Page 1 of 2 12 LastLast
Results 1 to 30 of 37

Thread: Can a Telescope Mirror be made of a reflective thin film?

  1. #1
    Join Date
    Jul 2016
    Posts
    14

    Can a Telescope Mirror be made of a reflective thin film?

    Hi all! I just joined, this is my first post.

    I am a PhD student in a polymer department and fascinated with astronomy. The more I read (and obsess) about contributing to the field as an amateur the more I am understanding about how telescopes are built and that for dim objects, size matters. Combining that with my knowledge in the field of plastics engineering I pose this question: Can the reflecting surface of a telescope be a thin film? Can we scale this up to make no only an super large mirror but the infrastructure affordable for the an backyard enthusiast?

    I have yet to invest my time in any optical math, not really my forte... but maybe a good place to start is: Is it possible to get a film with the desired shape: (I know this to be very much yes). How important is the fidelity of that shape? What is the tolerances for the shape of the mirror on your standard reflector? The sailing industry has become very good and engineering curvature into their sails... This should be doable... right?

  2. #2
    Join Date
    Jun 2007
    Posts
    5,718
    No optical reason it couldn't be, but producing and maintaining the required shape would be very tricky. The precision requirements are quite high: the Hubble mirror deviated from its intended shape by 2.2 micrometers at the edges.

  3. #3
    Join Date
    Jun 2006
    Posts
    4,738
    Quote Originally Posted by BenCherniawski View Post
    Hi all! I just joined, this is my first post.

    I am a PhD student in a polymer department and fascinated with astronomy. The more I read (and obsess) about contributing to the field as an amateur the more I am understanding about how telescopes are built and that for dim objects, size matters. Combining that with my knowledge in the field of plastics engineering I pose this question: Can the reflecting surface of a telescope be a thin film? Can we scale this up to make no only an super large mirror but the infrastructure affordable for the an backyard enthusiast?

    I have yet to invest my time in any optical math, not really my forte... but maybe a good place to start is: Is it possible to get a film with the desired shape: (I know this to be very much yes). How important is the fidelity of that shape? What is the tolerances for the shape of the mirror on your standard reflector? The sailing industry has become very good and engineering curvature into their sails... This should be doable... right?
    If you could get the shape just right while the plastics are flexible and then just 'freeze' everthing in place . . . .

    I must admit, I've seen some really low quality plastic optics.
    I'm not a hardnosed mainstreamer; I just like the observations, theories, predictions, and results to match.

    "Mainstream isnít a faith system. It is a verified body of work that must be taken into account if you wish to add to that body of work, or if you want to change the conclusions of that body of work." - korjik

  4. #4
    Join Date
    Aug 2006
    Posts
    3,368
    Quote Originally Posted by John Mendenhall View Post
    If you could get the shape just right while the plastics are flexible and then just 'freeze' everthing in place . . . .
    "just" freeze it in place? How would that work?

    The issue is not so much in getting a reflector the right shape in the first place, it is keeping it that way under operating- as well as adverse- conditions.

    How would you do that if every bump and vibration sends a ripple across such a thin shape like ripples in a pond? I can't imagine how you would stabilize such a thin film - unless you attached it to something massive and inflexible. In which case, what's the point?

    They tried making telescopes from pools of Mercury, spun to produce a parabolic curve, but the practical problems with vibrations were prohibitive and it was scrapped.
    Last edited by DaveC426913; 2016-Jul-26 at 12:10 AM.

  5. #5
    Join Date
    Jul 2016
    Posts
    14
    To the point on freezing, there are definitely ways to harden a polymer after spinning the monomers (or possibly molten polymer). Crosslinking is a solid example. Maybe even just temperature would be good enough, you'd definitely want to be below the Tg though. Above that creep (sagging) would be an issue.

    I read a few resources on this today mainly concerned with thin glass films and using adaptive optics... not the simplicity level i was hoping for. If I wanted to image Vis light, what is the maximun rms roughness i could tolerate? If hubbles mirror only needed a few micronss... thats a pretty tall order :/. Not even considering thermal expansion... lol

  6. #6
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    8,440
    Assuming this is in space, maybe there are tricks that could be used. Early mirrors were made by spinning molten metal about a vertical axis, forming a parabola.. A simple spinning disc in space should form a flat mirror. But suppose you made two parallel discs with a small spacing? There is the Casimi effect but you could apply charge. In fact you could apply a pattern of charge from a remote electron gun. The charge would cause deflection and a perfect parabola could be made and errors corrected. Charge applied that way would be very stable but you would need shields to keep out unwanted space noise. Or you would constantly correct the curvature with your electron gun. I am pretty sure that would work. Charge can be applied to better than 0.1 micron with some care.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  7. #7
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    8,440
    Quote Originally Posted by DaveC426913 View Post
    "just" freeze it in place? How would that work?

