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Title: Viewing Sunspots Using Galileo’s Projection Method

Date Occurred: March 19, 2006

Date Written: March 26, 2006

Written by: Joel T. Kant

Copyright (c) 2006

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A warning that I have heard from childhood is not to stare at the sun because it can permanently damage one’s eyes. I have never personally tested that by doing it, of course! Another common experiment to do when the sun is shining bright to burn a sheet of paper with a small spot of intense light created with a large magnifying glass. Burning paper with a magnifying glass is something I did several times as a child, such as when trying it as a method to start a campfire in Boy Scouts. It worked as long as the sun was bright.

Since it is dangerous to look at the sun with the unaided naked eye, what should then be common sense is never to look at the sun through a telescope! It can lead to blindness, for what should be rather obvious reasons. My Meade DS-80AT telescope has a big warning sticker on it about not using the telescope to look at the sun. The Autostar computer for that telescope requires getting through a warning message about not using the telescope to look at the sun before the computer can continue to be used.

Despite the danger, it seems the inventor of the astronomical telescope, Galileo Galilei himself, looked at the sun through his telescope! Much later in life, he did go blind, but what relationship this had to looking at the sun through his telescope is debatable. He only looked at the sun only at sunrise or sunset when quite dim, which would have kept the brightness down. Also, unlike a modern telescope even available at a common department store for a hundred dollars, the aperture on his telescope was tiny. (Reference: Tom Pope’s web site on Galilean telescopes, http://www.pacifier.com/~tpope/index.htm, then submenu http://www.pacifier.com/~tpope/Photo_Drawing_Comparison_Page.htm and also submenu http://www.pacifier.com/~tpope/Galilean_Optics_Page.htm#Eyepiece_Projection)

Whether his vision was damaged or not by looking at the sun through a telescope, Galileo then discovered a safe method of looking at the magnified image of the sun and made discoveries about sunspots. On Tom Pope’s web site, the discovery of this eyepiece projection method is attributed to Galileo’s friend Benedetto Castelli rather than Galileo Galilei himself. (Reference: Tom Pope’s web site on Galilean telescopes, http://www.pacifier.com/~tpope/index.htm, then submenu http://www.pacifier.com/~tpope/Galilean_Optics_Page.htm#Eyepiece_Projection)

Even in the twenty-first century, this eyepiece projection method of looking at the sun to see sunspots remains safe for the human eye if only the projected image is looked at and one never looks down the eyepiece of the telescope directly. A book by Patrick Moore from as recently as 2000 recommends the projection method as the only safe method to look at the sun. (Reference: Philip’s Atlas of the Universe, Patrick Moore, “Our Star: the Sun,” (c) 2000, Published Great Britain in 2000 by Philip’s, a division of Octopus Publishing Group Ltd, 2-4 Heron Quays, London E14 4JP, This edition produced for Borders, 2003, p. 152.)

However, this eyepiece projection method is not always safe for modern eyepieces of the telescope. The problem with the modern eyepieces is many, especially the expensive ones, are made of several types of glass often glued together. The heat of the sun when using a telescope with the eyepiece projection method could damage these eyepieces, such as by destroying the glue bond holding the different glasses together. If using a modern telescope, it is not recommended to use an expensive, valuable, modern eyepiece. (Reference: http://www.astro-tom.com/telescopes/eyepieces.htm, with e-mail astro-tom.com and Lake County Astronomical Society of Illinois, http://www.bpccs.com/lcas/Articles/sol-obs.htm, with e-mail for Webmaster: webmaster@lcas-astronomy.org, and Stockton Astronomical Society, Valley Skies, June 2001 Issue, “The Telescope Nut,” by Jeff Baldwin, http://astro.sci.uop.edu/~sas/Newsletter/TTN_SolarViewing.html)

The eyepiece projection method is not the method most sungazers today seem to prefer. What apparently produces the best, dramatic images of the sun is either a hydrogen-alpha filter before the aperture of a telescope or else a dedicated telescope with a hydrogen-alpha filter built in. The name of the filter refers to a spectral line of hydrogen. Most of the light in the spectrum of sunlight is blocked by an optical filter, but that line excited from hydrogen is let through at some level, generally designed to be safe to the human eye. A product that does this is the Coronado PST, with the PST supposedly standing for Personal Solar Telescope. (Reference: Described in Hot New Products for 2005, Sky & Telescope, Vol. 109, no. 1, January 2005, pp. 106-107.) It seems this gives dramatic and colorful images of the sun, with the corona clearly visible. The Coronado PST also has a list price of $499, which is a lot for me to spend even if the article claims that is less than a third the cost of the nearest competing product!

