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Dome rotation motor and gear question

I'm moving on to installing the dome rotation motor system. I was just looking carefully at the parts in preparation for its installation and I have a question. The motor shaft has a slot and a key as I would expect to see. The mounting hub for the gear does not have a slot. Instead, there is a set screw and the set screw is slightly bigger than the slot. This setup doesn't look right to me. Has anyone dealt with this and if so, what did you do? In case it's not clear, the gear will not fit on the shaft at all if the key is in the slot. The key must be removed and set aside.

I could just tighten down the screw in the slot to secure the gear on the motor shaft. To me, this seems like something that will fail, perhaps even over and over again. The easiest thing I can think of doing is to slightly grind down the tip of the set screw so that it will reach down into the slot. I think this will work but wanted to see if anyone came up with a better idea.

29 comments

29 Comments

Rodolphe

Feb 14, 2022

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can you post some pictures ?

My magnet is dead center with my dome slit. My sensor is a 0º (due north, as I don't use the NexDome wall but built my own small structure for it). My park position is 321.5 degree (charging pads).

in my application (TheSkyX) I do enter these 2 values. The home azimuth is the most important as it defines the absolute position. The Park position is also important to make sure the battery can be charged. but can only be set after the home azimuth is set.

Once the home azimuth is known and set, hone the dome, then use the buton to move the dome back to the charging pad and use the displayed position in TSX to set my park position :

https://s3.us-east-2.amazonaws.com/rti-zone-images/NexDome_Beaver_settings.png

This is true for any dome controller (I've written over a dozen dome X2 plugin so I kind of know ;) and I also wrote my own firmware for my own dome controller).

And this is on my own dome controller , same principle (which is what I now use in my observatory with my NexDome dome, but as said, all are using the exact same way, set home azimuth, all move are in relation to that position) :

https://s3.us-east-2.amazonaws.com/rti-zone-images/RTI-Dome_Settings.png

Steven Green

Feb 15, 2022

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Replying to

Rodolphe, I'll grab some photos when I get a chance to get out to the dome.

Steven

Rodolphe

Feb 14, 2022

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The magnet defines the home position. So if it's at 12º that's all you care about for position (aka dome goto done when the mount move the OTA to an object).

Saying the dome slit center is at 0º has no meaning because it rotates.. The magnet is centered in the dome slit.. and that's how it should be.

You do not need to move the controller to make the sensor be at 0º, you only need to know its position (12º here) and enter this as the home.

if your charger is at 0º, that's the park position

Steven Green

Feb 14, 2022

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Replying to

Rodolphe, that would explain a lot. I will relocate the magnet in the center of the shutter slit opening, so it sits at 0° while the rotator motor sensor sits at 12° in the Park position. When the magnet is directly over the rotator sensor, the dome will be at Home.

This should then send the dome slit center, from Home, to NCP position when I sync the dome to the scope.

Steven

Rodolphe

Feb 14, 2022

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The magnet is ON the rotating part of the dome, not on the controller, the SENSOR is on the controller. And this is the azimuth we care about.. the SENSOR azimuth.

Where is your dome controller located with the SENSOR ?

In the picture bellow I circled the home sensor.. this is what defines the home azimuth... this is what I labelled "NexDome Home Sensor" in my diagram above.

So, where is yours in relation to due north. This is what you need to set as the dome home azimuth.

Steven Green

Feb 14, 2022

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Replying to

My current setup is the rotator sensor is located at azimuth 12°. The dome slit center is located at 0° and the dome magnet sits directly above the rotator sensor at 12°, which is what I call the Home position.

What I think you're saying is to place the magnet under the dome slit center at 0°. If this was the case, the home sensor position would be at 12° azimuth, where the dome center slit location would be directly over the rotator home sensor. In order to align the shutter motor over the battery charger center (located at 0°) I would set this to park position to ensure my shutter motor is charging when closed.

Rodolphe

Feb 10, 2022

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I think we're getting lost somewhere.

If your SENSOR is due north, then the home azimuth is 0º.

But you did say the NexDome controller was on a post at an azimuth of 12º from North, and as the sensor is on the NexDome controller box, then the home Azimuth would be at 12º... or did you relocate the sensor (the magnet is just a point marking the middle of the slit... it is not the home azimuth)

Also, Home position and park position can be different.

Home position/Azimuth = position/azimuth in relation to due north of the SENSOR.

Park position/azimuth = position /azimuth in relation to due north of the charging pads.

