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Dome observatory

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#1 Celestial awe

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Posted 23 September 2020 - 09:02 PM

Hi,

   I am currently running all my equipment in a 10x20 roll off I build this spring but poured a 8x8 pad nearby where I want to build a dome eventually. I am still looking around and have most of my construction idea rolling in my head. However in motorizing the dome and sychcronizing the telescope to it I am lost. Can I buy a motor and design my own system or is it easier to buy a kit. How does the sychronizing work? If any one has some plans or pictures of the motors systems I would appreciate it.

 

Clear skies!!



#2 TOMDEY

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Posted 23 September 2020 - 09:35 PM

Hi,

   I am currently running all my equipment in a 10x20 roll off I build this spring but poured a 8x8 pad nearby where I want to build a dome eventually. I am still looking around and have most of my construction idea rolling in my head. However in motorizing the dome and sychcronizing the telescope to it I am lost. Can I buy a motor and design my own system or is it easier to buy a kit. How does the sychronizing work? If any one has some plans or pictures of the motors systems I would appreciate it.

Clear skies!!

The biggest driver should be your comfort and bandwidth re' building things. The roll-off is a great start, but building a dome from scratch, including the drive system... gets complicated. I did my own, using "relay logic" and wrote my own software to coordinate the dome and scope. I'd say (assumes your dome is not huge) --- to build it to a commercial drive system that you select before finalizing the design. Might be worth considering buying the entire turn-key dome and drive etc. and just installing it!    Tom

 

~click on~ >>>

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  • 29 Tom and KoKo in the 24-foot dome.jpg


#3 Stevegeo

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Posted 24 September 2020 - 03:53 AM

An 8 ft dome done right will move easily by hand.  I know mine does. After installing a drive motor with reduction gearbox, then speed controller, then wiring it up.... I found it was easier just sitting there viewing to reach over now and then and give the dome a shove.  With a 20 in opening  my  fork mounted   C11 tracking in the on position,  I haven't only to lean back slightly  only a couple times during a viewing session.  I took the drive off eventually ,  thought of making a push button relayed controller to spin CW or CCW. But that's for another day.  My doors however, are opened and shut by actuators, two at top and two at bottom , mounted on tool box drawer slides and lubed with mobile EP1, a synthetic  grease that does not get thick and stiff in the winter. A big plus.

Whatever you do, keep it simple.  You see the actuators at the bottom the doors are emt , bent and welded. Extremely  light too. Pic taken befor skinning with fiberglass.

1003181644a.jpg  

 

Stevegeo 



#4 kathyastro

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Posted 24 September 2020 - 04:50 AM

I bought an Exploradome.  The automation system they sold at the time was not very good, but they have changed suppliers, and the new one is apparently better.  I was dissatisfied with the old system, and I ended up designing my own.

 

If you purchase an automation system, there is nothing to do but install it.  If you build your own, it will take a bit of work to design it.  You have to like that sort of thing or it isn't worth doing it yourself.  I wrote my own synchronization software because I was running into vendor incompatibility problems, and the only way I could be sure it was done right was to do it myself.



#5 Jim in Sweden

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Posted 24 September 2020 - 12:59 PM

Kathy, can you elaborate on your approach to software for synchronizing dome and scope? Are you using a GEM, for example, and is the pier in the center of the dome? Do you program an offset for the tube or is that unnecessary? How often does the dome move while tracking a longer photo session? 



#6 kathyastro

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Posted 24 September 2020 - 01:55 PM

Kathy, can you elaborate on your approach to software for synchronizing dome and scope? Are you using a GEM, for example, and is the pier in the center of the dome? Do you program an offset for the tube or is that unnecessary? How often does the dome move while tracking a longer photo session? 

It is absolutely essential for any dome control software to know the exact geometry of the mount and scope relative to the centre of the dome.  The setup requires that the software know the location of the centre of the mount (defined as the intersection of the RA and Dec axes) relative to the centre of the dome, and the offset of the optical axis of the scope from the centre of the mount.  Even with the best construction in the world, you cannot expect the mount to be in the exact centre of the dome.  Millimetres matter.