If a sungazer prefers seeing the light generated by the entire spectrum of the sun, another common method is to use a special dark filter over the aperture of a telescope that blocks the vast majority of the incoming light. There are various advertisements in astronomy magazines for these kinds of filters, although I have never used one. These are far cheaper than hydrogen alpha filters. A warning for this method is that the filter be put on in some way that it cannot possibly fall off when using the telescope to look at the sun. If it were to fall off, it is back to the situation similar to using a magnifying glass to burn holes in a sheet of paper, but with the paper replaced by the retina of the eye! For this method, it is important that the bulk of the sunlight be filtered away before entering the telescope aperture because if a filter is put on the eyepiece end instead, intense heat might shatter the filter, which would then let blinding sunlight through. (Reference: Stockton Astronomical Society, Valley Skies, June 2001 Issue, “The Telescope Nut,” by Jeff Baldwin, http://astro.sci.uop.edu/~sas/Newsletter/TTN_SolarViewing.html and Philip’s Atlas of the Universe, Patrick Moore, “Our Star: the Sun,” (c) 2000, Published Great Britain in 2000 by Philip’s, a division of Octopus Publishing Group Ltd, 2-4 Heron Quays, London E14 4JP, This edition produced for Borders, 2003, p. 152.)

Another thing to avoid that might otherwise be overlooked is accidentally getting a glimpse down an unfiltered, unblocked finder scope. In many telescopes, the finder scope is simply a smaller telescope. This finder scope also needs to be made safe for observing the sun as well as the main telescope optical tube.

I decided to try the eyepiece projection method invented by Galileo Galilei or by his friend Benedetto Castelli. It does not show the corona like the hydrogen-alpha filters, but has the advantage of being relatively safe and very cheap. It will show sunspots. The only additional equipment was a couple pieces of white cardboard, although I soon added to that an empty black 35 mm film canister and a piece of masking tape.

Even with the eyepiece projection method, there is a danger if children are participating. If not very closely supervised, a child might sneak a peak in the eyepiece rather than look only at the image projected on white cardboard.

I have built a replica of a Galilean telescope. It features only two lenses, a plano-convex as the objective and a concave-concave as the eyepiece. It has none of the multiple glued lenses of some modern eyepieces.

A problem with my Galilean telescope is an extremely narrow field of view. I grew so frustrated when using it at night that I made a second Galilean telescope with a BB gun sight attached as an inexpensive finder scope. I have been far happier using the telescope after adding the finder scope, even if not remotely historically accurate to do that!

I wondered how to point this telescope at the sun without looking up at the sun, especially since it has a narrow field of view. I found a solution to my dilemma on Mike Weasner’s web site devoted to Meade ETX telescopes. This particular method described by Mark Gibbons does not depend on any particular type of telescope. I also had to adapt it slightly. (Reference: 35 mm film canister method of sun finder located at http://www.weasner.com/etx/menu.html, then submenu http://www.weasner.com/etx/techtips/2004/sunfinder.html, which brings up a post written by Mark Gibbons, mark@mrgibbons.com)

Mark Gibbons’ idea uses a common 35 mm film canister. I have mostly switched to using digital cameras, but I still have a 35 mm film camera I occasionally use for action shots when the time delay for the digital camera is unacceptable. This meant I had some 35 mm film canisters sitting around. Mark Gibbons recommended punching a pinhole in the center of the bottom of the canister. He had a film canister that had a top that was translucent. If his film canister is pointed up at the sun, a small and bright dot appears on the translucent top. By attaching the canister to his telescope, it served as a finder scope for the sun without ever having to look directly at the sun. (See Fig. 1)

Figure One: Film Canister With Pinhole

[Photograph by Joel Kant, 5:00 p.m., March 19, 2006]

