Mirror Cell material guidance
Posted 11 February 2013 - 10:08 AM
I am rebuilding my scope, repurposing the 16in f4.5 from my lightbridge into a completely new home. I have woodworking experience and tools and could even get into aluminum if necessary. I am looking for some inputs on the triangle cross bars of the cell. I plan to reuse the metal triangles of the LB, but need to complete the rest of the cell by hand. I was thinking of using either 1"x1" oak or poplar, or a square tube aluminum of 3/4" standard wall thickness. Any thoughts on strength or flexure between these two? The support bars won't be very long, probably areound 4-4.5 inches. Thoughts on which would be better suited? Thanks!
Posted 11 February 2013 - 10:51 AM
3/16" alum would be fine for the bars and triangles, but I think most people use 1/4"
once you put the kydex ring to hold all that sloppy mess together it becomes a springy...
Posted 11 February 2013 - 12:25 PM
Posted 11 February 2013 - 12:34 PM
the new metal cell...
Posted 11 February 2013 - 01:01 PM
... I am looking for some inputs on the triangle cross bars of the cell. I plan to reuse the metal triangles of the LB, but need to complete the rest of the cell by hand. ... The support bars won't be very long, probably around 4-4.5 inches.
The design of your cell is not at all clear to me from this. What supports the support bars?
Consider Pinbout's cell: it has two support bars, but they would be about 18 inches long in your scope size.
Posted 11 February 2013 - 01:45 PM
Posted 11 February 2013 - 02:04 PM
Posted 11 February 2013 - 02:29 PM
You are talking about these short support arms, but that is only part of the mirror cell, the central hub is an essential integral part. That is what is confusing to me.
Posted 11 February 2013 - 02:58 PM
Posted 11 February 2013 - 05:30 PM
Thanks! That is what I needed. I dropped this photo into paint to show you what I am thinking of. Please excuse the drawing... The parts that I have been discussing are labeled. Also note that this will be going into a full lower box so I am not constrained by the tube in the picture. What do you think?
If the cell picture is accurate:
then the existing mirror cell is one single cast/forged/etc. piece of metal.
And you are replacing the tube/tub base with a box?
You will notice that there is a very rigid hoop structure to which the cell is attached. You will need to replicate that structural frame in a new box to give the box rigidity and to support the mirror.
The traditional way of doing this is to make a "tailgate" - a rigid rectangular frame that supports the mirror/cell and which give the box additional rigidity when inserted and attached (see Pinbout's cell).
There are variations possible - sometimes the tailgate acts as integral cell (the collimation bolts are attached directly to tailgate cross-pieces), sometimes the mirror/cell are entirely separate and are simply supported by the tailgate, sometimes a very rigid integral structure is supplied by the "box" and no real tailgate exists.
The three support arms need to attach to some sort of rigid plate (the ring portion of the LB16 cell fills this role) if you want to keep the triangular geometry. Alternatives are to use two parallel cross beams like Pinbout, or "T" crossbeams, or crossbeam cross (plus) shape.
I can't recommend what will be stiff enough until a better definition of the whole system is available.
Posted 11 February 2013 - 10:30 PM
Posted 12 February 2013 - 10:24 PM
Only 6 support points for a 16" mirror per PLOP analysis. It also has a strap to support the weight of the mirror when not pointed straight up at the zenith. It uses a "tee" bar support beams made of aluminum and has top side collimation knobs that can be reached without having to get under the tailgate to reach them.
Posted 13 February 2013 - 09:59 PM
Posted 13 February 2013 - 10:31 PM
Posted 13 February 2013 - 10:39 PM
Posted 14 February 2013 - 07:47 AM
how many points, 18?
Posted 14 February 2013 - 07:58 AM
you could "hanging the cell" compressing springs between the aperature plate and the cell, making the collimation on the top so you don't have to run around the back to collimate the run up front to see if its working.
Posted 14 February 2013 - 12:01 PM
Posted 14 February 2013 - 12:09 PM
Posted 14 February 2013 - 01:28 PM
also it probably wouldn't cost much to get this waterjet cut. I had a 8in cell priced out at $40 for the plates $12 for the bars.
Posted 14 February 2013 - 02:15 PM
Posted 14 February 2013 - 03:36 PM
I like your hanging cell.
I would suggest that the mirror edge supports be connected to the stiff top plate rather than the hanging mirror cell plate.
That way any load they pick up will go directly into the stiff top plate and not through the more flexible collimation parts.
Posted 14 February 2013 - 03:51 PM
the aperture plate doesn't move.
the bottom support plate moves with the mirror.
the bolt that holds the sidesupport could go thru the the aperture plate and use it for another bearing surface but...
on another hand on my 8in travel dob I have the edge of the aperture plate to support the mirrors side. I put two teflon pieces at 90°. I'll accept any distortion from not being exactly on CoG. I install some thumbnut head on my socket cap head screws and sitting there collminating it was so easy. so fun. couldn't stop smiling.