What Counts As 'naked Eye'?

[pipnina]

On EP 2 (or was it 3?) of this year's Stargazing Live they mentioned that comet catalina would require binoculars, and not be visible naked-eye... But it's scraping mag 5 right now which is well within what people here would consider 'naked eye'

This set me thinking... I know that people have seen barnard's loop with a filter but no optics besides that, does that make barnard's loop a naked-eye object? And the HH (IC-434 specifically) is brighter than Barnard's loop, surely that would make the HH a naked-eye target, too... And the flame is brighter still... Yet we only ever describe M42 as naked-eye in that region...

I'm guessing that far more can be seen 'naked eye' than we (or maybe just I) think... Based on this text:

"a few unaided-eye observations were made using the filters hand-held and looking up at the sky (for Rosette, North America, California Nebula, Lambda Orionis complex, and Barnard’s Loop"

I sure would love skies dark/transparent enough to see that stuff without aid, though. It would be incredible. As it stands, I'd have to go abroad. Don't think anywhere in the UK has high enough altitudes or dark enough skies...

Another note, I've heard of people seeing THIS in 100mm fracs. It's strange how there aren't really any "professional"-looking images of the whole object. I'm guessing it's just obscure. (Anyone here want to give it a try? )

p.p.s It's quite nice being able to locate many of these objects, and remember their shapes + sizes from images/stellarium, then going outside and imagining how monstrously huge &/or far away these things are. Even if I can only see a gradiented grey-yellow glow across my whole sky with some white dots behind it, imagination is a powerful tool.

    ~pip

[John]

For me "naked eye" means without optical aid but I'd make an exception for glasses or contact lenses, unless someone has sneakilly UHC or O-III coated them.

[jason.p]

I suppose we really need a "standard" naked eye. People and conditions vary enormously. For instance, after I had located Catalina with the bins I was convinced I could see it naked eye, but I've never split Alcor-Mizar naked eye and that's supposed to be easy!

[Astro Imp]

Isn't this a case of aggregating the light from an extended object to arrive at the magnitude? A point source of magnitude 5 might well be easily visible naked eye but spread that light over an extended area and the surface brightness is much less.

[pipnina]

Isn't this a case of aggregating the light from an extended object to arrive at the magnitude? A point source of magnitude 5 might well be easily visible naked eye but spread that light over an extended area and the surface brightness is much less.

Is a comet not point-source anyway? Unless the magnitude measurements include the tails... I can see down to mag 4.3 from my front garden... Mag 3 from the back but still. I only have to travel 10 minutes down the road to get to mag 5~ (most recent estimate suggests posibly mag 5.34 but who knows at this point, my estimates keep changing ) And I live in a city, surely naked-eye comet catalina would be an easy object for most of us!

[Stu]

Is a comet not point-source anyway? Unless the magnitude measurements include the tails... I can see down to mag 4.3 from my front garden... Mag 3 from the back but still. I only have to travel 10 minutes down the road to get to mag 5~ (most recent estimate suggests posibly mag 5.34 but who knows at this point, my estimates keep changing ) And I live in a city, surely naked-eye comet catalina would be an easy object for most of us!

No, AstroImp is correct. A comet is not a point source, the light is spread over an area in the same way as a galaxy or other DSO is. It has a surface brightness depending upon this area, and so will be harder to see than a point source star. I've yet to see it from home (or at all for that matter

[Owmuchonomy]

I have made several observations of it and imaged it with my 4" frac. I would estimate it at mag 7 to 8. It is nowhere near a naked eye object. As of tomorrow it's moving away (from us) I believe.

[Peter Drew]

My understanding is the same as Alan in that the magnitude of an extended object such as a comet or galaxy has been derived by condensing the extended brightness down to a point source to such as a star of that apparent magnitude. Tis has the effect of making extended objects fainter than expected.

[Scott]

I always thought of naked eye as not magnifying with optic. John is probably right in that filters are a cheat

I find naked eye difficult in that the specs scatter the light

[brantuk]

Just a small point relating to the op - don't confuse naked eye with light polluted. It may be invisible due to street lights, mooon, or dawn - but it's still a naked eye object (to me at least)

[acey]

Very interesting question which raises a few issues.

There is no standard agreement on what “naked eye” means. Is it one eye or two? (We see fainter when using two). Is it sufficient for the object to be glimpsed with averted vision for a fraction of a second, or should it be seen for a definite period of time, perhaps directly? In the early 20^th century Curtis set up a black screen with a hole in it and looked for stars through that: by shielding the natural brightness of the sky he boosted his limit by 2 magnitudes. Should that count? And what about breathing pure oxygen, as has been done by Stephen O’Meara, apparently boosting his limit?

If stargazing were a sport then rules would have been agreed. You can’t catch fish with dynamite or improve your golf handicap using an air cannon instead of a club, but if you want to hold a filter in front of your eye and see what you can see, go ahead, no one’s complaining. Just be sure that when you say the Rosette Nebula was a “naked eye object” you explain the observing conditions clearly.

