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markstar46
member
Reged: 02/05/08
Posts: 13
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Hello to all. This is my first post here. I'm planning on building a new AL mirror cell for my 10.1" f/4.4 mirror and have moused around with PLOP for awhile. I want to be sure I'm reading everything correctly, so feel free to chime in and let me know if you see anything amiss.
Mirror dia. 254mm
Thickness 31.75mm
focal length 1123.6mm
sec. dia 63.5mm
Plop color plot > options > Z axis range = 0.0000275 (1/20 lamda)(Makes a HUGE difference in comparing different cells)
optimize for 6pt cell.
Data File:
;autocell for 6 point
diameter 254
thickness 31.75
focal-length 1123.6
n-mesh-rings 12
rel-support-radii 0.581121
num-support 6
basis-ring-size 3
basis-ring-min 0
obstruction-diam 63.5
optimize rel-support-radii 0 0.01
part bar 3 point 0 0 point 0 1
Results:
Vis P-V error: 4.12804e-6 (1/133 lamda)
Vis RMS error: 6.93418e-7 (1/793 lamda)
Three bars, 6 pads
Center-to-center dis between each pad = 2.9" long.
Center of bar located at 2.18" on x-axis and up 1.26" on Y axis.
Length from center of circle to CG of bars (hypotenuse)= 2.52"
Angle of above line (with x axis = 0): 30 degrees.
Radius of circle where center of all pads will lie = 5.05" x 0.581121 = 2.93"
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I think that's it . . . . Thanks in advance,
Mark
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Nils Olof Carlin
professor emeritus
Reged: 07/26/04
Posts: 690
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Mark,
I get the same results. The RMS error (the one measure that counts) is 0.7 nm - with a tremendous safety margin.
In your place I would move the circle of support points outward for a mechanically more stable configuration - you could move them almost all the way out to the edge and still go below a very conservative 2.5 nm RMS.
Don't forget the all-important edge support though. Your best choice is no doubt two supports at +- 45 deg from "vertical", and placed at the plane of the COG - see Robert Houdart's page!.
Nils Olof
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markstar46
member
Reged: 02/05/08
Posts: 13
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Hi Nils,
Both RMS & PV errors were astonishingly small. So small in fact that I was skeptical about the numbers.
Hadn't thought of modeling the support points wider. Regarding edge support, I plan on rtv'ing the mirror to the cell pads. I plan on building a mirror cell like this:
http://www.ultralightscopes.com/images/CellAsm.jpg
At present, it's supported by a 12pt AL cell I built years ago with a sling edge support. But the mirror flops onto the edge supports whenever it's tilted towards horizontal. In the near future, I'd like to rebuild the whole thing now that I've learned so much since 10 years ago.
Thanks for your reply,
Mark
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Olivier Biot
Amused
Reged: 04/25/05
Posts: 17260
Loc: 51°N (Belgium)
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Is your mirror made from Pyrex or from plate glass? And is its diameter 10" (254mm) or 10.1" (256.5mm)?
Here's the input file for plate glass:
Quote:
;autocell for 6 point
diameter 254
thickness 31.75
density 2.45e-06
modulus 6000
poisson 0.22
focal-length 1123.6
n-mesh-rings 12
rel-support-radii 0.580857
num-support 6
basis-ring-size 3
basis-ring-min 0
obstruction-diam 63.5
optimize rel-support-radii 0 0.01
part bar 3 point 0 0 point 0 1
This means PLOP computes an optimal 6 points cell for this 10" mirror if you put the 6 support points at 0.580857 times the radius of the blank. The visible RMS error of this cell configuration is 0.82nm.
If you decide to move the support points to 80% of the radius, then the visible RMS error increases to 2.1nm, and the visible P/V error to 11.8nm.
For the record, here's the PLOP input file for this simulation. Replace the value in red with any other value in PLOP to simulate other support radii:
Quote:
;autocell for 6 point
diameter 254
thickness 31.75
density 2.45e-06
modulus 6000
poisson 0.22
focal-length 1123.6
n-mesh-rings 12
rel-support-radii 0.8
num-support 6
basis-ring-size 3
basis-ring-min 0
obstruction-diam 63.5
part bar 3 point 0 0 point 0 1
Remember that the values I computed are for a 10" f/4.42 plate glass mirror, not Pyrex.
Cheers!
Olivier
--------------------
Computer entomology: study of computer bugs
Tal-200K (#199) with JMI NGF-Mini2M focuser on GEM3 • Astro-Tech AT80ED • Orion Sirius EQ-G with wireless EQDIRECT • Astro-Tech Voyager • Celestron Regal LX 10x42 • Helios 15x70
ATM 14" f/5 (redesigning) • ATM 10" f/6 Portable Truss (polishing) • ATM 10" f/25 Dall-Kirkham (optics)
AstroForecast
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Nils Olof Carlin
professor emeritus
Reged: 07/26/04
Posts: 690
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Quote:
Regarding edge support, I plan on rtv'ing the mirror to the cell pads.
Considering it is a small mirror, this could very well work, though this is far from ideal - judging from discussions, others have been happy with it. The main reason I am sceptical is that the forces parallel to the mirror plane are not mechanically determined - you might get all or most of the forces on 4 or even 2 of the 6 points.
Robert H's simulation for for a 18 point cell RTV-ed (ad I assume the forces are evenly distributed) would give about 5 nm RMS error at alt=0 (about ten times a simple 2 point support) - this is acceptable, but what would happen with a 6 point cell, I don't know. But if RTV-ing is important for mechanical reasons, chances are reasonably good that the performance will be acceptable even at mid-to-low altitudes.
Nils Olof
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markstar46
member
Reged: 02/05/08
Posts: 13
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Thanks for your assistance! The mirror is a 10.1", it's an old Coulter that I had refigured. I'm not sure what type of glass Coulter used, but I modeled it as Pyrex.
I'll use your example to play around with the support radii and see the changes. More opportunity to get better aquainted with plop. However, I don't intend to move them further out, as Nils suggested, since I do plan on gluing the mirror.
Thanks again.
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Olivier Biot
Amused
Reged: 04/25/05
Posts: 17260
Loc: 51°N (Belgium)
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You're welcome 
By the way, the differences between plate glass and Pyrex are not extreme. Certainly not for a 10.1" mirror.
Note that the PLOP simulations above were done for a 10" (254mm) primary, not a 10.1" primary (256mm). This shouldn't affect the computations too much either.
Cheers!
Olivier
--------------------
Computer entomology: study of computer bugs
Tal-200K (#199) with JMI NGF-Mini2M focuser on GEM3 • Astro-Tech AT80ED • Orion Sirius EQ-G with wireless EQDIRECT • Astro-Tech Voyager • Celestron Regal LX 10x42 • Helios 15x70
ATM 14" f/5 (redesigning) • ATM 10" f/6 Portable Truss (polishing) • ATM 10" f/25 Dall-Kirkham (optics)
AstroForecast
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markstar46
member
Reged: 02/05/08
Posts: 13
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Thanks to all for verifying my results.
I've been drawing out the cell in AutoCad and machining aluminum. Taking advantage of the cloudy weather to do some scope work.
Next thing to tackle is optimizing the focuser height vs. sec size vs strut pole length. I didn't quite understand all this when I built this scope 10 years ago, and it needs some rework.
Regards,
Mark
--------------------
10.1" f/4.4 custom truss Dob
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