Modern telescope mirror can be made extremely thin. First is mirror base support that is like wok pan (actually like pancake pan, when looked from upward), with hexagonal honeycomb shells. Then optical material is put to this “pancake pan”, and actual optical glass can be just few millimetres thick. This was used in Herschel space telescope, mirror base was carbon fibre (?), and optical surface silicon carbide. 3,5 m mirror weighted 240 kg, and SiC mirror was only few centimeter thick (or few millimetres, I don t remember). Herschel mirror base was manufactured around 2005 - 2006, so it is 15 year old technology. 5 m mirror made similar would weight about 680 kg (1500 lb), if weight of mirror increases cubic scale, so 3,5 X 3,5 X 3,5 = 43 and 5 X 5 X 5 = 125, so 125 : 43 X 240 kg is about 680 kg (coarse estimation, 5 m mirror weight can be even lower). This 680 kg and 5 m wide miror can use hexapod or altazimut telescope structure. Modern cars actually use carbon fibre or silicon carbide mirrors as rear view mirrors, they cost about few dozen dollars. When similar material is used in telescope mirrors it suddenly cost millions. Why? Epoxy is used widely in boats and cars, this 5 m telescope can use epoxy secondary mirror support. Or use aluminum, aluminum tube sructures similar seen in 4- 5 m and bigger telescopes as secondary mirror support can be used. But using already available commercial aluminum tubes that is used in different industries around the world, and telescope structure also using available aluminum plates etc. Using industrial off the shelf structural components, not specially made for telescope, because this specially for telescope manufacturing could cost millions. However any machine shop around the world can build those simple telescope structures if needed. Actually 5 m telescope is not so difficult to build if main mirror weights 680 kg or less, telescope is just like any other aluminum or epoxy structure. It is not even very expensive to build. Cars and boats use fine aluminum structures and epoxy (and cars have SiC mirrors) and they do not cost so much. Standardised design of telescopes of 4,2 -5 m class and then 3,5 m class, then 2,4 m and then gradually smaller, so that smallest are about 0,5 m. Spherical main mirror quickly polished using modern polishing methods. Series production of units, about from 100 units (5 m) to 1000 units or several thousands. Like car or boat or airplane production. Using preorders, when enough of orders for 100 5 m units is arrived, construction begin. Price of 5 m telescope is about the same like in making a car or boat. Mirror accuracy not so extremely sharp like real pro astronomical telescopes, those large cheap telescopes are for masses, amateur and hobby astronomers, like large Dobson telescopes are nowdays. Andrew Rakich has studied four spherical mirror telescope systems, the longer the focal length gets the easier mirror is to polish? Ultra compact fSona telescope can be base for cheap photometric telescope, when its focal length is extended its mirrors are easier to polish? And its field of view becomes smaller? Modern mirror polishing techniques are used in large 6 m or bigger modern telescopes only, not small amateur scopes. If their computer and measuring systems etc. are not suitable for making small mirrors, they still can make big mirrors fast and accurately. So instead of making one mirror every few years and then just waiting for orders during several years, those mirror polishing firms (Zeiss in Germany, REOSC in France, Stewart Mirror Labs in USA etc.) could make full series production of large telescope mirrors at very cheap price. Every year computers become faster, and new polishing methods for “free form optical surfaces” are invented, nowdays mirror polishing can be really fast even making several metre size mirrors. Polishing is automated (computers) so it is like industrial robot making a car. Hundred years ago amateur telescopes were 15 - 25 cm sized. Today amateur telescopes are still same size. However modern mirror making and polishing has progressed enormously. So now amateur telescopes of several metre mirrors can be build cheaply. But no one is making them. Also smaller than several metre telescopes could use those modern mirror polishing principles, smaller size, 0,5 m - 0,25 m amateur telescopes would then cost only few dozen euros, mirror material can be plastic (for cheap price) or soft optical