Sunday, July 10, 2016

Solar Adventures With Opera and Field Glass

If you've been reading the blog, you know that I've been thinking about "retro-astronomy" a bit. Recently, I decided to read, once more, Garrett Serviss' classic, "Astronomy With An Opera-Glass". My copy is over a century old now, so I find myself alternating between its delicate pages and an ebook version on my Kindle. The way it is written is a sheer delight, a peek at the way popular astronomy was handled at the time, and you can see within its pages how it was evolving.
In the last section of the book, Serviss deals with the Moon, the planets, and lastly the Sun.
The method that Serviss recommends for observing the Sun is the use of smoked glass over the eyepieces. This is pretty similar to the same method used by astronomers for some time at that point, though of course it wasn't the only one.
I could go into many reasons why this is pro and con. Today, of course, one could simply put a solar filter over the objective lenses and be set.
What puzzled me, though, was why Serviss didn't recommend solar projection.
Admittedly, with low magnification, the image would be small, but if the room where the image was projected was dark enough, the surface onto which the image was projected could be set far enough away so as to present a decent sized image.
The problem, though, is one of resolution. I've found that direct observation, by means of a solar filter and a decent sized instrument (even as small as 35mm) is best. For projection, the instrument has to be larger, at least 40mm, to present a clear image.
The other concern is the size of the spots that may be on the Sun. Smaller ones will not show up in low resolution projections. The day this was written, 10th July, 2016, the Sun only had three sunspot regions on its surface, and all of them were small.

An afocal shot of the Sun taken the morning of 10th July, 2016. Camera was a Kodak C180, telescope a Tasco 50mm 6TE-5 with an 18mm Plossl eyepiece. Processing done in Gimp. Image by the author.

This doesn't mean that solar projection with an opera or field glass can't be done, of course. These are merely some of the challenges. 
I decided to investigate this on my own. The first thing I did was to cobble together a mount on a tripod. For this, I used some corner brackets, a 4" and a 1 1/2" (100mm and 38mm). The larger bracket was bolted to the tripod head with a 1/4"-20 nut, and then the smaller bracket to the larger one, aimed backwards. This would be my main mount.
For each instrument used, a different method of attachment was required. This was done with a combination of steel straps, bolts, nuts, and coat hanger wire. The first instrument used, my old Lemaire opera-glass, was fairly straight forward, using just the steel strap and a bolt with a wing nut.

This glass has very low magnification, around 3x, so the projected image was expected to be small. For my "screen", a piece of Crescent Board was taped to another tripod head.

As expected, the image was small.

But it was sharp. It is very likely that larger spots can be seen here (though, of course, the current batch of small regions would not be).
The next instrument used was my Airguide 4x40mm field glass. This required both a strap and coat hanger wire.

While the objectives in the Airguide are achromats, they are not the best, especially on bright targets. The Sun is, of course, very bright.

The Sun's image was so bright, in fact, that it was hard to get a sharp focus. 

Is it likely that direct observation with a solar filter with this instrument would yield better results? Perhaps. That would be something to test at another time.
These tests, though, prove that it was possible to observe the Sun using a different, and safer, method that was available. More than likely, if anyone was to undertake this, it would have been best done either in the morning or at sunset, with the Sun at a ow angle, and projecting the image into a darkened room at a distance. More importantly, however is that it could be done.
Perhaps, soon, I'll attempt to try that method. For now, it is enough to know that it could have worked.