Default Sigma for Plate Solve

[GaryS]

Hi:
Last night I finally experienced an inaccurate slew - a situation that I'd prepared for by setting up all the plate solving components, including Astrotortilla and all the correct indices. I said to myself: "ahaaa, I finally get to use Plate Solve."

Well, Plate Solve failed repeatedly and each time I was told to adjust Sigma. The only problem is that I have no idea what the starting point, or default setting, for Sigma is so how high or low do I go from that unknown starting point? I checked the Settings Menu and found that "8" was shown in the 'Star Detection Noise Threshold' box. I don't know if that is a default setting or just a random number I may have left there when I was exploring and experimenting with the Sharpcap Settings menu.

So, my question is three-fold: 1) what is the default setting for Sigma and 2) when plate solve fails and I'm suggested to increase or decrease sigma, what are the usual ranges of adjustment up or down from a default (if there is one) and 3) how do I have any idea what sigma should be if there is not a default or any guidelines for adjusting Sigma?

I'm observing (EAA) on fast newtonians and using ZWO 178, 290 and 294 cameras under bortle 8 skies - if thats of relevance....

Cheers and thank you for any help you can offer!
Gary

[admin]

Hi,

The default value for the Sigma parameter is 50. There are some fairly helpful guidelines for how to adjust this in the documentation – see this page space https://docs.sharpcap.co.uk/3.2/#Plate%20Solving%20Tab .

If you watch the notification bar while the plate solver is happening you should spot the number of stars that it detects – something like this will appear

Solving : simplexy: found 128 sources.

If you happen to miss that because it goes past too quickly then you will also find it in the log. If you have set a low value Sigma and still are not getting enough stars then you should try increasing the exposure time and/or gain value to make the stars brighter.

Cheers, Robin

[GaryS]

Oh, thanks very much indeed Robin.
If 50 is the default, how do we know how much to lower or increase it to find more or fewer stars? Is there a range of star quantity to be within or does it boil down to trial and error? Perhaps this is part of the ‘art’ of SC rather than the ‘science’. Thank you for responding.
Gary

[admin]

Hi Gary,


As per the docs, less than 20 stars is almost certainly too few, more than 200 is almost certainly too many.


Cheers, Robin

[GaryS]

Thank you Sir!

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