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Optical configuration
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Over the coarse of my involvement with this group I have heard mention four different optical configurations: prime focus, Mersenne, folded and Gregorian. Each of these designs requires a different structure above the primary mirror. I would like to discuss some of the ramifications of each. I am assuming that the telescope will be able to point to all parts of the sky above about 10 degrees from the horizon. and be mounted Alt/AZ. Prime Focus This approach requires the tallest truss system, ~ 13 feet above the primary. It also requires a coma corrector as well as a rotator. all of which must be suspended at the prime focus location. The camera/corrector/rotator will have a small optical cross section. Baffling will be minimal but required. Mersenne This would require a truss system almost as tall as prime focus, ~12 feet above the primary. This will require fabrication of a convex parabolic element with the same focal ratio as the primary of about 6" in diameter. If it is going to be a Mersenne-Nasmyth configuration then it will require a two diagonal mirrors, both about 6" in diameter and support structure just above the primary and again at the Nasmyth position. Imaging will require a refracting telescope to focus the image without correction. The obstruction would be minimal on the same order as the prime focus design. Imaging will be tricky unless the system is well baffled to prevent outside light from reaching the first diagonal. Folded Vaughn has mentioned the availability of a 30" precision flat mirror that could be used to fold the primary beam back on itself. This would require a truss system with mirror mounting at ~7 feet above the primary and again at about 4 feet above the primary for the diagonal mirror mounting. It would require a coma corrector, rotator and focuser at the diagonal level above the primary. This will reduce the aperture by about 18%. Also good baffling will be required to prevent stray light from entering the system. Gregorian This will require a truss system that will be ~14 feet above the primary mirror. It will also require the fabrication of the Gregorian mirror and a diagonal mirror at the top end if it is a crossed system. It will require a corrector, rotator and focuser at the focal plane of the system. If a crossed system the upper weight will not be symmetrical about the optical axis. If a symmetric system then a diagonal mirror mounting will need to be above the primary mirror. Also good baffling will be required. What hasn't been discussed is a classical cassegrain system. The truss system would be about 12 feet above the primary mirror. it will also need a diagonal mirror mounting system. It would require the fabrication of the secondary mirror. Unfortunately in order to reduce the curvature of the focal plane a field flattening lens would need to be fabricated.. I have mentioned the need for a rotator. As I'm sure you know an ALT/AZ mounted telescope has a rotating field of view. There is also a portion of the sky that can't be reached due to the speed of the azimuth drive and/or the field rotator. This "forbidden" field is cone centered on the rotation axis of the azimuth drive and has a cone angle associated with the drive speeds: the slower the speeds the bigger the cone angle. Also mentioned in conversations is the use of a Barlow lens to increase the focal ratio of the primary and allowing the use of commercial coma corrector. Native, the primary has a focal length of ~3960 mm. adding a 2x Barlow brings it up to ~7920 mm. With the field of view decreasing from ~36 arc minutes to ~18 arc minutes for a 25mm sized sensor. From these possible designs it should be clear that a single truss system and its corresponding spider systems cannot be made to accommodate all of the designs. Further the pivot point of the altitude bearing to maintain balance will be vastly different for the differing amounts of weight above the primary. I think that the truss system needs to be fabricated from carbon fiber tubes. And perhaps include carbon fiber plates as mounting points for the correc
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3 and 4 poles with fixed base joined in a box at the apex. The box is introduced to help with the mesh. Poles are assumed to be "intermediate modulus carbon", 2" OD, 1/8" thick walls, 163 inches long. 10 pounds lateral force (44 N) applied. Elasticity modulus is a bit of a guess at 130 GPa and Poisson's ratio, also a guess at 0.3. Maximum displacement for the 4-pole design: .008 mm. Maximum displacement for 3-pole design: .1 mm. By way of comparison, a single vertical pole subjected to the same transversal force is displaced 57 mm. This is just a "black box" calculation and inputs need to be double checked, but as one might expect the 4 pole model suffers less deformation. But maybe the relatively greater deformation of the 3 pole is acceptable as a trade for reduced expense and light obstruction.
