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Novak Mirror Cell Modifications

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#1 TheBigEye

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Posted 07 December 2023 - 03:56 PM

I have an "old school" GEM-mounted 17.5" Newtonian - fiberglass tube, Novak mirror mounts, fully rotating rings, VFD-controlled synchronous motor drive. Definitely a "blast from the past." It's a great scope, too! I'm planning a couple of mods to the scope, starting with the Novak primary mirror cell.

 

A few years ago, Aurora Precision had done a couple of custom mirror cells for my 22" and 14" scopes. They are hands down the best mirror cells on the market in terms of design and workmanship. Initially, I was planning to replace this Novak cell with one from Aurora Precision. Unfortunately, the cost estimate for a new custom AP cell was beyond the budget of this now-retired astronomy enthusiast.

 

The 17.5" mirror is 1-5/8" thick and weighs 28 lbs. So, I'd like to modify the Novak cell to:

 

- Replace the tiny 18-point mirror rear supports (1/4" plastic nuts) with larger, round discs, to improve weight distribution.

 

- Replace the three edge support posts with three whiffletree pivoting edge supports.

 

 

Has anyone here ever done that? Would love to hear any experiences, get advice, ideas, or even let me know if I am crazy! TIA!

 

- Chuck

 

 

Attached Thumbnails

  • 2023 - Chuck with Scope Merged - Web.jpg
  • Novak Mirror Cell Retainer Support Detail Web.jpg

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#2 PeteDCard81

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Posted 07 December 2023 - 04:11 PM

My first thought would be to run PLOP on where ideally the mirror pads should be placed. Novak made his mirror cells way before Finite Element Analysis was conveniently available to ATMs with PLOP. Not likely to be at the calculated locations. The mods needed may not be too involved and rather straight forward.

 

Love the Novak cells. I have and use 6 of them.

 

 

Mark


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#3 Jon Isaacs

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Posted 07 December 2023 - 04:24 PM

How well do whiffle trees work on an equatorial mount where the gravitational orientation of mirror cell changes?

 

Jon


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#4 Jim45157

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Posted 07 December 2023 - 05:42 PM

JOHN PRATT ALSO MAKE A great cell


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#5 Dan Watt

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Posted 07 December 2023 - 07:52 PM

Pretty well in my experience. I use four of them (so eight points of contact total) evenly spaced apart. More discussion here: https://www.cloudyni...e-edge-support/

 

How well do whiffle trees work on an equatorial mount where the gravitational orientation of mirror cell changes?

 

Jon



#6 Pinbout

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Posted 07 December 2023 - 08:09 PM

 

- Replace the three edge support posts with three whiffletree pivoting edge supports.

yes, if your tube rotates on the gem, you'd need 4 edge supports

 

 

- Replace the tiny 18-point mirror rear supports (1/4" plastic nuts) with larger, round discs, to improve weight distribution.

have no clue what your talking about, need to see some kind of sketch. I like the nylon hex heads.


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#7 Dan Watt

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Posted 07 December 2023 - 08:22 PM

yes, if your tube rotates on the gem, you'd need 4 edge supports

 

have no clue what your talking about, need to see some kind of sketch. I like the nylon hex heads.

I think he means something like this. Personally I have similar hex heads on all of my cells, from the 6" to the 20". Not sure of the pro/cons one way or another. I haven't had any issues. post-3892-0-69820800-1670947201_thumb.jpg


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#8 Pinbout

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Posted 07 December 2023 - 08:23 PM

I think he means something like this. Personally I have similar hex heads on all of my cells, from the 6" to the 20". Not sure of the pro/cons one way or another. I haven't had any issues. attachicon.gif post-3892-0-69820800-1670947201_thumb.jpg

6 wiffle trees on the edge even better. grin.gif



#9 duck

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Posted 07 December 2023 - 09:11 PM

I believe the pictured 18 point cell achieves collimation adjustment with a screw thread on the beams.  This has the advantage over a moving frame in weight and allows a closer end-plate (important to minimize back focus in Cassegrains.)  I considered this too tricky for me.  I stuck with a moving frame.  The moving frame captures the mirror entirely, and the frame is locked by counter screws in the end-plate.  The Novack cell the OP posted is a moving frame.


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#10 Pinbout

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Posted 07 December 2023 - 09:31 PM

 

the pictured 18 point cell achieves collimation adjustment with a screw thread on the beams.

look closer. the collimation screws on offset from the beams. 


