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INDI Library v2.0.6 is Released (02 Feb 2024)

Bi-monthly release with minor bug fixes and improvements

Re:New Polar Alignment Scheme and Features

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I'll try yet again.
Yes, you can rotate the mount about any set of coordinates to correct for a polar align error. But the correction indication determined by using a reference star will depend on the position of the star determined in the coordinate system of the correction axes.
There are a number of positions which need to be avoided, for example for the normal Alt Az corrections a star at the zenith will be a poor choice for the azimuth correction because adjusting this axis will not move a star at the zenith. Similarly looking East or West won't work for the elevation correction. The optimum positions are fairly close to the meridian and away from the zenith. Avoid a band running overhead from East to West.

Your tripod leg length error will have the effect of rotating the mount about an axis defined by the other legs and in theory the required corrections could be determined in that coordinate system. You could then get a correction that could be corrected by adjusting tripod legs. That's going to be tricky to determine. Simpler to determine the error in the AltAz system and correct in that system.

Not having the mount level will make a difference but as I said previously the effect will be small for small errors, about one arc minute per degree of error or level.
3 years 2 months ago #66068
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I agree it is bad practice to choose a reference star in that band. But lets say you choose a star near the zenith. And the algorithm determines that a large azimuth adjustment is needed. When the corrected position is converted to HA/Dec it is more or less the same position as the reference star already. So the correction line will be very short or non-existent. And as you say, any error in the calculated position is small. So were you just being contrary for the heck of it ????
I'm actually more concerned about the calculated position being a moving target when far from the pole. Some folks need time to adjust alt/az and by the time they get there they could have moved to the wrong spot. So the position needs to continuously recalculated, if it isn't already.
3 years 2 months ago #66079
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I’ve been using it on Astroberry for the last couple of nights. It worked fine (north vision isn’t a problem, so I’m using the “classic” way of pointing towards Polaris).

The only glitch that I’ve found is that it doesn’t likes zoom changes. For example, after the correction triangle is draw over my image if I zoom in before clicking to move the triangle, then as I click it will reverse from left to right, invalidating the procedure.

If I zoom in before the triangle is drawn (before the three measurements finished) it works as expected.

It’s a minor delight to be carried on to a more consistent alignment. Thanks for this improvement!
3 years 2 months ago #66094
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Didn't mean to start a "frank exchange of views" here!

So as I understand the geometry and mechanics, the routine calculates correction vectors in the vertical and horizontal axes, and displays those as the base and height of a right triangle, the hypotenuse of which is the direct vector from "wrong" to "right". Right?

If the mount isn't level, adjusting, say, the azimuth will also introduce some movement up or down. Which will cause the star to not track along the outline of the triangle. When you adjust the altitude a bit, you can fix that.

Which is to say, my sloppiness in leveling might obviate to a small extent Hy's fine work in making things easier for me, unless by some burst of mathematical brilliance which I don't even think is possible, the routine could *also* deduce how far off from local vertical the azimuth axis is. But he's amazed me before so I wanted to check.
3 years 2 months ago #66112
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It is true that being unlevel will introduce some amount of error between Alt-Az adjustments, but it should be fairly small. Overall level is a non-issue as long as you get from one end of the hypotenuse to the other, which is defined by the original correction vector. Doing so should be all it takes to align the mount's axis of rotation with the celestial pole.
Level does matter when doing a drift alignment, but that is not the case here.
3 years 2 months ago #66120
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It will be a deviation between the Alt Az distance computed and the Alt' Az' steps needed to do the correction, yes. The endpoint is the same. And the needed correction vector doesn't depend on what you use to correct it. The AltAz is some helper coordinate system. The relevant ones are the two spherical coordinate systems of the mount axes and the celestial coordinates, and there is only exactly one correction vector to get them to match. You can compute it directly, without needing to go via AltAz, Alt'Az', or any other third coordinate system.
The only moment when a non-level mount would cause problems is if you compute (absolute) corrections for the AltAz system, and then blindly apply them in the Alt'Az' system and assume you have now corrected the error and have perfect PA. This is indeed not the case. But you would see that after those corrections the star is not at the position that the (unique) shift vector gave. But the whole process works in checking the progress live and in celestial coordinates. With a non-level mount you notice that the movement is not along the given directions and with different amounts. Without drawing those help lines one wouldn't even notice. The AltAz decomposition of the correction vector is a convenience for the user, but not needed/relevant for the correction process. For that, only the end position matters. And if you end up there, the PA will be done properly...
3 years 2 months ago #66124
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An injection of a bit of a Cloudy Nights vibe ;)
But Chris raised a very good point which is that if your reference star is at the zenith then no amount of adjustment in azimuth will move it. You could spin the mount 360 and see no change. Similarly at the E/W horizon the altitude adjustment does not move the reference star. So a modicum of care is needed with reference star selection.
And I'd be interested to know how long one has to make the adjustment before the correction point becomes invalid - say more than 1 arcminute off.
3 years 2 months ago #66132
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Yes, it might be a good idea to think about some sanity tests for the star used to correct the PA and pop up a warning if it is too close to the critical points. But you wouldn't want to pick one on the horizon anyhow I guess ;)

