Stars in daytime?

[Andy Perrin]

Stefano made the following suggestion:

During the day, we can not see stars and planets because the sky is bright enough (Rayleigh scattering) to cover them, but if you point a telescope at the biggest planets in daytime you can still see them

. In UV I cannot see the moon in daytime, and only managed to get some faint images. Now, in the NIR, Rayleigh scattering is much reduced, and this means that, maybe, stars could be seen in daytime. Can someone try pointing a camera directly to the sky, in daytime, with a long-pass filter (like 1000 nm), and see if he/she can see something? That would be crazy awesome.

 

We had the first super-clear blue-sky day in a long time here in Massachusetts today, so I gave it a whirl. Sadly, the answer seems to be "no," at least so far. But maybe others will have suggestions.

 

My setup was:

- Sony A7S (modified)

- EL-Nikkor 80mm/5.6 metal (which is my sharpest lens)

- Thorlabs 1100nm long pass filter (blocked only to OD4) rear-mounted using putty on the back of the lens

- 1000nm absorption glass (Chinese, no brand...) on the front to beef up the out-of-band blocking

 

First, a typical photo with this setup:

F/5.6, 4", ISO50

Processed in Aurora HDR from RAW

 

Next, a photo of the sky, around 80 degrees from the horizon (so, nearly straight up). I don't know what is making the strange pattern, but it's probably stray reflections, junk on my sensor (which needs cleaning...), etc.

F/5.6, 10", ISO100

Processed in Aurora HDR from RAW

 

The Aurora HDR software is extremely good at bringing fine details out of the shadows, so if there were any stars to be seen here, we would be seeing them. Instead it seems that the remaining Raleigh scattering is still sufficient to drown the stars out, even in the 1100-1150nm region which the camera is capturing (based on the previous experience with the solar panel).

[OlDoinyo]

4" at ISO 50 is not much exposure. At nighr you might not get much with that, and the stars are no brighter in the daytime. Try playing with exposure, selecting a small rectangle of sky, and jacking contrast to the max

[dabateman]

I still want to try this. But haven't had clear sky yet. I am hoping it works.

What wavelength of IR filter do you think would be best. I was thinking 800s as want to stay clear of any water absorbing peaks.

 

[Stefano]

Here https://www.google.com/url?sa=t&source=web&cd=1&ved=2ahUKEwipkL2u2sLnAhXTOcAKHSrPCaoQFjAAegQIBRAB&url=https%3A%2F%2Fwww.ultravioletphotography.com%2Fcontent%2Findex.php%2Ftopic%2F3617-infrared-with-a-solar-panel%2F&usg=AOvVaw35lDTUAUZGegkWkLVnC0Dc I had f/2.8, ISO 80 and 60 s of exposure as settings to capture those star photos. Try to make a longer exposure. Very helpful would be to try the exact same thing in nighttime (we are sure that stars can be seen in those hours). Keep in mind that I used a full spectrum camera with no filters, and that surely makes a difference.

[Andy Perrin]

4" at ISO 50 is not much exposure. At nighr you might not get much with that, and the stars are no brighter in the daytime. Try playing with exposure, selecting a small rectangle of sky, and jacking contrast to the max

That wasn't the exposure I used for the star photo. I used 10" at ISO100 F/5.6, which is easily enough for stars at night. The contrast is already to the max. Bear in mind that the Sony A7S is optimized for low light photos, it's among the best you can get. But the original image before tweaking contrast was mostly gray. The problem is a lack of signal to noise ratio for the stars due to remaining scattered sunlight.

 

Unedited (except for size reduction and JPEG) version of the above:

10" at ISO100 F/5.6

 

By all means, other people should have a try at this, but I can say that at 1100-1150nm, it does not seem to work. The humidity today was 26%, and the temperature was around 0C, with only a few scattered clouds near the horizon, as shown in the first photo of the original post. Conditions were ideal. My feeling is that unless you try it at high altitude, or another wavelength works better, then this is probably not going to work.

