Polarie counterbalance experiment

[Omaroo]

Looking at the Polarie, the entire weight of the camera and lens (or refractor) hangs off the end of a ball head fastened to the coupling disk on the front of the unit. While I'm not concerned about the lateral weight applying sheer force on the bearing, I am interested to know how much exertion the gearing mechanism is being asked to tolerate longitudinally - i.e. a twisting moment. The more weight the more we're expecting of the mechanics to keep the whole thing moving at the advertised rate, the less chance we have of tracking accurately.

The answer is, I think, a counterweight. I've seen a couple already produced by a couple of people and they look fairly sophisticated in comparison to what I envisaged. The coupling disk has an spring-loaded 1/4"x20 thread which exits its central face to run into the bottom of a ball head. I figured on using this as usual, but have it screw in to a dovetail which has threaded holes along its length to screw into. These holes, spaced one inch apart (for the moment) would allow you to mount whatever camera gear you wished, and then by holding up the bar between your fingers you could determine the balance point - and screw the coupler in to the nearest threaded hole. This would hopefully result in a unit that is balanced reasonably well around the axis, letting the motors keep moving without having to keep lifting the entire weight of your imaging rig. It's a very simple approach, and one that I think will work with the minimum of extra mass added.

Well... here it is - a half-hour's work in my garage. Works a treat :) Will try it out this week some time. The advertised weight limit of the Polarie payload is around 3.5kg, and I wonder if that is imposed by the manufacturer based on the motor strength and the systems ability to lift the weight to meridian and then the exact opposite on the way back down the other side - or bearing capacity. I'm thinking the former. In any case, if the motors aren't straining so much then maybe the batteries will last long than the four or sou hours I currently get.

All good fun anyway...

[multiweb]

hmm... looks good but dangerous IMHO. That's a potential lot of torque applied to the internal gears of the polarie with that long dovetail. Better not knock it accidentaly, or wind shear... I know the original system is not ideal for heavier loads but it's compact and kept close to the RA axis.

[Omaroo]

I disagree. Dangerous? LOL There's hardly any cross section in it - so wind isn't really a factor - I don't image in gale conditions. The bar is balanced remember - that's the whole point. It's easier on the gearing - not harder. If I knock it I'd stuff my image more than the unit. I have never knocked a telescope imaging rig in as long as I've been doing it as far as I can remember.

Not really concerned... unless a kid at a public night decided to hang off it.

In reference to the weight being kept close to the RA axis, it's not - even with a short ball head mechanism. Loosen off the thumb screws and just see how much twisting moment there really is when the camera rig is at 3:00 or 9:00 o'clock - it's incredible. With this, I can loosen both thumbscrews and the whole thing just hangs there - not exerting any rotational force.

[multiweb]

I understand what you are trying to do re:balance but keep in mind the polarie is designed for a 2kg load only. I suspect its internal mechanism and gearing are built accordingly. If you increase the distance of any load from the axis of rotation, balanced or not you introduce the potential for very important leverage. I'm just saying to be cautious as not to damage the unit. In contrast the Losmandy star lapse looks like a cut down GM8 so I suspect it's a lot more sturdy.

[Omaroo]

Under normal conditions, my system exerts far less damaging turning force than what you're running Marc. I believe that the weight restriction is, as I stated, imposed because of that twisting force through the gearing required to lift any more than 2kg. Without the added advantage of balance - i.e. your reference to distance of load from the axis of rotation - which I've dramatically reduced by doing this, its ability to turn more easily is increased. Think about it...

[multiweb]

We're talking about leverage here. Not balance. You know the noise the vertical blinds make when they get caught and jump a few teeth when you pull the string to close them? Good luck with it though :)

[Omaroo]

Leverage from what???? Someone hoiking on the bar? Not concerned. Keep away from my Polarie if you have a tendency to bump things... LOL :) This game is fraught with fragility - everything we use is precision equipment. You have to be careful - period.

The motivation here is in trying to achieve better tracking accuracy. I'm pretty sure that this will help do that.

