We can't see stars in the daytime, with one obvious exception. Or, maybe we can...

I thought I'd throw this out and see if anyone wants to play with it.

The sky has a specific and knowable spectral irradiance (fig. 3).

What if a filter were made to adjust the blue daytime sky to a low and even spectral irradiance level? [Big "if", I'm sure.] Couldn't we see some stars as a result? Would it not greatly improve the signal to noise ratio, as in electronic filtering? I don't really know, but I think it would.

A bright red star like Antares, or Betelgeuse, with strong reds would not be attenuated by the filter as much and it's light might stand out enough to see with a scope. Venus and an October Mars (this year at least) should work fine.

I do not know enough about filter manufacturing to suggest the effort would be worth the while. You might have to know the exact spectral irr. for each point in the sky you wish to observe to be truly effective. So, even it the idea is sound, it's likely "too much squeeze for the juice".

Googling produced nothing for me on this. Is it nuts, or what?

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

The best shot would probably be the red stars, as you said, since their spectral distributions are most different from very blue scattered sunlight. In a very clear sky, and most especially 90 degrees from the sun, adding a polarizing filter would also help (one of the tricks used to get the surface features on Titan from Cassini). The best contrast would likely be from a filter that simply cuts out the blue (more detailed statement requires plugging in numbers and the eye/detector's sensitivity curve). This is related to the frequent observer's strategy of starting in twilight with redder or near-IR filters and working to the blue as the sky darkens and loses the blue scattered sunlight component (and vice versa as dawn impends), which can add an hour to the useful length of a night. For people using digital cameras, this could be very powerful, since their chips have much better IR response than the eye (which resulted in my completely overexposing a shot of the rising Sun with naked-eye sunspot a few weeks ago - it was just barely visible to me through the haze, but so much deep-red came through that it swamped the camera response). For those who remember the fiasco of the Saturn-like image behind Hale-Bopp, that was the same thing -a very red star, aberrated by the telescope optics, which was too faint visually to appear on a comparison chart. Everybody gets burned by passband effects - the best we can do is to only let it happen once. But I digress, and am not sure my license is paid up.

I've picked out maybe three stars naked-eye in the daytime, all from very clear observatory sites (Sirius, Arcturus, and Vega - the stars, not the observatory sites). With our 16" on campus, it's easy enough to show people stars to magnitude 2.5 or so - we can get the brighter component of Albireo, but the other one is tough. (This is in the context of "showing binary stars to class members who won't come back at night"). At our latitude, that leaves Castor as the only workable daytime double star.

Thanks. Quite interesting. I would not have guessed you could get down to 2.5 mag. with a 16 inch.

Using red/IR filters at twilight near the horizon makes perfect sense.

I wonder if you have tried a H-alpha filter to see other stars (in daytime)? I am guessing our atmosphere is weak here and other stars might shine through at this low scattering wavelength.

My hope was to adjust the overall sky irradiance to a specific desired level of intensity for each wavelength and, somehow, canceling it out leaving the background light remaining. Admittedly, it is an idea half-baked, so I'll think more about before commenting unless someone sees a way to tickle the background light out of it.

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

But he's seen 1 mag. with a 1/4 inch!

Seriously, I think the BA's book argues that that is not possible.

Even from the bottom of a well????

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

How 'bout this enhancement...

With the special hypothetical filter on the scope, use a color ccd unit where each pixel has its own color filter (Bayer system, IIRC). The filter would cause the color pixels to produce the same electron response rate as each other (since the scope's filter adjusted each wavelength accordingly). Any additional rates would be from a luminous background object, right? Normal processing could take it from there (e.g. interpolation).

Take it to the next level.... Eliminate the hypothetical scope filter and use the sky irradiance data to work within the software. The irradiance data would dictate each pixel's rate for sky only which could be nulled producing black sky, as well as, reveal any luminous object (produced from the additional pixel respone).

Is there any inherent merit, Garret?

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

Not sure where all this speculation is comming from - yes you can - not many - and you have to know exactly where to look. I have seen Sirius Canopus and Alpha Centauri, oh and Venus - all as they crossed the merdian in broad daylight

Glen

Naked eye, right? As I said, I think BA would be interested in that info. Are there cites in the literature too?

It comes from the deep end of a shallow pool. I'm just an amateur but I have a hunch there could be something to it. I have been working with irradiance data lately to help resolve color issues, hopefully, and this idea popped into my head. I thought I would toss the idea out to see if anyone had any thoughts on it.

How did you do that?

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

Yes, that'd be very impressive for the naked eye.
A circular patch of sky an arcminute across (at the diffraction limit of dilated pupils) has an apparent visual magnitude somewhere around -3.5. Diffraction turns Sirius into a similar-sized spot on your retina, but two magnitudes dimmer. So you'd be detecting just a 16% difference in luminance between sky+Sirius and sky-on-its-own. The usually quoted cut-off is that you can't detect a "point source" until the luminance difference is >50% of the background luminance, so conventional wisdom puts all stars (bar the Sun!) below the threshold for naked-eye detectibility against the daytime sky.

Grant Hutchison

The process was fairly easy actually, though takes a small amount of planning. I'd read about, and attempted daylight naked eye observations of Venus, and just adapted the process to find the three stars at different times.

