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RE: [atm_free] Re: perforating primary

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  • Christo vd Merwe
    Mark Thank you very much for the detailed information and advice. I did read some books and web sources, but it’s good to get the opinion of someone who has
    Message 1 of 36 , Jul 1, 2011

      Mark

      Thank you very much for the detailed information and advice. I did read some books and web sources, but it’s good to get the opinion of someone who has recently built similar scopes.

      I guess that my best option is to core from the back to within a few mm and finish drilling from the front after figuring. I know that I’m going to stress a bit while drilling that last bit, but it has to be done. Hopefully the surface won’t scratch (much) around the perimeter of the hole due to me using a diamond hole saw.

      Christo

        

       

      From: atm_free@yahoogroups.com [mailto:atm_free@yahoogroups.com] On Behalf Of tnut55
      Sent: 01 July 2011 06:19 AM
      To: atm_free@yahoogroups.com
      Subject: [atm_free] Re: perforating primary

       

       

      Take a look at the Cassegrain Project by Dick Parker at ASGH.org. He made his 12.5" f/4 primary by rough grinding first and then coring to within .15". The mirror was finished and then the coring finished from the front.

      I would certainly prefer to make the mirror without the full perforation.

      Mark

      --- In atm_free@yahoogroups.com, Mark Whitaker <tnut55@...> wrote:
      >
      > Yes, there are a lot of considerations and these are generally spelled out in books on the subject.
      >
      >
      > First, you must be sure the glass is well annealed and internal strain is very low.  The pyrex disks we obtain from "reputable" sources are generally advertised to be annealed to a certain level.  If your glass is of unknown origin, there is always the possibility that the glass may crack when perforating if internal stresses have not been relieved.  There are quite a few sources on the web and in the books (I have mentioned many times) on how to do a polarized light test to check strain in the glass.
      >
      > Typically, the mirror is cored (either fully or partially) before grinding.  This is to prevent astigmatism due to stress released by the coring.  However, my first 8" cassegrain primary was already f/5 when I cored it.  Then I hogged some more to take it down to f/4.  Your faster mirror may be more susceptible to astigmatism so I would suggest you core now and finish the last bit of rough grinding after the core is completed.
      >
      > I prefer to partially core.  That allows one to finish the mirror using standard techniques and then finish the core after the mirror is complete.  There is always a risk of some deformation around the inner edge of the hole but I have not had any problem with my 8" and 10" mirrors.  This method is called the "Henry" method and is recommended for smaller mirror (Texereau).  The other disadvantages are that you must finish the core from the front and there is the possibility that you may scratch that nice, polished surface.  It is nerve wracking the first time.  Also, it is not always easy to get the holes perfectly aligned.  If you don't plan to mount on a baffle tube support or some moving primary mounting, that's OK so long as you have left enough hole to accommodate the baffle tube that will pass through the mirror later.  Of course plaster (or wax) is poured in the recess of the cored and left there until the core is ready to be removed.
      >
      >
      > The second method of completely perforating the mirror from the beginning is called the "Ritchey" method.  The core is then plastered back in and the mirror is fabricated as usual.  This is generally the preferred method for larger mirrors either because the blank is molded with a hole already or because there is less risk of astigmatism than using the Henry method on a larger mirror (so suggest the books).  Even then, there can be some retouching required after removing the core.  Some disadvantages of the Ritchey method are that the recess around the core on the front side must be respected because either grains of abrasive can get trapped and scratch the mirror at a later time.  Abrasive will also build up in the recess and may cause a depressed area at the center of the mirror.  LeClaire's book suggests removing the core altogether during the last two grades of abrasive to prevent this problem.  He also removes a circular area of pitch at the
      > center of the lap for the same reason (polishing by hand).  His example mirror is a 300mm f/3 mirror, similar to yours.
      >
      > I am currently making a 12.5" f/4 primary using the Ritchey method.  It is the first mirror I'll do by this method so I might have more to say about it after I'm done.  I am grinding/polishing by machine so LeClaire's methods will not completely work for me so it will be interesting to see what issues I encounter.
      >
      > Mark
      >
      >
      >
      > ________________________________
      > From: Christo vd Merwe <christo@...>
      > To: atm_free@yahoogroups.com
      > Sent: Thursday, June 30, 2011 2:01 PM
      > Subject: RE: [atm_free] Re: perforating primary
      >
      >
      >  
      > As mentioned here before, I’ve purchased a diamond hole saw via eBay. While I’m waiting for it to be delivered, I’m wondering about a few things.
      > To recap; I’m busy with a 12” f3.33 primary for a Cassegrain and it’s already almost at the required sagitta. The next step is to get the hole done. Therefore I’ll have to core from the front so that the mirror can be supported on the flat rear end.
      > 1. Is it possible to fine grind and figure in the usual way with the central piece cemented back with Plaster of Paris?
      > 2. If not, would it be better to drill from the back to within a few millimetres of the front face?
      > 3. Are there any special considerations I should be aware of?
      > 4. Will the plugged hole present any problems, firstly, fine grinding and secondly, figuring the mirror?
      > Thanks
      > Christo
      >  
      >  
      >  
      > From:atm_free@yahoogroups.com [mailto:atm_free@yahoogroups.com] On Behalf Of art.bianconi
      > Sent: 24 June 2011 03:50 PM
      > To: atm_free@yahoogroups.com
      > Subject: [atm_free] Re: perforating primary
      >  
      >  
      > Hi Christo!
      > You can buy directly from THK  Diamond  by going to their main web site:
      > THK Diamond Tool Company 
      > I used a combination of diamond and bi-metal holes saws to make the "fishmouth" cuts on 2" galvanized pipes. I then I then MIG welded them for the GEM head on the replica scope (See Photos section)
      > THK is fast shipping even thought they are in the Far East.
      > The products are flawless.
      > I cut a secondary flue in an existing chimney using one of their 5 inch hole saws and a hand drill.
      > It cut through two layers of brick and cement in less than 15 minutes. I would have done the job much faster if I did not have to stop and break out each plug with a chisel when the hole saw bottomed on the brick.
      > Art
      >
      > --- In atm_free@yahoogroups.com, "Christo vd Merwe" <christo@> wrote:
      > >
      > > I can vouch for it being low priced. Last night ordered a 53mm diamond
      > > coated hole drill via eBay and it's been posted already. It cost $6.50 +
      > > $5.00 shipping to South Africa.
      > >
      > > The place I bought from is http://stores.ebay.com/thkdiamondtools
      > >
      > > Christo
      >