    The issue is not so much in getting a reflector the right shape in the first place, it is keeping it that way under operating- as well as adverse- conditions.

    How would you do that if every bump and vibration sends a ripple across such a thin shape like ripples in a pond? I can't imagine how you would stabilize such a thin film - unless you attached it to something massive and inflexible. In which case, what's the point?

    They tried making telescopes from pools of Mercury, spun to produce a parabolic curve, but the practical problems with vibrations were prohibitive and it was scrapped.
    Spun metal mirrors were used for many years, for example in Ireland where the first good images of the moon were made. The cooled metal was polished every night or thereabouts to get rid of oxides. You can visit the site, it's a stately home, but sadly the telescope is not in use today.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  8. #8
    Join Date
    May 2007
    Location
    Earth
    Posts
    10,240
    Quote Originally Posted by DaveC426913 View Post

    They tried making telescopes from pools of Mercury, spun to produce a parabolic curve, but the practical problems with vibrations were prohibitive and it was scrapped.
    Not so fast: https://www.google.com/url?sa=t&sour...BLm5LQHIg7WzvA
    http://www.aeos.ulg.ac.be/LMT/instruments.php
    Information about American English usage here and here. Floating point issues? Please read this before posting.

    How do things fly? This explains it all.

    Actually they can't: "Heavier-than-air flying machines are impossible." - Lord Kelvin, president, Royal Society, 1895.



  9. #9
    Join Date
    Jun 2006
    Posts
    4,738
    Quote Originally Posted by DaveC426913 View Post
    "just" freeze it in place? How would that work?

    The issue is not so much in getting a reflector the right shape in the first place, it is keeping it that way under operating- as well as adverse- conditions.

    How would you do that if every bump and vibration sends a ripple across such a thin shape like ripples in a pond? I can't imagine how you would stabilize such a thin film - unless you attached it to something massive and inflexible. In which case, what's the point?

    They tried making telescopes from pools of Mercury, spun to produce a parabolic curve, but the practical problems with vibrations were prohibitive and it was scrapped.
    Wish I knew. Hey, if the OP manages it, he'll make a fortune.

  10. #10
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    8,440
    I was making aircraft mirrors from Melanine (Mylar) film aluminised. You heat shrink the fixed film to get a excellent flat mirror. Obviously we played with pressure to make curved surfaces but an electret version is feasible I am sure. In air the charge would not last long but in space it would be near permanent.
    Last edited by profloater; 2016-Jul-26 at 02:33 PM. Reason: correction in brackets
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  11. #11
    Join Date
    Jul 2016
    Posts
    14
    Great comments! I am not yet a proven member so it takes forever to reply!

    @Profloater - I can't really address the space telescope aspect mostly because I will likely never be able to afford that.. haha. But, I'll look into your comments about shaping a spinning fluid in microgravity. Mylar and melanine are actually a bit different. mylar is cool because it's the same stuff soda bottles are made out of (PET) just biaxially stretched to stiffen the plastic. And yes! They do metalize the surface of these films (think of the metallic emergency blankets that fit in your pocket). Melamine is a crosslinked aromatic network of carbons and nitrogens, think hard surface of laminate counters (Formica). Engineeringwise - quite different properties.

    @Swampyankee - Holy [redacted] that is an awesome link! This is clearly a proof of concept that a rotating liquid can create a high fidelity lens. Very interesting. could you use gallium or other alloy with a melting temp near Room temp (reduce shrinking)? and solidify it?


    Here's what i'm thinking. Today's telescopes scale exponentially (it seems) with the size of the main objective. In the largest observatories that objective is only ~5% of the total cost. I am assuming a significant fraction of the rest goes into the infrastructure supporting that mirror and preventing it from deforming. My main gripe with this ideology is that the engineers are fighting to control upwards of 70 tons of glass and maintain mirror fidelity/position. If we could channel that effort into a creative solution for an ultra low-mass reflective surface this would drastically reduce the cost (maybe, lol).