 

If you are interested in programming your own calculation, you will need to use three-dimensional vector arithmetic to do it.  The calculation is not difficult, but you need to be able to visualize the problem completely in three dimensions.  There is also a trap built into the ASCOM specification which trips up some commercial software vendors, and is the reason I did my own version.

 

The way my software works is that it interrogates the mount every ten seconds to determine its hour angle and declination.  It uses that information and its knowledge of the mount geometry to calculate what azimuth the dome needs to be at to centre the dome slot in front of the scope.  Then, it tells the dome to move.  It uses ASCOM drivers to communicate with both the mount and the dome.

 

I have it set to tolerate a 2-degree difference between the desired dome azimuth and the actual azimuth.  The difference changes more or less slowly, as the mount tracks.  Once the difference exceeds the 2-degree limit, the dome slews to match the calculated azimuth. 

 

Tracking an object from the northeast across the zenith to the northwest, for example, will produce a very low rate of slews, as the target climbs vertically up the dome slot.  As it approaches the zenith, slews will become more frequent.  I have seen slews every 30 seconds for targets near the zenith.  The rate drops off again as the target descends vertically down the dome slot.  This is in effect a digitization of field rotation.

 

My software is available at: https://www.dropbox....ncV2.0.zip?dl=0


Edited by kathyastro, 24 September 2020 - 02:16 PM.

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#7 duck

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Posted 24 September 2020 - 04:37 PM

I have a 20ft diameter home made dome.  Made from rolled 1" x 2" steel tubing and galvanized sheet metal gores.  Driven using a 1/2 hp 90vdc motor.  Minarik Electric chopper drive.  Home made H-bridge and logic circuits.  Home made azimuth encoder read using a JR Kerr servo drive (only uses the encoder reader).

 

I attempted to slave the dome to the telescope using s/w in Windows.  Not a good idea.  Windows will go off and do some other stuff right when it needs to stop the motor.  Now slave the dome occasionally as needed.

 

Regarding Kathy's comment on slaving to azimuth error...I slaved to keep the angle between the scope and a vector at the same elevation angle pointing to the center of the shutter opening less than some number.  This avoids excessive dome movement near the zenith.



#8 kathyastro

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Posted 24 September 2020 - 05:53 PM

I attempted to slave the dome to the telescope using s/w in Windows.  Not a good idea.  Windows will go off and do some other stuff right when it needs to stop the motor.  Now slave the dome occasionally as needed.

True.  I tried a DC motor and some Windows software to control it.  As you say, Windoze can be off looking somewhere else when it's time to shut off the motor.  You can't use Windows software to read the azimuth; you need a dedicated controller like an Arduino.

 

I ended up using a stepper motor driven by an Arduino.  The software running on Windows initiates dome movement.  It doesn't matter a lot if that happens 30 seconds late because Windows was snoozing.  Once the movement is initiated, the Arduino looks after the speed, timing, and pulse-counting to ensure the dome stops where it is supposed to.  I get 9 arc-second accuracy on the dome position, which is overkill, but nice to have.



#9 StarmanDan

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Posted 24 September 2020 - 10:39 PM

I'm about to build a domed observatory using a Sirius 2.3m dome and automate it using DPP Dome Automation.

http://www.dppobserv.../DomeDriver.php

#10 TOMDEY

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Posted 24 September 2020 - 11:23 PM

Yes to Kathy's up there. Vector analysis make it so much easier to visualize, analyze, and code up. The first trial runs are fun. Want to do that to make sure it behaves reliably, and as anticipated. My dome is massive as is my scope... to this day I add a layer of software conditionals to kill motions and flag if somehow questionable... and even got my finger on a kill-switch when slewing! Each axis slews simultaneously at 5 deg/sec... which is actually pretty fast, for a big/massive scope!

 

When I was in the military, we had some 5-axis gimbaled big things that would slew to any point in the hemisphere, lock and track --- all in less than a second. That's get out and stand back territory!   Tom



#11 duck

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Posted 25 September 2020 - 10:58 AM

Tom - have to consider the momentum transfer to the Earth!




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