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Mirror Cell material guidance

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#1 Guyonthecouch

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Posted 11 February 2013 - 10:08 AM

Good morning everyone!
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!

Justin

#2 Pinbout

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Posted 11 February 2013 - 10:51 AM

you could order the 1"x1" 1/8" wall thk 1018 steel square tubing from online metals along with the 1.25" x1/4" rec. flat cold rold for the sides all cut to length. build a wood frame to hold them and have someone weld them up for $30.

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3/16" alum would be fine for the bars and triangles, but I think most people use 1/4"

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once you put the kydex ring to hold all that sloppy mess together it becomes a springy... :grin:

#3 Guyonthecouch

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Posted 11 February 2013 - 12:25 PM

Thanks for the feedback Danny. I was hoping to stick with materials that I can find at Lowes or Menards. I would love to use flat metal stock as it would save vertical room in the box but I am trying to stay cheap and also trying to keep from commissioning anything out. I am really just trying to get this thing up and running by spring and it's my first build so I don't intend on it being a "final" rendition.

#4 Pinbout

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Posted 11 February 2013 - 12:34 PM

I made a wood cell before I made the metal one, I was going to keep the wood cell but it was too light and I couldn't balance the scope properly given I was rehab'in some old throw away scope.



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the new metal cell...

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#5 careysub

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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.

#6 Guyonthecouch

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Posted 11 February 2013 - 01:45 PM

I apologize for the lack of drawings, I know it makes it hard to picture in you minds eye. I will try to get some sketches up later. I am thinking about using the same geometry that is found in the original LB cell. Let me see if I can make sense of my thoughts so far: The original cell is an 18 point. Each pair of triangles is supported by a single crossbar that rests on top of the collimation tub (mirror basket). They are simply resting on top of a raised notch so that they can rock freely side to side. I would like to replace these. My thoughts are a 1" square box aluminum or solid square hard wood about 5 inches in length would do it. Instead of a nub, I would run a bolt through the middle. That bolt would go into a central board (simalar in design to a typical three point cell seen on 6-8 in scopes. The bolt would allow the bar (and triangles) to float, and would radiate outward beyond the crossbar. It would terminate in a post that would contain the collimation screws/springs. I really hope this makes more sense. I tried to find pics on the net of ATM cells and this is kind of a frankenstein of those ideas =)

Justin

#7 Guyonthecouch

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Posted 11 February 2013 - 02:04 PM

Danny, I really like the simplicity of the cell you created there. What size mirror is that? I am really doubting the ability of wood to support my 16". Do you think it's possible? I have also tossed around the idea of reinforcing the wood by wrapping it with fiber glass and resin.

#8 careysub

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Posted 11 February 2013 - 02:29 PM

The mirror cell looks like the picture here, I take it.

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.

https://www.cameraco...1.image-476.jpg

#9 Guyonthecouch

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Posted 11 February 2013 - 02:58 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?

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#10 careysub

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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:

http://a3.ec-images....2d82364513fa...

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.

#11 Pinbout

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Posted 11 February 2013 - 07:59 PM

astrosystems telekits use a wood tailgate

http://www.astrosyst...images/pmc1.jpg

someone else here made a solid wood tailgate with cutouts for vents, painted black on the inside and a reddish brown stain on the outside. it looked very nicel.

#12 Guyonthecouch

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Posted 11 February 2013 - 10:30 PM

I plan to make a box and ditch the tube. I like the design of the center hub system so I just used the image you sent to modify for what I was looking for. I don't think I specified the tail gate and that has probably led to confusion. I would like to do 3/4 in wood for it. (Thanks Danny!) The hub would sit above it, suspended by the collimation bolts. So basically, the entire hub would float. The cross bars would be held in place by the single bolt running through the COG perpendicular to the long axis. I hope this makes sense. On another note, I picked a piece of 3/4 in aluminum square tube. I will cut a piece to length and try to measure flexure. Don't know how I'll do this, maybe a caliper fixed in place?

#13 StarDusty

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Posted 12 February 2013 - 10:24 PM

There are several photos of my home made cell here:

http://www.clearskyo...emaking/7-16dob

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.

#14 Guyonthecouch

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Posted 13 February 2013 - 09:59 PM

Allen, that scope is awesome! I love the unique direction you took. I am very interested in your mirror supports. Is your 16 full thickness or is it 1.75 in? Are you seeing any give in the aluminum? If not, I would like to steal parts of your design as long as you don't mind. Unfortunately, I have a Mac, so I am not sure how to get my plop layouts. Still working that aspect. Thanks again sir!

#15 Pinbout

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Posted 13 February 2013 - 10:31 PM

post your plop I'll give you a full size pdf of the parts.

#16 Guyonthecouch

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Posted 13 February 2013 - 10:39 PM

I don't have plop at all. The site only has a windows version. Do you know if there is a Mac version available?

#17 Pinbout

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Posted 14 February 2013 - 07:47 AM

well what is your mirrors:

dia
thickness
focal length
secondary size
how many points, 18?

#18 Pinbout

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Posted 14 February 2013 - 07:58 AM

btw, here's a different take on the cell plate.

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. :grin:

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#19 Guyonthecouch

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Posted 14 February 2013 - 12:01 PM

My mirror is 406mm diameter, 44mm thick and is an f4.5. I plan on doing 18 point because I am reusing the stock triangles. But, if plop says something different, I have no problem adapting. I currently have a 3.5 in secondary, but I think I may use a 3.1 when all is said and done.

#20 Guyonthecouch

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Posted 14 February 2013 - 12:09 PM

I'm a big fan of that reverse cell. It never occurred to me to have springs on top. Now I can't get it out of my head!

#21 Pinbout

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Posted 14 February 2013 - 12:14 PM

this is what I get from plop.

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#22 Pinbout

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Posted 14 February 2013 - 01:28 PM

how's this for a full size pdf. you could take it to kinkos and have them print it on 24x36 and make sure they don't print to fit paper.

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.

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#23 StarDusty

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Posted 14 February 2013 - 02:15 PM

My 16" is not full thickness. Sorry I don't remember its thickness, but I can look when I return home. Feel free to borrow any idea you want.

#24 StarDusty

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Posted 14 February 2013 - 03:36 PM

Danny,

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.

#25 Pinbout

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Posted 14 February 2013 - 03:51 PM

the side support has to move with the mirror.

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.

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