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

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.

  • Similar Content

    • By StarMich
      I'm aiming for the best possible field illumination for my Explore Scientific PN208 f/3.9 imaging newtonian. The scope is 5 years old now and while doing maintenance I wanted to do something about the design flaws as well.
       
      With the current secondary holder design it's impossible to center the mirror beneath the focuser. My question is: is this really an issue when the entire primary is visible? I read about optimal field illumination when the secondary is centered, but this isn't achievable with this telescope.
       
      I attached an image showing the (very bad aligned and already degrading) secondary at the moment. This is as far down as I can go, I would need to replace the holder screws with longer threads to go further but I don't know if it would make any difference for field illumination?
       
      Thanks in advance,
      Michael
       
       

    • By runway77
      Hello, I am having a problem with a relatively expensive flat field eyepiece in a combination with a newtonian reflector. I am unable to achieve proper focus with that eyepiece because when I focus it on axis, the edges are blurry. When I focus it at the edge, the center field is out of focus. I don't want to mention the concrete type of this eyepiece because I received it with a spot on the bottom barrel, therefore I cannot be sure whether it has been dropped. This eyepiece has very good reviews on this forum. Optically it looks good and there is no rattle. My question is whether these flat field eyepieces are usable with newtonians as these telescopes doesn't generally suffer from the field curvature. Many thanks for help.
    • By Anthony RS
      I'm selling my my 70mm secondary mirror I used with my 150mm F4 newtonian for 100$ (final price). Shipping expenses (from Beirut, Lebanon) on me for free provided I use LibanPost for shipping. We can discuss using other shipping methods and maybe splitting the cost.
      The mirror is brand new, still have the original package and box, bought the telescope a month ago and used only twice. No scratches, not even dust (images attached). Excellent mirror, no issues whatsoever, 94% reflectivity and optimized for the offset of f / 4 Newtons, but it can be used up to f / 5. It's from TS, made in Taiwan, original price is 134 euros (including VAT) excluding shipping and custom fees. Link below:
      https://www.teleskop-express.de/shop/product_info.php/info/p10673_TS-Optics-Newton-Fangspiegel-D---70-mm-mit-Halter.html
      It does not come with the tilt screws and center screw though unfortunately.
      I can also sell you the spider vanes (1mm thickness) for 6 inch newtonian for 25USD. Also brand new but without the tilt screws and adjustment screws. https://www.teleskop-express.de/shop/product_info.php/info/p11056_TS-Optics-Spinne-fuer-6--Newtons---164-mm-bis-180-mm-Tubus-Innendurchmesser.html
      Let me know if interested.
      Clear Skies!
      Anthony
      <private email address removed>
       



    • By WiltsStarGazer
      Having previously got some reasonable images by mounting a smart phone to the eyepiece. I thought I would try for a more sophisticated set up.
      So last night I tried to get some pictures of the moon using a Canon Eos D450 connected to a Skywatcher 130p Newtonian via a T2 connection on the eyepiece holder.
      As the camera has automatic focusing built into the lens I thought I would have to adjust the focuser on the eyepiece holder to manually get a sharp image.
      Basically the telescope acting as a manually focused lens for the camera.
      But no joy, I just got a bright light which seemed to fill the camera view finder. I tried various settings on the camera, adjusting ISO and aperture etc,  I also had some extention rings for the camera lens so tired fitting those to extend the focal length but no better.
      I sure there are many palms being slapped against foreheads reading this but as you can tell I have no idea, although I do have some of the gear.
      Any pointers and/or advise would be gratefully received.
×
×
  • Create New...

Important Information

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