    For the amateur astronomy nerd (< 2 meter objectives) This also has some important implications; the first of which being cost of materials and the second being systems (cost of infrastructure to move/control this mirror). With all of the advances in these high strength materials (carbon fiber and kevlar come to mind) I would be surprised if we could not come up with a material or composite film that could hold it's shape well enough through +/-50degrees F. Can someone 'explain it like i'm five' what defines the tolerances on a mirror and what is the wavelength dependence of those tolerances? I'll dig up some thermal expansion coefficients and bounce this off some of my peers working on composites.
    Last edited by PetersCreek; 2016-Jul-26 at 06:29 PM. Reason: Masked language

  12. #12
    Join Date
    Jul 2016
    Posts
    14
    NOTE ACCIDENTAL REPOST (post lag is hard wait for):
    To the point on freezing, there are definitely ways to harden a polymer after spinning the monomers (or possibly molten polymer). Crosslinking is a solid example. Maybe even just temperature would be good enough, you'd definitely want to be below the Tg though. Above that creep (sagging) would be an issue. Additionally shrinkage is a common problem with resins and curable plastics, I will try to find out if someone has worked this out already.

    I read a few resources on this today mainly concerned with thin glass films and using adaptive optics... not the simplicity level I was hoping for. If I wanted to image Vis light, what is the maximun rms roughness i could tolerate? If hubbles mirror only needed a few micronss... thats a pretty tall order :/. Not even considering thermal expansion... lol
    Last edited by BenCherniawski; 2016-Jul-26 at 05:56 PM.

  13. #13
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,548
    The challenge is in keeping the optical surface within a couple of millionths of an inch of the desired shape. That is to keep the light at the focal point within about 1/10 of a wavelength of perfection. The trend in recent years at major observatories has been toward lighter structures and active control of the figure of thin, segmented mirrors, as opposed to the earlier practice of building it like a main battery turret on a battleship to support a thick, heavy mirror.

  14. #14
    Join Date
    Apr 2016
    Location
    Nether
    Posts
    706
    Wasn't there a recent project using inflated spheres with one part see-through and another part reflective surface on the inside? Might make for a stable carrier for ultrathin reflective surfaces..
    "Downwards is the only way forwards" Cobb

    Noting in science is proven actual

  15. #15
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    8,440
    Quote Originally Posted by BenCherniawski View Post
    Great comments! I am not yet a proven member so it takes forever to reply!

    @Profloater - I can't really address the space telescope aspect mostly because I will likely never be able to afford that.. haha. But, I'll look into your comments about shaping a spinning fluid in microgravity. Mylar and melanine are actually a bit different. mylar is cool because it's the same stuff soda bottles are made out of (PET) just biaxially stretched to stiffen the plastic. And yes! They do metalize the surface of these films (think of the metallic emergency blankets that fit in your pocket). Melamine is a crosslinked aromatic network of carbons and nitrogens, think hard surface of laminate counters (Formica). Engineeringwise - quite different properties.

    @Swampyankee - Holy [redacted] that is an awesome link! This is clearly a proof of concept that a rotating liquid can create a high fidelity lens. Very interesting. could you use gallium or other alloy with a melting temp near Room temp (reduce shrinking)? and solidify it?


    Here's what i'm thinking. Today's telescopes scale exponentially (it seems) with the size of the main objective. In the largest observatories that objective is only ~5% of the total cost. I am assuming a significant fraction of the rest goes into the infrastructure supporting that mirror and preventing it from deforming. My main gripe with this ideology is that the engineers are fighting to control upwards of 70 tons of glass and maintain mirror fidelity/position. If we could channel that effort into a creative solution for an ultra low-mass reflective surface this would drastically reduce the cost (maybe, lol).

