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34" F2.89 Large Fork Mount BHMO Project Build

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#26 ctcables

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Posted 04 December 2016 - 11:17 PM

added more carbon fiber layers over the nomax. Slow going but will be supper strong.

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#27 ccaissie

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Posted 13 December 2016 - 12:43 AM

Can you describe the sawing of the blank?

 

I have a 16.2" x 3.2" that could use splitting in two.


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#28 ctcables

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Posted 13 December 2016 - 12:49 AM

Can you describe the sawing of the blank?

 

I have a 16.2" x 3.2" that could use splitting in two.

AGI (Advanced Glass Industry) in NY did the cutting, I am sure they could do you yours. It is the only place I would go to have glass cut in the size mine was.



#29 ctcables

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Posted 13 December 2016 - 12:51 AM

More work done on the bearings and the cell. Video at this link https://www.facebook...43802765976808/

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#30 ctcables

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Posted 13 December 2016 - 12:51 AM

To cold out so I had to epoxy in the house, my wife was very concerned. I am very happy I did not drip any on the table. LOL

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Edited by ctcables, 13 December 2016 - 12:54 AM.

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#31 jtsenghas

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Posted 13 December 2016 - 10:12 AM

To cold out so I had to epoxy in the house, my wife was very concerned. I am very happy I did not drip any on the table. LOL

You're a brave man not to have used an additional drop cloth! Your Christmas nearly became a lot less merry....


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#32 ctcables

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Posted 14 December 2016 - 01:32 AM

Update the primary is complete. 2.30" thick, 97.5/34 = f/2.87.

 

stimated weight is around 142 lbs and is it right at 34" and truly round?  Right at 34.0" as close as I can tell.  Just center spotted it and the indicator showed it was nicely round to within a couple of

hundredths of a millimeter.  Making the cover now.  Hope to get it

crated tomorrow.  A big Thank You to Mike Lockwood. I need to get moving on my end so I am ready when it gets coated.



#33 Mike Lockwood

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Posted 14 December 2016 - 04:35 PM

Not going to get crated today - too windy and cold to open my shop door!  Less wind tomorrow.  But the mirror is now engraved, the sides are cleaned off, and it is covered.

 

This is one of those mirrors that I wish was mine!

 

You are going to have a lot of fun with it when everything is set up properly.


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#34 ctcables

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Posted 14 December 2016 - 09:39 PM

Thank you Mike, as I am one not to fear from asking help I am open to people going over my numbers. See photos. I plan on using a Sips system and have worked out what I think is right, newt wants a 5" focuser so not sure how the 2" sips is going to work out. Also 15" to top of tube seems to work well but please let me know if anyone has other ideas. I want to be sure that mirror to secondary and secondary to sips distance is dead on and not sure how to work the sips into newt.  Anyway thank you for any help / checking.

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Edited by ctcables, 14 December 2016 - 11:00 PM.


#35 Mike Lockwood

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Posted 14 December 2016 - 11:49 PM

The P2/SIPS fully illuminates the center spot of the field down to about f/2.0.  Above that you have a larger illuminated field, which I estimate at around 13mm in diameter for f/2.87.  This is quite sufficient for visual use.  Illumination falls of fairly slowly outside of that.

 

All that matters is the light cone as it gets to the first lens in the SIPS/P2.

 

Just ignore the SIPS and treat the system as if the focal plane is in the usual spot with no corrector, which is about 0.25 to 0.5" above the height of a racked-in FeatherTouch focuser.


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#36 Mike I. Jones

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Posted 15 December 2016 - 02:04 AM

I post this to only show what could be done with this scope with a Rosin hyperboloidal primary and 3-element corrector.  Not at all intended to try to steer you off your path, just FYI.  I can't help myself  :shocked:  :shocked: when it comes to Rosins.

 

Field is 1º circular (fully covers 35mm full-frame sensor with no vignetting), spectral correction is 0.365-1.0µm (full UBVRI) and max distortion of 2.6µ (about 1/2 pixel) at the extreme corners.  Primary is f/3.4 by itself, conic is -1.45447, or about 45% deeper than the equivalent paraboloid.

 

Don't do it this way!  You have a great project.  I'm looking forward to first light results.