glass, lightened mirror. Lightweight mirrors of modern type are used only in very expensive like Herchel space telescope, not in cheap amateur telescopes, why? Mirror material for bigger mirrors can be some glass ceramic, or Corning ULE, aluminum, or exotic like carbon nanotube mirror. Silicon carbide is made in siliconised SiC, SiC CVD, and HALSiC forms. Really cheap mirror is spin casted and not polished at all, NASA has made satellite system that uses spin casted 0,5 m mirrors that are not polished, mirrors are just coated reflective after spin casting, but mirror accuracy is just 6-8 micron RMS accuracy in mirror edges so those are awful bad mirrors. Those are f2 mirrors, so if mirror has f6 or f8 focal optical component length (minimum of f4 is needed), and uses four mirror optics like Rakich that doubles (?) that f4 - f8 focal length RMS becomes somewhat acceptable? So spin cast can be used in telescope mirrors without polishing, but optical quality is just barely acceptable for visual observation, and telescope is very long. Also secondary mirror must be very large, up to 30 - 34 % obtrusion. Secondary, tertiary and quaternary mirrors are polished, parabolic main mirror is just spin casted and not polshed. Telescope up to size of 2,4 m can use spin cast main mirror, but optical quality is extremely bad and telescope very long. If it is Dobson type it is so long that it needs some kind of anchor mass so that it does not turn over when turned. Offner relay based optics (?) and extensive baffling can be used in spin casted telescopes, if baffling little improves otherwise almost unacceptable bad optical quality. More effective spin casting systems are studied, and in USA is at least one firm that tries to make acceptable quality spin cast telescope mirrors. If focal lenght is very long those mirrors are good enough? Using Luc Arnold miltiple mirror Mersenne telescope of 36 connected mirrors even spin casted main mirrors can be used if focal length is extremely long. So 5 m diameter telescope that has 36 - segment Mersenne multi mirror telescope structure can use spin cast main mirrors without polishing. Additional spin casted mirror blank can be send together with telescope for those who prefer polish their telescope mirrors themselves. But it is enormous task to polish for example 2,4 m mirror by hand. Smaller like 0,5 m to 1,5 m are more easily polished by hand. However those spin casted mirrors are very long focal length so polishing is easier? But modern mirror polishing have become so fast and accurate that even several metre size mirror, if spherical, can be polished very cheaply using automated computerized mirror polishing machine. So if technology exists that makes amateur telescopes possible in several metre size, and telescope price cheap, why this technology is not used? It is “reserved” only for 100 million - one billion euro supertelescopes. However the same technology could enormously improve amateur astronomy, and would be win-win situation, telescope factories for big telescopes would get work and not staying standstill many years like they nowdays do, and amateur astronomers would get large telescopes up to 5 m size cheaply. LIghtened main mirror is the key, when main mirror is very light telescope is also light and cheap. And modern telescope mirror can be build to be extremely light. However this lightness is not used in commercial telescopes, only in real pro supertelescopes. Lightweight mirror makes light telescope, and telescope making is not rocket science, any machine shop can build its aluminum structure parts or use commercial off the shelf components, even hexapod support can be build using available commercial hydraulic components fine-tuned. Serial production of standard parts is second key to very cheap but big amateur telescopes. Polishing machine that would make very fast polishing of 1,6 m spherical mirrors for OWL telescope was proposed, but OWL was cancelled (and its polishing machine too?). If telescopes are made, not one or two at a time, but in series production of thousands of units, price per unit would be extremely low. It would be like car or boat production. So 5 m “amateur” telescope can be in same price range like car or boat. Using epoxy and aluminum structure like car or boat. No observatory building is needed, only light tent-type structure that was proposed for VLT telescope. It would be like circus tent and its price would be very cheap.
