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34" F2.89 Large Fork Mount BHMO Project Build

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#101 Achernar

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Posted 12 February 2017 - 10:34 AM

 

Impressive project, and well underway! 

 

I'm curious about the primary cell.  What is the construction? The components appear to be carbon fiber over a core I can't identify. 

Carbon Fiber over Nomax core which is used in plains and NASA, it is a fiber material that absorbs the epoxy and becomes very stiff but light and when combined with the top and bottom layer of carbon fiber it is hard to beat. I have added a better view of the layers and also added a better photo of my 8" secondary.

 

I'm not familiar with primary mirror cells made of this kind of material, I built a few from a combination of carbon steel, 6061 aluminum alloy and 316 stainless steel bolts and fasteners. Are you using carbon fiber composite to reduce the weight but maintain the stiffness, or are you using it to reduce thermal issues? It certainly won't have the coefficient of expansion that steel and aluminum do when made of carbon fiber composites.

 

Taras



#102 jtsenghas

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Posted 12 February 2017 - 11:37 AM

Are you using carbon fiber composite to reduce the weight but maintain the stiffness, or are you using it to reduce thermal issues? 

Taras

 

The answer is c), all of the above. 

 

These materials, when properly assembled with enough thickness can be extraordinarily light and stiff, and having a coefficient of thermal expansion very similar to that of glass helps too for some applications, like mounting secondary mirrors or maintaining dimensions of large members. 

 

I can see how this Nomex material, with its light and crush resistant structure for the cores of these parts is a very wise choice. The stiffness varies with the cube of the thickness of a beam, so just a few more millimeters of thickness dramatically helps at little additional weight. 


Edited by jtsenghas, 12 February 2017 - 11:38 AM.

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#103 ctcables

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Posted 19 February 2017 - 10:09 PM

Little bit more work done on the side supports, this is taking longer than expected. Lots of points to line up and make sure are just right. Looking forward to summer so I can get back to doing more viewing like the second photo. Just keep plugging away at it and I will get there.

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#104 ctcables

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Posted 25 February 2017 - 09:52 PM

Side supports almost done, just need to mount the rollers on the ends and mount the 90's to the cell. I think it is going to work very well and am happy with how it turned out so far.

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#105 ctcables

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Posted 10 March 2017 - 06:46 AM

Testing mirror on cell and side supports before drilling bolt holes. 2.29" to bottom of mirror and .99" to center of rollers. Right on COG. Also will post vids on FB page.  Good progress and happy.

 

cell test https://www.facebook...88278798195871/

 

completed cell  https://www.facebook...88278581529226/

 

 

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#106 ctcables

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Posted 20 March 2017 - 07:10 AM

Not much work done this last week but hope to get back to it as there is much to do.  This is a link to a better video of the mirror cell and a photo of the side supports.

 

https://www.facebook...93575320999552/

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Edited by ctcables, 20 March 2017 - 11:55 PM.


#107 ctcables

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Posted 24 March 2017 - 11:13 PM

Baking coils, this will settle the hot glue and make them flat before epoxy seals them in place.

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#108 ctcables

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Posted 25 March 2017 - 06:25 AM

Restiance on all 3 phases. Turned out good. need to work on dec next and magnets, lots to do yet.

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#109 mark cowan

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Posted 25 March 2017 - 02:50 PM

Darned impressive.  :waytogo:



#110 PPO

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Posted 25 March 2017 - 07:08 PM

Man, if this all works first light, you deserve the Nobel Prize :)



#111 Ptarmigan

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Posted 25 March 2017 - 07:15 PM

Cool. cool.gif waytogo.gif



#112 ctcables

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Posted 03 April 2017 - 03:55 AM

Some more work done to the base mount to bottom bearing. Link to vid. https://www.facebook...99947797028971/

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#113 jtsenghas

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Posted 03 April 2017 - 10:24 AM

You are MOST DEFINITELY not approaching a vast project in a half-vast manner! wink.gif


Edited by jtsenghas, 03 April 2017 - 10:25 AM.


#114 Mike Lockwood

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Posted 03 April 2017 - 10:47 AM

Your 8.0" m.a. secondary mirror is at the coater, FYI.


Edited by Mike Lockwood, 03 April 2017 - 07:32 PM.

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#115 sopticals

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Posted 03 April 2017 - 05:30 PM

Secondary size? I am using a 7.25" minor axis with my 33"f4.1

 

Stephen.(45deg.S.)



#116 Mike Lockwood

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Posted 03 April 2017 - 07:32 PM

I edited my post above.


