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Moon-Pleiades-Mercury Conjunction - An Oldie But Goodie


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nrothschild Neil is an expert in several areas, including camera support Registered since 25th Jul 2004
Sun 19-May-13 03:53 PM | edited Sun 19-May-13 04:53 PM by nrothschild

On April 26, 2009 the roughly 1 day 21 hour old moon fully occulted the Pleiades star cluster (M45). This was the view on the East Coast USA, shortly before 8pm EDST. At this point it is properly termed a conjunction, I think, not an occultation.

The actual occultation occurred two hours earlier, somewhere in the middle of the Atlantic Ocean, perhaps seen by a few lucky ocean cruisers who knew to look for it. Or perhaps they bought a ticket just to see it

The moon moves fast enough that it passes through the Pleiades in only a few hours or less. Meanwhile, the window for photographing a thin crescent with the Pleiades is well under an hour because you need the sky dark enough to record the stars, yet the moon and the Pleiades is rapidly sinking in the western sky. As a result, only thin slices of longitude see this event at any particular stage.

Each spring, for several months, early successive crescent moons pass by the Pleiades. The Pleiades lies about 5° north of the ecliptic. As I have discussed recently, the moon's orbit relative to the ecliptic is tilted by about the same 5°. The occultations can only occur when the moon is at or near its northernmost extreme. The precession of the tilt has an 18.6 year period. As such this is a very rare event, although there were several occultations a few successive years ago, during this favorable time of the cycle.

I have visually observed several of these occultations via telescopes, and imaged at least one. Personally I find it a better visual sight because the occultation obliterates much of the beautiful Pleiades asterism, making it unrecognizable. In many ways, I actually like this setup here, where I missed the actual occultation but got this very nice compositional setup.

The occultation or conjunction of a one day moon always occurs in late May. Although in principle it would be a neat image, in practice I am unsure if the Pleiades would well register on such an early moon so deeply embedded in the dusk sky glow.

The occultation/conjunction of the second day moon (roughly 48 hours old) occurs in late April or early May, depending on the particulars of the specific lunation (precise time of new moon). I think this is the best time to shoot this because the sky is just dark enough to record the stars but the moon is still relatively thin and as such does not tend to interfere.

By the third day moon, some time in or around March, there is enough moon to start interfering with the stars, similar to how any moon interferes with stars, especially nearby stars. This is still shoot-able, just not as ideal, in my opinion, as the 2nd day moon in April. By the 4th day I think it is very difficult because that moon is quite bright.

The 2nd day moon also sets up better landscapes at that magic time at late dusk where the terrestrial scene is still visible and the moon is low enough that ultra-wide angle fields of view are not required, and therefore a reasonable size moon can be imaged.

This first image was shot on a Nikon D200 and 24-70/2.8 AFS lens, at 62mm. Exposure was f/2.8 2.5s ISO 800.

Mercury is the bright "star" below the moon and the Pleiades, about 1/3 the way down to the horizon. A conjunction of the moon and Pleiades is a rare enough event, but to get Mercury in there was superb. Mercury can be a very challenging object to record as well as it did here.

Click on image to view larger version

This second image was shot with my D300 and 70-200 f/2.8 AFS/VR (original version), at 200mm. Exposure was ISO 1600 f/2.8 1s. I do not recall the details now but I think I had two tripods set up, with different cameras and lenses, such that I could shoot these two basic compositions without having to do complex gear swaps.

As you can see, thin high clouds interfered somewhat with these images. And as I recall, that limited my shutter speeds. At that point in my life I was bothered by "haze" from the moon when doing this type of image.

In later years I've evolved, and now, in retrospect, wish I had shot, or at least retained, some longer shutter speed images. And as I suggest above, this is an image I may not be able to replicate in my lifetime. In any event, I suspect even today I would have selected these exposures. But then I'll never know for sure

Click on image to view larger version

This 3rd image was shot with the D300 and 300 f/2.8 AFSII. Exposure was f/2.8 1s ISO 640.

I had all the heavy artillery out that night! Including a 500/4 for some high res lunar images not shown here.

Note that although the exposure was over one stop down from the 200mm image (ISO 640 vs 1600) I captured fainter stars, and better impressed them into the image. That is because the ability to record stars is a function of physical entrance pupil size (basically front element size), not focal ratio.

Where you need to extend focal length in order to increase physical aperture, as is typically the case, that must be balanced against the more rapid rate of celestial motion when shooting from a fixed tripod, as was done here.

Click on image to view larger version

As an aside, some of the "stars" at the top of the first image are almost certainly not stars at all. I cannot match them up to a star atlas. At some point in time I learned that street lights from the terrestrial scene below tend to record as stellar looking objects in the sky.

In particular the short line of 4 "stars" in the upper right, and the 4 more widely spaced line of "stars" just below the short line of 4. I think I have only seen this with my 24-70 but it is likely related to the focal length, not the lens (??).

I figured that out when studying a series of similar night images and noticing a similar formation that did not move with the other true background stars. The false stars hovered in the same place in the image. I was thinking about sending the shots to UFO Hunters to see if I could get some quick money for them

Some day I will make a more careful review of the entire sequence and clone out all the fake streetlight stars.

I pre-planned these images well in advance, by studying sky charts generated by SkyMap Pro with various focal length fields of view inserted into the map. You can do the same with the very similar and excellent Cartes Du Ciell app. I think it almost mandatory to do so, given the very short time window when the dusk dynamic range works best. That is not the time to be fumbling around in the dark, swapping lenses.

We have a very nice evening dusk planetary conjunction coming up in June. I will talk about that in a few days. For now, I want to try to get members thinking about how to do this, and where they might shoot them.

Note: edited to correct some general estimated dates for various age moons coinciding with a Pleiades conjunction or occultation.


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