Another promising mirror making technology is mirror replication. Googling “telescope mirror replication” or “optical mirror replication” brings many results. Quality is better than in spin cast mirrors. In netpage “ATM forum astronomy cnet” is “Opticast mirrors, optical and reality” text about spin cast and replicating mirrors. It mentions electroless nickel as perfect material for mirror polishing, but this material has also some difficult properties. Mirror replication means that mirror is not polished, when it is replicated it just is coated and ready for use. New technique can replicate even 8 m optical mirrors. If they don t need polishing that is big saving of telescope price, and actually even “amateur” telescopes can be build using spherical optics, long optical focal length (for making optics cheaper and more accurate) up to 8 m mirror size. They would be like cheap Dobson telescopes, but 8 m wide. If same cubic measuring method is used and compared from 3,5 m Herschel space telescope, 8 m mirror weights then 3 ton max., or actually much less, about 2 tons? Rigid mirror. Herschel had mass of 315 kg complete telescope and 240 kg main mirror. So 5 m telescope would have, much longer focal length than Herschel, about 680 kg (1500 lb) mirror and about 1100 kg (?) total weight (2500 lb), of which about 450 kg (1000 lb) is other mass than main mirror. Made from epoxy like Herschel. And then telescope mount that weights several tons, altazimuth or hexapod type. Made from aluminum or steel. 8 m telescope with 3 ton rigid mirror would have mass of 4,5 tons (?), 10 000 lb, and then supporting mount, altazimuth type would weight over 50 tons? Made from steel or aluminum. 8 m mirrors are too expensive (?) to be polished in amateur telescope, so mirror replication or spin casting is used. 5 m mirror can be polished, spherical surface. 8 m is over 2,5 times more surface than 5 m, so polishing costs more. Idea is to use standard mechanical parts, international standard specs are made for mechanical (and optical) parts of telescopes from sizes 0,5 m to up to 8 m. Then those standardised mechanical parts can be build around the world, in almost every machine shop, for telescope mount, steel or aluminum. Mechanical steel or aluminum parts can be made in India, China, South America like Brazil etc, and telescope buyer can complete his telescope from choosing different parts from different manufactures, and use those those parts from all around the world to build his telescope. Standard specs makes parts same, but quality and price may differ because there are several manufactures. Buyer can choose best quality / price option that suits for him. 5 m “amateur” telescope can be in price range of car or boat, 8 m will cost over million euros, or several million euros. But still 8 m is cheap enough that astronomy clubs around the world can buy one for themselves, when price is divided between astronomy club members. Even private persons can buy 8 m and over 50 ton colossus in their backyard, if they have over million euros to spend. Those “amateur” telescopes must have effective baffling, because they are used in enviroment that is not so “clear” like real pro big telescopes have that are in mountaintops. Those cheap large amateur telescopes have optical and mechanical quality not that super sharp standard that is used in professional supertelescopes, but build nearer to amateur astronomy specs. Mirror polishing firms that have capacity to polish large mirrors must wait for years to have orders, then they polish one mirror and then wait for years again. Some never have order for large mirror, altough they have polishing machine for it. All this time their skilled workforce and high-tech machines are without work. Instead of just waiting for years and not making money, they should churn out large spherical mirrors as fast as they can, 24 hours a day, 7 days a week, 365 days in year. And then sell those to amateur astronomers for their cheap but large telescopes. Full 24/7 series production of large mirrors, from 100 units to thousands of units production series, makes cost per mirror small if production speed is risen to maximum and optics spherical and optical quality “acceptable”. Large (and small) mirrors can also be left without polishing, if they are made using replication or spin casting. Bad quality of spin casted mirror can be improved if fSona optics is made to long focal length version, so optical requirements are easier? Polished mirror, spin cast mirror and replicated mirror all use epoxy mirror base, so weight is almost same. Buyer can perhaps choose between them, altough telescope mechanically is otherwise same (standard specs). Mirror surface can be some glass ceramic, ULE, aluminum, Zerodur or silicon carbide. Buyer can perhaps choose between them too. Small telescopes about 1,5 - 0,5 m, or even up to 2,4 m can use soft glass or electroless nickel perhaps. 