Next issue: magnitude. If someone says they have a “mag 5.5 sky” it means they can see stars to mag 5.5, according to whatever procedure they use. They might have great vision and a lot of light pollution, or have poor vision and be at a very dark site where others would be able to see past 6. Limiting magnitude is an expression of sky quality and observer acuity.

The limit refers to stars, which are point sources. Apparent magnitude is really a measure of illumination: how much light is being received from the source. Two sources can have the same apparent magnitude, but if one is large and the other is small, the large one will look fainter: its surface brightness is lower.

So you might be at a place where you can see stars down to mag 6, and there might be a mag 5 comet in the sky, but because it’s very big you can’t see it. It’s fainter than your limiting surface brightness.

Further point. Your limiting surface brightness is itself a function of size. The limit gets lower (fainter) as target size increases. A telescope at lowest magnification leaves the target surface brightness unchanged (assuming no light loss in the telescope, which is an idealisation). By making a nebula look bigger while not unduly reducing its surface brightness, a telescope makes it visible. A nebula that is easily visible in a telescope at lowest power can be invisible to the naked eye, because it is too small. A filter might boost its contrast sufficiently to make it visible without magnification.

Astronomy is not a sport like angling or golf, but it has some similarity to mountaineering. People are encouraged to attempt extreme feats. When I hear people say they’ve seen Barnard’s loop or M81 without a telescope I am frankly sceptical. It’s impossible to verify their claim independently, we just have to take their word for it (or not) based on their level of experience and credibility. During one of his earliest telescopic observations of the Moon, Herschel thought he could see trees on it. On one of the earliest occasions when I looked at the Ring Nebula, I thought I could see red in it. Experience taught Herschel otherwise, and it was the same for me. I’ve no doubt that some of the amazing feats that are claimed are true. I’m also sure a lot of them are like those stories anglers tell about the one that got away – wishful thinking.

[pipnina]

Very interesting question which raises a few issues.

There is no standard agreement on what “naked eye” means. Is it one eye or two? (We see fainter when using two). Is it sufficient for the object to be glimpsed with averted vision for a fraction of a second, or should it be seen for a definite period of time, perhaps directly? In the early 20^th century Curtis set up a black screen with a hole in it and looked for stars through that: by shielding the natural brightness of the sky he boosted his limit by 2 magnitudes. Should that count? And what about breathing pure oxygen, as has been done by Stephen O’Meara, apparently boosting his limit?

If stargazing were a sport then rules would have been agreed. You can’t catch fish with dynamite or improve your golf handicap using an air cannon instead of a club, but if you want to hold a filter in front of your eye and see what you can see, go ahead, no one’s complaining. Just be sure that when you say the Rosette Nebula was a “naked eye object” you explain the observing conditions clearly.

Next issue: magnitude. If someone says they have a “mag 5.5 sky” it means they can see stars to mag 5.5, according to whatever procedure they use. They might have great vision and a lot of light pollution, or have poor vision and be at a very dark site where others would be able to see past 6. Limiting magnitude is an expression of sky quality and observer acuity.

The limit refers to stars, which are point sources. Apparent magnitude is really a measure of illumination: how much light is being received from the source. Two sources can have the same apparent magnitude, but if one is large and the other is small, the large one will look fainter: its surface brightness is lower.

So you might be at a place where you can see stars down to mag 6, and there might be a mag 5 comet in the sky, but because it’s very big you can’t see it. It’s fainter than your limiting surface brightness.

Further point. Your limiting surface brightness is itself a function of size. The limit gets lower (fainter) as target size increases. A telescope at lowest magnification leaves the target surface brightness unchanged (assuming no light loss in the telescope, which is an idealisation). By making a nebula look bigger while not unduly reducing its surface brightness, a telescope makes it visible. A nebula that is easily visible in a telescope at lowest power can be invisible to the naked eye, because it is too small. A filter might boost its contrast sufficiently to make it visible without magnification.

Astronomy is not a sport like angling or golf, but it has some similarity to mountaineering. People are encouraged to attempt extreme feats. When I hear people say they’ve seen Barnard’s loop or M81 without a telescope I am frankly sceptical. It’s impossible to verify their claim independently, we just have to take their word for it (or not) based on their level of experience and credibility. During one of his earliest telescopic observations of the Moon, Herschel thought he could see trees on it. On one of the earliest occasions when I looked at the Ring Nebula, I thought I could see red in it. Experience taught Herschel otherwise, and it was the same for me. I’ve no doubt that some of the amazing feats that are claimed are true. I’m also sure a lot of them are like those stories anglers tell about the one that got away – wishful thinking.

I think everyone who replied to this thread gave good answers but this one sums them all up!

Cheers.

[ollypenrice]

Using filters against the otherwise naked eye is an interesting idea and not one I've tried. My own eyesight is on the diabolical side but, still, I'll try it. But Barnard's loop? I doubt it. Unfortunately the person with (I think) the best eyesight of anyone who has been to my place is currently otherwise engaged in life and hasn't been recently. However, should he re-appear I think 'naked eye and filter' would be a great test. I was perfectly convinced that he could reach mag 7. 

