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

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question about sidereal tracking w Ekos alignment

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Hi all,

I am wondering if the sidereal tracking in INDI is simply commanding a constant rate in the RA axis or if it is using alignment data (eg from Ekos) to command both motors to correct for mis-alignment at all times. The latter would be useful when taking a long exposure without a guiding camera.

Thank you.
8 years 6 months ago #5038

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It is just constant sidereal rate. The alignment data is used to improve pointing, not guiding.
The following user(s) said Thank You: Bernard GODARD
8 years 6 months ago #5039

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Thank you for your quick reply knro.

It would seem to be a useful feature though. Don't you think?
I wonder if I could achieve this guiding by sending a slew command every few seconds to my target coordinates. The only problem I can think of is that transitions from slew to and from sidereal tracking do not seem very smooth.

Alternatively a virtual guiding camera could also be created using as output the difference between the actual stepper position and the ones derived from the alignment model for the desired target at all times.
8 years 6 months ago #5048

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The alignment data corrects the mount's pointing, and guiding corrects the the mount's tracking. In principle, you can use the guide camera to capture images and solve them as they arrive and then figure out RA/DEC diff and issue guiding pulses accordingly; but you'd need a super computer to make that usable.
8 years 6 months ago #5050

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Are you talking about guiding relative to zero motion which would be more demanding? I am talking about guiding relative to sidereal tracking (or whatever best suits the target) and saying that a virtual guiding camera that just use the alignment data to compute correction to the mount tracking could help in case we don't have a guiding camera. This would be similar to a complex tracking model that uses the alignment data.

In summary:
- telescope is tracking sidereal
- software retrieves stepper motors state at time t (in theory this could be derived/predicted from tracking rate and guiding commands?)
- software computes stepper motors state at time t for given target and a given alignment
- software takes the difference to compute guiding commands

There might be a time delay that needs to be calibrated in the retrieval of the stepper motor state.
8 years 6 months ago #5056

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The problem with this approach, it assumes you have perfect gears, and everything is predictable. The reality is, guiding is how you overcome gear imperfections in the mount, it's NOT a method of overcoming alignment errors. It will give the appearance of correcting for 'very small' alignment errors, but if you guide perfectly using a misaligned mount, you get rotation of the field, and the whole field will rotate around your guide point over time.

With a very short focal length, and relatively short exposures, this can be masked, and give the appearance of a good shot, but, it'll only look good if your field rotation is less than half a star width over your exposure time. Using something like an 600mm focal length, and 5 minute exposures, you will get away with somewhat poor alignment and still produce a result that looks good. Change the circumstances a bit, use a telescope with 2000mm or longer focal length, toss in some narrow band filters and use 20 minute exposures to make up for the filter bandpass, then it wont work anymore. In the worst case scenario, using an off axis guider on a very long focal length system, the entire field will always be rotating around the guide star, which is by definition outside of your field in that case.

To get good shots unguided, you have to take a completely different approach. Start by getting your alignment as close to perfect as possible, then you have to develop corrections for the periodic error inherent in the mount, and apply those corrections blindly thoughout the exposure. It's a system that works really good in theory, and will generate perfect shots using a simulator with large periodic error. Then you go out into the real world and try it, only to discover, there are more things causing errors along the way. Flex in the mounting system, occaisional wind gusts. a little dust in the gears that causes PE to be slightly different now as compared to when it was measured, all sorts of things that make this route a complete and utter exercise in futility, with one exception. That exception is, using a system where the pixel size (measured in arcseconds) is large enough, small errors are lost in the system resolution.
8 years 6 months ago #5080

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Gerryr,

I know the proposed guiding from alignment data is far from perfect but I thought it would be better than nothing.

Since you are mentioning PE, do you know if there is any tool in INDI to record and replay PEC? I have done this in the past with my Synscan hand controller, but the idea is to replace Synscan by INDI-Eqmod.

Thank you,
8 years 6 months ago #5086

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Recording and replaying PEC is a great idea, but, in reality for a mount like a synscan (we have 3 of them), there are issues you need to consider.

The periodic error comes from many sources. First source is the gear on the motor, which meshes with another gear, with finally meshes with the gears on the RA axis. The result is complex curve that has the sum of 3 different periods overlapped with each other, one has a period of a few seconds, one a few minutes, and one 24 hours. To get it 'right' to the point of knowing how it will behave over a full evening of tracking, you really have to spend a whole night measuring. Then, release the clutch, make an adjustment on the telescope, and re-tighten the clutch, things are no longer in sync. Now you have to start over measuring things.

The reality is, in my opinion, trying to correct for PE on these mounts without any feedback at all, is indeed worse than nothing. You are essentially steering blind making corrections for errors that you may or may not have correctly identified, and will often end up just making things worse, and even if not worse, not nearly as good as you want them to be.

File Attachment:


I have done plots for the EQ6, and here is the end result. Blue line is unguided data where I did a full plate solution on every image from the guide camera to get a fairly good measurement on the periodic error. I was experimenting with guiding algorithms at the time, and the orange line is the result after guide corrections were applied, and I dont remember offhand which methodology this graph was for. The important detail to take away from that graph, periodic error is periodic, but, it's NOT smooth and it's not a nice sine wave like a lot of folks seem to think. In some parts of the curve it's a relatively slow movement, and in others it's a much faster movement.
The following user(s) said Thank You: Anders Lange
8 years 6 months ago #5089

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