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07/10/2007 01:26:59 PM Last updated


The IMSS # 2428 Telescope Optics  and Telescope  Tube Length
This section presents the  details and a discussion of the optics that are presently in the IMSS INV #2428  telescope and  the optics we used in replicating this telescope.

Below  is a schematic, not to scale, for a visual aid to help our understanding of what the eyepiece and objective physically look like and an optical ray diagram of a Galilean type telescope. The path of the rays of light through the lens shapes were reproduced from a 1860 text on Natural Philosophy. We modified the curvature of the lens surfaces from symmetric curves to no symmetric curves.

Galileo's Telescope
Two: small, thin, pieces of glass,  held up to the eye, how simple can it get?

We have found in the drawings of lenses of Galileo's telescope such as IMSS 2428, from at least 1800 up to today,  configured as a symmetric double convex lens for the objective and symmetric double concave for the eye piece. This is not what Galileo was suppose have used in this telescope.  Rather he was supposed to have used a "non symmetric or plano?" convex lens for the objective and most likely a "non symmetric plano concave lens " for the eyepiece. There is a double concave in it at the moment.

Is this difference in the drawings of the optics compared to the optics Galileo, was supposed to have, used important?
The authors believe that it is.  If your in the business of science, reporting it, or doing it, one knows how important accuracy is.  The very essence of science and scientist reputations are  staked on the accuracy of their work.  This  example of  a technical error or misunderstanding  about  the very heart (the lenses that make it work)   of a  scientific  instrument  of such significance historically and internationally should be cleared up.   Maybe it is time for  those of us who are looking forward to celebrating the 400 year anniversary of the high powered telescope to set the record straight. A further benefit of drawing them to be as accurate as possible  would be to keep the critics of science at bay.   The  critics of science are known to use any error or detail no matter how small to discredit science. Why give them the opportunity to use this to justify their position? See

Our choice and reasons for the optics we used in our replicas and discussion on how this impacts other dimensions of the telescope:

Conflict in the modern measurements of the exiting lenses at IMSS:
One of the many departures often made in building replicas of Galileo telescope is to compromise in the focal lengths and the shapes of the lenses. The objective lens in Galileo's # IMSS 2428 telescope have been reported in the literature to be 98.0 cm in one case by Greco et al in 1992 and  95.6 cm as measured by Ronchi in 1923, (please see our references).  Using either of these focal lengths requires nonstandard focal lengths which cost hundreds of dollars more as compared with the much cheaper standard of one diopter or 100 cm.  Use of the less expensive lens leads to a domino affect of compromises.

a)      It changes the over all length of the telescope by almost an inch.

b)      This in turn alters the location and size of the gilt decorations on the body of the telescope. 

c)       It also changes the optical power! 

Where does it stop? If these telescopes are to be used to tell the powerful story of the Galileo telescopes and teach people how science works, then to be trustworthy, we must follow the first basic rule of science: accurate repeatable testable measurements.  This must also apply to our replication of the dimensions and characteristics in building this symbolic telescope!

Our final choice for the objective lens:
We used a plano convex lens as in the original and chose the 1992 Greco et al 980mm focal length rather than the 1923 value of 956 mm by Ronchi et al. There are three reasons for this decision.

1st.  The overall length of the body of the telescope is too long to focus the telescope at infinity using a 956 mm objective with an eyepiece of 5 cm focal length. To accommodate this objective, the eyepiece would have to have a considerably shorter focal length resulting in powers greater than 20.(see below on in this web site for how to determine the over all length)

2nd.   If one takes the refractive index of Ronchi's calculation and his measured radius of curvature one calculates the focal length to be 980mm rather than 956mm suggesting an inconsistency within the data of the reference.

3rd.  Greco, et al measured 980 mm. Since He etal had the benefit of Ronchi’s measurements because he references Ronchi  and his was different  we feel that it can be safely assumed he would have checked his data to be sure his measurements were correct  to reconcile any differences for the earlier measurement.

Based on these reasons we chose a 980 mm focal length for the plano convex lens for the objective of our replica.    

The Eyepiece.
The current lens in the leather covered telescope IMSS 2428 located in Florence, Italy, is not the Galileo original. It is estimated that the original eyepiece was lost some time during the eighteenth century.   We used the same focal length as is reported for the original today, based on our arguments of telescope length and our own measurements as described below.  However for the shape we made an independent decision to use a plano concave eyepiece instead of the biconcave lens as is presently used in the current telescope.