So in the diagram bellow, The home azimuth is not a 0º because the SENSOR is not due north.

So all you need to figure out is what is the position of the home SENSOR in relation to due north

When the magnet, place centered to the slit opening, is on top of the sensor then the slit is a the home position. All other dome move are in relation to the home position.

Steven Green

Feb 14, 2022

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Replying to

I have not moved my rotator magnet. It sits over the rotator motor closest to the NCP where I have the dome slit centered at 0° azimuth. So home position for the dome is 12° and scope center is at 0° azimuth. So in the Dome control settings section, I need to list the dome home position to 12°?

Steven

Rodolphe

Feb 10, 2022

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Using the telescope to asses the location of the origin (the sensor) is also not correct as it doesn't take in account a lot of the OTA offsets from the axis of the mount (aka what is needed in application and often call "dome geometry").

The sensor is the 0 point for the internal step counter.

The controller knows how many steps it needs for a full rotation, and of course it knows how many steps it needs to move per degree.

This 0 position for the step counter can be anywhere, BUT as this is the 0 for step, the controller need to know what this azimuth is.

So if you put the sensor 12 degree east of north, the 0 step is at 12 degree.

Let's say (not a real value) that you have 10 steps per degree. if you are at home (so at 12 degrees) and tell the dome to go to 180 degrees. The controller calculate that it needs to move (180-12) * 10 steps clockwise to land on 180 degrees.

So 180 - 12 is 168. It will move 1680 steps. That new position in steps is 1680 , or 168 degrees .... plus 12 ... so it is indeed at 180 degrees.

So what is REALY important for accurate pointing of the dome is :

1 - Proper value for the home azimuth (aka the position of the magnet)

2 - Proper value for the steps per degrees (of proper number of steps for a full rotation).

#2 is set by default for a 2.2m NexDome but can be refine via calibration.

#1 is really what you have to figure out. I set mine (I have a structure with the dome on it, not the original NexDome round wall) to 0 degree using sun noon when I was building my observatory. You can also use sun noon to figure out actual north (absolute 0 degree position in space) and then measure how far the sensor is from north. This distance can then be converted to degree which is your home azimuth.

now if that value is already known (looks like 12 degrees for all of you), then this is your home azimuth. And of course the magnet itself need to be centered on the slit, not anywhere else.

If you relocate the sensor, then you need to update the home azimuth to the new value so that the positioning calculation can be accurate.

Steven Green

Feb 10, 2022

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Replying to

Rodolphe, this statement confuses me: "if that value is already known (looks like 12 degrees for all of you), then this is your home azimuth. And of course the magnet itself need to be centered on the slit, not anywhere else."

My dome slit at home is azimuth 0° on the NCP where my mount/OTA is polar aligned, which is lined up with my shutter motor charger. The Dome zero point is now at azimuth 12°, which is the closest wall post to accommodate the rotator motor.

To understand what you've said that If I move the magnet to the center of the slit and send it to home, and then I sync the dome to the scope azimuth at 0° (and my dome geometry is correct) then the dome slit should move to azimuth 0°. So in order to charge the shutter motor battery park position has to be at 0°.

Is this correct?

Marc Buie

Feb 10, 2022

I get exactly the same angles on my system. I started with the fiducial (magnet and sensor) and park position at 0. I did have to peel off the sensor and move it to better align. I'm not sure why this needs to be attached with more than sticky tape. The screws seem overkill. Anyway, move the telescope (with good pointing) to 180 (due south to the meridian). Visually center the dome. The difference between that reading and 180 is the position of the fiducial. For me it was also 12 degrees. I then moved the park location to 12 as well just so it parks on the fiducial. But, the park location can really be where ever you like. I do notice I don't get perfect alignment away from the southern meridian. I have a fork-mount system and it's not perfectly in the center of the dome. I expect I just have to measure the offsets and it will work better.

Rodolphe

Feb 09, 2022

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The dome azimuth AT HOME , aka when the magnet on the dome (middle of the slit) is on top of the sensor, is not 0 in your case, it is 12 degree. All move are then in relation to this position, the home position.

So you home Azimuth MUST be set to 12 degrees or all goto are off by 12 degrees.

Steven Green

Feb 10, 2022

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Replying to

Rodolphe, thanks for the feedback.

