The Wipe module detects, models and removes any source of unwanted light bias.
The Wipe module's main purpose is to eliminate unwanted light in an image and establish a neutral background.
Unwanted light may come in the form of gradients, colour cast or light pollution.
Other issues that the Wipe module may ameliorate are vignetting and amp glow;
Strictly speaking, Vignetting is not an additive light source and the correct course of action is to apply flat frames during sub frame calibration. That said, reasonable results can be achieved using Wipe's "vignetting" preset.
Note that while part of Wipe's job description is 'establishing a neutral background', this doesn't necessarily the background is colourless. It simply means that the colour channels are now bias-less, however colour calibration of the channels by the Color module is still required.
It is of the utmost importance that Wipe is given the best artefact-free, linear data you can muster.
Because Wipe tries to find the true (darkest) background level, any pixel reading that is mistakenly darker than the true background in your image (for example due to dead pixels on the CCD, or a dust speck on the sensor) will cause Wipe to acquire wrong readings for the background. When this happens, Wipe can be seen to "back off" around the area where the anomalous data was detected, resulting in localised patches where gradient (or light pollution) remnants remain. These can often look like halos. Often dark anomalous data can be found at the very centre of such a halo or remnant.
The reason Wipe backs off is that Wipe (as is the case with most modules in StarTools) refuses to clip your data. Instead Wipe allocates the dynamic range that the dark anomaly needs to display its 'features'. Of course, we don't care about the 'features' of an anomaly and would be happy for Wipe to clip the anomaly if it means the rest of the image will look correct.
Fortunately, there are various ways to help Wipe avoid anomalous data;
Bright anomalies (such as satellite trails or hot pixels) do not affect Wipe.
Once any dark anomalies in the data have successfully been dealt with, operating the Wipe module is fairly straightforward.
By default, a setting is selected that performs well in the presence of moderate gradients, colour casts or bias levels.
If the gradient is found to undulate stronger, a higher 'Aggressiveness' setting may be appropriate. When using a higher 'Aggressiveness', be mindful of Wipe not 'wiping' away any medium to larger scale nebulosity. To Wipe, larger scale nebulosity and a strong undulating gradients can look like the same thing!
If you're worried about Wipe removing any larger scale nebulosity, you can protect this nebulosity by masking it out, so that Wipe doesn't sample it.
Because Wipe's impact on the dynamic range in the image is typically very, very high, a (new) stretch of the data is almost always appropriate so that the freed up dynamic range that used to be occupied by the gradients and/or light pollution can now be put to good use to show detail. Therefore, a global re-stretch using the AutoDev or Develop module is almost always required.
Having to 'Keep' the result and switching to 'AutoDev' or 'Develop', just to see the result, is a bit tedious. Therefore, switching on a courtesy 'Temporary AutoDev' operation allows you to see the result.
A number of controls for advanced use and special cases are available.
The 'Corner aggressiveness' lets the user specify a different aggressiveness value for the corners of the image. This can be useful if gradients become stronger in just the corners and can help ameliorate vignetting. The 'Drop off point' determines how far from the center of the image the 'Corner aggressiveness' starts taking over from the main 'Aggressiveness' parameter. At 100% for the 'Drop off point', no effect is visible (e.g. only the main 'Aggressiveness' parameter is used) since the' Corner aggressiveness' only comes into effect 100% of the way between the center of the image and the corners.
The 'Precision' parameter can help when dealing with rapidly changing (e.g. undulating) gradients combined with high 'Aggressiveness' values.
The 'Mode' parameter allows for the selection of what aspect of the image should be corrected by Wipe;
Any other tools on the market today simply approximate the visual likeness of such star spikes and 'paint' them on.
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