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A year ago last February, through the offices of Mark Cornell, I began plans to move the former CTI telescope from UNM to Magdalena. One may wonder why John McGraw would hand the keys to the telescope over to me when there are so many gifted and experienced astronomers around. The simple answer is that Prof McGraw had to move the telescope somewhere and no one else would take it. My promise to John McGraw was to bring the telescope to my property in Pi?on Springs and build an observatory for it. After discussions with Mark and John Briggs, my plans were to start by reconstructing the CTI as nearly as possible but to eliminate the Paul-Baker UTA in favor of a simple detector at prime focus -- something actually forced upon us by the disappearance of the tertiary mirror and the suspicious crack in the secondary that Eric Toops noticed. Eric has been an enthusiastic supporter of the project with the goal eventually of converting the telescope to a solar telescope since the mirror is made from ULE glass. Hence the genesis of the idea to construct a Gregorian UTA when the time came. In particular, Eric offered to store the CTI components and offered space to work on it. I agreed and had Donny Chavez drop the scope off at Eric's school instead of Will Vantwoud's warehouse next to my place, and there Eric and Danny and I built the shelter for the CTI in Eric's "telecourt". The principal challenge from the beginning has been to enlarge the swath of sky accessible to the scope. How much could the telescope be tilted? The changeover from a water-based support to an air-based support enabled John McGraw to tilt the scope ~5°. I would like to thank Dan Gray and Roger Ceragioli who have encouraged us to go further, that we may not be able to get all sky but we could get *some* sky. The examples of the sister MMT mirrors and even the MRO have been guiding lights, and some of the people associated with the original MMT and Spacewatch have been helpful. Finally, it would be great to get Joe Houston involved to see what enhancements to the original design could be made to expand the reach of the cell we have. Nobody would know better than he. Finally, I agree with Mark's down-to-earth approach: start tilting and keep tilting until the image starts distorting. Roger says not to worry, the mirror won't break. (Many thanks to John Briggs for introducing me to Roger.) So how should we tilt the scope? A primitive alt-az design. We need a rotating base. The industrial table is nice, but if the scope is destined for a dark sky location, I don't see how it could be moved. Possibly we could go with Eric's idea to support the weight on the 4 ft slewing bearing and provide motive force with a smaller slewing bearing if moving the big table proves impracticable. For altitude movement, both John and I have drawn up several designs for a structure, any of which should work. Find the center of gravity, connect the structure to the cell with one of Eric's slewing bearings, and we should have a start. There is much more that could be done, but I find even the simplest plan quite daunting: (1) a primitive alt-az with quasi-tiltable cell, (2) detector at prime, (3) observatory at a dark sky site, done with sufficient care to realize the potential of the mirror, done with sufficient dispatch that the oldest among us can enjoy the fruit from the effort, done with an eye to future development.
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Biography of our interested friend, Joe Houston
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Hi Folks-- We have quite a spectrum of talent already interested in the 72-inch project! But one person who has communicated with many of us, but whom only I've been fortunate enough to meet in person, is Joe Houston in California. I believe he's 80 years old now, and health issues restrict him from travelling. However, he's one of the generally most enthusiastic people I've ever met. He has created a monograph related to telescope making and the Cold War. I'm presenting it for him at the Stellafane Convention in Vermont next month. I'll paste the abstract below. In the course of all this, he sent me a copy of his CV, and I attach it. It's very interesting. His original interest as a student at UT was binary stars, under no other than famous George Van Biesbroeck! His experience with telescopes and optics is unusually vast. I encourage you all to communicate directly with him: hrajeh@... His own first idea was that the 72-inch should become a Gregorian. But I gather he's been open to many suggestions! In any case, he likes hearing about any progress, and there's no doubt regarding the depth of his wisdom. Note that we should see an announcement shortly, reminding us about an astronomer's breakfast in Datil tomorrow the 15th, at 10:00 AM. --JWB. ### Plowshares into Swords: How Astronomers and Telescope Makers Helped Win the Cold War Joseph B. Houston, Jr., and John W. Briggs Joseph B. Houston, Jr., is a senior optical engineer who is long familiar with Stellafane and with many of its leaders and their careers in optics through the Cold War. Among diverse achievements in his career, Houston invented and patented the laser unequal-path interferometer (LUPI) and demonstrated it at Stellafane in 1970. Houston has prepared a detailed monograph highlighting the contributions of telescope makers to the Cold War and to the ongoing progress of science and technology, drawing much on his own unique experience. John W. Briggs will present an outline of the monograph, the full draft of which will be available and engaging for the entire Stellafane community. Houston is a past president of the Society of Photo-Optical Instrumentation Engineers (SPIE) and of the New England Section of the Optical Society of America. Briggs is a past-president of the Antique Telescope Society and is now Secretary of the Alliance of Historic Observatories.