Steven

Steven Green

Feb 09, 2022

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I remember way back when I first assembled my dome and that the rotator motor, I'm on my 3rd, would wobble up and down in the track, fall out of the track etc. I ultimately stuck a bolt in the center of the dome floor, loosened every bolt on the walls and kicked and pushed the walls and posts to ensure I had a stable diameter from the center point of the dome. I measured at both the base of the dome and at the top of the wall. Then I tightened everything up, rechecked the roundness of the dome and bolted the dome down into my concrete pavers on the concrete pad. It really helped with the track staying engaged with the rotator motor gear, still had to tighten the motor tension to the track, but the dome moved much better than it had and doesn't disconnect with the track.

I do have a question about the rotator motor magnet position. I have the dome slit center pointed to the CNP, within a degree or two, but the rotator motor is on the closest post about 12 degrees east of the NCP. When I calibrated the dome, I set the azimuth to zero, since the dome slit center is at azimuth 0. My PA of my OTA/Mount are pretty close to the dome slit centerline. Dome to mount tracking is pretty good except on the southwest side of the dome where the mount is off by about half a dome slit opening. Since I'm using a C11 Edge, my view is partially obstructed. I have measured and remeasured the geometry of the dome with a tape measure and laser measure. My numbers are consistent. Should I set the dome azimuth to 12 and the scope to zero?

Thanks,

Steven

Marc Buie

Feb 10, 2022

Replying to

When I built the walls, I made sure the bottom of the wall was circular. I thought the top would take care of itself. Boy was I wrong about that. There were two clues of a problem. First, the one equipment bay I has was flying high. The outer edge was 2" off the deck. Also, the dome rotation on the wheels was very bad. Jumping out of the track, thumping hard at joints, and in general not very happy.

After a lot of thought I came up with an idea that worked really well for me. Get 7 small 90-deg angle brackets with holes big enough for the bolts at the base of the wall. Also get 7 identical ratchet straps. The hooks for the straps didn't fit in the angle brackets so I got 7 identical S hooks to connect from a hole to the strap. Finally, I got a 2" diameter steel ring. The ring goes in the center of the dome. The straps connect from the ring to each of the brackets at the base of the wall. Bring the straps in to identical lengths as measured from the ring to the wall at each of the 7 posts. At this point you want the base of the wall free to move on the foundation. In my case, I left one post anchored but wanted the other 6 to move as needed. Once all straps are the same length, you can can then crank them in uniformly, one click at a time. As I did this, the equipment bay began to drop. I cranked the straps in until the bay just settled on the foundation. While this was going on, I did jiggle the dome a bit to let it settle into position. Some of the posts moved more than 3-4" to its final location. Once it reached it's final position, I did some final macro rotation on the either building to get it centered on my foundation (only an inch or two). After checking that the straps were still all the same length and under nearly equal tension, I bolted down the bottom of the posts. At this point, the dome rotation was nearly perfect.

It was hard to get this picture but I think you can get the idea. Here the straps have not yet been tightened but you can see how they go from post brackets to the central ring.

This is after it's all tightened up and round. Note that this procedure will allow each post to rotate somewhat if it wants to. That turns out to help line up the wheels with where they need to be. Also note that if you wish, you can center the ring on a fiducial point to place the center of the dome where you like. In my case, the deck is just barely big enough, with only a couple of inches of reveal outside the walls to the edge on the short side. Due to the equipment bay, the dome center ended up about 2 inches north of the center and within 0.5" east-west -- plenty good enough for the dome.

bgwynne

Feb 07, 2022

Yes, I would love to hear about dome circularization!

Marc Buie

Feb 07, 2022

Well, I hope I don't jinx myself here, but, today was a bit of a breakthrough day. First, I learned that the manual switch is very touchy about working right. The left side of the rocker switch is mostly fine, the right side can be _very_ challenging to find the exact spot to engage the motion. Given that realization, I decided to move on to computer control.

During manual testing, I had noted the magnet sensor didn't ride in a good place to pick up the fiducial magnet. I tried rotating the post to line them up and that adjustment made the dome rotation much, much worse with many disengagements of the gear from the track. I did two things. 1) put the post back where it wanted to be after I learned how to circularize the dome, and 2) detached the sensor and re-attached it further back to where it should be.

