Revisiting the lateral support for my 14 inch f/5 telescope

Time to resume working on my 14" f/5 telescope project.

The mirror cell parameters have been computed with PLOP. A standard 6 points cell it will be, with support points at 106.9mm. Here are a couple images of the cell - the discussion can be found on Cloudy Nights:

In this picture and the next ones I designed a 45° whiffle tree, just to have an idea about the extent of such cell. As you will see in the lateral support calculation table further down, such complex support is not needed for my 14 inch f/5mm with 24mm edge thickness.

Regarding the lateral (edge) support, I get the following figures from Robert Houdart's calculator:

ConfigurationDescriptionRMS
at COG
RMS 1.2 mm
above COG
180° cable sling cable on half the mirror circonference 0.6 nm 2.6 nm
90° edge bearings 2 support points at 45° from vertical 1.3 nm 4 nm
120° edge bearings 2 support points at 60° from vertical 5.4 nm
45° whiffletree 4 support points at 22.5° and 67.5° from vertical 0.7 nm 2.7 nm
mirror glued on 18pt cell
the back of the mirror is glued to an 18 points mirror cell
17.4 nm  

This table confirms the lack of need for a 45° whiffle tree lateral support. This also means that I have less hardware protruding at the bottom of the cell, which is good Smile

According tp Robert Houdart's computations my mirror has the following properties:

Position of COG: 10.93mm from back of mirror
Mirror Sagitta: 4.45mm
Mirror Mass: 4.45kg (if it were a Pyrex mirror)

First I'll try to see if I can further simplify my design, and if I can reduce the real estate used by the cell. Indeed, the smaller the cell footprint, the lower my primary mirror can go in the final telescope.

Now it's (almost) time to start making the mirror cell. This means I need to find someone who can make the cell for me since I cannot weld not do I have access to a metal working machine shop.

Olivier Biot's picture

Update

There was an error in the lateral support performance table posted in my initial post, hence I recomputed the figures and updated the table accordingly. As a consequence, I will move away from the 45° whiffle tree support in favor of the simpler, leaner and smaller 2 support point at 90°.

Post new comment

The content of this field is kept private and will not be shown publicly. If you have a Gravatar account associated with the e-mail address you provide, it will be used to display your avatar.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Allowed HTML tags: <a> <em> <strong> <cite> <code> <ul> <ol> <li> <dl> <dt> <dd> <img>
  • Lines and paragraphs break automatically.

More information about formatting options

By submitting this form, you accept the Mollom privacy policy.