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14.7" f/3 Dob/RFT

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#26 mark cowan

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Posted 30 October 2019 - 07:16 PM

ZL, you might want to post your results for checking - screen shots, settings, etc.


 

#27 Augustus

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Posted 05 November 2019 - 07:07 PM

Mirror is done - it's now technically a 14.5" f/2.9 - and I got Tony's absolutely fantastic secondary and holder (THANK YOU!). 

 

Right now my finances are mostly focused on getting a Paracorr II and I really need to sell my CGEM, but once that's all done I'll be getting started on the scope. Not that there's really much to actually build. Goal is to do it once and do it right instead of scrapping yet another build.


Edited by Augustus, 05 November 2019 - 07:09 PM.

 

#28 coinboy1

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Posted 07 November 2019 - 08:39 PM

That is going to be an awesome scope! Do you have a DPAC image of the primary?


 

#29 Augustus

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Posted 07 November 2019 - 09:26 PM

That is going to be an awesome scope! Do you have a DPAC image of the primary?

Si, this is what Steve sent me.

 

msg-25105-0-04837200-1572636127.jpg


 

#30 Augustus

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Posted 08 November 2019 - 01:51 PM

To be completely honest I really can't figure out the exact quality from that image (I know it's good) so I'd be interested in what y'all think. 


Edited by Augustus, 08 November 2019 - 01:51 PM.

 

#31 PrestonE

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Posted 08 November 2019 - 03:36 PM

To me the outer 20% or so looks rough at the 2:30 and 7:30 positions...in

the dark bands...hint of it at 10:30 11 ish also...

 

Looking forward to what others have to say.

 

Best Regards,

 

Preston


 

#32 stargazer193857

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Posted 08 November 2019 - 04:10 PM

To me the outer 20% or so looks rough at the 2:30 and 7:30 positions...in
the dark bands...hint of it at 10:30 11 ish also...

Looking forward to what others have to say.

Best Regards,

Preston


Following the blurred lines from top to bottom, they are not even. The dark areas are wider or tighter in spots, but I have no idea how to interpret it. I was looking at 5:30 where it suddenly gets tight.
 

#33 stargazer193857

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Posted 08 November 2019 - 04:14 PM

To be completely honest I really can't figure out the exact quality from that image (I know it's good) so I'd be interested in what y'all think.


From 4:30 to 6:30, the bands are tighter than everywhere else. I'm guessing you saw that though and wanted an expert interpretation of what that means. Since that is very good, then that is a sensitive test I need to learn.
 

#34 coinboy1

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Posted 08 November 2019 - 11:15 PM

Well straight lines is a good indicator of good correction for DPAC, it looks pretty straight to me. DPAC is 2x as sensitive to errors, perhaps the slight roughness in the outer edges is amplified in this test. I think for an F/3 (or F/2.9) Nova Optical did a great job considering the price point. 


 

#35 Augustus

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Posted 26 November 2019 - 09:19 AM

Mirror is a week or so from delivery, just ordered the cell and a drum shell for the UTA. I bought Albert Highe's book which has been giving me lots of ideas, as is the Sky Valley ultralight 13.1" I currently have in my possession. 


 

#36 Augustus

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Posted 27 November 2019 - 06:14 PM

So the drum shell got here fast. 1-day shipping I guess? It's only 15.5" inner diameter. Already at the perfect length for the scope at 14" long. If it were a couple inches shorter I'd have less stray light protection below the focuser, which I find to be crucial.

 

Full loaded my UTA is going to weigh about 8.5 to 9 pounds with an eyepiece and Paracorr (those 2 make up about three pounds of that weight, LOL). That's about as much as the primary mirror! Good thing the scope is short and my bearings are BIG (26" crescents, already have 'em). My mirror box should be able to get away with being quite short at only about 10" tall according to my rough math.

 

My guesstimate for the total scope weight (not including eyepiece/Paracorr) is about 45 pounds. This will be one heck of a scope!


 

#37 grzesznypl

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Posted 01 December 2019 - 04:15 PM

Any pictures of your design and/or progress during construction??? 


 

#38 stargazer193857

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Posted 01 December 2019 - 04:48 PM

I like how he bought a drum shell instead of building a UTA. Why work harder than you have to? Suddenly I'm questioning whether my design is good enough to justify the work. Depends I the price of the drum.
 