Then, I installed all the software per the Lunatico instructions. There was a bit of fumbling there but after a full calibration and setting a proper azimuth on the home position, the motion and tracking now works perfectly under computer control with no other adjustments. I did daytime tracking and dome following and it all seems to work really well. I'm anxious to try it out at night now. I still don't trust the gear and track but, during a 360 rotation, I don't see _any_ variation in gear engagement depth, just a slight vertical wander that stays within the gear. Time will tell if this is robust but things look good for now without any special action like you and others have described. Perhaps I should write up how I circularized the dome. That may be an important consideration.

bgwynne

Feb 06, 2022

Oh, one other thing, user Rory Clarke did a presentation on his "Nexdome Experience" on the Astro Imaging Channel on youtube in the latter part of 2021 sometime (September???) and he recommended removing the wheel that is right by the dome motor because it seemed like the dome was riding up on it there due to friction so he just took it off, so I did that too.

It seems like the horizontal 'play' in the dome is larger than the depth of the geartooth/track interface unless you really JAM the motor/gear up against the track so that it's actually PUSHING the dome away just enough that the play can't happen.

Even after doing all this (well haven't tried anything since the wire) but I would have a problem with the gear skipping when the panel with the open shutter would go by. Maybe this was a coincidence but it happened often enough that now I manually move the dome to the approximate location I'm going to need it making sure that the shutter panel 'goes the other way' before engaging 'slave to telescope'.

If during imaging the slit has to go past the motor so be it but until then…

Bill

bgwynne

Feb 05, 2022

WellI guess that's the"charm" of this particular product; we all get to do our own little bit of engineering to fill in the gaps! KInda fun and kinda not.

I have the same problem with the gear pulling away, had to absolutely CRANK the motor into that coupler to bring it close enough and just yesterday I ran a guy wire to a sunken stake in the earth on the outside of the dome to stabilize the dome wall vertical support bar cause it was moving all over the place when the dome was rotating. I also don't let the dome rate any faster than 1/3 of maximum speed.

Marc Buie

Feb 05, 2022

Replying to

Interesting idea about guy wire. I can't go to the ground due to safety issues but I could do a wire and a stay (like sailboat rigging) and anchor to the deck. But, that pre-supposes the wall is what's moving and not something odd with the dome. I don't know (yet) in my case but it's another thing to keep a close eye on. Actually, I thought I had the dome moving pretty well but that was before mounting the motor. To get the magnet to line up I had to rotate the post and I fear this has now mis-aligned the wheel. It's noticeably rougher when spinning that before. I'm inclined to try rotating the post back (more like letting it relax, actually) and then relocating the magnet. As you so aptly put it, "kinda fun and kinda not". No matter what, I've very grateful for the sharing of ideas on this forum.

bgwynne

Feb 02, 2022

This is my full wiring scheme for the installation.

Marc Buie

Feb 05, 2022

Replying to

That's quite a setup. One idea I'm working out is how to power the shutter without using any A/C power. I found a flexible solar panel that I intend to mount directly on the dome so that the wires stay above the rotating interface. It's a panel that Dakota batteries specifically recommend for charging their battery which is enclosed in the shutter mechanism. No charge controller required for this low of a power setup. As for the rest, I once had such ambitious plans but the dome requires enough hands-on care that I pretty much gave up on the stuff that is needed for a more autonomous installation like you have. Also, I don't need to control as many lines and that power distribution box you have is a bit pricey. I think I can set up a couple of relays and use my existing raspberry pi to control the relays for a much lower cost.

Interestingly enough, last night I was on the sky and on a whim thought I'd try the dome rotation system again. I haven't changed a thing but I plugged into power and the dome motor worked! I was very surprised. Now I'm on to the next problem of a slightly out of round dome where it pulls away from the gear at certain locations. Maybe someday I'll figure out why it didn't run last weekend when I first plugged it in.

bgwynne

Feb 02, 2022

The Box I use is the Digital Loggers DC-3.

As so far as determining voltage I went by what the output was from the wall wart they supplied with the unit. It was 12v so I figured I could come straight off the batteries. I checked with the NexDome folks and they said that would work just fine so I did it and it does.

Don't know why yours wouldn't work; I don't know about the light patterns, never had it powered up with the case off since it worked from the get go.

Can you operate the dome under software control? Via your shooting app or the ASCOM Device Hub Software?

Bill

bgwynne

Feb 02, 2022

I had a terrifically hard time getting that power plug into the dome rotation motor as well. At home it barely worked, out at the observatory it was a complete no go. I ended up opening up the control box, removing the jack entirely, running the wire in thru the hole and attaching it straight to the header on the circuit board. Just follow the wires from the jack and all you need is a tiny screwdriver. Been perfect ever since.

I too am running on solar power but have a DC power management box hooked up to the load side of the charge controller. That way I can turn on or off 8 different observatory subsystems over the internet. It's the Digital Loggers DC-3.