    • art.bianconi
      Guy I recently had occasion to have a disk trepanned from a larger mirror. The mirror was parabolized and coated. Since the intention is to refigure the mirror
      Message 36 of 36 , Jul 18, 2011
        Guy I recently had occasion to have a disk trepanned from a larger
        mirror.

        The mirror was parabolized and coated. Since the intention is to
        refigure the mirror from F-8 to F-15, I had no concerns for the surface
        or the coating.

        I watched as the machinist zeroed the disk on the platten in the water
        and then stood there in awe as the water jet scribed a precise cut, all
        in about 2.5 minutes!

        On close inspection with a 60x scope, the surface was flawless! The
        geometry was also perfect.

        When I consider the time spent making large custom diameter hole saws;
        the time spent setting up a vertical mill; the time spent cutting the
        disk, the time spent cleaning the mill of abrasives, I've pretty much
        concluded that I'll not likely ever use a hole saw to treppan again!

        Art


        --- In atm_free@yahoogroups.com, "Mark Christensen" <mjcw500@...> wrote:
        >
        > Art,
        >
        > Diamond corer or not the idea of coring glass (to use the
        > A.S. term), which itself produces abrasive particles -
        > the glass, on an expensive piece of gear would scare
        > the Willies out of me as well.
        >
        > Sounds like a job for a cheapie (but not too cheap - you
        > don't want a lot of wobble) drill press. I regularly abuse my
        > oldie from Harbor Freight (1980s vintage), even using it
        > as a (manual) polishing and figuring station for 10 inch
        > and smaller mirrors since it has a rotary base and
        > adjustable height.
        >
        > Guy,
        >
        > I'd like to add my thanks for the link to the Hong Kong
        > coring tool company as well: I needed to get several of
        > these and was on the verge of spending $50+ apiece for
        > them from other sources.
        >
        > Mark Christensen
        >
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