    For the amateur astronomy nerd (< 2 meter objectives) This also has some important implications; the first of which being cost of materials and the second being systems (cost of infrastructure to move/control this mirror). With all of the advances in these high strength materials (carbon fiber and kevlar come to mind) I would be surprised if we could not come up with a material or composite film that could hold it's shape well enough through +/-50degrees F. Can someone 'explain it like i'm five' what defines the tolerances on a mirror and what is the wavelength dependence of those tolerances? I'll dig up some thermal expansion coefficients and bounce this off some of my peers working on composites.
    spinning a fluid in microgravity is not the same as spinning in gravity but I guess you are thinking cheap terrestrial. Thin films make great flat mirrors but with gravity and other distortion they would not be good for telescope work unless corrected. If you imagine a sandwich of say three films, the outer transparent as a wind shield, the back as a ground plane, the centre as an electret you could form a neat curved surface which can be adjusted. Although now my electron gun has to penetrate the outer shield. The material is indeed Mylar, I was typing too quickly and it was 45 years ago.
    I did make a very simple concave mirror that could set fire to paper at twenty feet when playing about. Have you played with electrets made from thin films? They are quite easy to make and the charge will cause distortion of a thin film next to a ground plane. In low conductive environment the charge on an electret is stable.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  16. #16
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    8,440
    Quote Originally Posted by BenCherniawski View Post
    NOTE ACCIDENTAL REPOST (post lag is hard wait for):
    To the point on freezing, there are definitely ways to harden a polymer after spinning the monomers (or possibly molten polymer). Crosslinking is a solid example. Maybe even just temperature would be good enough, you'd definitely want to be below the Tg though. Above that creep (sagging) would be an issue. Additionally shrinkage is a common problem with resins and curable plastics, I will try to find out if someone has worked this out already.

    I read a few resources on this today mainly concerned with thin glass films and using adaptive optics... not the simplicity level I was hoping for. If I wanted to image Vis light, what is the maximun rms roughness i could tolerate? If hubbles mirror only needed a few micronss... thats a pretty tall order :/. Not even considering thermal expansion... lol
    A mylar mirror to look at is as good as glass, the surface is way better than 1 micron. If you make a round frame and bond mylar both sides, heat shrink out the wrinkles you will have what looks like a perfect cicular specular mirror. A little vacuum pressure will make both surfaces concave spherical which can be used in a reflector T but to get a parabola is more difficult. At that level you can make a good mirror in half an hour if you have some aluminised mylar. Other polymer mirrors are already very common and easy to find. It is commonplace to aluminise them for domestic and scientific work.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  17. #17
    Join Date
    Jul 2016
    Posts
    14
    Quote Originally Posted by profloater View Post
    A mylar mirror to look at is as good as glass, the surface is way better than 1 micron. If you make a round frame and bond mylar both sides, heat shrink out the wrinkles you will have what looks like a perfect cicular specular mirror. A little vacuum pressure will make both surfaces concave spherical which can be used in a reflector T but to get a parabola is more difficult. At that level you can make a good mirror in half an hour if you have some aluminised mylar. Other polymer mirrors are already very common and easy to find. It is commonplace to aluminise them for domestic and scientific work.
    I was thinking about a vacuum as well. Ultimately, I have my doubts about how this will turn out. My major issue with the vacuum is any variation in film thickness or local mechanical properties will manifest in pretty significant deviations in the final surface. Any heat treatment in the surface is changing how the polymer chains' conformations, entanglement, and alignment and thus varying their mechanical properties. In an extreme case we could imagine pulling vacuum on a film with pieces of tape scatter across it (the tape will not expand like the rest of the film). Best case scenario, we (likely) have a very elastic rubber with very uniform material properties that can be precisely stretchered. Metalizing that may be difficult... and any barometric changes would directly influence the shape of the mirror (you'd need a feed back loop for the vacuum pump/pressure). Vacuum forming a surface is still in the back of my mind.. just not my prime candidate.

  18. #18
    Join Date
    Jul 2016
    Posts
    14
    Quote Originally Posted by Hornblower View Post
    The challenge is in keeping the optical surface within a couple of millionths of an inch of the desired shape. That is to keep the light at the focal point within about 1/10 of a wavelength of perfection. The trend in recent years at major observatories has been toward lighter structures and active control of the figure of thin, segmented mirrors, as opposed to the earlier practice of building it like a main battery turret on a battleship to support a thick, heavy mirror.
    Did a bit of digging you are very right! Seems like 1/10 wave is a good place to start (For UV-Visible light, 20 to 70nm)ish

    QUOTED FROM: "http://www.bbastrodesigns.com/ratemirrors.html"

    The wavelength that our eyes is most sensitive to is green light which has a wavelength of 500 nanometers or 0.5 microns or (25,400 microns to the inch) 0.00002 inches or 20 millionths of an inch. Often a mirror is rated in red light at 750 microns. This means that to our eyes, an error rated using red light will be 50% greater than stated. We want the mirror to focus light to a fraction of this size.

