Hi All 16" Mirror Cell Ideas

[Saturnine (Jeff)]

Want to tap into the knowledge base on here. Building an 400mm (16") F4.5 newt / dob with a Hubble Optics sandwich mirror and I'm looking for suitable mirror cell designs and also trying to get my head around Wiffle Tree and sling supports.
Have looked at many designs online, Mel Bartels etc and most of the info seems a bit vague to me (or maybe I'm the vague one) and most of the images I've seen so far are not detailed enough for me.
It will be an 18 point floating cell, that part is relatively straight forward to construct, it's the edge support that I need to get my head around.
Thanks in advance for any design tips or insights that will help.

Cheers
Jeff

[sfarndell (Scott)]

H Jeff. Do you use fusion 360? I've designed a binodob with whiffletree supports for a 14" hubbleoptics mirror. You can leverage off that. PM me your email address.
Cheers
Scott

[Saturnine (Jeff)]

Thanks Scott, sent you a PM.

[Satchmo]

I don't think you would need any more than a 9 point cell , given that the mirror is somewhat lighter than a solid mirror of the same thickness. A 9 point cell works fine for a monolithic 16" X 40mmm thick mirror .

[Saturnine (Jeff)]

Thanks for the input Mark, a lot of the designs that I looked at varied between 9 point and 18 point cells and was thinking that the sandwich mirror might flex more easily than a full thickness mirror, therefore may need more support.

[Satchmo]

Originally Posted by Saturnine

Thanks for the input Mark, a lot of the designs that I looked at varied between 9 point and 18 point cells and was thinking that the sandwich mirror might flex more easily than a full thickness mirror, therefore may need more support.

Thats a fair assumption , though it should have less self weight deflection due to its lighter weight . The strength of the glass sandwich comes mainly from its thickness, and I assume these mirrors are at least 40 to 50mm thick , and structurally the fused mirrors should behave like a monolithic mass.

So my guess is it should be a little stiffer mechanically than a solid blank of the same aspect ratio. It's better to pull off a really good cell with less points than not get quite right a more complex one . The edge support on those mirrors with just front and back plates at the edge looks trickier . Maybe just a single cable sling between plates at the C of G ?


I would be interested to see the Fusion 360 results if you get any .

[sfarndell (Scott)]

I don’t think an 18 point cell is much more onerous than a 9 point cell and you do have lower tolerances the higher the number of points. That being said, it’s worth building your cell as accurately as possible.

Here is my cell for my 14” hubbleoptics mirrors. I used 5mm aluminium for the whiffletree and the bearings are positioned to be at the centre of each layer of the sandwich mirror. Works fine for me.
Orange knobs are for collimation - Makes it easier having them on the front.

[Saturnine (Jeff)]

Thanks for the input guys and for the photos, has given me more food for thought. The biggest question is, do I go for 9 or 18 point cell, both are doable from a mechanical perspective, though the 9 point cell is more simple obviously and from what Mark has said, should be sufficient support for the weight of a sandwich mirror.
With the edge support, am thinking that instead of rollers, just having nylon pads a little longer than the thickness of the mirror. Should be slippery enough to allow the mirror to move without too much friction that may distort the mirror.
Happy to take onboard any other insights or suggestions that may be forthcoming.

Cheers

[gb44 (Glenn)]

Having had astigmatism due to nylon side supports in 16 and 18inch plate mirrors I certainly prefer the idea of just a single cable sling between plates at the C of G . Simple and adjustable.

GlennB

[Satchmo]

Another question is is the back curved or flat, I'd guess these mirror s are slumped during fusion ? Your support structure will have to accommodate this .

Single points edge support at 90 degrees works well up to a monolithic 20"X 1.5"thick mirror. There are unique astigmatism cancelling properties with a 90 degree support . It is really only below 30 degrees or so that the edge support really mattered too much . Ensure that the edge pressure on both front and back plates is even , but if the whole thing is mechanically square its not much of an issue .

Scott's cell looks very nice and the edge support deals with the face plates .

The thing with cells is that you can start simple and get more sophisticated if you need to .

The finest Mars opposition detail I've ever seen was through a 14" F5 plate glass mirror Newt of 30mm thickness mounted in the scope with a solid ring of 1" Velcro squares at the 60% zone and nothing else . I transported this scope on its side for years and never had any issues with the mirror coming loose. The idea started as a bit of fun but it worked superbly . A wooden spatula was all that was needed to prise the mirror off for coating once .

[Saturnine (Jeff)]

Thanks again for the imput , insights etc. I'm almost sold on an 9 point cell, the triangles will be 6mm aluminium plate, mounted to an aluminium and plywood base. Although the velcro idea sounds so simple, I'm assuming that it was adhesive backed velcro.
Another question is what materials to use for the pads on the triangles, nylon, teflon, cork or felt or something else, have read via Cloudy nights, some conflicting views on some . Also, not sure I'm comfortable with metal bearings of the wiffle tree being in contact with the glass, to my mind there is a chance of chipping the glass when bouncing around when being transported.
Scott, I like the collimation knobs being where you have them, makes the task much easier than crawling around on he ground.