As for the duration of validity: The PAE is a constant, it doesn't change as the mount/sky rotates. Therefore the correction vector (in sky dRA and dDEC) is also constant. It only depends on the sky coordinates of the reference star. So the only thing that will change in time is the AltAz decomposition of that vector. So the effect would be similar to the one of a non-level mount: Movements done by turning the Alt/Az knobs of the mount would no longer move along the plotted lines, but the endpoint still is the same. So as long as you end up on the proper endpoint you can take as much time as you want...
3 years 2 months ago #66138
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The PAE does not change but I would expect the target position to rotate around the reference star at sidereal rate on the sensor. Same as field rotation.
Anyay I just did a back of envelope calculation. Similar to field rotation the rotation amount is proportional to the length of the vector which has the length of the PAE. So lets say 10 arcmin. And at a pixel scale of say, 1"/pixel the vector is 600 pixels long. After 1 minute the correct target position would move through an ARC of 15' or 0.0044 radians so r.x = 600 * 0.0044 pixels = 2.6 pixels
So the nett error in the alt az corrections would be 2.6" i.e. the resulting PAE would be 2.6" if the adjustments moved the star precisely to the target position - which is insignificant.

If the PAE was, say, 60' the resultant error would be 15.7" and still fairly insignificant. And if it then took 10 minutes to adjust, instead of 1 minute then the error would be 2.6'
So in that case the user would still a see a PAE of 2.6' even after doing a "perfect" polar alignment - still not a major problem except for the OCD.
So I would conclude that the rotation is not a significant problem. But maybe something users should be aware of that they should perform the corrections in good time.
The following user(s) said Thank You: Peter Sütterlin
3 years 2 months ago #66142
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I really don't see this as an issue. Since, even if a small error is incurred by taking a extra time to adjust, it will still be better than it was. Meaning, if you do two or three iterations, as one should, you will converge on an excellent alignment. With each adjustment being smaller, and quicker than the last.
3 years 2 months ago #66143
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Darn! Yes, you're right of course. Didn't think of the fact that due to PAE the sky coordinate grid on your camera rotates :blink:
3 years 2 months ago #66145
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Its not caused by the PAE (unlike field rotaion) - just the normal rotation of the sensor with an EQ mount. If, when the camera is pointing to the east (HA 18h) the top of the sensor is pointing terrestrial east. At HA 0h the top of the sensor point upwards in terrestrial. When the camera points west (HA 6h) the top of the sensor is pointing terrestrial west. So if you need to adjust in azimuth to the east on the mount then the target point on the sensor is above the reference star is the camera is poiting at HA 18h, to the left of the reference star when the camera is pointing at HA 0h and below the reference star at HA 6h.
As I've shown it is not a significant problem but its always worth doing the maths to be sure. No doubt someone will post one day something like "I did a PA using Ekos and got to < 0.1 arcminute PAE, then I checked in Sharpcap and PHD2 and it was DIFFERENT!!!!" :)
Last edit: 3 years 2 months ago by Ken Self. Reason: Pedantry about camera orientation
3 years 2 months ago #66149
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