1,5 - 0,5 m can use plastic surface in optical mirror (cheap). Mirror base can be aluminum, silicon carbide or epoxy up to 8 m, but then weights differ so three classes of mirror weights is needed? Simply using only epoxy base, perhaps graphene hardened, can be the standard? Graphene hardened epoxy or just epoxy can be used in telescope secondary mirror supporting structure also (so whole telescope without altazimut or hexapod mount is made from epoxy, but telescope must have also mount, and that is build from steel or aluminum so complete telescope with mount is much heavier than just few tons). In text “Michael Ashby about cellular glass glass foam mirror” is that glass foam (glass - ceramic foam) mirror can be made. It is porous but in mirror surface can be thin layer of non- foamed glass. In text “The future of filled aperture telescope: is a 100 m feasible” is fast mirror making methods, but text is about 20 years old. No separate mirror base is needed if mirror is made from aluminum or silicon carbide, mirror then supports itself and does not need extra base structure for that, support is inbuild in mirror itself. Perhaps price is then smaller? Multi mirror or segmented mirror cannot perhaps be build and aligned / operated cheaply, so not suitable for cheap telescopes? Mirror also must be (?) rigid, adaptive optics with their electric motors under mirror are too expensive (?) in cheap telescope construction. Cheap 1,5 m telescope with plastic mirror surface (spherical optics) can be just about few thousand euro price, or even less. Only main mirror in all standardised telescope classes needs to be spherical, secondary, tertiary etc. mirrors can be some other optical form, or spherical aberration corrected using lens. Spin casted mirrors are parabolic. Telescope mirror polishing capacity of several meter class is in Russia and China, and smaller mirrors can be polished in India, Bulgaria etc. so polishing cost can be low if standard specs for cheap but large (“amateur”) telescopes are used and series production. Also aluminum mechanical telescope parts class (not epoxy) is suitable, telescope weight is then bigger but price smaller? Main mirror base can be epoxy or silicon carbide. Aluminum parts can be made in machine shops around the world, but epoxy parts are expensive. Telescope mount in all cases, up to 8 m, is steel or aluminum anyway.
Also those cheap mass produced telescopes of 5 m - 8 m class can be used to form cheap supertelescope. OWL was 100 m planned, if 150 units of 8 m telescopes is optically coupled like VLT they have same surface area as one 100 m mirror. If one unit costs million euro and optical connections 50 million that is only 200 million comparet to OWL 1000 million price tag. And 150 individual units is much more flexible system than just one mirror, they can be used as one or as maximum of 150 individual telescopes.
Mirror polishing firms have expensive and elaborate machinery, best high tech in the world, optical measurement and computer systems, but those machines stay standstill many years making nothing, about every 5-10 years they have order for mirror large enough that those machines are used. Some firms never have orders for large mirrors, altough they have machinery. That cannot be so profitable business. When they receive order I think they overprice their polishing costs in order to compensate losses of many years when that machinery is not used. So it is situation where those who order telescope mirror polishing for large mirrors lose and also mirror polishing firms lose when their machinery is not used. But true win-win situation is when that machinery is used every day at its maximum production capacity making series production of large 5 m - 8 m mirrors, not only for professional astronomers but astronomy hobbyists also. There is enough of astronomy clubs and rich private persons in the world that production runs of 1000 units or so can be made. When mechanical parts of those large amateur telescopes is outsourced to some machine shops in India or China or Brazil also mechanical parts are cheap. Series production of standard parts helps bring cost down like car or airplane production. Mirror can be made nowdays light enough, and telescope structure also if epoxy composites are used. Airplane aluminum is another alternative for light structure, but normal aluminum will do in cheap telescopes.
Another way to make lightweight mirror: mirror is so thin that it bends in gravity, but does not use adaptive optics (electric motors behind mirror keeping mirror up to shape). Instead mirror is let to bend and optical distortions are corrected by computer program. CCD camera picture is corrected by computer that corrects visual distractions caused by mirror bending. First, mirror is tested when assembled in telescope and distractions caused by mirror bending are measured in different telescope angles, until database is made that includes all distractions in all mirror positions in all telescope angles. Then when telescope is used and mirror bends, optical distractions are corrected using this database. Or another way: sensors are in back of mirror that measure mirror bendind, like electric motors in adaptive optics, but those are just sensors measuring mirror bendind, and that sensor data computer uses to make distorted picture corrected when that comes from CCD camera or other observing apparatus. Picture quality (accuracy) may not be so good, but when mirror is so light that it bends in gravity it makes telescope structure much lighter than rigid mirror so telescope is much cheaper to build. Low picture quality is acceptable in amateur telescopes, so 8 m thin lightweight mirror amateur telescope can be build at cheap price. How much optical distortion is acceptable and how light mirror can be made so that computer can correct picture to acceptable quality I don t know, but this is just one idea. Both database of optical distortions that is collected when thin mirror is assembled to telescope and telescope is tested, and sensors behind mirror when mirror / telescope is used, can be used together to correct distorted picture as best as possible. There is thing called “Anamorphic stretch transform” (AST) that is used with warped images and data compression. Perhaps AST can be used to correct distorted images of lightweight mirror that bends in gravity.