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#117 jtsenghas

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Posted 03 April 2017 - 08:22 PM

Wow! 8"!

 

That is more of an autocollimation flat than a secondary! 



#118 hakann

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Posted 04 April 2017 - 05:45 PM

34" and at f/2.8 and 8" diagonal ?
Did I miss something here.

#119 PPO

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Posted 04 April 2017 - 05:55 PM

34" and at f/2.8 and 8" diagonal ?
Did I miss something here.

It all depends on the size of field illumination needed, and back focus amount needed. In my 16.5" f/2.9 I using a 3.1" secondary, and its barely big enough to fully illuminate a small area in the center, if the focal point is near the edge of the incoming light path.



#120 hakann

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Posted 04 April 2017 - 06:22 PM

I'n not aware of that yet, but as I 'learned' draw up light cone and has not the paracorr tube in the primary, it is way bigger on a 25" at almost the same focal.
So I' m little lost.
In my 18" f/4 I need almost a 4" be safe according to many and my own line drawings.
And you told me a 7.5" was to small on my idea vs fast 25". And I noted that to vs ray lines.

As I heard it if to small diagonal, primary get's less but human eye don't can see it anyway or huge vignett is not a problem either.
So why really go big diagonal as less % obstruction the better.
But I need more info here to understand it.

Edited by hakann, 05 April 2017 - 03:34 AM.


#121 Alpollo

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Posted 04 April 2017 - 06:50 PM

While I agree less obstruction would be nice, IMO the default baseline for a good telescope would be a fully illuminated field, as the smaller obstruction also gives away light by not bouncing it to the eyepiece or camera. Nobody has a television in this day in age with Vignetting, so I'll admit I don't understand purposely designing it into a custom telescope.

 

As all designs have compromises, a little contrast loss in a Newtonian is reasonable one. Consider a fully illuminated field as a "regular" secondary, everything else is undersized.

Two scopes of the same diameter and f/ratio don't necessarily have the same size secondaries to have FIFs. Lets say one used a DSLR and one used a Webcam. The one with the smaller chipped webcam can get away with a smaller secondary and still be fully illuminated.

 

In your case Hakann, you can always build your scope in Newt for the web it might be easier to see whats going with your Paracorr compared to manually drawing ray traces.


Edited by Alpollo, 04 April 2017 - 06:51 PM.


#122 PPO

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Posted 04 April 2017 - 07:16 PM

Sizing of secondaries is extremely simple. Just divide the distance from the spot on the secondary where the optical axis intersects, to the focal plane, by the f/ratio of the primary mirror. That gives you the minimum clear minor axis of the secondary for 100% illumination in a spot in the center of your field. Its the law of similar triangles we all learn in high school geometry.

 

If you want a larger 100% illumination field, make the secondary bigger, or move focal plane closer. For slow mirrors the place where optical axis intersects the secondary is very close to the center of the clear aperture, for fast mirrors you need to account for the secondary offset formula.

 

But this is the basic way to figure it out without software or ray tracing.



#123 ctcables

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Posted 04 April 2017 - 08:53 PM

Information from NEW for the web.

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Edited by ctcables, 04 April 2017 - 08:57 PM.


#124 Mike Lockwood

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Posted 04 April 2017 - 09:43 PM

Sizing of secondaries is extremely simple. Just divide the distance from the spot on the secondary where the optical axis intersects, to the focal plane, by the f/ratio of the primary mirror. That gives you the minimum clear minor axis of the secondary for 100% illumination in a spot in the center of your field. Its the law of similar triangles we all learn in high school geometry.

Exactly.  This is how I do it too, then I add a bit.

 

21"/2.89 = 7.27", which is comfortably less than 8.0", which is what the secondary is.

 

So, let's let the O.P. continue his work without second-guessing his telescope design.


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#125 ctcables

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Posted 04 April 2017 - 10:12 PM

Do not mind input, things that are now fixed, Primary at 34" x 2.3" quartz  F2.89, Secondary 8", 18pt mirror cell with 360 support and 38" tube ID 40.5 tube OD.  this will not change at this point.  Plan on using SIPS system. I have used Mel Bartell's telescope design along with Newt for the web to get the best design I can. Still debating on doing a full tube for the secondary 38id by 20" in height or use baffles and do a 8" ring with angled secondary spider to make a smaller foot print for transport. 2 sample photos. leaning toward something like what Tom uses with his Spica Eyes.

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Edited by ctcables, 04 April 2017 - 10:13 PM.

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