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#11 duck

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Posted 07 December 2023 - 10:21 PM

My bad.  That's the moving frame shown in the pic.  The end-plate (or whatever) not shown.


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#12 TayM57

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Posted 07 December 2023 - 11:45 PM

My first thought would be to run PLOP on where ideally the mirror pads should be placed. Novak made his mirror cells way before Finite Element Analysis was conveniently available to ATMs with PLOP. Not likely to be at the calculated locations. The mods needed may not be too involved and rather straight forward.

 

Love the Novak cells. I have and use 6 of them.

 

 

Mark

 

Hi Mark. I remember you. I bought a pyrex 17.5" blank from you which I ground, polished, and figured to f/3.4 with Swayze's help. First light will be this winter, around February.


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#13 TheBigEye

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Posted 08 December 2023 - 03:50 PM

My first thought would be to run PLOP on where ideally the mirror pads should be placed. Novak made his mirror cells way before Finite Element Analysis was conveniently available to ATMs with PLOP. Not likely to be at the calculated locations. The mods needed may not be too involved and rather straight forward.

 

Love the Novak cells. I have and use 6 of them.

 

 

Mark

Hi Mark - I think using PLOP would be ideal, depending on if the triangular plates would have to be modified or replaced entirely. I would be weighing that against the fact that PLOP calcs are absolutely essential to the new large, thinner mirrors that require support across a much larger surface area. This old school mirror is 17.5" X 1-5/8". I appreciate your observation and will take that into account.  Thanks!   



#14 TheBigEye

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Posted 08 December 2023 - 04:10 PM

How well do whiffle trees work on an equatorial mount where the gravitational orientation of mirror cell changes?

 

Jon

Hi Jon - Thanks for your comment. I wondered that same thing at one time. I had a mirror cell for a 14" f/3.5 done for me by Aurora Precision a few years ago. It was a thin-profile mirror, installed in a round tube, GEM-mounted. Nathan used whiffletree supports all the way around and they worked perfectly. The weight distribution transfers from the back of the mirror to the edge, as the scope is moved vertically down from the zenith, which can cause deformation of the optical surface. The whiffletree design assures that the weight is distributed equally between the edge contact points, instead of being biased to one side or the other.  It's even more complicated in a GEM configuration because you're also introducing changing forces on the mirror edge due to the necessity of rotating the OTA. This is especially the case with large, thin-profile mirrors. Aurora has a great treatise about this on their website.     



#15 TheBigEye

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Posted 08 December 2023 - 04:13 PM

JOHN PRATT ALSO MAKE A great cell

Hi Jim - Yes he does! I think he just does Dob-style mirror cells though . . . I'm running a GEM.



#16 TheBigEye

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Posted 08 December 2023 - 04:32 PM

yes, if your tube rotates on the gem, you'd need 4 edge supports

 

have no clue what your talking about, need to see some kind of sketch. I like the nylon hex heads.

Thanks for commenting! The second picture in my original post shows the detail of the support triangle with the tiny plastic support points. Admittedly, the existing plastic heads have supported the mirror adequately for (probably) over 4 decades. But I've never seen such small supports for a big mirror like this. Dan Watts' picture (I think that's Nathan from Aurora Precision in the photo), shows the larger pads for rear mirror support and that's what I'm after. The other issue I'm having with the Novak cell is that when the OTA is pointed in a more horizontal position, the weight of the mirror transfers from the rear support points to the edge post studs. When that happens, the rear support pads at the top of the mirror lose close contact with the mirror's rear surface. If I rotate the tube, sometimes the triangle pads of the top support arm will slip out of position. So, hoping a larger support pad surface, coupled with whiffletree supports, will solve the problem.     



#17 PeteDCard81

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Posted 08 December 2023 - 10:53 PM

Hi Mark. I remember you. I bought a pyrex 17.5" blank from you which I ground, polished, and figured to f/3.4 with Swayze's help. First light will be this winter, around February.

Hi Taylor,

 

Glad to hear from you and congratulations on finishing the mirror. Nice to know that you made a mirror out of the blank. From your profile picture looks like you are holding that mirror.

 

F/3.4 is pretty fast. I figured a 12.5" F/3.92 and that challenging enough.

 

 

Mark




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