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Using Wipe with Flats

Posted: Mon Sep 16, 2019 4:33 pm
by saublestarman
Sorry if this question has already been answered elsewhere...

I always use flats when I calibrate and stack my light frames. Are there any suggestions for how to adjust the wipe module when using flats? Specifically, I wonder if the default Aggressiveness setting of 75% should be significantly reduced to take advantage of the Flats and prevent the wipe module from reducing any nebulousity. I guess I am assuming that the 75% setting has been chosen as a best practise for users who don't use flats.


Re: Using Wipe with Flats

Posted: Tue Sep 17, 2019 1:21 am
by admin
Hi Peter,

Gradients are not necessarily caused by uneven illumination of the imaging plane. E.g. gradients may constitute a real recorded signal in your light frames.
Skyglow (whether artificial or natural) can (and usually will) cause this uneven illumination that flats do not correct for.

If you are absolutely certain your data is free of Skyglow, you can use the Narrowband preset. It is so name, because Skyglow does not tend to be such a problem with narrowband data. You can just try the two presets (gradient vs Narrowband) and see what works best for your data and whether you are noticing any difference.

In general though, StarTools' Wipe module tends to be more robust than the more naive modelling implementations found in PI (ABE/DBE), APP etc.
It uses (by default) the entire image as an input, rather than user-specified samples; the gradient model construction and sampling is left to a more objective algorithm, rather than the user.
As such it is much less susceptible to obscuring small-to-medium scale detail by leaving signal mired in skyglow, and you cannot successfully (globally) stretch or recover what doesn't stand out;
NewLRGBComposite_simple_ST15_RC6.jpg (97.5 KiB) Viewed 1290 times
(example of naive implementation in another application vs ST, only global stretching used, no local processing)
As you can see, robust gradient/bias removal is much more important than you might think when it comes to bringing out the fainter detail.

The flipside, however, is that Wipe is very sensitive to dark anomalies (e.g. dust donuts, smudges). Especially with properly flats-calibrated data, this should not be an issue however.

Hope this helps!

Re: Using Wipe with Flats

Posted: Tue Sep 17, 2019 2:01 am
by saublestarman
Thanks for the detailed reply. I will probably stay with the default wipe settings but I will experiment with the narrowband preset to see it's effect.

Startools is a great program