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Daytime Stellar Astronomy?

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#1 GregBuchholz

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Posted 27 February 2012 - 08:04 PM

The recent post about Venus at 1PM got me thinging about seeing stars (besides the sun) during the day. Wikipedia thinks that the blue sky has an apparent magnitude of about -4.0. The sky is blue because of Rayleigh scattering, which is proprotional to the wavelengh of light to the fourth power. So what if you had a filter that cut off everything shorter than 680nm. That might cut down the daytime "sky glow" by a factor of 9. It is also interesting that Sirius (magnitude -1.47) is about 10 times less bright than the blue sky (2.512 ^ (-1.47 - -4.0)). Of course, the filter is going to cut some of the light from Sirius, etc.. So first order we might not be able to see any stars with the eye at noon, but what about using instruments in the infrared? Do they have a chance?

Thoughts?

(Supernova SN 1006 was apparently a magintude -7.5, visible in daytime).

#2 Tony Flanders

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Posted 28 February 2012 - 05:43 AM

Wikipedia thinks that the blue sky has an apparent magnitude of about -4.0.


That's not phrased properly. What Wikipedia says is that the naked-eye limiting magnitude at noon on a clear, cloudlesss day is -4.0. The sky itself is much, much brighter than that -- probably around magnitude -24, I would guess.

I'm skeptical of that claim; I bet that Jupiter is visible at noon under the correct conditions. But what's a magnitude or two between friends?

So first order we might not be able to see any stars with the eye at noon, but what about using instruments in the infrared?


As soon as you introduce instruments, you're playing a different ballgame. Stars down to 3rd magnitude or fainter are visible through a telescope at noon when the atmosphere is really clear -- even before introducing infrared.

But yes, the sky is dramatically less bright in infrared. Professional observatories often do critical infrared observing during twilight and moonlight, when the sky would be too bright for visible-light astronomy.

#3 archer1960

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Posted 29 February 2012 - 08:55 AM

[quote name="Tony Flanders"][quote]Wikipedia thinks that the blue sky has an apparent magnitude of about -4.0.[/quote]

That's not phrased properly. What Wikipedia says is that the naked-eye limiting magnitude at noon on a clear, cloudlesss day is -4.0. The sky itself is much, much brighter than that -- probably around magnitude -24, I would guess.
...

The Sun has a magnitude in the -20's, so the sky itself can't be nearly that high. I would guess that -4 or so is probably about right (or at least somewhere in the single digits). It's easy to see the moon during the day if the phase is right.

#4 bassplayer142

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Posted 29 February 2012 - 09:44 AM

I read somewhere that in a few months, Venus will be visible during broad daylight. I'm sure if this is observed it would shed some light on this topic.

#5 Tony Flanders

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Posted 29 February 2012 - 01:46 PM

I said:

The sky itself is much, much brighter than that -- probably around magnitude -24, I would guess.


You responded:

The Sun has a magnitude in the -20's, so the sky itself can't be nearly that high. I would guess that -4 or so is probably about right (or at least somewhere in the single digits).


I think you're confusing total (integrated) brightness, surface brightness, and limiting stellar magnitude.

Total brightness is a measure of how bright an object is when all its light is counted together. Surface brightness is the intensity of the light per unit area. Naked-eye limiting stellar magnitude is the brightness of the faintest star that's visible to the unaided eye.

The Moon is visible during the day because its surface brightness is higher than the sky's surface brightness. But because the sky is 180 degrees across and the Moon is 1/2 degree across, the sky's total brightness is on the order of 100,000 times greater than the Moon's total brightness.

The Sun's magnitude is -26.7. At sea level when the Sun is overhead, 12% of that light is scattered away. Figure that half of it is scattered to outer space and half down to Earth, and that means that 6% of the Sun's light ends up in the blue sky.

6% is 3.0 magnitudes, yielding a total magnitude for a perfectly clear blue sky of -23.7. Pretty close to my guess of -24, considering that I just guesstimated!

As everyone is aware, the light from the blue sky is extremely bright. I have no trouble threading a needle standing outside in the shade of a tree, with the needle illuminated only by light from the blue sky.

#6 ChipAtNight

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Posted 29 February 2012 - 02:16 PM

Venus was easy to find on Feb. 25, 2012 from about 1:00pm EST on. It was very close to the Moon and flashed in a out naked eye and easy to find with Binocular, doing some public outreach that day we helped about 15 people find Venus in the daylight.

#7 Jb32828

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Posted 29 February 2012 - 06:59 PM

At some point didnt someone get a photo of the ring nebula in broad daylight with like a 14" or 16" Meade SCT? I remember reading and seeing the photo at some point here in this forum.

#8 GregBuchholz

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Posted 29 February 2012 - 07:27 PM

With the right keywords, Google is our friend. Here's the link to the previous discussion that mentions the Ring Nebula in daylight. And here's the picture of M57 after post-processing some video (scroll down to the bottom of the page).

#9 GregBuchholz

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Posted 29 February 2012 - 07:30 PM

Total integrated brightness doesn't seem like the quantity we want for computing the signal-to-noise ratio for trying to find objects in a sea of sky-glow. What is the correct technical term for the brightness metric that we should be using?

#10 Andrev

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Posted 01 March 2012 - 04:51 PM

Hi guys.

