Galileo's Original Telescopes: New measurements of  their dimensions  with special optical rulers.
Click here to see the errors in the literature about the optics Galileo used in his telescopes

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 Jim & Rhoda Morris   781 245 2897   K1ugm@comcast.net

 Recently  (12/4/2008) new and more accurate measurements of the length of both telescopes were made by IMSS    The new   lengths for  IMSS inventory 2427 is (1273 mm not 1360mm) and IMSS inventory 2428  is (927mm. not 980mm). This is very good news for all of us especially those that build models of these famous telescope. We all appreciate the very careful and good work done by the staff at the IMSS. Thank you IMSS.

A little  history; As we progressed through the task of making  creditable replicas of Galileo's surviving telescopes for  major museums and other educationally institutions the  IMSS staff were very kind and helpful  in every aspect in our literature search by sharing their records and giving us access their data. There was  sufficient ambiguity in the  physical dimensions of Galileo's telescopes IMSS 2427 and 2428 reported in the literature to justify making measurements of our own.  Basically the published physical dimensions for the length of the telescopes when compared to the dimensions required by the reported optics were inconsistent and would give significant blurring of the images if used to make replicas.    These errors were too large to be ignored in building and using our replicas. It was clear that the lengths had to be checked out with new measurements. So with the help of Dr. Giorgio Strano - Curator at the IMSS  we  made measurement  of  the instrument, while they were still in the case, by a variety of techniques  on both the IMSS 2427 and 2428 telescopes. Our measurements did show clearly that both telescopes were considerably shorter than reported.  The  methods that we employed are abstracted below. and  are reported in more detail. Click here for more fun in the detail of these experiments.   In part as a result of this work IMSS reviewed the lengths of both telescopes out of the case  and up dated  the lengths with new and more accurate results. These new data are reported  on their very  lively web sites. Again thank you IMSS for your good work it is so very helpful. to all of us. and a wonderful example of scientist working together to achieve the best data. for our and others  projects to replicate these instruments. WE have made very fine replicas  for the the IMSS in Florence Italy, Griffith Observatory, the  Adler Planetarium, and President L. B. Johnson's Library and Museum in Austin, TX. two of our replica's IMSS inv #2427 and 2428 are on the exhibit at Beijing Planetarium  Beijing, China

(The IMSS at Florence Italy has acquired 3 versions of our replicas of IMSS 2428: one full telescope,  a second constructed in two halves to show the interior workings of the instrument, and a third consisting again of two half models less the outside leather cover to demonstrate the details of the novel wood construction. We understand that our replica will be loaned out for an exhibit at the Pitti Palace some time in February 2008)
 

On our trips to the IMSS we took up the challenge to make our measurements of the original instruments through the display case windows in the exhibit  hall before normal exhibit hours.

The windows of the display case are of adequate optical quality to make useful optical measurements. For these measurements  we designed and built our own special portable optical rulers  which are described below. We compiled  two sets of data, one in 2005, using a single telescope on a substantial tripod, the second in 2007, using two microscopes  connected to a rail and ruler. A pictorial description is presented below for both techniques. A comparison of the two sets of data showed an agreement to within + or - 3 mm.
 

Length Measurements

In the beginning of our project, making the most precise replicas of Galileo's telescopes, we found that the dimensions in the literature just  don't add up	The portable measuring stand we assembled and used to measure Galileo's  telescopes at the IMSS in Florence, Italy in September 2005	Making the actual measurements of the original telescopes in their cabinet with the telescope on a tripod method at the IMSS, September 2005.  Here we are measuring the diameters. Over its 400 year history, the leather covered telescope has become less round and no longer has a truly  circular shape.  However we have vertical and horizontal photos from which we have  made diameter measurements. It would be valuable to have measurements of the circumference at various stations along its length, but this was not possible on either of our visits.

Below Rhoda, Jim and the staff at the IMSS, July 2007, reviewing and comparing the data in the literature and records on the lengths of IMSS 2427 & 2428 telescopes.

Rhoda and staff checking out IMSS inventory catalogs at the  for information on Galileo's telescopes		Rhoda and Dr. Strano, curator of the IMSS , making a visual overall comparison of our replica of 2428 with the original Galileo instrument.

