I have learned that it is not possible to watch the upcoming Mercury transit with the plain eye (using special filter glasses).
Is it possible to watch the transit with improvised devices (like a camera obscura)?
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$\begingroup$ Yes it is, viewing a transit is identical to viewing an eclipse because you still need to see the sun but without looking directly at it with your naked eye. I answered a question on viewing an eclipse here (astronomy.stackexchange.com/a/13950/10437) which gives a similar method to a camera obscura. $\endgroup$– DeanApr 22, 2016 at 15:11
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$\begingroup$ @Dean: My concern is the size of planet Mercury: Will it be visible? $\endgroup$– Sir CornflakesApr 22, 2016 at 15:40
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Mercury's angular diameter on transit day will be 12 arcseconds. A camera obscura using a 12 mm aperture could resolve it; one lens from +0.75 diopter reading glasses, if you can get them, will project a bright 12 mm image of the Sun at a distance of 1.33 m. Note that a larger aperture or a shorter focal length will make the Sun image hotter than direct sunlight unless you add a filter.
Test with sunspots before relying on it for Mercury. If it counts as improvised, projection with 7x35 binoculars easily showed me the 2012 transit of Venus.
Update: I tried both of the above with the Nov 2019 Mercury transit. With handheld wobbling, neither showed it. Someone else's 50mm binoculars did, probably because they were mounted on a tripod.
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1$\begingroup$ Binocular projection worked for me. In the dry run yesterday, I saw a sunspot. Today there were clouds, but fortunately not all the time. We saw a second spot that changed its location slowly :-) $\endgroup$ May 9, 2016 at 17:56