Wipe: Light Pollution, Vignetting and Gradient Removal
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.
- Gradients are usually prevalent as gradual increases (or decreases) of background light levels from one corner of the image to another. Sources may include the or a nearby street light.
- Colour casts are a tint of a particular colour which, contrary to a gradient, affects the whole image evenly.
- Light pollution is the presence of a persistent haze of (often) coloured light, caused by urban street lighting.
Other issues that the Wipe module may ameliorate are vignetting and amp glow;
- Vignetting manifests itself as the gradual darkening of the image towards the corners and may be caused by a number of things.
- Amp glow is caused by circuitry heating up in close proximity to the CCD, causing localised heightened thermal noise (typically at the edges). On some older DSLRs and Compact Digital Cameras, amp glow often manifests itself as a patch of purple fog near the edge of the image.
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.
Preparing data for the Wipe module
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;
- A 'Dark anomaly filter' parameter can be set to filter out smaller dark anomalies, such as dead pixels or small clusters of dead pixels, before passing on the image to Wipe for analysis.
- Larger dark anomalies (such as dust specks on the sensor) can be excluded from analysis by, simply by creating a mask that excludes that particular area (for example by "drawing" a "gap" in the mask using the Lassoo tool in the Mask editor).
- Stacking artefacts can be cropped using the Crop module.
Bright anomalies (such as satellite trails or hot pixels) do not affect Wipe.
Operating the Wipe module
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;
- Correct color and brightness; removes both colour and brightness bias across the image.
- Correct color only; removies color casts but does not impact brightness bias.
- Correct brightness only; retains color but corrects brightness bias. This mode is useful when processing narrowband data, or data that was not acquired on earth (for example Hubble Space Telescope data).