We offer three rational for this decision.

1st. The original objective of the telescope is plano and the second, longer paper covered Galileo telescope, IMSS 2427, uses a plano convex lens for the eyepiece.

2nd.  There is a fair consensus that Galileo made  a number of his own lenses, and it would have been easier and faster for Galileo to make  the simpler  plano-concave lens.  He’d have only had to grind and finish one rather two surfaces.  Grinding is faster than polishing and he would have had only one surface to polish.

3rd  There are significantly fewer problems going the plano concave route - problems like getting the two radii lined up etc. which, in our opinion, Galileo would have certainly known about from his experiments.  

Of course if our choice is incorrect, it is simple to change the eyepiece lens to a concave-concave configuration.

The overall length of the telescope is determined by the optics:

There is an inconsistency in the length of the IMSS #2428 telescope as we measured it and as reported in the literature. It is reported that the length of the telescope is the same as the focal length of the objective, namely 98cm. This does not leave room for the eyepiece when focused on distant objects.

Total length of a telescope focused for infinity = Focal length objective + Focal length eyepiece + lens holder over hangs.

NOTE the overall length will vary, of course, depending on how far the lenses have been extended for focusing. It appears that both the objective lens holder and the eyepiece lens holder can be used to extend the optical length for terrestrial focusing (see figure 9).  This “draw” distance may be about 10 cm each or 20 cm for both lens tubes combined.
The present telescope at IMSS has an objective that is plano convex with a focal length of 980 mm and a diameter of 37mm. The eyepiece is biconcave with a focal length of -47.5mm and a diameter of 22mm.
Therefore the optical length of the telescope for an object at infinity would be the sum of the focal lengths, namely

             (980 -47.5) = 932.5 mm.  Add 12 mm for the lens holder over hangs = 944.5 mm overall

This is the calculated value for the overall length of the IMSS telescope when both lens tubes are fully inserted for viewing at infinity. We have measured the scope at IMSS (with the apparatus described earlier and a video showing its operation) to be 945 + or - 5mm. It appeared that both tubes were fully inserted when we made our measurement.Our best guess at present is that it can open 20 cm more giving a maximum length of 1145 mm for focusing on nearby objects.

The catalog of early telescopes by Albert Van Helden, page 30 lists the telescope's overall length as 980 mm which is off by about 35 mm or nearly 1 3/8 of an inch different according to our measurements.

The objective has a focal length of 980 mm lens which is not a standard production item and therefore an expensive piece to purchase from out side sources. In stead we purchased four 1000mm fl. lenses and shortened them to the to 980mm fl . .
We tested the process with plane glass blanks and found that we can make changes in the shape at a rate acceptable to changing the focal length of our 1000 mm. lenses to 980 mm. See photos below

General details of the equipment assembled to shorten the 1000 mm focal length lens to 980 mm.
Our objective lens focal length of 980 mm is not a standard production item and is therefore an expensive piece to purchase from outside sources. Instead we purchased 1000mm FL lenses and shortened them to the 980mm FL.   We tested the process with plane glass blanks and found that we can make changes in the shape at a rate acceptable to changing the focal length of our 1000 mm. lenses to 980 mm. See photos below.

Here is a web site that explains how the lens testing method that we are using works

Optical diagram of lens testing apparatus

Close up of the lens holder and optical flat

We tested the polishing machine using a 2 inch diameter 1/8" thick plane glass blank. with 4 minutes of polishing we took off about 46.4 millionths of an inch and converted it to a convex surface. The photo above shows the interferogram of our glass blank with the interference lines use to get this measurement. It's going in the right direction with reasonable speed. All we have to do is get good spherical surface and not overshoot the focal length of 980 mm.

close up of the light source and the knife edge

Lens Grinding/Polishing Machine

Lens polishing /grinding machine that we've put together for this project.

All photos and written material are by Jim & Rhoda Morris unless noted otherwise. Free personal and educational use and reproduction is encouraged--- Acknowledgment is appreciated; all commercial rights are reserved  HOME PAGE  
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Copyright 04/18/2007 Jim & Rhoda Morris