Monday, May 13, 2013

The Galileoscope As A Solar Observation Instrument

One of my favorite telescopes from an experimenter's standpoint is my Galileoscope. It has been used with my various video astronomy cameras, my Meade Lunar and Planetary Imager, afocal photography, and of course traditional astronomy. I've been looking at ways to use it for solar work, which is tricky, and of course somewhat risky.
(Warning - looking at the Sun with any astronomical instrument, without the proper filters and tools, is dangerous, period. Not only is it dangerous to your eyes, it is also potentially damaging to your instrument, as we will soon discover)
It was actually Galileo himself who made some of the first solar studies, and used his telescopes in this extensively, primarily through lens projection. These observations were almost always made with the Sun low on the horizon, either at sunrise or set. Contrary to legend, these are not what led to his eventual blindness.
I already have a homemade solar filter that fits the end cap of the Galileoscope perfectly, however due to the straight through nature of the telescope, it is somewhat risky to use, primarily because once you move your eyes away from the eyepiece, you are looking straight down the telescope, and at the Sun itself. What is needed is a shield.
I made a shield from a 10" x 10" (250mm x 250mm) cardboard. In the center of this square, I cut a 2 1/4" (56mm) circular hole that allows the shield to fit smoothly over the back of the Galileoscope dew cap. It should fit flush against the back of the dew cap extension.
Initially setup, it worked great with my solar filter; I normally use a neutral density filter as well, and this fits over the eyepiece.

Here we see it with an afocal setup, which sadly did not work, primarily because the camera has a hard time with focus (wanting to focus on the eyepiece or the shield; you cannot disable the auto-focus on some models). Aiming the telescope proved a little tricky, but was not too difficult.
But what about solar projection?
This is where caution is needed. Even with the Sun at a 30° angle above the western horizon (a point where its energy is cut almost in half), the amount of sunlight coming down is more than enough to damage the inside of the telescope, and especially the eyepiece. The lenses weren't damaged, but the internal face of the eyepiece was.

Still, it worked. I used a circular plastic to-go food container as my screen

One thing I'd recommend, however, is to move the screen back. Optimally, using the 20mm (Plossl) eyepiece, I'd recommend between 12" to 14" (300mm to 350mm) distance from the eyepiece to the screen.

At 8" (200mm); the image is small but bright.
At 12" (300mm), the brightness drops, but contrast improves, allowing for more details.
Using this technique, you can clearly see sunspots. 

The sunspots

Based upon my experiences today, I'd recommend the following to anyone who wants to do solar projection with the Galileoscope - 
  • Use another 20mm eyepiece, one that uses metal in its construction. This should prevent the sort of damage we witnessed to our Plossl.
  • To help in aiming the telescope, a hole might be cut in the shield that lines up with the sights. This hole should be no more than 1/4" (6mm) in diameter. Try not to look directly into the hole, instead look to see if the sunlight that comes through it projects over the sights and onto, say, the screen or even your hand. 
  • Try to focus the telescope before aiming for the Sun. Find some very distant object and preset the focus, not only to simplify the operation but to make it safer for equipment.
As usual, I will share any additional findings and improvements I may make.
But remember, if you choose to try this, remember, again, be careful. And as always, have fun.

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