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Seeing monitors
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I am working on moving one of the DIMM seeing monitors off the FOAH hilltop today and reactivating it for use at other locations. The DIMMs been sitting idle for a while so it might prove a chore. --JWB.
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It would be wise to test the candidate tertiary mirror that Eric has for flatness. It won't be hard. One way is to use it with, say, a Celestron 14 tube assembly, set up in autocollimation (this is easy) with an artificial star and knife edge. If the mirror isn't flat, it will become very obvious, fast. If the optic in hand isn't good, we should learn that as soon as possible, and proceed accordingly. I was supposed to go to Texas Tech University this week for a consulting job, but because the weather forecast was poor, they suggested that we delay it. I plan to be here also the first week of July, when Vaughn plans his return. Vaughn suggests we move the gantry crane from my place to Eric's telecourt when he returns, and that sounded good to me. (I've moved it before, but it's kind of a pain!) --JWB.
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Do we have chain needed, or need to get what? guessing 2 small chains and 4 heavy duty links going between 2 rings on each side then use hoist on each side.. I can move the 3 phase generator outside.. Anything else??
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As Jerry said, the current cell for the 72" is not suitable as a cell for an all-sky telescope. So two questions arise. How much of the sky would be accessible with the current cell? Can the current cell be modified to expand its reach? Just as a SWAG (scientific wild-ass guess), we could aim for a 35 degree tilt with appropriate radial support and still not have the pistons fall from their bases. The attached photo shows the effect of a 45 degree tilt without pressure. From photos it appears that the MMT mirrors rested on a bladder covering the entire base of the cell. It might be possible to replicate that approach with the cell we have -- though I don't know how we'd source an air mattress for a 1203 lb mirror. That aside I also wonder what price range we'd be looking at if we wanted an entirely new cell.
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Intermediate goals for the 72" telescope
When John McGraw handed me the key to the 72" telescope storeroom, he emphasized that the 72" telescope would never be an all-sky telescope. He had used it as a transit telescope tilted 4.8 degrees from zenith and feels its use is limited to about that. But as I gathered information and opinions from numerous astronomers, I have come to think the mirror and cell we currently have can be used to make a "part-sky" telescope. Many people have suggested that we begin by building an alt-az structure with detector at prime focus. This could be done with minimal investment to allow testing of the 72" mirror and structure control strategies, all the while with an eye to a future where a new cell could allow greater tilt, and secondary mirrors could make the telescope more convenient to use. For example, Jerry's design of the base could go both ways. So could an upper cage that could hold a secondary mirror or be the base for a pyramid to prime. It might also be good to mention that Eric's interest in the mirror has always been to use it to construct a Gregorian solar telescope, another reason we might want to check the scope at prime focus.
