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14.7" f/3 Dob/RFT

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#1 Augustus

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Posted 29 September 2019 - 11:02 AM

It's been just about a year since construction started on my 20". That scope was a mistake, to be honest. It was unwieldy, had poor optics, and the ladder was a massive inconvenience. Setup and transportation was also a nightmare. The 16" wasn't much better, but at least it had good optics and actually fit in my car. I had a 12" Dob for a while that was okay but it wasn't the most optimized design and the aperture wasn't large enough to satisfy me. Now I've just got the C11, 10", and a C8 to play with. I need a bigger scope! 

 

After a lot of work on concepts ranging from 13" to 24" in aperture, I've decided upon a 14.7" f/3 with a 0.8" thick quartz Nova Optical primary. This scope design has a number of advantages compared to some of the other concepts I had mulled over:

  • Fast delivery & construction times
  • Low cost
  • Ability to use many COTS parts as well as recycle stuff from previous/abandoned projects to save on cost
  • Light total scope weight (aim is under 50 lbs)
  • Traditional truss design w/Aurora parts still works without high weight or other compromises
  • Fast cooldown time

The two biggest drivers in my decisions on this particular scope and configuration, however, have really been Logan's busy schedule and my uncertainty about college. It would really suck to start on a 24" only for Logan to run out of time or me out of money or space. 

 

I'm again aiming for a weight below 50 pounds, and will be using a single-ring aluminum UTA. Construction will start sometime this October once I've wrapped up completely on the 10" f/3.2.

 

 

 

 

 

 


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#2 MitchAlsup

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Posted 29 September 2019 - 01:22 PM

I am aiming my 13" F/3 at 35 pounds, not including table.


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#3 Venator

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Posted 29 September 2019 - 03:09 PM

Awesome! Looking forward to the watching the build. My 14.7” F/4.24 0.77” thick dob I’m currently building has the CAD model giving me a 60lb overall weight. I’m trying to save weight where possible, but definitely not pushing the limits by any means. You should totally be able to hit 50lbs, especially with a single ring UTA.
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#4 ctcables

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Posted 30 September 2019 - 07:25 AM

this is my 14.7 F4.5 quartz supper light.

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#5 Augustus

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Posted 26 October 2019 - 05:53 PM

Mirror is now in the polishing stage, and I've more or less finalized the design. The OTA is little more than a mirror cell, trusses, and a simplistic UTA.......



#6 Aperturefever

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Posted 27 October 2019 - 04:31 AM

Good call. The 24" can wait and won't go anywhere, believe me. A spot-on 14 will serve you well. College will keep you busy enough - a hassle-free scope will be a great escape from the pressures of study.

#7 stargazer193857

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Posted 27 October 2019 - 10:56 AM

Would the 20" fit on your car if it were f3 and thin mirror?
How much are you paying to get the 14.7" figured?
Sounds like a fantastic scope.

#8 Augustus

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Posted 27 October 2019 - 12:07 PM

Good call. The 24" can wait and won't go anywhere, believe me. A spot-on 14 will serve you well. College will keep you busy enough - a hassle-free scope will be a great escape from the pressures of study.

Would the 20" fit on your car if it were f3 and thin mirror?
How much are you paying to get the 14.7" figured?
Sounds like a fantastic scope.

24" would've been 3/4" thick, and the ultimate killer has really been lack of a blank. I've seriously considered picking mirror making up again and building a grinding machine and Bath IF to make it happen - only reason I haven't already is that it's not cost-effective or time-effective for these relatively small mirrors. With a string strut design and f/3 primary I could conceivably get the scope down to 65 pounds - and it would probably be able to fit folded up into a passenger seat with room left for my eyepieces. The 10" can do this as will the 14.7.

 

Go look at some of Steve's ads for some of the other quartz mirrors he's done and I think you can figure out the price. Good deal but I believe he's out of blanks for these, I got the last one.


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#9 avarakin

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Posted 27 October 2019 - 11:00 PM

24" would've been 3/4" thick, and the ultimate killer has really been lack of a blank. I've seriously considered picking mirror making up again and building a grinding machine and Bath IF to make it happen - only reason I haven't already is that it's not cost-effective or time-effective for these relatively small mirrors. With a string strut design and f/3 primary I could conceivably get the scope down to 65 pounds - and it would probably be able to fit folded up into a passenger seat with room left for my eyepieces. The 10" can do this as will the 14.7.

