Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

GTom

Slumped mirrors?

Recommended Posts

I am thinking on grinding my own lightweight mirror (first f4 16", later f4 24"). The lightest and cheapest option is to get a thin blank and slump it in a decent kiln.

Anyone has longer term experiene with slumped mirrors? Overall doesn't seem to be more work than a normal (not pregenerated) blank.

Do I have to grind the backside as in case of normal flat back mirrors to avoid astigmatism? How do people support a convex back while grinding? Does it make sense to grind a hole in the middle for additional support? I am remotely considering a convertible Newtonian/Cassegrain system anyway...

Edited by GTom

Share this post


Link to post
Share on other sites

Hello, I have slumped my own blanks in a kiln but gave up due to the expense of running the kiln. To properly anneal a large blank takes many hours of controlled cooling which is expensive. You then have the problem of supporting the meniscus blank during grinding which is not straight forward.  It is more economical to go the monolithic blank route which is well tried and tested. Hope this helps.

John

  • Like 1

Share this post


Link to post
Share on other sites

Thank you for your thoughts John. The ultimate goal is a portable big Dob like Mel Bartels. Portability is an absolute must for me, a conventional 1.5" thick ("thin") 24" is already too heavy.

Share this post


Link to post
Share on other sites

I agree with John. Trying slumping a couple of 12" disks, one was about 6mm thick the other 10mm (?) Used the services of the local ceramic group and their Kiln.

Making a "proper" mould to slump into proved difficult, temperature control/ annnnealling became a PITA!

Supporting the disks for grinding - sand bags/ air bags/ various disks of carpet didn't work for me.

An alternative was to vacuum distort a thin blank (which already had a "reasonable" finish) - supported on an edge O ring within a closed cell and hand vacuum pump connected to the rear. Yes, I think it would have eventually worked, getting around f10.

Don't even think of stretching a Mylar mirror and vacuum!!!!!

 

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Recently Browsing   0 members

    No registered users viewing this page.

  • Similar Content

    • By JonCarleton
      It figures!  First clear night in ages and the pollen is as thick as..well, something really thick.  Up to now, I have been fairly careful to avoid dust getting down the tube, but pollen is going to be unavoidable for the next several weeks.
      So...what does one do?  Are there some do and don't things I should be aware of?  It looks like my mirror comes out fairly easy (SkyWatcher 10" ).  I'm guessing  "chuck into the dish washer on the pot scrubber setting" is probably a really bad idea.
      What then, is the plan?  What chemicals/detergents should be used...or maybe all chemicals should be avoided?  Is it a no-touch surface?  Is there perhaps some magic spray that one uses?  I do have a can of electronics air that I plan to use from a safe distance to persuade away loose pollen, and that may be enough for the present.
       
    • By Kevin Francis
      Purchased the Orion Astroview 6 three years ago.  Found out later that I could not attach my Canon EOS XTi and obtain focus.  Everything I saw online required sawing and drilling the OTA.  That makes the scope a single purpose telescope.  After reviewing the primary mirror holder I created a holder that moved the mirror 25 mm.  This way I can keep the scope ready for visual observing and sell later if I desire.  If you're interested in saving a few dollars in this hobby check out the link below.
      How I Moved Prime Focus: Link to the detail.
      Photo from this telescope

    • By markastro
      Hi,
       
          Looking for a 12" DOB (or larger) which can be transported to Prestwick, Ayrshire. Any brand, as long as the telescope is optically sound and in good working order. I'm a reliable buyer who has bought and sold on this site before.
       
      Thanks,
       
      Mark
    • By rorymultistorey
      So I picked up a classic... a 1970's - 80's edmund optics f6 150mm newtonian...

      ... like this one but mine has a different mount. It was cheap. Very good condition. The focuser is pants. The mechanics of the secondary holder is (IMHO) brilliant and apparently the primary is 1/10th wave.
      But its f6 and I trhink I'd rather swap it out for a faster synta f5 mirror. .  The thing is its a one shot job bc to make the f5 mirror work I will have to saw off a good few cm from the barrel of the scope.
      So the question is: Is a high quality f6 mirror better than a faster synta f5 mirror for wide deep space astrophotograhy?
       
      All comments gratefully received. 😉 
    • By AstroRuz
      Skywatcher 150/1200 f8 planetary Newtonian.
      Bought this originally intending to set up a planetary rig but circumstances call for sale.
      Will come with eyepieces and a collimation eyepiece. Not used by myself and has seen very little use. Mirrors in good condition
      Collection only
      £50



×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.