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Question re 6in f8 truss ota and struts

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#1 Sol Selegna

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Posted 27 February 2021 - 03:17 PM

Hello everyone, I found a CN article from Beri about an 8in f6 truss dob that he built. Here is the link: https://www.cloudyni...-projects-r1038

 

I'm interested in a similar design but using a 6in f8 mirror instead. How could I go about figuring out the upper and lower OTA measurements and the truss length for this smaller 6in mirror?

 

Thanks!



#2 stargazer193857

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Posted 27 February 2021 - 03:54 PM

Hello everyone, I found a CN article from Beri about an 8in f6 truss dob that he built. Here is the link: https://www.cloudyni...-projects-r1038

I'm interested in a similar design but using a 6in f8 mirror instead. How could I go about figuring out the upper and lower OTA measurements and the truss length for this smaller 6in mirror?

Thanks!


Just past halfway down, there is the skywatcher style strut scope, made in 2005. I wonder how far it would have spread.
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#3 Pinbout

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Posted 27 February 2021 - 04:53 PM

6” f8 is easiest for a sonotube

 

but If ya need to dismantle a strut would be cool.

 

For length, you need two measurements, primary to secondary and secondary to focal plane. Simple grin.gif

 

so if you make the inside dim for your mirror box at least 8”, your 2ndry to focal plane dim has to be at least (8”/2) + focuser height +1/2 focuser travel.

 
Now start draw


Edited by Pinbout, 27 February 2021 - 04:54 PM.


#4 Sol Selegna

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Posted 27 February 2021 - 07:30 PM

Just past halfway down, there is the skywatcher style strut scope, made in 2005. I wonder how far it would have spread.

 

Hi Stargazer, that is the scope that I'm interested in figuring out measurements on. 



#5 Sol Selegna

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Posted 27 February 2021 - 07:38 PM

6” f8 is easiest for a sonotube

 

but If ya need to dismantle a strut would be cool.

 

For length, you need two measurements, primary to secondary and secondary to focal plane. Simple grin.gif

 

so if you make the inside dim for your mirror box at least 8”, your 2ndry to focal plane dim has to be at least (8”/2) + focuser height +1/2 focuser travel.

 
Now start draw

Hi Pinbout, it's good to hear from you! grin.gif

 

Let me ask you a question regarding the 6" f8 sonotube: if I were to build the 6"f8 sonotube OTA but then cut the finished product into two smaller pieces that could be clamped together for viewing (for example, 24" primary OTA and 24" secondary OTA), would that affect the overall performance of the scope? 



#6 Pinbout

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Posted 27 February 2021 - 08:47 PM

If you did that, I would add a layer of fiberglass on the outside of the sonotube. But I would first bondo all the seams and sand it smooth. 



#7 Sol Selegna

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Posted 27 February 2021 - 09:11 PM

If you did that, I would add a layer of fiberglass on the outside of the sonotube. But I would first bondo all the seams and sand it smooth. 

Got it. Thanks Pinbout! smile.gif



#8 Sol Selegna

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Posted 18 March 2021 - 04:21 PM

If you did that, I would add a layer of fiberglass on the outside of the sonotube. But I would first bondo all the seams and sand it smooth. 

Hi Pinbout, I hope that you've been well! I'm using Newt for the web to build the 6in f8 and it looks like I'll need a 50in ota.

 

I've called quite a few places (including white cap) and the only options are either 48in or 12 feet.

 

I read one of your posts where you said that it was okay to duct tape two sonotubes together. That's what I'm thinking about doing, but I just wanted to ask if there was any other things to keep in mind or watch out for?

 

Also, it's tricky finding tubes with matching ODs. I'm thinking to wrap heavy duty paper around the smaller tube to match the larger OD, since I'm going to fiberglass the entire OTA anyway. Does that sound like a good idea? 

 

Thanks!



#9 Pinbout

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Posted 19 March 2021 - 07:39 PM

Not duct tape. I spliced 2 tubes together. I cut big fat tabs from left overs from 2nd tube. Something like (3) 4”x8” pieces. Glued them to the inside of the smaller section first. Then butted up against the 1st tube and glued the tabs to inside of the 1st tube.

 

then bondoed all seams then sanded smooth. 
 

nice and strong joint

 

and any mounting holes I glue small cutoffs to create double wall support.

 

F4E427C9-DB4A-460A-8890-36A2B1AEC077.jpeg


Edited by Pinbout, 20 March 2021 - 01:50 PM.


#10 Sol Selegna

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Posted 22 March 2021 - 01:30 AM

Not duct tape. I spliced 2 tubes together. I cut big fat tabs from left overs from 2nd tube. Something like (3) 4”x8” pieces. Glued them to the inside of the smaller section first. Then butted up against the 1st tube and glued the tabs to inside of the 1st tube.

 

then bondoed all seams then sanded smooth. 
 

nice and strong joint

 

and any mounting holes I glue small cutoffs to create double wall support.

 

attachicon.gifF4E427C9-DB4A-460A-8890-36A2B1AEC077.jpeg

Hi Pinbout, thanks for the description and the picture. That tube looks great! And also, thanks for the advice on double walling the mounting holes. smile.gif




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