Mike

 

34in f2_89 Rosin 3LFC Rev 2 prescription.gif

 

34in f2_89 Rosin 3LFC Rev 2 layout.gif

 

34in f2_89 Rosin 3LFC Rev 2 spots.gif

 

34in f2_89 Rosin 3LFC Rev 2 MTF.gif

 

Attached File  34in f2_89 Rosin 3LFC.len   1.28KB   37 downloads


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#37 ctcables

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Posted 15 December 2016 - 03:08 PM

Mike J that is interesting but yes there will be no changes to the primary or the secondary mirror. However Mike Lockwood dose have a 34" F/3.3, 2.5"-thick QUARTZ - a one-of-a-kind blank for a special telescope on hand. Just saying LOL



#38 ctcables

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Posted 18 December 2016 - 08:33 PM

I made a jig for the support drill holes, turned out very well.

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#39 ctcables

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Posted 18 December 2016 - 08:34 PM

another photo

 

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#40 Mike Lockwood

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Posted 22 December 2016 - 11:45 AM

Chris, I think this short article about the SIPS will be useful, and is relevant here given your design work.

 

http://www.loptics.c...SIPS_facts.html


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#41 ctcables

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Posted 27 December 2016 - 12:32 AM

Well this took more time than I thought it would LOL. Had to drill out the center point of all the cell triangles so I could put in a floating bearing. First drilled through the back side carbon fiber and through the nomax up to the front side carbon fiber. I then filled the 5/8" hole with epoxy to give to make the hole strong. I then re drilled the hole to the same depth on all the parts. Now that I have a solid foundation for the bearing to sit in I pressed the bearings into place with a thin coating of JB Weld. It has all worked out very well. Now the cell support is completely floating on the center point of the part. This will add no resistance to the back side of the mirror.

 

 

Link to bearing press vid.

https://www.facebook...51252408565177/

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#42 JM La Galette

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Posted 27 December 2016 - 03:37 PM

Ctcable,

I don't want to be unpleasant to you, please take this as honest feedback. I can remove these comments if needed.

 

I think you do not work Nomex (not "nomax") and sandwich the best possible way. I'm concerned about the results, my main comments being:

- the honeycomb used for the cage looks to be with hexagonal cells, this is why you had to partly cut them to be able to form to the shape. Instead of this, it is recommended to use rectangular shapes rather than hexagonal. However this should be fine according to the rather large diameter of the part.

Cellule%20Hexa%20vs%20cellule%20rectangu

 

- The carbon skins for the cage look to be "un-continuous" (the Nomex too but this is less than a problem), if true the sandwich effect will be canceled. Actually I don't understand /see well how this is made, I may be wrong.

 

- On your last post above, you mentioned having added some epoxy around the hole to make it stronger. This is a good idea, but this has to be done before the hole is drilled in order to have adjacent Nomex cells filled with what is called a syntactic foam (blend of epoxy and microsphere). In your case the bearing will only be bonded to a small and thin aramid paper around the hole, whereas the forces of your mirror will tend to shear everything through the triangle.

 

Example:

BEFORE DRILLING: no carbon skins

nidalurempli.JPG

 

AFTER DRILLING: with carbon skins, place to put the insert (bearing in your case)

 

syntactique.jpg

 

My overall feeling is your mirror cell / support is too weak versus the massive 142Lbs mirror.

 

Jean-Marc


Edited by JM La Galette, 27 December 2016 - 04:52 PM.

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#43 mark cowan

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Posted 27 December 2016 - 05:25 PM

Might want to make a spare part and destruction test it...


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#44 ctcables

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Posted 27 December 2016 - 06:05 PM

Ctcable,

I don't want to be unpleasant to you, please take this as honest feedback. I can remove these comments if needed.

 

I think you do not work Nomex (not "nomax") and sandwich the best possible way. I'm concerned about the results, my main comments being:

- the honeycomb used for the cage looks to be with hexagonal cells, this is why you had to partly cut them to be able to form to the shape. Instead of this, it is recommended to use rectangular shapes rather than hexagonal. However this should be fine according to the rather large diameter of the part.

Cellule%20Hexa%20vs%20cellule%20rectangu

 

- The carbon skins for the cage look to be "un-continuous" (the Nomex too but this is less than a problem), if true the sandwich effect will be canceled. Actually I don't understand /see well how this is made, I may be wrong.

 

- On your last post above, you mentioned having added some epoxy around the hole to make it stronger. This is a good idea, but this has to be done before the hole is drilled in order to have adjacent Nomex cells filled with what is called a syntactic foam (blend of epoxy and microsphere). In your case the bearing will only be bonded to a small and thin aramid paper around the hole, whereas the forces of your mirror will tend to shear everything through the triangle.

