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This website presents digital images taken through a telescope made from two 1-inch diameter singlet lenses. Such a telescope is an approximate optical replica of that used by Galileo Galilei for his epoch-making astronomical discoveries of 1609-1611. Although we have not personally had the pleasure of looking through Galileo's telescope, we believe our photographs are a close approximation to what Galileo saw. Even if they are not, they show the remarkable detail that can be seen through a small and very simple telescope constructed from good quality singlet lenses.
The replica telescope consists of a 25.4 mm diameter plano-convex objective of 1000 mm focal length stopped down to 23 mm and a 25.4 mm diameter plano-concave eyepiece of -50 mm focal length giving a power of 20X. The objective is mounted at the front end of a piece of 1.7-inch diameter PVC tube, while the eyepiece is mounted on a slightly smaller tube that slides into the rear. The lenses, both of whose flat surfaces face the interior of the tube, are held in place with plastic computer desk cable bushings obtained from a hardware store. Several baffles were placed inside the PVC tube to suppress reflections from its shiny interior surface. Since baffles were unknown in Galileo's time, Galileo's telescopes did not contain them, but their effect is no different from using a larger tube with a less reflective interior surface.
The whole is mounted piggyback on a classic 8" Celestron Schmidt-Cassegrain telescope as shown here. Besides providing a clock-driven tracking platform, the piggy-back mounting affords two additional advantages: (1) the Celestron's finder scope and setting circles can be used for locating objects and (2) an extension tube inserted into the Celestron's eyepiece holder provides a convenient means for supporting the digital camera behind the eyepiece of the Galilean telescope.
The following pictures were all taken with a 3.3 megapixel Olympus Camedia C3000Z digital camera operating afocally (photographs taken through the Galilean telescope eyepiece using the normal Camedia lens focused approximately at infinity) as shown above. The digital camera substitutes for the observer's eye and shows approximately what a human observer would see. In addition to the components shown, the live video output from the camera is displayed on a small television monitor. This greatly assists in fine-tuning the pointing and selecting moments of good seeing (i.e., moments of minimal atmospheric blur). To eliminate vibration the camera is always operated using its remote control. With the camera lens set at its maximum zoom (optical+digital), the image scale on the CCD is 0.73 arc-sec per pixel. All of the photos have been cropped and some have been reduced in scale for more rapid downloading to this page. Clicking on most of the reduced photographs will either show a larger image or display additional photos and information.
The pictures of the Moon, Sun and Venus are reproduced (and were usually shot) in black and white because the neutral grays more accurately reproduce how these very bright subjects are seen in the eyepiece. Otherwise the camera and/or filter (in the case of the Sun) impart a color cast which has little or nothing to do with the visual appearance.
It should be emphasized that the lenses used in this replica, although simple singlets, are still high quality commercial lenses. Although they have not been tested, we believe their surfaces are near diffraction-limited (i.e., close to perfect spheres), as, we believe, were Galileo's. Attempts to reproduce these results with a telescope constructed from low quality or inappropriately shaped lenses, such as drug store magnifiers or eyeglass lenses, will almost certainly lead to disappointing results. The long focal length objective is particularly critical since nearly its full aperture is used. For more information about the telescope, see Parts and Assembly of Present Telescope and our suggestions on How to Make a Galilean Telescope.
Although a Galilean telescope uses, by definition, a negative eye lens, some of the following photos were taken after substituting a positive singlet eye lens of the same power (Keplerian mode). The reason for doing this was to demonstrate the field of view, as shown below :
Left: An area near the Moon's center photographed on June 6, 2003 through the telescope operating in the Galilean mode. Only a small area, centered on the deeply shadowed crater Albategnius, is visible. Among Galileo scholars, Albategnius seems to be the favorite candidate for the crater that inspired the large feature depicted in Galileo's famous engravings of the First and Last Quarter Moons, one of which is reproduced in the next section. The field of view is a circle approximately 14 arc-minutes in diameter. Although this is a very limited field, different parts of the moon can be seen by moving one's eye around the eyepiece. With a 23 mm diameter negative eyepiece roughly five 14-arc minute fields can be seen in this way across any given diameter. Right: Photo taken on the same evening using a +50-mm focal length plano-convex eyepiece. When the telescope is used in the Keplerian mode (positive eyepiece), the entire field of approximately 55 arc-minutes diameter, more than the angular diameter of the Moon, is seen in a single view. Albategnius is near the center of this view, and rather prominent if you click to see the enlargement. The area viewed through the positive eyepiece is not noticeably affected by moving the eye around the eyepiece. Additional photographs illustrating the difference in field with terrestrial subjects are given on a separate Galilean Optics page. Galileo originally thought the field of view of his telescope was strictly proportional to the diameter of its objective (front lens), but in fact the field visible in Galileo's telescopes depended on a number of other factors including the size and position of the observer's pupil.
