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20" f/3.3 Dob in 9'x12' observatory...closer to final plan.

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#1 Cotts

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Posted 20 August 2021 - 03:30 PM

Thread on potential, but ultimately rejected plan is here...

 

The pier is precisely in the centre of the 9' x12' building.  The south wall folds down to a height of 4'.  The other three walls are 7' high.

 

The plan:  Have the existing pier, currently 4' high, cut 12" above the floor.  Weld an 8" square x1/4" steel plate on to the newly exposed 12" high pier.  Bolt to this short plate a 1" thick plywood 'sandwich', 25" in diameter on which to set the feet of the dob.... This will raise the dob about 13" above what it would be sitting on the ground. 

 

A matching plate is to be welded to the bottom of the cut-off portion of the pier so that the original 4' height, great for GEM mounts, can be restored at some future time...  (My EQ6 will be used out on the lawn on my Losmandy HD tripod occasionally with the Mewlon 210 while the dob is in the obsy....) The dob will be somewhat easily moved to my van if I am going to a star party (2022 I'm lookin' at YOU!)

 

With the south wall folded down I will be able to bring the full 20-inch aperture to bear on objects down to about -25º declination.  This is a compromise, I admit.  But non-seated viewing is something I shall not stand for!!!! 

 

The Eyepiece height at Zenith will be about 73" and, when pointed as low as the south wall allows, about 48".  These heights will allow seated viewing with my Catsperch chair.

 

The side walls are 7' high.  Pointing the scope due east or west is going to be difficult. My scale diagrams show that I will have to keep the scope pointed above about 60º elevation to the East and  West.  (I'll be able to "cheat" some with the scope pointed into the NE or NW corner.....) I will have to plan sessions to get most of my targets within about +/- 1 hour of the meridian.. This is when they are at their best anyway....  I also will have to be careful with the motorized slewing to not have the scope hit the side walls....

 

I've been asked before, why bother with a native f/3.3 scope if you raise it a foot above the floor?  My response is that the f/3.3 allows me to raise it for use in the observatory and still see a goodly portion of the sky and still be seated while doing so, in the relative comfort of an observatory with walls that will block much of the wind..  An observatory also allows for almost zero set-up or tear-down time.  Far more pluses than minuses..Anything that makes it EASY to get out observing...

 

Feedback is always welcome!

 

Dave

 

 

 

 



#2 greenstars3

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Posted 20 August 2021 - 03:50 PM

Dave

 

I built my RoR with 6 1/2 ft walls and then bought a 15 " dob, obs was designed 12x12 ft for an 11" cat 

the problem I ran into was seeing any thing at 45 degrees or less because the mirror is at the base of the scope

I then purchased a lift table from Southworth that will handle 700 lbs and lift the scope about 20 inches so I can now observe the planets with the dob (lift table powered by a cordless drill)

Much to my dismay the lift table was not rock solid so the scope had the shakes, solved that with some brackets that hold up the lift table and make the system ridged (rock steady) .

All the problems were solved and I can now enjoy the sky down to about 10 to 12 degrees

 

Robert 



#3 Cotts

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Posted 20 August 2021 - 04:01 PM

Lift table would rest on the floor (wobbly) rather than the pier (1500lbs concrete 4'down...). 

 

Dave




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