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INDI Library v1.9.8 Released (29 Sep 2022)

Bi-monthly INDI Library released with new drivers and bug fixes.

For those with focus issues

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Hi!

So, I have no problems in moving to those positions and positions beyond them (in the 12000 range) manually.

Magnus

Celestron C11, Skywatcher 100 ED Pro
Losmandy G11
Atik 383L+, ASI294
2 years 6 months ago #51267

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Some focus problems are caused by outliers - hot pixels and extended sources such as galaxies. Would these be more effectively removed by changing the full field HFR determination to use the median rather than the average HFR value. This will cause extreme values to be ignored quite naturally.
2 years 6 months ago #51273

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Replied by Giles on topic For those with focus issues


Or even better discard the 5% smallest and biggest outliers and take the mean of the remaining detections.
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2 years 6 months ago #51275

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With 50+ stars you might as well do a winzorized sigma clipping. Or maybe even linear clipping. For only one 'pixel' the computational overhead is not noticeable.
openSUSE Tumbleweed KStars git INDI git
GPDX+EQMOD, CEM60EC, ASI2600/1600/290mini+EFW+EAF
2 years 6 months ago #51277

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Better? Why? How does an arbitrary change to the data make it better?

In actual fact the current code has an undocumented feature where the full set of HFRs is averaged, the standard deviation is determined, values that are more than two standard deviations from the original mean are removed and a new 'average' is determined.
This is statistically dubious, not only does this reduce the mean but the amount by which it is reduced will depend on the data. Maybe extended objects will be included sometimes but not in others.

The median is at least a valid way to determine a central value.
2 years 6 months ago #51278

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What benefit does this have, other than be more complex?

From a look at the code the HFR array is already sorted so getting the median is easy.
2 years 6 months ago #51279

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Replied by Giles on topic For those with focus issues


If that is what is in the code then I would leave it as it is, in a binomial distribution 2 standard deviations within the mean would equate to 95% of samples, with those outside being discarded, so pretty much it is already discarding outliers then.

I'm not sure whether using an actual median value would be better in all cases, perhaps it would be nice to have the option to configure it either way.
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2 years 6 months ago #51280

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Replied by Eric on topic Re:For those with focus issues

I wonder if the HFR of stars on a field is always a Poisson distribution. There are probably not enough of them in a focus frame to determine that and use that statistical property.

I am trying to introduce PSF in the CCD simulator to better test the different focus algorithms. SExtractor support will be there too relatively soon. We also need to display an interface for the SEP settings.

We also need to better document the features in the tooltips that appear in the UI.

Anyone has unused work days to spare? ;)

-Eric
HEQ5-Pro - Atik 314E - Orion ED80T - DMK21 on Orion 50mm
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2 years 6 months ago #51282

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Not really. This is standard, called sigma clipping. And the winsorizing I mentioned is an improvement on this for a better determination of the standard deviation. Both methods are quite well analyzed and accepted in statistics for removing outliers.
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GPDX+EQMOD, CEM60EC, ASI2600/1600/290mini+EFW+EAF
2 years 6 months ago #51283

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Something I heard of at work was that people were analysing the waste from gold mines to see how much residual gold was left and if it was worth reprocessing it using more modern methods.

Lots of analyses were done, and some came up with negative amounts of gold. You can't have less than nothing so they were set to zero and the results averaged - and there was enough gold to be worth processing. I'm not sure how much waste they got through before they discovered the error of their ways.

It looks as if this is just the same, one side of the distribution is being clipped and that will inevitably affect the average value.
2 years 6 months ago #51285

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Hi!

I am nowadays using full field focusing, and rather long exposure times to get good SNR (4-10 secs, depending on filter). But I do have one issue that I am unsure of how to handle:

There is about 250 tics offset between my V filter and the LRGB filters. Often, I start the night with a run of variables, using the V filter. Then I want to switch to LRGB imaging. THis is where my issue is. I have problems getting Ekos to handle the offset here, and find a good focus 250 tics less than it was on, on the last variable target. I have an initial step size of 300, but that is OUT, and I need to move inwards. If it started in, instead of out, this would not be a problem. As it is, it goes 300 tics OUT, and since it is off focus to begin with (not in the CFZ), I often end in a very bad focus situation.

I've tried using a short focus-job inbetween here, but it often does not help. It sure gets the right filter in place, but not the right focus position. And offset in the filter dialoge is not what solves this.

I use Polynomical, SEP for star identification. Any ideas on how to handle this better?

Magnus

Magnus

Celestron C11, Skywatcher 100 ED Pro
Losmandy G11
Atik 383L+, ASI294
2 years 5 months ago #52205

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Replied by S on topic For those with focus issues

I'm not at a computer, so my memory may be wrong.... But I think you can specify the approximate offsets between filters in the filler focus widow, which I guess should be able to start you v filler in a better position.
2 years 5 months ago #52214

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