 

Example:

BEFORE DRILLING: no carbon skins

nidalurempli.JPG

 

AFTER DRILLING: with carbon skins, place to put the insert (bearing in your case)

 

syntactique.jpg

 

My overall feeling is your mirror cell / support is too weak versus the massive 142Lbs mirror.

 

Jean-Marc

I understand your concerns and have addressed this. 1 the nomex is surrounding a continues layer of carbon fiber and because the radius is so large it works well when the second layer of carbon segments are filled in to be followed by one more layer of nomex. Wish I had rectangle shape but quite expensive and the stuff I have is surpluses cheep.  as for the support I removed the paper from the center to give it about a 1.5" diameter before filling it with epoxy then drilling it out with a 5/8" for the bearing.  Put small film of JB Weld on the outside of the bearing before pressing into place.  The hole on the top side is 3/8" so not only is the bearing locked in on all sides but has a double layer of carbon fiber it would need to push through to fail.  I have tested the support with 40lbs of weight  on it and no problem. This gives me a tested weight for the 18pt setup of 240lbs so it should be good to go. Might try one to fail just to see at what point that is as I have a scrap part to do this with. The tube is very strong and I have not done the second layer yet. Used a hammer on it the other day looking for soft spots but it is solid. Might not look the best but it is very strong.



#45 JM La Galette

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Posted 28 December 2016 - 10:08 AM

OK good to hear you managed to adress the issues. I'm however not sure hammer is the best tool to test your scope :D , I mean the most important is not impact damage tolerance but rather flexural stiffness and bending. In that sense this can be "strong" as you said, but not rigid/stiff enough. We are not talking about inches here, but mils or tenths of mils. Looking at the cell thickness I wonder what the deflection will be under a 142Lbs weight.

 

Anyway if you're happy this is what is the most important, clear skies and good luck fo rthe remaining work,

 

Jean-Marc



#46 ctcables

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Posted 28 December 2016 - 05:47 PM

seeing is believing, photo of parts test and link to vid of parts test. Very happy with it. 

 

https://www.facebook...52016715155413/

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  • part te3st.jpg


#47 mark cowan

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Posted 28 December 2016 - 07:12 PM

:goodjob: :waytogo:



#48 jtsenghas

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Posted 29 December 2016 - 10:14 AM

Jean-Marc, I believe the cell will perform excellently as is, provided the edge support is appropriate and appropriately placed for whatever deflection these components suffer. I believe your concerns, though well-intentioned and diplomatically expressed, are groundless.

It matters not at all if one method of back support deflects more or less than another, because it is the final static balance of load that matters in the end. That's what this PLOP analysis is all about, after all.

The actual amount of deflection of a mirror cell is orders of magnitude larger than the deflection of the glass, after all, regardless of any incremental differences in the design and construction of any single component. These components clearly will be trivial in weight compared to the glass and will pivot close enough to their centers of gravity that their ratios of forces should be very close to that calculated by PLOP regardless of their individual deflections. Any differences would be absolutely negligible with respect to collimation and focus. Support would be the same.

Carry on with this excellent build! I'm sure this cell will be worthy of Mr. Lockwood's fine optics!

Edited by jtsenghas, 29 December 2016 - 10:36 AM.

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#49 JM La Galette

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Posted 29 December 2016 - 03:36 PM

J.T,

 

I understand the theory, unfortunately the reality of these big mirrors is unforgiving. The reason why professionals make mirror cell pretty stiff is not groundless either.

What can be afforded with small mirrors is no longer possible on bigger ones (actually when R4/t2 ratio becomes large, here 15800).

First the flexions on the triangles will probably never be homogeneous, as accuracy needed to achieve perfect balancing and triangle support positioning on the base plate are pretty high, and if finally they are, the edge supports to mirror contact points will probably move with these (hypothetically homogeneous) flexions. Depending on design (we don't have all details here, but the base plate is a clue), differential flexions may occur also during collimation, which may also ruin optical quality.

 

Well, this may become an endless discussion, I'm not sure I want to go deeper into it , would be more friendly face to face with a beer in hand, but this is definitely a good topic.

 

I agree on your conclusion, carry on ctcable, looking forward to your first light,

Jean-Marc


Edited by JM La Galette, 29 December 2016 - 03:37 PM.

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#50 mark cowan

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Posted 29 December 2016 - 03:42 PM

Chris is scaling up from his 20" project SFAIK and he's not crossing some kind of invisible barrier here.  If nothing else the attention to detail on this design borders on the obsessive, so I wouldn't be concerned about it...




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