The July 2007 repetition of our original 2005 measurements of the length of the Galileo's original telescopes using a new version of our optical ruler designed especially for this task of making the measurements from outside their cabinets. The optical ruler  is composed of two microscopes with cross hairs. One microscope views the left hand side of the telescope the other the right side. They are connected with a centimeter scale.

Below are some photographs of the equipment and measurement process.

Corrections had to be made to our measurements for 2428 because it was not parallel to the case. This amounted to a fraction of a millimeters in over all length. The two arrows show the rotated position of the 2428 which is above the arrows. The objective end of the telescope is pointed at the camera.

On Jim's  80 th birthday and after after completing  the  delivering of several of our replica's to the IMSS we with with Dr. Strano, curator of the IMSS,  made one more fast measurement by a third technique of  the length of the  IMSS 2428 telescope.

We did this  using a draftsman right triangle, a tape measure, and a few strip of black tape. It was a bit crude certainly  fast and accurate to a few mm..

Using the  right angle drawing triangle shown above  to obtain a simple rough measurement of   the  length of Galileo IMSS 2428 telescope while it's   still in its the display case. This was used to make a quick estimate of the overall length of the 2428 telescope during  our visit to the  IMSS delivering three of our replicas telescopes to them

The photo above shows the process from the telescope's point of view while positioning the edge of the right triangle to one end of the telescope. The narrowest end the drafting triangle (the blue line in the foto) is place on the surface of the display case window and slid back and forth while sighting down the right angle edge of the triangle at the telescope. . When the edge  is registered correctly on the end of the telescope a piece of black tape is fixed to the display cabinet's window at the intersection of the right angle intersection of the drafting instrument marking that end of the telescope. The process is repeated at the other end of the telescope and the distance between the tapes is measured with a tape measure.

This view the triangle is too far to the right of the end of the telescope.	For this view the triangle is too far to the left of the end	In this view The triangle is very close to being on target at the end of the telescope and will be marked with a strip of black tape by a colleague while the observer maintains the position of the drafting square.

 

When the  drafting square  is registered a piece of black take is fixed to the display cabinet's window marking the end of the telescope and the observers move on to the other end.

The telescope is cantered in its case so that it is not parallel to the window.. Additional measurements were made from the side of case making it possible to correct  this potential error.

Again we very much thank the staff  of the IMSS in Florence Italy for their help and patience during this projects especially Dr. Giorgio Strano  Curator of IMSS who is on the left of the photos  above helping with the measurements. The photo on the right shows the team  including Rhoda getting data to correct for the angular displacement about the vertical plane of leather telescope.

The data in the literature  generated for the focal length of the objective of 2428 and 2427  around the first quarter of the last century by Ronchi is 95.6 cm and that by Molesini etal in the last quarter is 98.0 cm. We had chosen to favor the latter data because it is the most recent and must certainly have been check because the authors must have been aware of the difference and checked their results carefully. We authors' are looking forward to a resolution of these figures which maybe done in the near future. It is important fro a number of reasons among them the eye piece nub! The eyepiece nub? Side view of the eyepiece nub on the original. IMSS. c 1930  overall view showing  of Guilio Cipriana replica now at Adler Planetarium.  No nub. as an  eyepiece End view of the nub on the original. IMSS c 1930  Eyepiece and its decorations of Guilio Cipriana replica now at Adler Planetarium. Note  No nub. as an eyepiece The authors believe that there is a convincing set of arguments that can be made for getting rid of the little paper covered eyepiece holder, that we call a nub, and move the lens back into the original mount. this would shorten the optical length about 21 mm  which would then accommodate a 956 focal length objective  the data  that Ronchi reported in 1923. The telescope would then be short enough to accommodate this lens and still be able to focus at infinity.   In fact the old inventory data at the IMSS records a total telescope length of 0.92meters.  We assume that this measurement is without the nub, since it is so much shorter than our measurements of the original telescope length which includes the nub.  If this assumption is true  then it is further evidence that the nub  was not considered a part of the original parts list.  It is also consistent with the overall lengths we measure with the nub since adding the nub length of 1.6 cm to the 0.92 meters gives us an overall length of 93.6 cm, a value that falls within the range of  measurements we have made on the lengths of the original, namely from- 93.3 to 93.9cm.   The little piece now holding the eyepiece is out of style with the rest of the decorations of the red leather telescope. It has been reported the present eyepiece lens s not an original. Guilio Cipriana in building his replicas in the 1920 and  30's did not include it in his replicas. Why not?  He appears to have had  access to the original because his art work  while not exact is similar. His  construction details, such as the diameter along the length  of the telescope, do not follow the original that closely but have been good enough to satisfy a number of museums around the world for their collection. It seems worth while to see if their is any information available about Cipriani and his  work at Arcetri, Italy in the 1900's . that could shed light on this issue. The little nub just does seem right. Its time to clear this issue up for the up coming year of astronomy. in the mean time we have made some pieces to hold the lens in the exiting mount along with a leather decoration to convert the lens so that it would be  keeping the style of the objective end..

