Interpretation of drift graph for ALT/AZ mount

[roelb]

Are the axis in the sharpcap drift scatter graph for a ALT/AZ mount equal to x axis = azimuth and y axis = altitude?
Note: the camera is aligned to the horizontal plane (RA)

[admin]

Hi Roel,

is this the scatter graph in live stacking showing the relative positions of the frames or the one in the feature tracking tool (or perhaps even another one that I'm not even thinking of right now...).

In both of the two I mention above, the axes are not Alt/Az or RA/Dec, but horizontal and vertical pixel offsets in the image - a point plotted at X=+10, Y=+5 means the image has moved 10 pixels to the right and 5 up.

cheers,

Robin

[roelb]

I mean the drift graph in the live stacking.
I understand that's the drift in px from the image.
But, may I conclude that this drift is coming from the mount, where the x-axis corresponds to the horizontal (AZ) movement of the mount?
And the y-axis corresponds with the ALT movement?
Is this drift a measure of the mount tracking accuracy?
I ask this because I want to check the tracking accuracy of my new Evolution mount, which has now spring loaded worm gears.
I don't apply polar alignment. It's pure a ALT/AZ operation.
Is field rotation also in play?
If so, which other possibilities offer SC to check the mount accuracy?
Thanks in advance.

[admin]

Hi,

ah, OK...

the orientation in terms of which is alt and which is az depends on how your camera is aligned on the telescope - unless you have deliberately set the camera up so that the image of a terrestrial object is the right way up then most likely the orientation will be random. It's not too hard to sort this out though - set a short(ish) exposure and high gain so you can see stars even with <1s exposure, then move the mount slowly in alt and then az and see which way the stars move. If you want you can use this to rotate the camera until azimuth is left/right and altitude is up/down.

In terms of field rotation, it does have some effect - with rotation, the amount of movement varies from one part of the image to another. To take account of this, SharpCap works out the movement at the image center as the best reference point. If you want to avoid the effects of field rotation to simplify things, test when pointing east or west at low to mid altitudes - that gives the lowest field rotation rates.

Another way to do this would be to use PHD2 which is perfectly designed to measure/record tracking accuracy.

cheers,

Robin

[roelb]

To be sure:
My camera is oriented horizontal, then the axis correspond to:
x: AZ
y: ALT

I have attached some examples.

20 x 15 s AZ 43° NE - ALT 78

NGC 7380 - Wizard Neb in OpCl-AZ 43° NE-ALT 78°.jpg (126.81 KiB) Viewed 686 times

50 x 15 s AZ 180° S - ALT 19°

NGC 7293 - Helix PlNeb-AZ 180° S-ALT 19°.jpg (150.45 KiB) Viewed 686 times

25 x 15 s AZ 220° SW - ALT 68

NGC 6995 - Eastern Veil Neb-AZ 220° SW-ALT 68° .jpg (166.75 KiB) Viewed 686 times

30 x 15 s AZ 240° SWW - ALT 60°

NGC 6960 - Western Veil Neb-AZ 240° SWW-ALT 60°.jpg (124.29 KiB) Viewed 686 times

65 x 2.5 s AZ 280° W - ALT 77°

NGC 6826 - Blinking PlNeb-AZ 280° W-ALT 77°.jpg (112.56 KiB) Viewed 686 times

38 x 3 s AZ 270 °W - ALT 53°

NGC 6791 - OpCl-AZ 270 °W-ALT 53°.jpg (114.93 KiB) Viewed 686 times

Looking at the last 2 graphs (W direction), may I conclude that the error in the ALT direction is higher then in the AZ direction?

My new Evolution mount has spring loaded worm gears.
Does that improve drift figures?

Thanks in advance looking at this questions.

[admin]

Hi Roel,

there certainly do seem to be more traces with vertical drift in them than those with mainly horizontal drift, so that would tend to suggest that the altitude was drifting more than the azimuth. It's interesting to note that two of the traces (#2 and #5) seem to have stabilized after some initial drift - you wouldn't really expect that to happen if the drift was being caused by poor alignment of the mount, so I'm not sure what is going on there.

I don't have experience of the Evolution mount, but I would expect the spring loaded worms to largely reduce backlash rather than improve tracking - I think! You are probably better off searching on some of the more general astronomy forums to see if there has been discussion of this sort of tracking with the mount in question previously, as that might have come to some useful conclusions.

cheers,

Robin