How to use HSL settings? How to neutralize green in OSC image.

Mabula goes into the HSL here more, maybe that already helps? https://www.astropixelprocessor.com/community/main-forum/processing-with-narrowband-only-to-get-a-color-image/#post-21291

That link is not really helping me. It seem this is a preset for a range of the color? I don't understand what this means "background + 2*noise - 25% range" or how it works, or how to use it. Carol

Hi,

I hope this is the type of explanation you are looking for.

The HSL selective colour tool provides a means of adjusting the colour values in an image file by:

a) selecting a colour (or the All or a Grey scale options) via the Color selector

b) selecting pixel brightness range  using the Range selector tool.

Mabula has provided several preset options which I assume he feels will be most useful for the average user, but a user has the option to refine the selected values using the sliders below the preset selection.

The presets are arranged by brightness levels, low to high. Thus if you are wanting to modify the very darkest elements of your image then select one of the options:    '0 - background'  thru to   '0 - background  + 3 * noise'.  Read this latter preset expression as 'pixels with a brightness value in the range 0 (ie black) to the background brightness value plus 3 times the noise level value'.

The value of the background level of your subject image is given below the range selector tool.  In the image I am currently looking at the (sky) background is 0.00257 on a scale 0 - 1. so very dark areas.  I image in monochrome and the noise values for each channel (shown in the FITs headers) are typically less than 0.00005, so very small values but as most images are rather dark, so potentially selecting a large number of image pixels.

Next up are the presets for selecting mid-range pixel values, ranging from    'background - 25% of range'   to 'background + 3 * noise - 75% of range'.   These ranges would typically select the pixels values of, for example, emission or reflection nebulae, or outer regions regions but probably not the very bright cores.

Next, if you are wanting to modify the values of stars and possibly bright galaxy cores, you would select from one of the high value ranges, e.g: '25% of range - 100% of range'   to   '75% of range - 100% of range'.

Lastly, if you say you are wanting to remove an overall colour cast in an image there is the preset:  '0% of range - 100% of range',  which would select every pixel level having an element of the selected colour. In one of his videos focused on the HSL tool, Mabula gives an example of the use of the 0% - 100% preset for removing Green colour from an image on the basis that Green is only rarely found in astro images.

I suggest the best way to become more familiar with the applicability of the presets is to load an image into the HSL tool then experiment by selecting from the various preset options.  By pressing the button 'show colour selection' the image viewer will change to displaying a mask image reflecting the relevant selections.  The  areas of the image covered by the white areas of the mask will be modified by subsquent use of the calculate button while the black pixels will not.

Mestutters, thanks you for this explanation. This is what I was looking for.

Thank you @mestutters! Excellent overview.