[colinbm]

Apparently if you look up a tall chimney or a mine shaft you can see the stars....

[Andy Perrin]

Apparently if you look up a tall chimney or a mine shaft you can see the stars....

Haha, sure, at NIGHT! (Or else I think this is fantasy.)

 

ETA: It's been tested! (And of course it doesn't work. But experiment is king.)

https://www.livescie...rs-daytime.html

 

Interestingly, that article says that sometimes especially bright stars can be seen with magnification. Maybe what I should have done is use a longer lens...

[colinbm]

Thanks Andy....:-)

[dabateman]

Well I now think we have this all backwards. I was thinking about solar light. We need the exact wavelengths with the maximum solar penetration without absorption from anything in the atmosphere. That means I think we need 490nm to 500nm.

Look at this image I randomly pulled:

The peak max through everything is about 490nm.

So time to test our Suba #729 or #727 Lee filters. They peak at 500nm to 490nm.

Unless anyone has an OIII filter. That might be the best and what some Astrophotographers may still have on there scopes when they see stars in daylight.

[Stefano]

You want to have the least sunlight with the least scattering and the maximum atmospheric penetration. I would suggest using 1200 nm (but silicon cameras shouldn’t be able to see that far), or alternatively 1050 nm.

[ulf]

My bet is on a short exposure time while still having a rather good darkening of the sky by a filter. Long-pass >800nm?

 

Using a light-sensitive tele lens gathering more light is also a plus, especially if it is pointed at some known bright stars.

During the day I think the air turbulence can smear the light from small objects like stars, especially during longer exposure times.

[ulf]

Without knowing any details I think that at least some versions of FW in the Sony A7S "eat stars" in the raw images trying to hide any sensor defects.

[dabateman]

Will have to try both ways. My thoughts were that if the sun is a model star. Then the maximum wavelength emmission from the star would be 500nm. So if I want to capture the brightest stars, I would need 500nm.

Ultimately, I don't think this will work. As Astrophotographers would be doing it.

[Stefano]

Red giants peak towards red/infrared. If we can see the sun, we should also be able to see other stars, since the intensity (“concentration”) of light is the same regardless of the distance. The reason why stars are not blinding when seen through a telescope is because the atmospheric turbulence makes them appear bigger. The moon is much less bright than the sun, but it is still visible during the day.

 

Again, I think that it is mandatory to try the same experiment in nighttime, and see if stars can be seen in the infrared in that moment. They should.

[Andy Perrin]

Without knowing any details I think that at least some versions of FW in the Sony A7S "eat stars" in the raw images trying to hide any sensor defects.

That only happens in bulb mode, even for those cameras that are affected. You can’t take photos of stars for over 30sec without star trails for most lenses. (Google “rule of 500”.) I’ve done a fair bit of night photography, including Milky Way, so I’m not totally new at this.

[Stefano]

Can someone plot Rayleigh scattering, solar spectrum, atmospheric absorption and camera sensitivity, all multiplied together? This way we can find the sweet spot.

[Andy Perrin]

You want to have the least sunlight with the least scattering and the maximum atmospheric penetration. I would suggest using 1200 nm (but silicon cameras shouldn’t be able to see that far), or alternatively 1050 nm.

I used 1100-1150nm above.

[OlDoinyo]

Diffuse sky radiation from scattering seems to level out beyond 850 nm. My suggestion would be to use a filter blocking below this wavelength, and to expose so that the sky is almost (but not quite) blown out. Then select the target area in the image and jack the contrast all the way up with the Levels adjustment (just below clipping.) If any stars are imageable, they would most likely be the class K, M, and N bright stars--think Arcturus or Betelgeuse. O and B stars such as Rigel, Alnilam, or Mintaka are going to be tougher, as the filter will dim them more.

 

Low ISO will provide less noise, and image stacking may help pull detail out.