European space agency ESA has made BIAMOS bimorphic mirror project for space telescopes. Idea is that it is not main mirror but secondary mirror that is used for active optics, this secondary mirror can corrcet gravity bending, optical aberrations of main mirror and even manufacturing defects of mirror, all at the same time. So main mirror can be thin and it bends in gravity, smaller secondary mirror uses piezoelectric or electric motors in active optics behind mirror to correct picture that is distorted by gravity bending and optical aberrations. Instead of computer program that I proposed, this uses active optical element to correct gravity bending and all other optical aberations. Then main mirror does not need correcting electric motors, thin secondary mirror has piezolectric etc. small motors in small mirror. That kind of system is suitable for ground telescopes with thin mirrors but without correcting electric motors in main mirror. So cheap lightweight amateur telescopes with very thin 8 m mirrors can be build. Astronomical cameras use active optics, so instead of using it at the camera, active optics can be in secondary mirror and it corrects not only athmospheric aberrations but gravity bending of main mirror also. Price is about the same as nowdays when active optics is used in the camera only. There is “Ultra-light telescope mount” (Kurita), japanese project for 2,5 m 2 ton main mirror telescope that weights 7 tons (5 ton telescope, 2 ton mirror). When this ultra light telescope is (theoretically) using 8 m mirror of 2 tons telescope weight would be over 50 tons? Beryllium can be used in mirrors but it is both expensive and dangerous, so not so suitable for 5 - 8 m amateur telescope? Other mirror materials are “Continuous fiber reinforced ceramic”, reaction-sintered SiC, reaction-bonded SiC, Ceraform SiC, AL/SiC, C/SiC, and fused silica (but that is expensive also?). “Fabrication of electroless nickel plated aluminum freeform mirror for”…, “Ultra-precision polishing of electroless nickel”…, “All-spherical catodioptric gregorian optical design for”…, US Pat 20150241677A1 “Aplanic four-mirror system”, “Flexible mirror for space telescopes” eurekalert (org) 2018. “Cold-shaping of thin glass foils as novel method for mirror processing”. “Super mirrors at lower cost - RTI”. “Mike Carambat unusual telescope designs”. Chinese patent CN10438404 “Surface metal mirror polishing device”. Patent DE202012004116U1 “Means of play-free movement of optical components”.
There is book “Unusual telescopes” and “Weird telescopes page” netpage, where is “Anderson-Newton” type whose optical aberrations are corrected by mechanical bending of main mirror (Carl Anderson). “Unusual telescope mounts” are in cloudynights netpage. “Topology- optimization based light weight primary mirror design of a large aperture space telescope”. Maksutov-Sjogren telescope type (Tore Sjogren).