[dannat]

Chris Greg Walton has already designed a counterweight bar forthe polarie (for vixen). I saw it we'd at ASV meeting, beautiful bit of kit I just didn't get a pic

The orig camera platform is removed, it's a bar which can pivot about the hole where polar scope goes (polar scope can stay in)

Not sure if he's a member here but someone from MPAS might have his email for a pic or 2

[multiweb]

Yep, the length of the bar is the issue IMO. If you get the bar a lot shorter and place your camera assembly and an equivalent weight opposite at the same distance from the RA axis you won't have any issues with accidental knocking. It will also track better. I have experienced this first hand with a G11 loaded with a C11 and hyperstar. After talking to Peter Ward at the time he advised to have a heavier counter weight placed closer to the RA rather than a lighter one down the shaft. This way the whole system was more compact, less torque involved and the tracking was improved with less vibrations introduced by higher momentum with the previous arrangement.

[Omaroo]

Yes, definitely. I tend to shove my heavier counterweights up my shaft to reduce flex and move that inertial mass in closer too. This is an argument - bumping it is not.

The design of what I have here is geared around what you're saying anyway. The holes go to both ends... Once I have a known weight that I'll expect to work with, I'll attach an appropriate weight at the other end of a shorter bar, yes. That's why i went the whole length with the drilled/tapped holes. Adjustability.

[Omaroo]

Thanks Daniel. I had this stuff lying around, and it's simple to produce. There's no milling or turning involved. It'd be interesting to see what he's done though, for sure. I was after simplicity, but would pay to have a nicer arrangement if they were built.

[multiweb]

S||t happens. :)



Shorter dovetail will be much better. As is you've also occulted a large chunk of sky should you choose to do widefield with a 25mm lens or wider.

I'd also be interested to see pics of Greg Walton's solution if they can be posted in this thread.

[Omaroo]

Again - the problem with a shorter bar... is extra weight. We're trying to keep that to an absolute minimum. Flex isn't an issue - we don't have 20 or 30 pounds dangling off the end.

Greg's is easy to make of you have access to a lathe and mill. It's what I would have done too. The way he has it, the weight is positioned well back towards the back of the unit too so that the camera's COG and Polarie bearing are on the same plane.

[multiweb]

Any pic?

[Omaroo]

Get ready for a problem for you.... the counterweight shaft is 280mm long to offset a 6kg package without a high total weight. I agree with this approach.

I haven't uploaded the entire document without Greg's permission.

[multiweb]

:lol: Ok I can see the problem now.

[Omaroo]

Quite. The point of making it is to accommodate other equipment - not just the same camera/lens combination every time. My FS-60C/OM-D combo weighs 2,824g whereas the camera/75mm is 1,109g.

[gregbradley]

Interesting work Chris. I'd like to hear how it works out.

There is a Vixen tripod for the Polarie that has a counterweight shaft. Perhaps look at that as well for ideas.

Greg.

[multiweb]

I believe that's an Orion ED80 on the pic. 600m FL or there about. In my experience anything over 200mm FL providing you nail focus will need guiding both in RA & DEC. And I did a lot of experimentation with various lenses. I see the polarie as a very light weight unit with a sinple ball joint and a DSLR for widefield. That's its strength and that's what it was designed for. Mods to max out the load are not really useful. Anything heavier or longer FL requires a small mount at the minimum. I'll be interested to see the pics coming out of the rig when loaded up though but I suspect you'd get trailing.

[Omaroo]

The scope won't be used for star fields Marc - just for solar/lunar.

[Omaroo]

Greg - isn't the Vixen-supplied Velbon tripod itself counterbalanced? It has nothing to do with the Polarie as I understand it. It's just so that you can set up the tripod in a way where it is less likely to topple over.

[gregbradley]

Oh I see. I thought it was counterbalancing the camera/lens.

Well that's not so useful.

Greg.

[dannat]

That is an ed80 Greg had on there, he tested it briefly he tod me & the polarie handled it , at the Asv meeting e had a long tele lens on it maybe 300mm (seemed like an older lend, not the compact ones you get today)

Hi counterweight shaft was long, I think it was so he could rotate it fully with contacting anything

[OzEclipse]

The Polarie can hold more than it's original rating.

A rep on the vixen mailing list recently posted :

From:vixenoptics@yahoogroups.com on behalf of mrstarguy_usa (j.deis@vixenoptics.com)Sent:Wednes day, 22 August 2012 3:12:33 AMTo: VixenOptics@yahoogroups.com
Vixen's Polarie Star Tracker has been re-evaluated and it is now confirmed that it will hold 7 lbs of equipment. Lets get out there with our cameras and take some amazing photos!


So some of the concerns expressed in this thread about weight limits need to be rethought.

Joe