 

Go look at some of Steve's ads for some of the other quartz mirrors he's done and I think you can figure out the price. Good deal but I believe he's out of blanks for these, I got the last one.

Steve is grinding his last blank for my mirror. He charges $600 for making mirror from customer supplied blank. I happen to know someone who has a 14.7 blank. Please PM me if you are interested. 

Alex



#10 tommm

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Posted 28 October 2019 - 11:45 AM

24" would've been 3/4" thick, and the ultimate killer has really been lack of a blank. I've seriously considered picking mirror making up again and building a grinding machine and Bath IF to make it happen - only reason I haven't already is that it's not cost-effective or time-effective for these relatively small mirrors. With a string strut design and f/3 primary I could conceivably get the scope down to 65 pounds - and it would probably be able to fit folded up into a passenger seat with room left for my eyepieces. The 10" can do this as will the 14.7.

 

Go look at some of Steve's ads for some of the other quartz mirrors he's done and I think you can figure out the price. Good deal but I believe he's out of blanks for these, I got the last one.

Doubt it would fit in a passenger seat - mirror box would need to be at least 26" I.D. and at least around 27.5" O.D. regardless of wood or metal structure. Put a tape measure to your passenger seat and you will see that is pretty big.  I have a scope with 22.4" mirror and it's mirror box is about that size. It's surprising how much room it takes up in my hatchback.



#11 coinboy1

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Posted 28 October 2019 - 12:06 PM

Wow F/3! Thats going to be very thin at the center of these already thin quartz mirror blanks. Your going to need great mirror support!  Steve Dodds made me a 14.7" F/4 (Clear aperture of 14.5" because of the bevel) and its very lightweight and thin. You will notice the astigmatism if the mirror is not properly supported. I did an 18 point for mine designed in PLOP and 3D printed the mirror cell components.  

 

BTW Steve Dodds of Nova Optical tests his mirrors with DPAC and mine showed an excellent star test. He makes great mirrors at an affordable price!

 

Your scope will be an awesome scope once completed. I saw your 10" F/3 and I have noticed how much your engineering and construction skills have improved over the years so I have no doubt it will be an awesome scope when completed. 

 

Zane, PM me if your interested in a 120mm minor axis quartz secondary with 3D printed secondary holder for your scope. Your going to need a big secondary at F/3... 


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#12 Augustus

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Posted 28 October 2019 - 03:49 PM

Wow F/3! Thats going to be very thin at the center of these already thin quartz mirror blanks. Your going to need great mirror support!  Steve Dodds made me a 14.7" F/4 (Clear aperture of 14.5" because of the bevel) and its very lightweight and thin. You will notice the astigmatism if the mirror is not properly supported. I did an 18 point for mine designed in PLOP and 3D printed the mirror cell components.  

 

BTW Steve Dodds of Nova Optical tests his mirrors with DPAC and mine showed an excellent star test. He makes great mirrors at an affordable price!

 

Your scope will be an awesome scope once completed. I saw your 10" F/3 and I have noticed how much your engineering and construction skills have improved over the years so I have no doubt it will be an awesome scope when completed. 

 

Zane, PM me if your interested in a 120mm minor axis quartz secondary with 3D printed secondary holder for your scope. Your going to need a big secondary at F/3... 

PM sent.

 

I ran PLOP and found that I would get around 1/15-1/16 wave of distortion with the thing (depending on whether I input 14.7" or 14.5") with a 6-point cell. Nathan at Aurora confirmed that I could fit this mirror in their AZ 14.5 cell - which would be a $600 investment with shipping, fans, and the retainer kit, and provides a nice base to build a relatively minimalist mirror box around. 



#13 stargazer193857

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Posted 28 October 2019 - 06:22 PM

Doubt it would fit in a passenger seat - mirror box would need to be at least 26" I.D. and at least around 27.5" O.D. regardless of wood or metal structure. Put a tape measure to your passenger seat and you will see that is pretty big. I have a scope with 22.4" mirror and it's mirror box is about that size. It's surprising how much room it takes up in my hatchback.