 

I understand some supernovae have been visible to the naked eye in the daytime, but they are a whole lot brighter than normal stars.

[dabateman]

Can someone plot Rayleigh scattering, solar spectrum, atmospheric absorption and camera sensitivity, all multiplied together? This way we can find the sweet spot.

 

I think it ends up being 660nm.

Best camera sensitivity.

A plateau in the solar spectrum, seems to be between 656nm and 675nm.

Slightly less rayleigh scattering of that more darkish red wavelength.

 

This is not my data but one I found here somewhere. Correction Jonathan provided it.

[JMC]

That's one of my graphs David. Measurements taken in Surrey, UK. Can't remember time of year or day, but can check if needed.

[dabateman]

Stacks I might test

Tiffen 29 + Bg40 2mm = 600nm to 680nm

Lee 787 + BW486 = 650nm to 720nm

Lp720 + BW486 = 700nm to 720nm

2mm Ug1 + lp720 = 700 to 800nm peak max 750nm.

Lee 87 + Grb3 = 730nm to 820nm

Bn850 = 820nm to 880nm

[Stefano]

I did try this. All images f/2.8, ISO 80, 60 s exposure. Full spectrum Panasonic DMC-F3. The almost full moon lit up the sky, and this affected the results.

 

No filters.

 

IR with my ugly pen ink filter (it isn't sharp at all, but I managed to capture something).

 

IR+UV, 3 mm thick ZWB1 layed on the lens.

 

Well, I didn't have a lot of luck. I surely would have expected much more from the pen ink filter.

 

I really do believe in this experiment. Andy, when you can and your sky is clean, try to expose for a really long time, or try to point near the north pole, so stars don't move around a lot. You MUST see something, the sky is so dark at 1150 nm.

 

Anyway, we need to explore better how deep your camera can see into IR in another topic. We are already almost sure that you can go beyond 1100 nm, can you even reach 1200 nm? That would mean seeing the next water absorption peak, at 1205 nm. Maybe your camera can go surprisingly deep into NIR, who knows.

[Andy Perrin]

I really do believe in this experiment. Andy, when you can and your sky is clean, try to expose for a really long time, or try to point near the north pole, so stars don't move around a lot. You MUST see something, the sky is so dark at 1150 nm.

But I DO see something! What I see is a pure gray sky, which is the remaining Raleigh scattering. So long as the Raleigh scattering is brighter than the stars, no amount of exposure will make them stand out, because it's like a recording of someone whispering in a rock concert. You can turn up the volume but you won't hear the whisper because the music is always louder.

 

The question is whether with high magnification, the brighter stars might compete with the scattering better. I will try again with a 200mm lens. (The challenge becomes that at 200mm, the longest I can expose for is 2.5 sec without star trails.)

 

Anyway, we need to explore better how deep your camera can see into IR in another topic. We are already almost sure that you can go beyond 1100 nm, can you even reach 1200 nm? That would mean seeing the next water absorption peak, at 1205 nm. Maybe your camera can go surprisingly deep into NIR, who knows.

No, I'm fairly sure it's gone by 1200nm. It's also very hard to be sure you aren't seeing light leaks with nearly the entire range of the camera blocked. 1150nm was already nearly gone with the solar panel. Remember that silicon goes TRANSPARENT at 1200nm. You can't capture photons when your light buckets have no bottom!

[Stefano]

No, I'm fairly sure it's gone by 1200nm. It's also very hard to be sure you aren't seeing light leaks with nearly the entire range of the camera blocked. 1150nm was already nearly gone with the solar panel. Remember that silicon goes TRANSPARENT at 1200nm. You can't capture photons when your light buckets have no bottom!

A silicon filter would be great to test this. Does the sensitivity simply drop to almost zero but never zero, or does it reach absolute zero at a point? Anyway, we are getting off-topic. We should open a new one and maybe Andrea could move this posts there.

[Andy Perrin]

Answer over in the other thread.