The focal length of 2428 objective (Ronchi 956 mm ref 6a) and  (Greco etal 980 mm ref 7). The focal length of the eyepiece of 2427 (Ronchi-48.8mm) and  (Greco etal-47.5)

 Length 

925mm. plus or minus 3 mm. corrected typo error 12/05/08

Bottom line on length: 12/05/08 IMSS has made new measurement for both telescopes that will appear on their web site shortly.

For the #2428 telescope we measure the overall length to be 925 mm plus or minus  3 mm.  This value is consistent with the value we obtain from our measurement of the barrel (which agrees with the IMSS barrel length) and the two lens housings all added together.  IMSS and a NASA affiliate, STScI, report 980mm, a difference of 45 mm 1  3/4 "or nearly 5%. of the total length.  The overall length  depends, of course, on the position of the eyepiece and objective tubes and they were pushed all the way in when we made our measurements.  For the #2427 telescope the offset is 95 mm or almost 4 inches or 5.5 %.

The importance of these numbers for the total length is that when one uses the longer lengths and the focal lengths for the lenses published for these telescopes it is an optical impossibility for the telescopes to focus at infinity! If they are made to the published data the view of the planets and stars will be blurred out of focus images.

Another point using our measurements it is possible to use the current lens without it present little holder which may  also not be original. Doing this one could use  objectives  lens that has  Ronchi focal length measurements of 956mm The end result is a telescope that dimensionally does not have this out of style extra piece holding the eyepiece and it will still focus at infinity.
  

Click on the picture to make it bigger

A review; The distances between the eye piece and the objective lens, in a Galilean design, is calculated from the sum of the focal length of objective and and the focal length of the eyepiece. Remember that Galileo used a negative lens i.e., -47.5mm. so the values are subtracted.  (See standard text book on optics)

Both IMSS and we, as partners in science, strive for accuracy in our work. We understand that errors such as this can and will be copied over and over again with repetition like a domino effect.  They will even find their way into other publications to which people will look for good data on the Galileo telescopes such as NASA's  affiliate, STScI, which reports on Hubble data and  identifies itself as "the Formal Education Group of the Space Telescope Science Institute's Office of Public Outreach," see Hubble.                                         
 

International Year of Astronomy 2009

The IYA  impact of this dimensional difference for both Galileo's telescopes is that builders and enthusiasts will be absorbing these errors when they make there own copy of these very historical and beautiful telescopes for their IYA activities.  If they use  the longer dimension and published optics for the lenses they will find that their telescopes will not focus at infinity.   They will not be able to replicate the Galileo observations that he made in 1609 and later. 

 It's not good for the "image" of sciences if we can't pin down how and where the image is focused for the worlds oldest, simplest, and historically significant telescopes, instruments we want to use as teaching tools for science.

On the other hand the process described above is an excellent example of science at work!  It starts out with a problem, an anomaly, or inconsistency to resolve and one has to determine the most probable cause.  Remembering Galileo's 1610 letter to the Tuscan court, substantial effort had to be spent honoring  the first principle of science, that is, to be right.  By searching the known data and making repeated measurements and with the help of other professionals in the field, we achieved consistent data.  So replicas can be made that will be working telescopes capable of operating the way they did for Galileo and to serve as the teaching tool to tell the "truth" he wanted observers to see in the heavens  The story of  building Galileo's instruments  is a classic example of how Galileo and his new methods of  science can be brought into action.