Note: to the eye, the images with the positive and negative eye lenses have essentially the same 20-power scale and resolution; you simply see much more at one time with the positive lens. The right-hand image shown above appears to be smaller and to have less resolution only because with the positive eye lens, the digital camera was zoomed out and the image scale much reduced to show the full field of view of the telescope. If taken with the same camera zoom and reproduced to the same scale, the photographs taken with the positive and negative singlet eyepieces would be nearly indistinguishable over their common field (see example) . Note also that to the eye the Keplerian view is inverted (upside down and reversed) whereas the Galilean view is erect. The Keplerian view shown above has been inverted to aid in comparison with the Galilean view.
Left: The Moon at First Quarter, photographed on June 7, 2003 using the positive eyepiece. The large faint crater low on the terminator (most evident in the enlargement) is Deslandres. The deeply shadowed crater below it, half way to the Moon's south pole is Maginus. Center: Engraving of the Moon at First Quarter as it appears in Galileo's book Sidereus Nuncius (1610). Right: An area slightly below the Moon's center photographed through the negative eyepiece on June 7, 2003. Albategnius is just above the middle of this photo. The three large craters of descending size to its left are Ptolemaeus (top), Alphonsus and Arzachel. Deslandres is the large crater at bottom left.
The small region of the Moon depicted in the right-hand photo is essentially the same as that shown above under "Field of View". It appears different because of the change in lighting. In the previous photo the terminator is a little more to the right making Albategnius more striking. As the terminator sweeps to the left, Albategnius becomes less prominent while the neighboring chains of craters, previously hidden in the shadows, are revealed. Galileo was fascinated, as are many modern amateurs, by the changing appearance of the craters from night to night. An example of the Albategnius region at the Moon's Last Quarter (i.e., with lighting from the opposite side) is shown in the center photo in the following section. For additional examples of Galileo's observations of the Moon and other astronomical objects, see our Photo-Drawing Comparison page.
The identification of the features shown in Galileo's lunar engravings is rather controversial. Adding to the confusion is the fact that due to an apparent rocking motion of the Moon (known as librations) the orientation of the Moon's feature relative to the terminator (the line dividing light from shadow) can be quite different from one lunar cycle to the next. Galileo left no hint of the date depicted, so it is impossible to be sure how closely this engraving corresponds to what he actually saw. The First Quarter Moon seen by Galileo could have looked substantially different from the one we photographed on June 7, 2003; but it does not seem possible to find any modern photo that matches any of Galileo's Moon drawings in every detail. Because Galileo could see only a portion of the Moon at any one time, his engravings are evidently mosaics of many small fields like the photograph shown at the right. This presumably accounts for some of the distortions in scale and position that are evident when the engravings are compared to modern photos, but it does not seem sufficient to account for all of them. They seem to be more schematic impressions of the kinds of details that Galileo saw at the different phases.
The most easily identifiable feature in the present engraving would seem to be the dark oval shown ringed by chains of bright mountains near the top. This seems to be a fairly accurate representation of the Sea of Serenity (Mare Serenitatis), the oval just to the right of the terminator in our photo for June 6, as it appears a bit before the First Quarter. Its identity seems to be confirmed by Galileo's juxtaposition of it with the three connected dark circles that look like Mickey Mouse upside down. The largest of these would be the Sea of Tranquility (Mare Tranquillitatis), site of the first human moon landing. Unfortunately, the Sea of Serenity is always well to the right of the Moon's centerline. Even with the most extreme librations it can never straddle the terminator at First Quarter as shown in the engraving. An alternative interpretation is that the oval Galileo shows on the terminator is the much larger Sea of Rains (Mare Imbrium), which can indeed straddle the terminator at First Quarter, as it does in our June 7 photo. Unfortunately, the mountains surrounding Mare Imbrium never give the impression of the nearly complete circle shown in the engraving. It seems possible that Galileo may have combined impressions from sketches made at a variety of phases to produce a single engraving, forcing the terminator to make a straight line down the middle.