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I thought maybe I could archive some detail about a point that's been bothering me. The image "cti_cell_detail.png" shows how the telescope cell is supported at four corners. At each corner two vertical plates hang from the gray square which in turn is supported by the red bars. The vertical plates at a corner attach to the cell by two horizontal plates, one extending below the cell, one attached to the side of the cell at the top end of the vertical plates. The top plate attaches the cell to the vertical "wings". There are 2 or 3 1/4" bolts through the wall of the cell to the horizontal plate and vertical bolts from the horizontal plate to the wings. This part seems primarily to function as a spacers although it is the only point where the shear strength of bolts is deployed. The bottom plate upon which the cell rests attaches to the wings by four larger bolts. And that seems like a weak point. If two of the corner supports are removed and the full weight of mirror, pistons and UTA are added to the cell, totaling upwards 3000 pounds, will eight vertical bolts provide sufficient support? Would it be possible to reinforce those points with, say, "L" brackets?
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Starting with the dimensions of Jerry's drawing: and Vaughn mentioned availability of low COE 3" carbon fiber tubes, 1/4" walls. This structure could support a secondary mirror at Jerry's prescribed height or a "teepee" structure attached to the top of the upper rim for a detector at prime or beyond. As before points of view available from FreeCAD or Step files.
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Testing rotary tables
4
We have the giant rotary table -- the Vaughn-o-Table -- at Eric's Telecourt. Soon we'll try lifting it from the trailer using a gantry crane that will be moved from my place to the Telecourt. I've been wondering how to test the condition of the huge worm and worm-wheel it must contain. One idea would be, once we have it resting on the concrete, to simply set up a tripod-mounted Celestron 11 or something like that on it, and look at a star transiting the meridian, where the motion is mainly right-to-left. If we also had something like a high-torque microstepped stepping motor connected to the worm shaft, with an adjustable speed, we could adjust the speed to the near-constant speed necessary to track the star in transit. Then we could simply sit and watch for how smoothly the table tracks. This would be a very simple and somewhat crude test, but it would be so easy to do, I don't see why we don't try it. I understand that the worm wheel has 360 teeth. I expect the change in azimuth of a star (on the meridian and near the celestial equator) is ballpark like its sidereal motion, or 15 arcminutes/minute, or one degree in four minutes. So we would want to be driving the azimuth worm slowly, and in first order, one rotation in four minutes. The only problem is that I don't happen to have the right kind of big motor or high-power controller right now that I'd like to use for the test. I just built and delivered one as a consulting project at LSU. I can start looking for another large surplus stepper. Eric might have another option for a controllable motor. We should measure the torque required to turn the shaft right now. Anyone got a spring scale? Evidently the torque is a bit different depending on turning direction. If we can confirm that the table was made by Giddings and Lewis, then maybe we can eventually find documentation for it. It's probably possible to pull the worm and its shaft out of the whole thing for cleaning and inspection. I'd love to see the guts of the whole big thing cleaned and re-greased! --JWB.
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thoughts on mount design
Here is a possible design for a alt-az support structure for the 72". Assuming the 72" table works out. And assuming that we could keep two of the current attachment structures. Assuming the "wings" would be cut short to allow altitude motion. Not very realistic about the slew bearings. FreeCAD and Step files available there's interest. Mark
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Can one combine a coma corrector and Barlow lens? From Sam Brown's book, formulas for a Barlow, the focal length F of the negative lens and its position A inside the focal plane of the optical system determines the magnification M and the distance B between lens and the new projected focal plane. (see attached) My question is what does a coma corrector do to the system focal plane? I'm a little confused because the 2" Paracorr type 1 has a "best" distance between lens and detector, but it still can be brought to focus when the detector is not in the best position. It would be nice to have a ray trace diagram for this corrector, but I don't know where one can be found. Thanks.
