Imaging needs good balance - Yes or no?

[swag72]

Looking around the net (bad move I know!!) I come across comments that perfect balance on a mount is not a good thing for imaging. I have read to balance the RA to the East - Does this mean to have the RA slightly heavier towards the weights? Also, if it's best to have the DEC slightly off balance, in what way? Back or front heavy?

Perhaps I've misunderstood things and got it wrong and you do benefit from a good balance?

[steppenwolf]

It is absolutely right that you should first achieve a 'perfect' balance BUT then offset that balance slightly once you have acquired your object. However, it is not quite as simple as making the weight end heavier - rather, you have to decide whether the telescope of the weights are 'rising' (i.e. on the east of the mount) and make the rising side slightly heavier. So:-

1. If the weights are rising, slide them a fraction DOWN the shaft

2. If the telescope is rising, slide the weights a fraction UP the shaft

What you are aiming for is the motor drive 'driving' all the time with no risk of the system teetering between driven and undriven.

[brianb]

The reason for balancing "east heavy" is so that the weight acts to keep the backlash out of the drive ... the drive will then run at a more predictable rate than if the drive is acting as a regulator for a gravity driven scope, since there will inevitably be random "sticking". This means you want the counterweight arm heavy when imaging east of the meridian, and the scope arm heavy when imaging west of the meridian. The easy way of arranging this is to have the counterweight slightly light and add a small extra counterweight when imaging east of the meridian.

Dec axis slightly off balance is good for the reasons given above. But which way it is off balance doesn't matter - if your alignment is good you can't predict which direction guiding adjustments in Dec will be. It is well worth while making sure that the movement is as smooth as possible, and that unreasonable amounts of backlash are mechanically adjusted out of the drive.

[swag72]

Looking at the mount Steve, how do I know which is the East side? The side nearest to the East?

[steppenwolf]

Indeed, the side nearest the east is the one that needs to be heaviest - But also think about how the RA axis rotates when tracking an object, the side that is rising up towards the sky is the side that is on the east and thus the side that needs to be made heavier. We can't talk so easily about 'right' and 'left' because we need to determine where you are standing and to do that we need to discuss 'north' and 'south' of the mount so it makes sense to discuss east and west instead!!!

Edited for moronic spelling!

[swag72]

Ok, got that. Let me just be absolutely sure I'd get it going the right way.

If I am imaging and the weights are nearest the East, if I look at my target in Stellarium for example, and it is moving upwards in the sky, then my scope side would need to me heavier?

[brianb]

and it is moving upwards in the sky, then my scope side would need to me heavier?

No. Object's altitude increasing, scope moving down relative to weights, weights need to be heavier. Object's altitude decreasing, scope moving up relative to weights, scope needs to be heavier.

[jgs001]

No... think of it another way... ignoring the east west... whichever bit is rising needs to be a touch heavier than the side sinking...

If your target is in the east, the chances are, the scopes will be to the west and higher, than the counter weight which will be east, and lower, therefore the weight end of the axis needs to be a little heavier...

[roundycat]

Or, to put it another way! If you are looking in the same direction as your scope and the weights are to your left they should be the heavy bit.

Dec balance is a bit more tricky and it depends on your polar alignment (PA). Your PA will never be perfect so Dec movements tend to be in the same direction, either north or south. Balance whichever way keeps the gears in mesh, same reasoning behind RA balance. Mine works more smoothly with the objective end heavy but yours could be mis-aligned in the opposite direction so it might work better camera end heavy. Try both and look at your guiding smoothness. Do it at a reasonable value of Dec, don't have the scope pointing straight up, it is virtually impossible to balance out backlash then. Don't set any backlash adjustment in your software. Eradicate it using gravity, it is very reliable.

Dennis

[swag72]

Or, to put it another way! If you are looking in the same direction as your scope and the weights are to your left they should be the heavy bit.

Good call, I like that, nice and easy!

Dec balance is a bit more tricky and it depends on your polar alignment (PA). Your PA will never be perfect so Dec movements tend to be in the same direction, either north or south. Balance whichever way keeps the gears in mesh, same reasoning behind RA balance. Mine works more smoothly with the objective end heavy but yours could be mis-aligned in the opposite direction so it might work better camera end heavy. Try both and look at your guiding smoothness. Do it at a reasonable value of Dec, don't have the scope pointing straight up, it is virtually impossible to balance out backlash then. Don't set any backlash adjustment in your software. Eradicate it using gravity, it is very reliable.

Dennis

So when using the PHD graph to checking alignment, I should have a go with different balance on the Dec - What will the graph be looking like if it's weighted the right way - Smooth? As opposed to bumpy if the wrong way round?

Also is there a better time to test this out - When checking towards the meridian or east/west?

[roundycat]

can't speak for PHD. If you are out of alignment Dec drift will be either North or South. The corrections applied will be the opposite of the movement to keep it all pointing to the same place. You can either drift align to measure Dec drift, take a couple of exposures five minutes apart and measure the movement of one of the stars from image to image or just try the two different modes of balance and then suck it and see.

The word 'balance' is thrown around rather glibly. What is really meant is 'out of balance' and the out of balance force should be big enough to overcome stiction in the mount. The Dec motor doesn't have to work too hard so it should not be overloaded.

Do this at about Dec = 0 degrees and somewhere around 1-2 hours before the meridian.

Dennis

[MikeWilson]

You'll know if you get a backlash problem because your stars will look like this:

._._.

[swag72]

My stars don't look like that, just still got a little rotation that I need to eliminate.

[roundycat]

Obviously I can't comment on your field rotation without seeing an image. All I can say is that, as a test, I once deliberately mis-aligned my AP by about 3 degrees and saw no field rotation.

You often hear about this but I have never seen it in any published work. More likely coma or plain old poor guiding are the cause. For any star distortion to be the result of polar alignment bad enough to cause rotation the star field in the image will rotate around the guide star. When you see this effect in your pictures it will be very obvious. Not to be confused with coma in the corners!

Dennis

[ollypenrice]

As a teacher I always wanted instructions to relate to first principle so, Sara, look at your mount. You can stand where you like in this version. When it is running it is being pushed round by a motor. If you were pushing it with your finger instead, (you are only allowed to push, not pull) which side would you have to push? Scope or weights? That is the side that needs to be heavy so that themass you are pushing is always offering a resistance to your finger, never balanced so that it sometimes falls away from it till your finger catches up. That is backlash...

Dennis, I reckon I get field visible rotation in long subs if not well aligned. I'm intrigued by what you say. Also if I use Astro Art 4 to stack, it does not rotate but only does x-y aligning and there in a five hour run rotation seems to show clearly. Or until now I have assumed it was that. I use sub stacks of an hour a time if this happens due to mounts getting nudged etc.

Olly