Now please understand we were not seeing a strong hard light source like we did with Venus. With the three stars found the image kind of swam in and out of view being visible maybe 50% time. Everytime you blinked you had to try and bring them back in again.

The only real tip, if you can call it that - is to have a really good reference point to cut down the amount of sky you are trying to look at. Example with Canopus, I used the corner of an awning that hung from the back of my house. Alpha Centauri was sited right at the exact apex of of the roof of my home, and Sirius between to power lines that ran to my house from the back lane.

After a couple of minutes, once the stars moved away I could not find them again, and I suspect the eyes were gettting too strained anyway.

On the day we observered Venus I couldn't help but wonder if that wasn't where a fair number of UFO sittings came from, it seemed a lot more substantial than during the night, I'm thinking there was some sort of optical effect in play with that.

MG

Semi funny (to me) story that relates to this. I had just learned that Venus could be seen naked eye in daylight and had a good bit of fun showing it to people at work. One morning, I was off work and went out to get a 9 am peek at it without going through that hassle of actually knowing where it was to start with. I knew it was between the Sun and the Moon, at about 1/3 the Moon sun distance from the Moon. I looked for about 10 minutes without luck, so I went in to get the binoculars.

I began sweeping the sky and found a bright object. The problem is, it was green. I pulled down the Binocs and green spec vanished. I scanned the sky again and there it was. About where Venus should be, but green. I’m very much a novice, with no formal training of any kind, but I knew that for an object to show green through the daylight atmosphere it must be amazingly bright. I thought that I had found a supernova. I looked at it for a while longer with the binoculars and decided I needed to get the scope on it. It was a 4.5 inch newt but it was all I had at the time. I looked all over for the little green spot, but I couldn’t ever see it with the telescope. I could spot it very quickly with the binoculars.

After about 15 more minutes of this I had to give my girlfriend a ride to work. When I got back, I pulled out the Binocs, and it was still there. I grabbed the scope again and set it up as carefully as possible. I knelt down to match the angle of the scope to use the peak of the roof as a reference point. And still couldn’t find it. I finally decided that while it was easy to spot in the 7x50’s, the 6x30 finder may have been too small to pick it up.

45 minutes after first spotting the object, I got the scope on it. It didn’t seem to have moved much. It was still 1/3 of the way from the Moon to the Sun. It was still green. It was in focus in the little 4.5” and I could clearly make out the string hanging down from it. It was an escaped balloon that had somehow managed to hover in the same place, more or less, for 45 minutes.

On the up side, at least I confirmed what it was before calling someone to report my find and become "famous" : )

Are you in the Southern Hemisphere, or merely in the Tropics?

-- a jealous North American who has never seen Alpha Centauri in his life.

Somebody who does lots of observing will know this better than I. Either Sky & Tel, or Astronomy had a post that referred to looking through a long metal pipe at the blue sky, and you could see planets, and bright stars if you knew where to look. It cuts the scattered light, and improves contrast.

Anyone notice the ghostly Jupiter in the latest S&T? Very nice image of Jupiter taken during daylight.

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

It had never occured to me that Jupiter might have a high enough surface brightness to be an interesting daytime object, until a freshman student asked once. We pointed the telescope, and there it was. Sort of a Cheshire cat - the cloud belts and zones appeared near the disk center. but limb darkening made the planet fade away into blue sky well short of the limb. It's joined the list for showing to daytime school groups when Venus and Mercury aren't visible.

(Anybody else here see Don Parker's pictures of Jupiter taken during the Galileo probe entry, to document its weather after the loss of incoming orbiter images? They were the only visible-light ones I saw, being as Jupiter showed up only about 16 degrees from the Sun at the time. Even getting IR images from Mauna Kea was no picnic for the IRTF people).

That's pretty cool. The S&T article, I think, recommended viewing early in the morning when air turbulence is minimal. However, I don't remember any mention of the use of an orange or red filter. Surely, these would have helped. Did you have a chance to use a filter?

Grant's post still intrigues me. Can we combine both the sky's glow and Jupiter's magnitude into a net magnitude? Assuming -3.5 for the sky and -2.5 for Jupiter, this should yield a net of -3.9 magnitude, or about 45% brighter than the -3.5 sky. [using m[combined] = m1 - 2.5*log(2.512^delta m +1).] I suspect a possible physical explanation would state the sky is augmenting Jupiter's light, so it can be considered accumaltive, though bluer, of course.

I am also curious on how to calculate surface brightness of objects given their magnitude. It is my understanding the magnitude represents the integration of the surface brightness, thus producing a magnitude value equal to what the object would have if squeezed down to a single point object. If so, what is the formula for this?

More specifically, I would like to have a formula to determine the increase in magnitude necessary to produce photopic vision of a nebula. 10 lux (cd/m^2) would be dandy. Even 5 lux might be adequate (mesoptic vision). The Dumbell would be a perfect example [also serving nicely as a personal metaphor. ] I have not dropped the colorscope idea; albeit, I am surprised no one seems to have done this considering the afforability of large aperature Dobs.

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

I have seen Jupiter in daylight numerous times now, generally within an hour of sunrise or sunset. It is best if you can sight off the Moon.

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