Artificial Lunar Eclipse

[solissydney (Ken)]

How to create a shadow, similar to a partial eclipse, on the full Moon ?.
I need to take an image of the full Moon, together with a few stars on the same image.
If, i could create an artificial shadow on part of the Moon would making the inclusion of a few stars close to the Moon much easier than just taking care with the exposures.
I have tried creating a shadow by placing a small black piece of cardboard close to the lens, and also further away, even a couple of meters away. It needs to be in focus in order to be sharp and that I have been unable to achieve.
Any ideas on how to create such a shadow?

[Wavytone]

Can't. You need a real earth, with an atmosphere, about 230,000 kms from the moon.

You'd do better to rely on some decent photos others have taken during real lunar eclipses; some do show a few stars.

A large part of your problem stems from the extreme range of illuminations involved. Prior to the penumbral phase the moon is 100% fully illuminated and the backreflectance is high, making it a bit brighter than an "ordinary" full moon, and in a decent photo of this there is NO chance whatsoever of recording any stars.

As it goes into the umbra the illumination drops by many orders of magnitude to the extent that while fully eclipsed you can photograph magnitude 6 stars in close proximity to the Moon. Between 100% full and fully eclipsed the change in brightness easily exceeds the dynamic range of digital cameras.

Lastly, the reason I suggest you need a real earth to do this is the effect of the atmosphere which scatters reddish light into the earths shadow, causing the colours we see in a real lunar eclipse. Short of using a real photo I don't see how you can simulate this with a crude mechanical mockup.

It can be simulated in computer software fairly well, though most of the simulations I have seen were based on real eclipse photos.

[Poita (Peter)]

Maybe a Coronagraph would work, I was just wondering this myself last week.
http://www.considine.net/dgroski/promscope/

[solissydney (Ken)]

Building a Coronagraph could be another weekend's project, Well, maybe a few weekends.
Yes, I am aware that one can't create a shadow on part of the Moon. I learned that soon enough last night. But when you mentioned the word Corona I sat up this morning and took notice.
Thanks for that tip.

[naskies (Dave)]

Originally Posted by solissydney

If, i could create an artificial shadow on part of the Moon would making the inclusion of a few stars close to the Moon much easier than just taking care with the exposures.

I can think of two issues with trying to image stars close to a bright moon: (1) exposing for the stars will grossly overexpose the moon and lead to major sensor blooming on a DSLR, and (2) the glare from the moon will drastically brighten the surrounding sky like light pollution.

Without creating a "real" shadow on the moon as Wavytone suggests, I think you'd only really be able to control #1 and not #2.

I have tried creating a shadow by placing a small black piece of cardboard close to the lens, and also further away, even a couple of meters away. It needs to be in focus in order to be sharp and that I have been unable to achieve.

Think of it like in normal photography... if you're focused at (effectively) optical infinity, a piece of cardboard close to the lens - or even just a couple of meters away - is sooo far away from the focal plane that you won't be within the depth of field.

If you want a sharp edge, you'd probably either need to use an object further away (e.g. edge of a mountain in the distance), or use an occulting disk very close to (or even directly on) the sensor.

I wonder if using a narrowband filter like Ha would help? You'd drastically cut down on moon glare while still letting a lot of star light through.

Originally Posted by Wavytone

Between 100% full and fully eclipsed the change in brightness easily exceeds the dynamic range of digital cameras.

Going by my exposure values for the Jun 2011 lunar eclipse, the difference in brightness from full moon to total eclipse was over 15 stops...

Short of using a real photo

Here's a great example.

[Nightshift]

Originally Posted by Wavytone

Can't. You need a real earth, with an atmosphere, about 230,000 kms from the moon.

Actually, that would be miles, Klms would be 384,400.

I disagree, I think it could be done using something black and large, say a lid from a plastic garbage bin (or bigger) placed much further away, say 30-50 metres and then use a long focal length lens and zoom right in, this is how they make the moon look rediculously large in movies, place the actors 100 metres from the lens in front of a rising moon and zoom right in. Of course this is tricky if you want the moon at a high altitude.

Good luck, Dennis.

[Wavytone]

Sorry, must have had a blonde moment LOL...

Well... here's one way to do it, good enough to try, if you have a zoom camera.

Start with a good quality gloss print of the fully eclipsed moon complete with starry background, pinned flat at the end of a long hallway (or similar), and a camera on tripod at the other end. Zoom the camera in on the moon so it sees just moon and stars (and not your hallway).

Note this assumes the background in the photo is really good dense black, and the reason to use gloss is to reflect light away but not into your camera whereas matte won't appear such a dense black.

You could edit the shot to add a few stars (throw in the Pleiades for good measure); the example linked below would do nicely.

Set up a lamp (your "sun") behind the camera, ideally a frosted incandescent bulb rather than a pinpoint (quartz-halogen) source as you want soft shadows, not a harsh edge. If you want to make this realistic, your "sun" should subtend 0.5 degree as seen from the moon (do the math to figure out how far that means for a light bulb).

As you suggest, use a circular disk of paper (a garbage bin lid is larger than you need and unnecessarily heavy) halfway between the lamp and the print of the moon to produce a diffuse shadow mimicking the earths shadow. The illuminated part is effectively the penumbra, the shadow the umbra. A rough guess suggests the disk will need to be 1.5 - 1.7X the diameter of the moon as it appears on the print, to give a realistically curved penumbra. Move the disk closer to the lamp to soften the edge of the shadow.

With some experimentation I would expect you could take a photo showing stars and a partially "eclipsed" moon.