I have the dome motor hooked up to one of those circuits, not using the wall wart power supply they sent with it.

Marc Buie

Feb 02, 2022

Replying to

I already did the same thing you did with the power lead inside the box. I have a wire going straight to the header on the circuit board. The power comes from my solar controller load output so it does have some current protection and there's no sign that I've overloaded the system. With the power connected, the status LEDs inside are flashing in a pattern but the system is unresponsive to the manual control switch to activate the motor. I see a couple of things on the control board. First, it's made by Lunatico and I've worked with them before. Second, the label next to the power header shows +27V on the hot side. I'm wondering if this board requires higher voltage or if it's supposed to be good with 12 VDC. If your board also shows this same voltage value printed on the board and works on 12 V I can set this possibility aside. Below is a picture of my board. If the original image comes through, you can zoom in on the power header and see Vin=+27V to the right. You see it here with the original power lead disconnected and my own 16 AWG wire coming in through the hole.

As I said, when in this state the lights are blinking a pattern (T = top LED, M = middle LED, B = bottom LED), upper case means the light is on, lower case means the light is off:

1) TmB

2) Tmb

3) tmB

4) tmb

and this pattern repeats over and over. It's roughly 1 second per step (maybe a little shorter).

I'm curious, though. What is the DC power management box you are using? I've been looking at either Web-Relay or RigRunner for this but those are fairly pricey. If there's a parasitic load for this dome rotation control system then I will need to have a way to shut it off remotely.

Marc Buie

Feb 02, 2022

I know I should have bench tested the motor before installing, but, I didn't. Once installed on the dome wall post and aligned (all went well), I then proceeded to power it up. I don't have A/C power in my dome, only 12 VDC (solar power and a battery). I have to use a different cable to connect to the power port but I checked the supplied cable against another known socket and found a cable to match that I could use to connect to the battery. To my surprise, I wasn't able to plug into the power socket. The pin in the socket is too big. Did anyone else have a similar problem? I contacted NexDome about this and their response was to "push harder".

For those of you with a working circa 2020 version system, does your dome rotation power supply indicate 12 VDC @ 5A or something else (this is what mine shows)? I ask because after connecting to a battery, the manual control switch does nothing and I'm trying to figure out what's going on.

bgwynne

Jan 25, 2022

You definitely want that screw in the slot. Mine went right in but if you had to file down the tip of the screw (or widen the slot in the motor shaft) you should do that. Based on my installation and seeing comments from a number of others the gear will end up pretty close to the end of the motor shaft so that would make widening it a touch easier.

And while we're on the subject of the gear motor I have found it very difficult to get the gear to mesh with the track reliably. I'm now of the opinion that if you can very slightly bend the L-shaped motor bracket in a way that pushes the motor outward from dome center, that will keep you from having to just absolutely CRANK that bolt from the motor bracket to the coupler like I'm doing. I also took the wheel off here.

The horizontal 'play' of the dome as it rotates is more than the depth of the track teeth/gear teeth interface so you need the gear pressing the dome outward so it won't just wander away.

Anybody have problems with gear mesh when the open shutter panel goes by the gear? Something about that panel is misshapen just enough to de-mesh there. I now manually move the dome to the approximate spot where I'm going to be and make sure the direction I choose takes the open shutter panel the other direction from crossing in front of the dome rotation motor, even if I have to go 270 deg to do it.

Then I engage "slave dome to scope"and it locks in.

My 2 cents

Bill Gwynne

Steven Green

Dec 27, 2021

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Marc, are you saying the set screw (key), is wider than the slot in the motor shaft? I did find it odd that the motor shaft came with a metal key that held the yellow plastic shaft cover in place yet the gear sleeve is round with a set screw.

I haven’t heard anyone mention a problem with this. The gear sleeve on mine is round and all I did was align the set screw across from the motor shaft slot and tighten it down once mated.

I’ve had the gear tensioning bolt come loose which resulted in the rotator gear loosing contact with the track. I fixed that with blue Loctite. Steven

Marc Buie

Dec 27, 2021

Replying to

The diameter of the set screw is greater than the width of the slot. Thus, when tightened, you get a very small depth of penetration to encourage the gear to stay put. You can also avoid the slot altogether and just depend on friction of the tightened screw to hold it in place. That seems even less desirable. There is a reason to use a key on a drive shaft but I find it odd that the key is meant to be thrown away as far as I can tell.