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Shifting center of mass of RC to fit in a dome

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#1 kiwiguy

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Posted 08 December 2023 - 09:04 PM

Hi,

 

I'm trying to see if I can fit a 14 inch RC on an AP1200 mount in an 8 foot NexDome!

My friend is selling his scope and I have it on loan to test out.  I know it will be tight, but I only do remote imaging (from my backyard).

I know of someone who has a CDK14 on an AP1100 mount in a NexDome.  

https://rouzastro.com/observatory-2/

 

I also found this very helpful spreadsheet that says it's possible (in theory).

https://planewave.co...ght-calculator/

 

How much can I shift the center of mass to effectively shorten the scope extent?  2-4 inches would make a huge difference.

I'm thinking you could get a 24 inch mounting plate and attach compensating weights, but what are the practical considerations?

 

Also I might need to shift the mount position relative to the dome.  The calculator says 6 inches.  I'd rather not move the dome as it is secured and sealed down.

Has anyone done this?  I'm thinking a steel plate with 1/4 to 1/2 inch thickness to move the 6 bolt rat cage that is on the pier.

 

Any advice or experiences would be appreciated.

Otherwise I'm staying with my little 90mm refractor :-).

 

Thanks,

David

 



#2 TOMDEY

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Posted 08 December 2023 - 09:15 PM

The best thing would be to shorten the pier; otherwise it's tail wagging the dog, always a severe compromise.    Tom


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#3 kiwiguy

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Posted 08 December 2023 - 09:33 PM

My pier is effectively only 24 inches.  It's a 36 inch pier, but I have the NexDome walls a foot higher than normal (now 5 feet).

Does that help the situation?

 

Also I don't image below 30 degrees. I think I would be limited to 20 degrees, which is fine.

 

Thanks!

David



#4 speedster

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Posted 10 December 2023 - 03:25 AM

You would be surprised how often we do this sort of thing:

 

PierL350sm.jpg


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#5 speedster

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Posted 11 December 2023 - 01:14 AM

Moving the pier over 6" is probably not difficult.  Post some pics and I can give suggestions.  With an AP1200 (or any of the AP mounts) you can get rid of the rat cage entirely and that will pick up a few inches for you.  Again, post pics and I can give suggestions on how to do that. 



#6 kiwiguy

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Posted 11 December 2023 - 01:41 AM

Thanks!  Here are some views.  The side is a bit fisheye because I didn't have much room.

 

2023-12-09 14.57.34.jpg

2023-12-08 15.54.38.jpg

 



#7 speedster

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Posted 11 December 2023 - 02:59 AM

To lose the rat cage, you can remove the mount, white mount plate, 6 rat cage nuts/bolts, and the center port cover.  On the bottom of the mount will be the AP 1612FSA adapter that attaches with eight 1/4-20 socket head cap screws.  Bolt the 1612FSA directly to the top plate of the pier. If you want to go this way, I can send you a template for the locations of the eight holes.  Rather than threaded holes that have to be rather precise, drill 5/16 through holes and use nuts from below.  The bolt pattern for the 1612FSA is a 4.615" radius so double check that top plate to see where the bolt holes fall in relation to the pipe below.  By drilling 8 holes, you lose the rat cage, nearly half the deflection of the pier, and get about 3" more clearance.  There is no necessity whatsoever to level an EQ mount so there is no need for a leveling function of a rat cage. 



#8 kiwiguy

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Posted 12 December 2023 - 12:53 AM

Thanks speedster for all the info about avoiding the rat cage.  Guess I didn't need to worry about making the plate level.

If I need to move the mount about 5.5 inches south to center the scope rotation within the dome, can I just make a pier offset plate?

How thick should the plate be?  I think the rat cage plates are 3/8 inch.

 

I was initially thinking of making an offset plate like that pictured below (that matched the rat cage spacing).

 

Thanks again!

Attached Thumbnails

  • plate1.jpg


#9 speedster

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Posted 12 December 2023 - 02:02 AM

If you are bolted to the slab rather than an isolated foundation, just cut the bolts of and install new bolts per this video:

 

https://www.youtube....uoTmcDZs&t=838s

 

If you have an isolated foundation and it's large enough to move the pier over and still be on it, do the same thing.

 

If you have an isolated foundation and it's not big enough to move the pier over and still be on it, then do an offset plate.  Bending moment is max at the base so that is where you most need the beef.  We use 1" plate to get our low numbers.

 

The bolt pattern in your drawing doesn't look like the bolt pattern of your pier?  Typically, you would do a plate with one pattern that matches your existing anchor bolts so you could use your existing anchor bolts to mount the plate.  Then, another bolt pattern in the plate that matches your pier holes and that pattern would be located 6" off.  The holes for the new pier location would be drilled and tapped for bolting the pier to the plate in its new position.  Your potential problem is the pier in its new location interfering with the plate's anchor bolt holes.  You'll just have to lay it all out on paper and see where things fall.  It may be that turning the pier 90 degrees would miss all but one existing anchor bolt and you would need to put a hole in the bottom plate of the pier to go over it.  Hard to say without seeing the base of the pier you now have and some dimensions.  If you rotate the pier, remember to rotate the bolt pattern of the top plate the same amount.  This sounds more complicated than it really is.    


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#10 kiwiguy

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Posted 18 December 2023 - 12:20 AM

Thanks for the suggestions @speedster!  In theory the isolated foundation has space to move the mount, though that would be my last resort.

 

Quick update.

I was able to get the scope in the dome (with some help).  It fits (sort of), and is balanced with no optical train yet. 

Ideally the scope will move back even further, but I need a longer rail first.

Either the dome or mount still needs to move as the motorized shutter is too close in the north, with about 8 inches spare in the south.

Height isn't an issue.

 

I found the folllowing thread and the first photo gives an idea of the counterweight solution I was thinking about:

https://www.cloudyni...limit-found-it/

 

Thanks again!

Attached Thumbnails

  • 2023-12-17 14.33.05.jpg



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