    Danjon-Couder condition 2 or Raleigh Criteria: Maximum wavefront error must not exceed a quarter wave and, for the majority of the mirror's area, the defects should be appreciable less. Some experienced planetary observers feel that 1/10 wave gives a slightly perceptibly better image, but since most mirrors are over-rated it's not possible to make this statement unequivocally.

    Danjon-Couder condition 1: The radius of the circle of least aberration should be comparable with that of the theoretical diffraction disk and, on the average, the transverse aberrations should not exceed the diffraction disk radius.

    Marechal Limit: 1/14 wavefront RMS.

  19. #19
    Join Date
    Jul 2016
    Posts
    14
    Here are the major hurdles in making this a reality:
    1) Form the correct shape
    2) Metalize
    3) Hold that shape under its own weight
    4) Mount
    5) Continue to hold the correct shape

    1) Use a rotating pool of resin to create the surface (not sure if it can be done perfect enough) but if not, possibly try a more complex molding process using a rotating dense liquid as the template for a negative mold?
    2) no problem to metalize. This process is already established and polymer films will readily take a metal coating.
    4) Haven't really gotten to this part... not a huge concern though. it can definitely be done.
    *3/5) This is my major concern: can we keep that shape? I am encouraged by the post from Swampyankee where the structure they use to support and spin the rotating pool of mercury is MADE FROM FOAM AND KEVLAR!!! They mention it meets the specifications for thermal and mechanical properties... Can this work for one of my films???


    QUOTED FROM: http://www.aeos.ulg.ac.be/LMT/instruments.php
    The container is a very important component of the system. It must be light and rigid. It is made with Kevlar laminated over a foam core (Hickson, Gibson & Hogg 1993). Moreover, this container must fulfill strict specifications of rigidity, temperature stability , etc. in order to guarantee a final shape as perfect as possible and to avoid the formation of wavelets over the mercury surface.
    See: http://www.aeos.ulg.ac.be/upload/container.jpg

  20. #20
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    8,440
    how big is your target diameter?
    Here is one link to the 72 inch in Ireland
    https://amazing-space.stsci.edu/reso.../scopes/rosse/
    it was a spun reflector plus polish.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  21. #21
    Join Date
    Jul 2016
    Posts
    14
    Quote Originally Posted by profloater View Post
    how big is your target diameter?
    Here is one link to the 72 inch in Ireland
    https://amazing-space.stsci.edu/reso.../scopes/rosse/
    it was a spun reflector plus polish.
    Interesting. They spin the glass in the arizona glass lab as well. Amazing tech to do it. I will target 25 centimeters first to check tolerance and better understand the molding process. Then if all goes well, i'd love to see a fleet of 2 M mirrors watching the sky. Thats still a bit of a ways off though

  22. #22
    Join Date
    Jun 2007
    Posts
    5,718
    For producing a good parabolic shape in a film, electrostatic force might be better. A set of ring electrodes would suffice if the film properties are uniform, the rings might have to be divided into sectors to account for variations. But...calibration to produce a good shape would be tricky, stability over time would be problematic (the film relaxing over time or otherwise aging), and this is basically an electrostatic speaker/microphone and would be subject to acoustic noise.

  23. #23
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,548
    Quote Originally Posted by profloater View Post
    how big is your target diameter?
    Here is one link to the 72 inch in Ireland
    https://amazing-space.stsci.edu/reso.../scopes/rosse/
    it was a spun reflector plus polish.
    Are you saying they spun the mirror blank during the casting process? It has always been my understanding that these speculum mirrors were cast in molds with convex bottoms to yield a matching convex surface close to the desired final curve. Do you have any references?

  24. #24
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,548
    Quote Originally Posted by Hornblower View Post
    Are you saying they spun the mirror blank during the casting process? It has always been my understanding that these speculum mirrors were cast in molds with convex bottoms to yield a matching convex surface close to the desired final curve. Do you have any references?
    My bold. That should have been concave.