The identification of the large crater Galileo shows below disk center is especially controversial. Galileo's description (in Latin) is usually translated something like this: "almost in the center of the Moon there is a cavity larger than all the rest, and perfectly round in shape. I have observed it near both first and last quarters, and have tried to represent it as correctly as possible." He also says he was impressed by being able to see the far wall illuminated by sunlight before the boundary of sun and shadow has moved half-way across the bounded plain. Many believe it is the crater Albategnius shown larger in size (to emphasize its structure) and lower in position than in real life. Albategnius is indeed very striking when it falls on the terminator, as in our June 6 photo shown in the previous section. In the June 7 photo, the much larger, though less striking, crater Deslandres is prominent. The curving bright lines which Galileo shows emanating from the top and bottom of his large crater might even match the shape of the bright ridges seen immediately above and below Deslandres in our photo (click on the left photo to see an enlargement). Unfortunately Galileo emphasizes both in the drawing and in his description that the floor of his large crater is bright on the side away from the Sun and dark on the side towards it. While this is true of small bowl-shaped craters, the floor of Deslandres is so large and flat that the Moon's curvature always causes it to be bright on the side towards the Sun and dark on the side away from it, as can be readily seen in our photo. Moreover, neither Deslandres nor Albategnius ever appears on the terminator at the same time that Mare Serenitatis is illuminated the way Galileo depicts it at the top of the engraving. A completely different crater, Maurolycus, is prominent on the terminator at that time (about a day before First Quarter). So, in the absence of a definite date it may never be possible to determine exactly what Galileo was trying to show. For additional examples of Galileo's Moon drawings, and the speculation about when and how they were made, see our separate Moon Page. We also have a page giving predictions of future dates on which the Moon will resemble, in lighting, the way it looked on the dates when Galileo is said to have made his various drawings. If you have a chance to see the Moon through a telescope on one of those dates, you may see something similar to what Galileo saw.
Left: The lunar highlands, with Clavius prominent as the large crater (deeply shadowed and with smaller craters inside it) straddling the terminator. The terminator is the broad transition from dark to light at the left in this photo. The sharp line at the bottom is the Moon's edge or "limb". The Moon's limb is not visible in the center and right photos. The small, very high contrast crater above and to the left of Clavius is Tycho, the crater from which prominent bright rays appear to radiate when the Moon is full. Center: Going down the terminator in this photo, Hipparchos and Albategnius are the two craters at the top, while Werner and Aliacensis are the deeply shadowed craters near the bottom. The last quarter illumination, with the left side of the Moon in sunlight and the right side in shadow, is opposite to that in Galileo's engraving and in the previous photo of Albategnius. The large crater at the top of the peanut-shaped structure just to the left of Albategnius is Ptolemaeus, with Alphonsus being the smaller part of the peanut, and Arzachel the prominent crater just below them. This is nearly the same region of the Moon as the photo illustrating the field of view (above), but with the lighting from the opposite side. The section of Galileo's watercolor painting of the Moon shown on our Photo-Drawing Comparison page appears to illustrate this region with nearly the same lighting shown here. Right: The circular basin of Mare Imbrium (crossed by terminator on the left) with Plato prominent near the top. The Montes Apenninus are seen curving out from Eratosthenes near the bottom.
Left: A composite of six 1/5th sec exposures of Jupiter taken on February 14, 2003 showing the northern and southern equatorial belts as dark bands running horizontally across the disk of the planet. Galileo did not notice the belts on Jupiter. Their visibility here has been enhanced both by enlarging the image and increasing the contrast as explained on the Mars Page. Although Jupiter was 45.2 arc-seconds in apparent diameter at the time of this exposure, as viewed at 20 power it is so bright that it appears to the eye only as a tiny, nearly featureless white ball.
Right: The four Galilean moons of Jupiter (from left: Europa, Callisto, Io and Ganymede): a 16 sec exposure taken on Nov. 28, 2002 at approximately 2:10 am PST. For the moons to be visible, the disk of Jupiter must be highly overexposed bringing out the halo of violet light that results from chromatic aberration. A normal exposure of Jupiter's disk has been pasted over the original overexposed image to indicate the true scale. Click here or on the photo for additional details.
Left: Saturn - an average of ten 1/2-second exposures taken shortly after midnight (PST) on Dec. 1, 2002. The inset shows a single 1-sec exposure of a bright star taken under similar conditions, demonstrating how close Saturn is to the size of the diffraction disk of the Galilean telescope. The disk of Saturn is approximately 20 arc-seconds across, and appears very tiny in the eyepiece. Galileo was never able to decipher the nature of the rings, whose appearance changes as Saturn tips towards and away from the Earth. This particular view is very similar to the image he published in The Assayer (1623): see the Photo-Drawing Comparison page. Click here or on the image to see photos of Saturn's moons, and to get a more accurate impression of how small Saturn actually looks through a 20-power telescope.
Center: Venus on April 25, 2004 - an average of eleven 1/200th sec exposures taken in the evening sky at 7:11 pm PDT. Venus was 33% illuminated and 33.5 arc-seconds in diameter. Galileo's observations of the changing phases of Venus convinced him that Venus orbited the Sun rather than the moving in a circle between the Earth and the Sun, as was generally believed at the time. Click here or on the image for more Venus photos.
Right: Composite of ten 1/15th-sec exposures of Mars taken on July 30, 2003 from 2:10 to 2:12 am (9:11 UT; Central Meridian = 3°). Mars was 22 arc-seconds in diameter. The south polar cap is visible at the bottom of the image, as well as a bright feature in the upper left on the sunlit east limb. As with Jupiter, Mars was too small and bright for Galileo to detect any surface detail of this sort. Click here or on the image for more Mars photos.
In his initial astronomical experiments with the telescope, Galileo attempted to piece together a number of drawings of extended star fields. These drawings, which were published in Sidereus Nuncius, cover areas much larger than that visible at any one moment through the eyepiece of the Galilean telescope. The image at left is an attempt to recreate the experiment by pasting together some twenty small star fields around the Pleiades star cluster. Click here or on the picture for more detail about our observation of the Pleiades.
At the suggestion of his student, Castelli, Galileo looked at Mizar, the middlemost of the three bright stars that form the handle of the Big Dipper, and was easily able to resolve it into two stars of unequal brightness with centers separated by a distance of approximately 15 arc-seconds. Galileo and Castelli assumed this was a chance alignment of two stars of similar brightness at quite different distances from the Earth, and hoped to see a periodic annual change in their spacing change due to parallax. Our modern understanding is that these are two stars at the same distance with the dimmer one very slowly orbiting the brighter one. Their relative motion is so slow that their current separation is scarcely different from when Galileo observed them in the 1600's. Click here or on the picture for more detail about this observation.
Galileo also observed the Trapezium region in Orion.
In a notebook entry from 1617 he described and clearly drew the three brightest stars visible in this 4 second exposure.
The camera reveals a faint fourth star 9 arc-seconds above and to the left of the star on the extreme right
(you may need to adjust your monitor to see it).
This star was apparently too dim for Galileo to see.
Click here
or on the picture for more detail about this observation.
Left: The full Sun photographed on June 11, 2003 using the 50 mm focal length positive eye lens (Keplerian mode). Three spot groups are easily visible in the thumbnail shown above, and a fourth in the enlargement of this photo. Right: The two spot groups near the right limb photographed with the 50 mm negative eye lens (Galilean mode). The large complex approaching the west limb (sharp line at right) is NOAA Group 0375. The isolated spot at 8 o'clock is the lead sunspot of NOAA Group 0377, also visible in the full disk photo.
Left: November 25, 2002. The sharp line on the right is the West solar limb. Right: June 4, 2003. The east solar limb is the sharp line at 10 o'clock. The sunspot group is NOAA 0375, the same as that shown approaching the west limb in the previous pair of photos, which show it in a later stage of its development.
Left: NOAA Spot Group 0486 coming over the West solar limb (sharp line at 7 o'clock). Right: NOAA 0484 near disk center. Both photos are approximately 1/250th sec exposures taken at 12:28 PDT on October 25, 2003.
Note: all solar photos were taken with a piece of Baader Planetarium mylar filter material placed over the entrance lens. Galileo primarily observed sunspots using eyepiece projection. Their changing speed and shape as they passed over the limb convinced him they were thin, flat objects on the solar surface. Examples of Galileo's sunspot drawings can be found on the Photo-Drawing Comparison page.
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Images (unless otherwise credited) © Tom Pope and Jim Mosher
Last modified: May 22, 2006