Another way to make lightweight mirror is membrane mirror / deformable mirror. Googling “membranel mirror telescope” brings many results. In the 1970s amateur astronomers tried to make “mylar mirror” telescopes from thin plastic shaped with vacuum or air pressure. That kind of mirror is like air mattress, so it is extremely light. Membrane mirrors can make possible even 10 m amateur telescopes, because their main mirrors are extremely light. Optical aberrations can be corrected using adaptive optics in secondary mirror. Membrane mirrors also at least in theory have more accuracy than spin casted mirrors, so making them obsolete, and nearing accuracy of replicated mirrors. If accuracy is even distinctly near accuracy of replicated mirrors, membrane mirror makes them obsolete also, because membrane main mirror is so extremely light. Spherical optics in main mirror is best in membrane mirrors? Or parabolical? Secondary mirrors etc, can be conventionally made. Cosmoquest netpage 2015 “Can a telescope mirror be made of thin film?”, researchgate “Has anyone considered making a telescope mirror by evacuating a space between two sheets of glass?” “A new membrane mirror for infrared telescopes”, photonics (com) netpage “Lab achieves mirror technology milestone”, “Large aperture holographically corrected telescope”, “Membrane primary mirror for a telescope with a real-time holographic corrector”, “Proceedings paper - stretch membrane with electrostatic curvature SMEC”, “Membrane mirror evaluation of APERTURE” 2018, “DOMinATE: a deployable optical membrane telescope”, “A concept for a membrane floating mirror” 1998, “Hybrid electrostatic / flextensional mirror”, “Modular inflatable composites for space telescopes”. Instead of making space telescope, membrane or inflatable mirrors can be used in ground telescopes, amateur telescopes up to 10 m diameter with extremely light main mirrors. Accuracy is not so important in amateur astronomy that it is in professional telescopes, so up to 10 m membrane main mirrors would suit ideallly to astronomy hobbyists, and many have tried to build large mylar mirror telescopes, but those homemade attempts are not even near what professional manufacturing of membrane mirrors can achieve nowdays, in series production of 100 or 1000 units of at least some way acceptable quality. So 10 meter telescope that astronomy hobbyist can buy and place it in his backyard, is possible. What accuracy of those inflatable air mattress type or other (rigid) types of membrane mirrors have, is almost non-important, main thing is that ultra light main mirror telescopes can be build with very large 8 - 10 m diameters. Those who want accuracy can order heavier telescope with heavier polished normal main mirror. But also price of telescope then escalates. So space telescopes is not the best way to use ultralight membrane mirrors. “Large optics: space telescopes may inflate”. Ultralight main mirror also means ultralight telescope structure for 8 - 10 m diameter. When telescope structure is lighter, it is also cheaper. Cheap even astronomy hobbyists to buy.
About cost, in text “Construction and optical testing of inflatable membrane mirror” 2015 is sunlight collectors that are made 0,15 mm accuracy and price of about 20 dollar for 0,8 m disc weighting about 2 kg. 10 meter mirror would then be 3000 dollar price and weighting about 1 ton (if weight increases in cubic scale). Altough 0,15 mm accuracy is no good for optical telescope (it needs 150 nanometer accuracy minimum), something about weight and price can be estimated from this. 10 m mirror that weights so little, and manufacturing costs of mirror can be hundreads of thousands of dollars, it is still enormously cheap for 10 m class astronomical telescope. So cheap that private persons can own them. If price of whole telescope with heavier polished mirror, or with light membrane mirror, is over million dollars or several million dollars for 8 - 10 m telescope, astronomy clubs can still buy them, when cost is divided between club members. Every astronomy club can then have own 8 - 10 m telescope.
About spherical main mirror and/or Offner relay optics: Gebelein spherical mirror telescope patent, and “Three-mirror anastigmat telescope with an unvignetted focal plane”, “Characteristic investigation of Golay9 telescope with a spherical primary mirror”, “Compact two-mirror schemes for telescopes with a fast spherical primary”, “Design of two spherical mirror unobscured relay telescopes using nodal aberration theory”, “Spherical primary optical telescope (SPOT)”, “A new configuration for a 10-metre optical/infrared telescope”, “Configurations for an extremely large telescopes with spherical and aspherical mirrors”, RIVMOS space telescope, DAWN space probe used “Shafer telescope combining inverted off-axis Burch system and Offner relay”, “Compact, three-mirror anastigmat, with reflective lens”, “Two axis Offner relay HERO”, “Scanning Offner relay simplifies fine pointing of large telescopes”, “High speed 2D Hadamard transform spectral imager”, “Rotatable Offner imaging system for ellipsometric measurement”, “Design and construction of an Offner spectrometer based on geometrical analysis of ring fields”.
About telescope baffling: “A new type of compact well-baffled telescope configuration”. Short length multi mirror telescope Luc Arnold: A multiple mirror telescope concept for a very compact…"