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Measuring astronomical seeing is one aspect in evaluating sites for the observatory. There has been quite a lot published concerning how exactly to take the measurements in such a way that measurements acquired by different observers and instruments can be compared. Both Mark Cornell and John Briggs have experience with these measurements. This is a description of my thinking on the matter. In his astronomy course lecture notes on seeing and turbulence Steve Majewski links to the Fundación Galileo Galilei (FGG) web page describing the "differential image motion monitor" (DIMM). https://www.tng.iac.es/weather/dimm.html Using the dimensions of the telescope and a "Hartmann mask" with narrow prism over one aperture, one uses formulas derived by Fried and Sarazin & Roddier to obtain a "full width at half maximum" (FWHM) from the variance of the distances between split images of a star. These images are obtained in very short exposures, 5-20 milliseconds, which effectively freeze the atmospheric distortion at a given instant. So, basically, the proceedure is to capture several hundred short expostures, estimate the center of each image, compute the distances between the centers, and compute the variance of the distances over the sequence. Then one converts the variances into FWHM using Sarazin and Roddier's formula. The site with the smaller FWHM is preferable, all other things being equal. There are numerous refinements, and one has to consider how far to take them, of course. Here is one exposure of Arcturus at 10 milliseconds: Albar Garcia De Gurtubai Escudero at FGG kindly shared their source for prisms and I was able to source prisms to the same spec from the same company. Since the minimum order was three prisims, I have three. John Briggs has (at least) one, so we could run up to four monitors simultaneously.
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We can glean a good deal of information from the Smithsonian video that Eric found. (MMT mirror clip) The lifting found in UA's Optical Sciences warehouse appears to be the one that appears in the video. Here is a still from the video: And here is a close-up of the lifting ring from November 2022: After the original 1.8m MMT mirrors were decommissioned, one was taken over by the UA Spacewatch. This still from the MMT video and the photograph from Spacewatch provide some information how the mirror support was designed and complements the description in the MMT project description by Beckers et al (1981). The video still shows a structure in the center of the cell that purportedly communicates the vertical orientation of the mirror to the radial chains. The Spacewatch photo shows the peripheral support structure more clearly. Also, the Spacewatch lifting ring seems to be different. This photo shows the 72" CTI mirror in the final cleaning stage with the lifting ring in the background.
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Thinking about 2800 lbs evenly divided resting on the ends of 2 4"x4" steel tubes, .313" thick walls, 100 inches long. The tubes rest on a steel disk ~47" diameter. Modeling in imitation of Vaughn's Lidar mount. FEM estimates 0.4 mm deflection at each end.
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When we detached the mirror cell from its support structure this past weekend, we learned a bit more how the cell weight is distributed. Suppose that we replace the vanes that support the cell with tangent planes like this: If we put all the weight on the bottom of the bracket, then forces look like this: But what we (Eric, actually) discovered was that the cell is actually supported by bolts on the sides as well. So an altitude support bracket could incorporate a side support as well. Something like this, depending on where the center of gravity falls with respect to the bolt holes already in the side of the cell:
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Coma corrector designs from Mark Ackermann and Roger Ceragioli
Mark Ackermann put in a lot of thought into a coma corrector for a single mirror system with the 72" as primary. He asked a lot of questions about performance goals and came up with a 3-lens Wynne-type design. There are diagrams to indicate expected performance and a specification chart that I sent to Optimax. Somehow he started calling us MRO and I didn't disabuse him. https://drive.google.com/file/d/1Kk5Z7pejdtFgJil7-FmdM2x4tph2r7it/view?usp=share_link The Optimax quote was only for the glass, without the coatings that would be needed and, of course, without the housing: https://drive.google.com/file/d/16xqBDJcgbTJgd5ZjvDQ12LUULtmehxr3/view?usp=share_link Roger Ceragioli designed a corrector along the lines of a Tele-Vue double doublet. He said the corrector would transmit only 80% of the light, but that he already has three of the lenses. However he wants to see a working telescope before he makes it. Here is a ray-trace and spot diagram: https://drive.google.com/file/d/1ky5dWdUhCg6Vpr2LqquFwIUdOPjIorxD/view?usp=share_link https://drive.google.com/file/d/1ODBNbAJM68fbbK-GxH49mI4S9NP9p-YL/view?usp=share_link There are more details, of course, and Bratislav Curcic was kind enough to send some designs, but I was unable to get a quote from the company he recommended. But this is basically how far I got.
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