  25. #25
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    8,440
    Quote Originally Posted by Hornblower View Post
    Are you saying they spun the mirror blank during the casting process? It has always been my understanding that these speculum mirrors were cast in molds with convex bottoms to yield a matching convex surface close to the desired final curve. Do you have any references?
    No they spun the very heavy turntable with molten metal till it cooled. This makes a good parabola. Then they polished the oxides off the solidified metal, regularly. I found that link easily and it was because I visited the site a few years ago that I remember being impressed. The Earl made excellent drawings of the moon and even estimated its surface temperature correctly although at the time he was ridiculed by scientists. The site in the middle of Ireland is far from ideal because it is often foggy, but that was his castle. At the time it was one of the largest reflectors in the world. I work in resins now and have considered making a resin equivalent although as a sound reflector rather than a telescope. I was thinking of making six ft diameter grp reflectors for that purpose .
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  26. #26
    Join Date
    Jul 2005
    Location
    Massachusetts, USA
    Posts
    21,840
    You can imagine that in a decade or two we might start having the nanotechnology capabilities to make a pair of giant sheets and attach them to a giant ring, have one sheet transparent at relevant wavelengths and the other reflective, and then put some dry Helium between the sheet to expand them. If they are simple sheets they will make a spherical mirror, but with some engineering of the material it could make a parabolic one.
    Forming opinions as we speak

  27. #27
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,548
    Quote Originally Posted by profloater View Post
    No they spun the very heavy turntable with molten metal till it cooled. This makes a good parabola. Then they polished the oxides off the solidified metal, regularly. I found that link easily and it was because I visited the site a few years ago that I remember being impressed. The Earl made excellent drawings of the moon and even estimated its surface temperature correctly although at the time he was ridiculed by scientists. The site in the middle of Ireland is far from ideal because it is often foggy, but that was his castle. At the time it was one of the largest reflectors in the world. I work in resins now and have considered making a resin equivalent although as a sound reflector rather than a telescope. I was thinking of making six ft diameter grp reflectors for that purpose .
    I learn something every day. Now that I think of it, I remember reading decades ago that large speculum blanks were cast with their backs concentric with the optical surfaces, to give uniform thickness and better stability with temperature changes. My educated guess is that spin casting with the blank face up would be the only practical way to do this. For one as big and heavy as the Leviathan, I am guessing a steam engine with a governor as the motive power.

  28. #28
    Join Date
    Jul 2016
    Posts
    14
    Been a busy few days. Just submitted a paper though, so not totally a loss! I had a few more thoughts on this project.

    I think it could be possible to roughly shape a thin film ( +/- 10 microns? lol maybe?) and attain the parabolic shape through clever film design, like variable thickness or material properties. Maybe you could cast the film on a spinning parabola to get the correct thickness and shape to begin with? Fine adjustment could theoretically be done using piezoelectric forces to help control the conformations of the mirror (there are piezoelectric polymers too). I want to avoid this complication all together and attempt to generate a near perfect surface right off the bat. Glass offers this, but is far too heavy. So i am thinking polymer composites offer a light, ridged, and cast-able option that can meet nearly all of these criteria. I haven't totally given up on the ultra thin film idea, but i feel it is less plausible than my other option.

    I was talking with my engineering buddies and asked what they thought. They said likely a open cell foam core would give the needed weight reduction and would synergize well with my idea of a carbon fiber and epoxy composite. Essentially, build the shell light and strong, with a basic parabolic shape on top, then cast a thin layer of resin over it while spinning to give the "perfect" surface, metalize and see how it looks in an interferometer. With this method, a 72 inch mirror could weight less than 40 pounds... Crazy right?

  29. #29
    Join Date
    Jul 2016
    Posts
    14
    Quote Originally Posted by profloater View Post
    No they spun the very heavy turntable with molten metal till it cooled. This makes a good parabola. Then they polished the oxides off the solidified metal, regularly. I found that link easily and it was because I visited the site a few years ago that I remember being impressed. The Earl made excellent drawings of the moon and even estimated its surface temperature correctly although at the time he was ridiculed by scientists. The site in the middle of Ireland is far from ideal because it is often foggy, but that was his castle. At the time it was one of the largest reflectors in the world. I work in resins now and have considered making a resin equivalent although as a sound reflector rather than a telescope. I was thinking of making six ft diameter grp reflectors for that purpose .
    Any chance you know how they dealt with the thermal compression after cooling? Also, do you happen to know any resins with ultra low cure shrinkage?

  30. #30
    Join Date
    Jul 2016
    Posts
    14
    Sorry to post so much,

    If anyone is interested, this details a bit on how mirrors are measured very accurately (interferometry): http://www.rfroyce.com/testmethod.htm
    &
    This is an NASA article on thermal expansion coefficients of epoxy composites: http://ntrs.nasa.gov/archive/nasa/ca...9810023033.pdf
    (glass, for comparison, has a CTE of ~5 x 10-6/K or C)

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •