb.
NGC 6210 in Hercules.
NGC 6543 in Draco. One of the most vivid. The Cat's Eye.
NGC 6826 in Cygnus. The Blinking Planetary.
NGC 7009 in Aquarius. The Saturn Nebula. Another contender for most vividly colored.
NGC 7662 already mentioned Blue Snowball in Andromeda.

Expect blue-green color when seen. Moonlight will affect the view.






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