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Is there a FWHM filter used in Starlight Live Stacking?


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When using Starlight Live to stack with my Lodestar X2,  I've noticed that on the Stacking Tab Starlight Live reports FWHM under status for each sub, along with #matched stars and other info. I wonder if there is a way for the user to specify a maximum FWHM in pixels for a sub to be included in the stack, so that a sub with trailing due to wind or poor tracking would be automatically rejected by the stacking algorithm? I know that a user can undo the last sub if one notices a poor sub. However, sometimes I'm not very attentive:happy11:, and I'll miss a poor sub. Would be great if the software could automatically do the rejection by checking for a maximum FWHM. Perhaps this is already built in, but I'm not aware of it. Thanks for any insights.

Edited by emustafa
change minimum to maximum
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Hi Errol,

Great idea - I will see what I can do for V3.2. I am thinking a control on the stacking tab which when set to 0 turns this feature off, set to a value grater than 0 then if the mean FWHM of the new exposure is greater than the specified value then the exposure is dropped. The suggested usage is to set to 0, take a few exposures to get a feel for the FWHM value for a given night and then set the limit.

Paul

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Hi Paul,

Thanks again for implementing the FWHM filter in v3.2.  I have been using it for a while now and it is a big help with automating the process of rejecting low quality subs. One interesting observation I have made  is that  occasionally a sub in which the stars are definitely oval shaped will pass the filter and not be rejected, even if I tighten up my FWHM threshold. This tends to happen when I hit a bad part of my worm cycle. If I tighten the threshold even further to eliminate such subs, I end up rejecting a lot of  perfectly good subs as well. I'm sure the filter is working as advertised, but I wonder if there is some way to refine the FWHM filter so that it additionally looks at the ratio of FWHM in the directions of an  elliptical/oval shaped star's principal axes  for  perhaps the two or three brightest stars in the FOV, and then rejects a sub for stacking if the major/minor axis FWHM ratio is significantly above 1.0? For perfectly round stars, the major/minor axis FWHM ratio would be 1.0. I have no idea how hard it would be to implement something like this, but I suspect that the computational overhead could be managed if it was only applied to the 2 or 3 brightest stars. Just a thought for your consideration  :happy11:.

 

 

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Hi Errol,

Good suggestion - I will have a play and see what I can come up with. I suspect this will be something along the lines of an extra stat in the stacking status - a metric of mean roundness and then another control to allow you to set a limiting range. I don't think this will be too hard to implement...

Hi Dom,

Mean FWHM is a stat output from the live stacking for each new exposure on the measured average FWHM of the stars in the image (well stars that have passed the star fitting algorithm). FWHM is a measurement of the size of the stars in the exposure. It is dependent on atmospheric conditions, the stars themselves and your equipment. Generally, for a given target, it should roughly be the same between exposures. If its getting larger as a sequence of images is being taken then either seeing is deteriorating, focus is slipping or movement in the mount has occurred during the exposure.

Max FWHM sets a limit to the mean FWHM for a given exposure. Set to 0 then this feature is off. Set to a value, if the exposure mean FWHM is above your value then the exposure is rejected and not stacked. The main idea here was to catch exposures containing mount hiccups but as per Errol's post some extra work is needed on this front.

FWHM is also useful on the focus / alignment / framing mode and can be used to focus the scope. Basically, point at a star and align with the centre of the reticle on SL and adjust focus until the measured value is at a minimum. The value will be dynamic but you get a feel for when you are at the optimum minimum position after a bit of use.

 

Hope that helps!

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Hi Paul,

Thanks very much for considering this idea! One complication that may arise is that if the image suffers from significant aberrations, e.g. due to aggressive focal reduction, then the star shapes in the outer parts of the field of view will exhibit some eccentricity (comet or egg shapes) and would probably skew the overall value of your metric of mean roundness across the field. Maybe just concentrating on stars in the central part of the FOV would get around this potential issue.

Anyway, I think it's fantastic that you are open to looking into these sorts of suggestions. Makes me very excited to be a user of the software.

 

 

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I have found the new FWHM setting to working great. Handles keeping wind gusts, poor tracking, etc out of the stacking without me having to keep and eagle eye on the monitor. 

Now if Paul could add a setting to exclude exposures with a satellite/plane streaks!  :happy7: The down side of not having to keep a close watch on every single exposure is I have miss these now and have to start the stack over,

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