The top of my film canister was opaque. I saw no spot when I tried this method using my top. What I did instead is simply cut a piece of common paper masking tape into a circle and stick it on the top of the canister. I could see the spot of light coming from the pinhole easily through the masking tape itself. (See Fig. 2)

Figure Two: Small Spot on Masking Tape

[Photograph by Joel Kant, 5:00 p.m., March 19, 2006]

I covered the finder scopes that I put on one of my Galilean telescopes. I actually have two on that replica Galilean telescope, so I made certain both were covered. One is the BB gun sight. Another is made from lenses taken from a 4X pair of binoculars, but the BB gun sight works better as a finder scope. I had these both covered. I attached the 35 mm film canister.

I also stuck a piece of cardboard with a hole over it at the eyepiece end of the telescope to shade where I would look with the another piece of white cardboard at the projected image of the sun.

The 26X magnification of this telescope gave a nicely magnified image of the sun, yet with the whole disk visible. (See Fig. 3)

Figure Three: Image of Sun Using Replica of Galileo’s Telescope

[Photograph by Joel Kant, 5:10 p.m., March 19, 2006]

I decided to try this method as well with my Meade 60 mm Compact Refractor. I have a Meade WA (wide-angle) 18 mm Plossl eyepiece that I like, so I did not use that because of the risk of cooking it, as I mentioned earlier. However, at almost the same focal length, I have a 17.5 mm Kellner eyepiece that I almost never use. The field of view is narrower than the 18mm Plossl, yet the image does not seem as sharp. I felt the 17.5 mm eyepiece was expendable. I taped a 35 mm film canister finder on this telescope as well. The magnification of this telescope with this eyepiece is 20X. (See Fig. 4) This little Meade telescope has no finder scope, so blocking that was not an issue.

Figure Four: Two Telescopes Set Up For Projection Sun

[Photograph by Joel Kant, 5:08 p.m., March 19, 2006]

The Compact Refractor with this eyepiece also gave a nice view of the full disk of the sun. The image was considerably brighter than from the Galilean replica, although I could see the same sunspots with both. (Fig. 5) When I used my digital camera to take pictures of the sun’s projected image, the photos did not seem to have the contrast that appeared to my eye.

Figure Five: Image of Sun Using Meade 60 mm Compact Refractor

[Photograph by Joel Kant, 4:53 p.m., March 19, 2006]

The 17.5 mm Kellner eyepiece seemed undamaged when I was finished.

I tried playing with zooming in and adjusting the contrast of the digital image to better show the sunspots. (See Fig. 6)

Figure Six: Zoomed In and Contrast Adjusted Sun Image

[Photograph by Joel Kant, 4:53 p.m., March 19, 2006]

REFERENCES

Tom Pope’s web site on Galilean telescopes, http://www.pacifier.com/~tpope/index.htm, then submenu http://www.pacifier.com/~tpope/Photo_Drawing_Comparison_Page.htm and also submenu http://www.pacifier.com/~tpope/Galilean_Optics_Page.htm#Eyepiece_Projection

Philip’s Atlas of the Universe, Patrick Moore, “Our Star: the Sun,” (c) 2000, Published Great Britain in 2000 by Philip’s, a division of Octopus Publishing Group Ltd, 2-4 Heron Quays, London E14 4JP, This edition produced for Borders, 2003, p. 152.

http://www.astro-tom.com/telescopes/eyepieces.htm, with e-mail astro-tom.com

Lake County Astronomical Society of Illinois, http://www.bpccs.com/lcas/Articles/sol-obs.htm, with e-mail for Webmaster: webmaster@lcas-astronomy.org

Stockton Astronomical Society, Valley Skies, June 2001 Issue, “The Telescope Nut,” by Jeff Baldwin, http://astro.sci.uop.edu/~sas/Newsletter/TTN_SolarViewing.html

Hot New Products for 2005, Sky & Telescope, Vol. 109, no. 1, January 2005, pp. 106-107.

35 mm film canister method of sun finder located at http://www.weasner.com/etx/menu.html, then submenu http://www.weasner.com/etx/techtips/2004/sunfinder.html, which brings up a post written by Mark Gibbons, mark@mrgibbons.com

THE END

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