I don't know why someone would want to put it in a back seat, when that means fewer passengers. Why not the trunk? Poles fitting in the trunk is another nice aspect of fast mirrors.
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#14 avarakin

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Posted 29 October 2019 - 08:57 PM

PM sent.

 

I ran PLOP and found that I would get around 1/15-1/16 wave of distortion with the thing (depending on whether I input 14.7" or 14.5") with a 6-point cell. Nathan at Aurora confirmed that I could fit this mirror in their AZ 14.5 cell - which would be a $600 investment with shipping, fans, and the retainer kit, and provides a nice base to build a relatively minimalist mirror box around. 

Zane,

 

This sounds too good to be true! Are you sure that 6 point cell will support it?

 

Alex


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#15 Augustus

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Posted 29 October 2019 - 09:13 PM

Zane,

 

This sounds too good to be true! Are you sure that 6 point cell will support it?

 

Alex

I have ran and reran PLOP. 1/15 wave distortion is not exactly ideal for a mirror cell but I'm confident I could probably modify the Aurora cell to use those little circular points that Mel Bartels puts in his, if that's even necessary.

 

Something else to keep in mind is that quartz deforms less under its own weight than Pyrex or plate glass does, so in theory the distortion should be less than what PLOP suggests.

 

This being said, $600 is $600 and I might just build my own 18-point cell....



#16 avarakin

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Posted 29 October 2019 - 09:21 PM

Thanks Zane.

$600 for a cell which may or may not work is maybe too much. Did you think of building your own 6 point cell?


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#17 mark cowan

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Posted 30 October 2019 - 01:39 AM

Hmm, I get 1/10th wave PVW for a quartz 14.7x0.8" f/3 and 100mm secondary on 6 points, 6.27 nm RMS surface.

 

Optimized through 3D Plop it improves to only 1/12.4 wave PVW and 4.66 nm RMS surface.

 

The latter is probably usable though.



#18 Augustus

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Posted 30 October 2019 - 12:42 PM

Hmm, I get 1/10th wave PVW for a quartz 14.7x0.8" f/3 and 100mm secondary on 6 points, 6.27 nm RMS surface.

Optimized through 3D Plop it improves to only 1/12.4 wave PVW and 4.66 nm RMS surface.

The latter is probably usable though.


My secondary is 120mm

#19 mic1970

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Posted 30 October 2019 - 12:50 PM

Good luck.... can't wait to see it.



#20 Steve Dodds

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Posted 30 October 2019 - 01:10 PM

The center is .5" edge is .8" but quartz is stiffer, does PLOP take that into account?


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#21 Augustus

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Posted 30 October 2019 - 01:59 PM

The center is .5" edge is .8" but quartz is stiffer, does PLOP take that into account?

No, that's why I'm saying it's probably better than what PLOP is giving me



#22 Venator

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Posted 30 October 2019 - 02:30 PM

I thought PLOP takes material into account if chosen? If I remember correctly, there is a drop down menu option in there somewhere for material, with fused silica / quartz as an option. Does this selected option not take the difference in material stiffness and density into account when calculating the cell?
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#23 mark cowan

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Posted 30 October 2019 - 04:47 PM

No, that's why I'm saying it's probably better than what PLOP is giving me

Yes you have to select it.  

 

With 120mm secondary, I get 1/10.1 wave PVW and 1/12.7 wave PVW for 3D (Z88) calculation, little difference.

You must be doing something wrong somewhere...


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#24 Augustus

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Posted 30 October 2019 - 05:22 PM

Yes you have to select it.  

 

With 120mm secondary, I get 1/10.1 wave PVW and 1/12.7 wave PVW for 3D (Z88) calculation, little difference.

You must be doing something wrong somewhere...

Found the setting, now I get 1/16 wave PV



#25 stargazer193857

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Posted 30 October 2019 - 05:48 PM

The similarity between them backs up that the resilience of fused silica to scratching has to do with its strength, not its stiffness. That is protective for dropped boxes, but not helpful for holding figure.


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