Interesting subject. Last week for the first time of my life (almost 52), I spotted and saw Jupiter during daytime. That was really fun to see. A very pale disk but clear enough to see the two main band. As for Venus, very spectacular during day and much more than night time. I love looking at Venus during day.

Does Sirius would be visible during day ? I have to check that.

#11 _Z_

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Posted 01 March 2012 - 06:01 PM

Back in 2004ish the moon occulted Jupiter and was near Venus in broad daylight. Using the moon I was able to see find both of them, Venus was easy naked eye and Jupiter was definitely visible in the telescope.

Attached Thumbnails

  • 5099895-moon and jupiter in daytime 2302 edited and croppedA.JPG


#12 Cathal

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Posted 01 March 2012 - 06:25 PM

I've seen Saturn and Jupiter during the day, with my 8" newt on lxd-75.

I've also seen both with the ETX-70 that I used to have.

#13 Andrev

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Posted 02 March 2012 - 06:54 AM

Hi.

Oh wow, what a nice sight !!! I have to check if there is another Jupiter's occultation in a near future, I would like to see that. And now you just make me curious to try Saturn in daytime. Its definitely my goal in the next days.

Andre.

#14 jumna

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Posted 03 March 2012 - 03:08 AM

I've been trying to find cloud markings on Venus during the day this past month, so I'll grab a quick look during the afternoon if I have a chance with a #47 or a skyglow filter and sometimes see a fleeting...something. I'll also check Jupiter just for fun, having also grabbed it's RA/Dec from Stellarium. Most of the time the seeing for me is poor during the day, sometimes terrible, rarely it's a little good. I guess that's the only way to see Jupiter at zenith for awhile.

#15 jgraham

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Posted 03 March 2012 - 11:16 PM

Last spring I set my LXD75 AR-6 outside in full daylight to do some alignment checks. Just for yucks, once I had the mount aligned I used the GoTo to visit several bright stars. I was able to easily see Sirius, all of the 7 brightest stars in Orion, as well as Castor and Pollux. I could also easily see Castor's companion. Based on that I estimated that I could see stars down to about magnitude 3. Pretty neat!

#16 izar187

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Posted 04 March 2012 - 12:08 AM

Saturn and it's rings are telescopically visible during the day.
Muted like Jupiter, but just as cool to see.

I'm not sure about Mars. Hmm...

#17 Andrev

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Posted 04 March 2012 - 07:38 AM

Hi

Now my friends you just make me curious o see that. I never imagined I would be able to the stars during daytime. Beleive me its my next exercise in the coming days.

Andre

#18 Cotts

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Posted 04 March 2012 - 09:07 AM

Interesting thread but there seems to be three separate topics here.

1. Naked Eye Daytime viewing.

2. Telescopic Daytime viewing.

3. Mallincam Daytime viewing.

Also, magnitudes per square arc second is a very good way to discuss sky brightness - kind of an accepted standard these days.

Dave

#19 Andrev

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Posted 05 March 2012 - 07:46 AM

Hi

Weather seems good today to have a try at some daytime stars. I'll keep you informed on that.

Andre

#20 Andrev

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Posted 05 March 2012 - 06:34 PM

Hi.

Now my friends I can tell you I'm totally excited about what I seen in my 14" scope during the last two hours at -11 C in my dome feeling -18 (0 F) with the wind. Starting at 4pm, my first target was Mercury and what wasn't my surprise to see it !!! WOW, that is a first for me and a 20 years gap since I watched Mercury in a scope. What a nice sight. I watched it periodically from 4pm to 6pm until it was too low in the airmass.

The sight was so nice that I called a friend to join me and he accepted because he is starting in astronomy. He brave the cold weather for one hour watching Mercury, Venus, Jupiter, the moon.

But another first for me and once again the excitation was at the highest peak when I spotted a couple of stars after my first sight at Mercury around 4pm. Geez, that is very very cool and they are very bright and easy to see. That is a great experience for me and now that will become a very nice attraction for the public at the star parties...

Thank you to all of you for introducing me to this new direction of the hobby and the great moment I experienced with my friend. He also was very impress to see all this while the sun is still ubove the horizon.

Andre.

#21 kw6562

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Posted 06 March 2012 - 01:02 PM

Some of us discussed this in a thread in the Science of Astronomy forum - link --Keith

#22 Nadrek

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Posted 06 March 2012 - 09:58 PM

Does anyone have experience with using various filters during the daytime to enhance planetary or deep sky (double star?) astronomy?

I'm quite curious, particularly about some of the OIII or HAlpha filters.

#23 PJTramdack

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Posted 13 March 2012 - 06:59 PM

I understand that the biggest problem of naked eye daytime viewing of the bright planets and Sirius is the focus of your eye. Your eye supposedly focuses naturally at about a few hundred yards whereas you need to focus at infinity to see the object. That is why it is much easier to see Venus or Jupiter if they are in the same area as the daytime moon. Once your eye is focused on the moon the planets are easily visible. I can't remember the trick for focusing at infinity on a blank sky.

#24 buddyjesus

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Posted 13 March 2012 - 10:07 PM

I focus during the daylight by using projection on the sun and getting the edge of it as sharp as possible. I dont know how it would be done naked eye though if your eye doesn't do it automatically.


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