Olly

[LukeSkywatcher]

Using filters against the otherwise naked eye is an interesting idea and not one I've tried.

I try these things at home, so you dont have to. Ive tried holding both UHC and OIII filters up to my otherwise naked eye and it simply doesnt work. You do need the magnification also. I really thought that the OIII would work and allow me to see the Veil,Rosette etc. They work great with my scope.

In theory, a sound idea really...........but not happening.

[pipnina]

Using filters against the otherwise naked eye is an interesting idea and not one I've tried. My own eyesight is on the diabolical side but, still, I'll try it. But Barnard's loop? I doubt it. Unfortunately the person with (I think) the best eyesight of anyone who has been to my place is currently otherwise engaged in life and hasn't been recently. However, should he re-appear I think 'naked eye and filter' would be a great test. I was perfectly convinced that he could reach mag 7. 

Olly

Using filters against the otherwise naked eye is an interesting idea and not one I've tried.

I try these things at home, so you dont have to. Ive tried holding both UHC and OIII filters up to my otherwise naked eye and it simply doesnt work. You do need the magnification also. I really thought that the OIII would work and allow me to see the Veil,Rosette etc. They work great with my scope.

In theory, a sound idea really...........but not happening.

I tried using my OIII filter by itself to try and help me see the north american neb, it didn't work because I ended up looking at a reflection of my eye. I figure that I'd need to make some sort of tube to put the filter in front of to cut out stray light. I would have thought it to work on the NA since it's supposedly an easy naked eye target from good skies. I'm yet to see the nebula.... (couldn't see it in my binoculars, or naked eye with the filter.)

What I really need is to pester the council ask the council nicely to try out this "Part-night" lighting thing that exeter is doing... A surprising number of people in exeter were for it... I was expecting a resounding "TURN THEM BACK ON"

[acey]

Filters are very reflective (because they only transmit small wavebands) so any ambient light bounces back into your eye. I've found them ineffective for naked-eye viewing for that reason. At a pristine site it might work, some people evidently think so. Fixing the filter in a cardboard tube and looking through that would be an improvement. But I'd rather have some lenses in there as well, that certainly improves things.

Edit: your post beat mine, pipnina, highlghting the same problem and likely solution.

[swamp thing]

The trick is to cup your fingers around the filter so it acts like an eyecup blocking out extraneous light. Simples

[jnb]

As far as I'm concerned naked eye means no optical aid. That does mean that naked eye will vary depending on location. So M33 for me is a naked eye object, if I observe from another location it is not.

[jetstream]

The trick is to cup your fingers around the filter so it acts like an eyecup blocking out extraneous light. Simples

It took me about 10 times trying the filter naked eye to figure this out Steve... I wish you would just tell us the tricks first

I found that doing this works for the NA neb, as its so huge in the first place, obviously from a very dark site. I sure like the way it looks through a telescope better though...

[brantuk]

Hmmm..... wow - that got a response lol

All I meant was - if I can see it unaided then it's a naked eye object. The Milky Way is naked eye and I know I can see it from a darker site - but here in town it's impossible with all the lights. Most folks can see M42 so that's naked eye - and M31 is the only galaxy visible naked eye to anyone.

But whether they can be seen at all depends on all the usual seeing criteria as well as individual vision abilities. I can only see 6 of the 7 sisters - but some folks can see 7 - so to me that's a naked eye object even though I can't see it. Great discussion though.

[pipnina]

Hmmm..... wow - that got a response lol

All I meant was - if I can see it unaided then it's a naked eye object. The Milky Way is naked eye and I know I can see it from a darker site - but here in town it's impossible with all the lights. Most folks can see M42 so that's naked eye - and M31 is the only galaxy visible naked eye to anyone.

But whether they can be seen at all depends on all the usual seeing criteria as well as individual vision abilities. I can only see 6 of the 7 sisters - but some folks can see 7 - so to me that's a naked eye object even though I can't see it. Great discussion though.

I don't even know where to look for the other 2 sisters. I only recognise the saucepan shape as the pleiades ^-^

[LukeSkywatcher]

M31 and M42 are both "naked eye objects"  from my observatory (A.K.A~my back garden). To say i can "see them" is a bit of an overstatement. I cant "detect" them with my eyes. Both are small,faint and diffuse..........but visible.

In saying this, i remember one night under a full Moon i went to look at M42 with my scope. I couldnt see,detect a single whiff of it. It was as if it had vanished. That was very strange. It should have been right there in the FOV.

[cotterless45]

Just one word missing, "contrast".

I was very sceptible about seeing dso's by eye until my first visit to a very dark site. It's just stunning to lay back and see M33, the Cygnus rift and so many puffs of clusters by eye.

I think the best weapon of choice to augment viewing is a blackened toilet roll tube. Make sure it's an empty one , or you're going to either look odd or desperate !

Nick.