Guyonthecouch
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Reged: 05/02/11
Loc: Washington D.C
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Mirror Cell material guidance
#5673736 - 02/11/13 10:08 AM
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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
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Pinbout
Post Laureate
Reged: 02/22/10
Loc: Montclair
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Re: Mirror Cell material guidance
[Re: Guyonthecouch]
#5673816 - 02/11/13 10:51 AM
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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.
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... 
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Guyonthecouch
member
Reged: 05/02/11
Loc: Washington D.C
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Re: Mirror Cell material guidance
[Re: Pinbout]
#5673963 - 02/11/13 12:25 PM
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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.
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Pinbout
Post Laureate
Reged: 02/22/10
Loc: Montclair
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Re: Mirror Cell material guidance
[Re: Guyonthecouch]
#5673982 - 02/11/13 12:34 PM
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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.
the new metal cell...
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careysub
Pooh-Bah
Reged: 02/18/11
Loc: Rancho Cucamonga, CA
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Re: Mirror Cell material guidance
[Re: Guyonthecouch]
#5674026 - 02/11/13 01:01 PM
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Quote:
... 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.
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Guyonthecouch
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Reged: 05/02/11
Loc: Washington D.C
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Re: Mirror Cell material guidance
[Re: careysub]
#5674094 - 02/11/13 01:45 PM
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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
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Guyonthecouch
member
Reged: 05/02/11
Loc: Washington D.C
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Re: Mirror Cell material guidance
[Re: Guyonthecouch]
#5674129 - 02/11/13 02:04 PM
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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.
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careysub
Pooh-Bah
Reged: 02/18/11
Loc: Rancho Cucamonga, CA
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Re: Mirror Cell material guidance
[Re: Guyonthecouch]
#5674162 - 02/11/13 02:29 PM
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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.cameraconcepts.com/store2/images/T/1.image-476.jpg
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Guyonthecouch
member
Reged: 05/02/11
Loc: Washington D.C
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Re: Mirror Cell material guidance
[Re: careysub]
#5674190 - 02/11/13 02:58 PM Attachment (9 downloads)
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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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careysub
Pooh-Bah
Reged: 02/18/11
Loc: Rancho Cucamonga, CA
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Re: Mirror Cell material guidance
[Re: Guyonthecouch]
#5674506 - 02/11/13 05:30 PM
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Quote:
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.myspacecdn.com/images02/149/68655dce0c5141a2a2d82364513fa...
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.
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Pinbout
Post Laureate
Reged: 02/22/10
Loc: Montclair
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Re: Mirror Cell material guidance
[Re: Guyonthecouch]
#5674714 - 02/11/13 07:59 PM
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astrosystems telekits use a wood tailgate
http://www.astrosystems.biz/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.
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Guyonthecouch
member
Reged: 05/02/11
Loc: Washington D.C
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Re: Mirror Cell material guidance
[Re: Pinbout]
#5674923 - 02/11/13 10:30 PM
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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?
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StarDusty
sage
Reged: 10/02/07
Loc: Parsippany, NJ
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Re: Mirror Cell material guidance
[Re: Guyonthecouch]
#5676716 - 02/12/13 10:24 PM
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There are several photos of my home made cell here:
http://www.clearskyobserver.com/index.php/20-telescopemaking/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.
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Guyonthecouch
member
Reged: 05/02/11
Loc: Washington D.C
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Re: Mirror Cell material guidance
[Re: StarDusty]
#5678449 - 02/13/13 09:59 PM
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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!
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Pinbout
Post Laureate
Reged: 02/22/10
Loc: Montclair
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Re: Mirror Cell material guidance
[Re: Guyonthecouch]
#5678496 - 02/13/13 10:31 PM
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post your plop I'll give you a full size pdf of the parts.
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Guyonthecouch
member
Reged: 05/02/11
Loc: Washington D.C
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Re: Mirror Cell material guidance
[Re: Pinbout]
#5678510 - 02/13/13 10:39 PM
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I don't have plop at all. The site only has a windows version. Do you know if there is a Mac version available?
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Pinbout
Post Laureate
Reged: 02/22/10
Loc: Montclair
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Re: Mirror Cell material guidance
[Re: Guyonthecouch]
#5678907 - 02/14/13 07:47 AM
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well what is your mirrors:
dia
thickness
focal length
secondary size
how many points, 18?
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Pinbout
Post Laureate
Reged: 02/22/10
Loc: Montclair
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Re: Mirror Cell material guidance
[Re: Guyonthecouch]
#5678919 - 02/14/13 07:58 AM Attachment (5 downloads)
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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.
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Guyonthecouch
member
Reged: 05/02/11
Loc: Washington D.C
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Re: Mirror Cell material guidance
[Re: Pinbout]
#5679305 - 02/14/13 12:01 PM
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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.
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Guyonthecouch
member
Reged: 05/02/11
Loc: Washington D.C
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Re: Mirror Cell material guidance
[Re: Guyonthecouch]
#5679316 - 02/14/13 12:09 PM
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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!
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Pinbout
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Reged: 02/22/10
Loc: Montclair
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Re: Mirror Cell material guidance
[Re: Guyonthecouch]
#5679332 - 02/14/13 12:14 PM Attachment (5 downloads)
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this is what I get from plop.
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Pinbout
Post Laureate
Reged: 02/22/10
Loc: Montclair
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Re: Mirror Cell material guidance
[Re: Guyonthecouch]
#5679479 - 02/14/13 01:28 PM Attachment (11 downloads)
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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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StarDusty
sage
Reged: 10/02/07
Loc: Parsippany, NJ
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Re: Mirror Cell material guidance
[Re: Guyonthecouch]
#5679589 - 02/14/13 02:15 PM
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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.
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StarDusty
sage
Reged: 10/02/07
Loc: Parsippany, NJ
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Re: Mirror Cell material guidance
[Re: Pinbout]
#5679757 - 02/14/13 03:36 PM
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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.
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Pinbout
Post Laureate
Reged: 02/22/10
Loc: Montclair
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Re: Mirror Cell material guidance
[Re: StarDusty]
#5679783 - 02/14/13 03:51 PM
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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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