Feature Tracking Calibration "Failed due to inconsistent scales from Axis X"

[Borodog]

Where X is either 0 or 1. What is causing this error and how do I avoid it?

I am still trying to use feature tracking for lunar and planetary imaging and it is still hit or miss and still frustrating. Sometimes the calibration succeeds, sometimes it fails. I don't really know why in either case. Same mount. Sometimes the error is something about "ve" or something on the DEC axis (the CGEM has a gearbox between the DEC motor and worm gear with an unavoidably large amount of backlash), but this error is new.

Even when the calibration succeeds and the mount is supposedly tracking, the drift is sometimes unacceptably large, as in arc-minutes, and I don't know why. It's frustrating when I know something like PHD2 is capable of tracking to ~1" at 3s intervals. PHD2 calibration never fails, can always get the mount to guide, even with the large DEC backlash, and keeps it within an arc-second or so, even with the too-heavy 1100 EdgeHD.

PS. I still love SharpCap.

[admin]

Hi,

this is one of the reasons I have a lot of respect for the PHD2 guys... Getting decent calibration out of very variable mount hardware like they have is a big achievement.

The most likely cause of the problem you are seeing is that SharpCap is observing different movement rates in RA depending on which direction it asks the mount to move. If the difference is big enough, SharpCap will give the error you are seeing as it is unsure about which of the two rates is correct.

The underlying cause is *probably* that, for instance, when SharpCap asks the mount to move at 4x sidereal rate in the + or - RA direction, the mount is turning off tracking, then moving at that rate, then putting tracking back on when the movement stops. That means that the stars will appear to move at 3x sidereal rate in one direction and 5x in the other.

The two possible things to try are to set the speed of movement to 'guiding rate' which will use pulse guiding to nudge the mount. This is slow, but should not be affected by the problem described above. Alternatively, pick a higher rate (16x sidereal for instance), since the two different observed rates will be 15x and 17x, which are similar enough relative to each other to not trigger the problem.

If none of this helps, please send me a log of a failed calibration session and I will see what I can work out.

cheers,

Robin

[Borodog]

Thanks, Robin. I will try pulse guiding. I didn't know that wasn't what it was doing; it seems like that is what I want, no? It seems like turning the tracking off and back on every time you want to correct may be what causes the jerky guiding with SharpCap feature tracking, as opposed to PHD2, which can keep stars within microns on the sensor indefinitely.

[admin]

Hi,

certainly the tracking on/off could be part of it. Some mounts don't have this problem and can happily add movement on top of tracking, others cannot.

SharpCap also takes a different approach of allowing a larger error to build up before trying to pull the target back to center. This is really designed for video imaging, where constant small shifts could be an issue compared to more infrequent, larger shifts. Anyway, you can tinker with this by adjusting the dead zone size, which is how far the image is allowed to move without a correction.

cheers,

Robin

[Borodog]

Unbelievably frustrating time trying to get feature tracking to calibrate last night. No options worked. Tried image features on the Moon, tried ROI on Jupiter. Tried every combination of speed and "initial time" and everything else I could think of. I have no real idea what is going on with it to try to fix something because the interface is too cryptic to understand what is wrong. Is it a balance issue? Is it the DEC backlash? Who knows.

I tried to record a video but it wasn't recording the dialog box. I'll try to dig up a log file.

[Borodog]

GuidingLog_2022-11-01T19_19_46-17788.log

(147.68 KiB) Downloaded 49 times

GuidingLog_2022-11-01T20_14_32-15552.log

(7.81 KiB) Downloaded 44 times

[Borodog]

By the way, *I* can see the target drifting on the screen and manually nudge it with the ASCOM controls on the screen just fine. There is nothing wrong with the mount. Very frustrating.

[admin]

Hi,

I can see two main causes for the failures that you are seeing here.

1) The movement is too fast, meaning there are not enough data points being measured before the required amount of movement has been completed. You should be able to improve this by reducing the 'Initial Step Size' setting from 1 second down to 0.2 or 0.3 or something. On my side I will look into allowing the scan to continue moving further to accumulate more points if necessary

2) The very different scales that are coming out for movements in opposite directions - this shows up with all the 'scalings of ... and ... differ too much' messages in the log. Frequently it looks like 1 unit of movement in +ve RA gives about twice the actual offset as one unit in -ve RA. This is possibly backlash related - perhaps the total amount of movement required to complete the calibration is just insufficient to clear the backlash in the mount movement, so the adjustments are always operating in or around the backlash zone. Diffiucult to be sure though.

let me have a dig to see if there is anything I can improve and come back on this.

cheers,

Robin

[Borodog]

I tried taking the initial step size down to 0.5" but not lower. I'll try 0.2" or 0.3" next time.

As I've said before, the CGEM has a gear box between the DEC motor and worm gear. The backlash is just unavoidably large. There is nothing I can do about it. PHD2 measures the DEC backlash at various values, but always greater than 7-9000 ms. But it can guide.

Because you are not trying to keep the image to < 1" on both axes, it would be really, really, really, really, really wonderful if there was an option to just have SharpCap guide in only one direction in DEC. During the Monitor phase, it should just watch which way the planet is drifting. Then during the calibration phase, just let it keep going until it clears the backlash and can figure the axes out. And then only guide in the appropriate direction in DEC, so there is never a reason to reverse direction and take up the backlash.

[Borodog]

Last night I did 2 things different. First, I made a mechanical adjustment by moving one of my 11 lb counterweights farther out to keep the mount more east heavy and keep the RA worm gear better engaged. Not sure it made any difference but it shouldn’t hurt.

Second, I dropped the initial step size setting down to 0.3”. Worked like a charm. Thank you very much.

I still think this part of the interface is very cryptic if you have errors you are trying to debug. I had tried changing all of the settings including that one, but I only varied it by a factor of 2 greater and lower and so missing the magic number. If the error text were in plain english instead of jargon and had helpful suggestions that might go a long way.