#39 Augustus

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Posted 01 December 2019 - 04:56 PM

Any pictures of your design and/or progress during construction??? 

Nothing is actually built yet. I am waiting for the mirror and cell


 

#40 stargazer193857

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Posted 01 December 2019 - 08:09 PM

Quartz has a modulus of elasticity of 76 GPa, and a density of 2.2. compare that to Soda lime which is 72 GPa and density 2.5. So it should deform less. If you can afford it, it is better.
 

#41 Pierre Lemay

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Posted 02 December 2019 - 08:18 PM

I like how he bought a drum shell instead of building a UTA. Why work harder than you have to?

This is the ATM forum. We don’t « work harder than we have to ». We choose the level of work we wish to build ourselves and proceed to have fun, and learn, by making it. Some people purchase most of the parts and assemble them in a working telescope, but some of us actually like to make most of the telescope parts before assembly. Depends on one’s goal I guess. 


 

#42 Augustus

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Posted 02 December 2019 - 08:22 PM

This is the ATM forum. We don’t « work harder than we have to ». We choose the level of work we wish to build ourselves and proceed to have fun, and learn, by making it. Some people purchase most of the parts and assemble them in a working telescope, but some of us actually like to make most of the telescope parts before assembly. Depends on one’s goal I guess. 

I buy the parts that I either can't make or simply aren't worth the time and effort when I could use said time and effort making other parts.


 

#43 Pierre Lemay

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Posted 02 December 2019 - 08:31 PM

I buy the parts that I either can't make or simply aren't worth the time and effort when I could use said time and effort making other parts.

For my upcoming 28 inch project, the only parts I will purchase will be the coma corrector and eyepieces. I guess you cold call me a little bit « intense »?


 

#44 ccaissie

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Posted 02 December 2019 - 09:03 PM

The pinching of the lines in the DPAC Ronchi might be due to a number of factors...

So how may lpi is the grating?  Even at 50lpi, I'd say this mirror will be all you'd hope for.


 

#45 stargazer193857

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Posted 02 December 2019 - 09:41 PM

The pinching of the lines in the DPAC Ronchi might be due to a number of factors...
So how may lpi is the grating? Even at 50lpi, I'd say this mirror will be all you'd hope for.


Are you saying it is very difficult to make a mirror like that and he is lucky it is this good?

Some mirror makers won't sell thin mirrors unless they sell the cell too. They don't want the buyer using a bad cell and blaming the bad stars on their mirror.
 

#46 Oberon

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Posted 03 December 2019 - 04:10 AM

Some mirror makers won't sell thin mirrors unless they sell the cell too. They don't want the buyer using a bad cell and blaming the bad stars on their mirror.

Sounds emminently sensible.


 

#47 a__l

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Posted 03 December 2019 - 05:34 AM

There is a curious hobby. May be no less interesting than building a telescope. And almost certainly cheaper. But requiring some understanding of the process. Name Bath Interferometer.
I recommend the author to become a master in this. I think the forum will help him.


 

#48 a__l

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Posted 03 December 2019 - 06:33 AM

If it were a couple inches shorter I'd have less stray light protection below the focuser, which I find to be crucial.

 

This is not crucial. An easy solution to the shroud.

What is really crucial, it is the strength and rigidity of it.
First, hold the clamps well. The second, hold the secondary well.

And both hold focuser well.


Edited by a__l, 03 December 2019 - 06:58 AM.

 

#49 happylimpet

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Posted 03 December 2019 - 06:51 AM

For my upcoming 28 inch project, the only parts I will purchase will be the coma corrector and eyepieces. I guess you cold call me a little bit « intense »?

 You making your own bolts and refining your own aluminium from Bauxite etc? Good man!

 

EDIT my apologies for being a bit of a dick.


Edited by happylimpet, 03 December 2019 - 10:03 AM.

 

#50 stargazer193857

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Posted 03 December 2019 - 10:29 AM

There is a curious hobby. May be no less interesting than building a telescope. And almost certainly cheaper. But requiring some understanding of the process. Name Bath Interferometer.
I recommend the author to become a master in this. I think the forum will help him.


Doesn't that require a big expensive flat, or is that a different test?
 


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