Additional peaks on HaRGB stretch histogram

[tomato]

I have recently added some 510 mins of Ha data to an existing RGB image by splitting the RGB image into it's respective channels then combining this with the Ha channel using APP's combine channel tool, option HaRGB 1.

After separating the stars out in PI with StarXterminator and stretching it with the Histogram Transformation tool in PI, in addition to usual RGB peaks I am seeing 3 RGB 'spikes' to the left of the main peak. These are not there in the RGB image alone so I am assuming they have appeared as a result of adding the Ha channel in. I have seen something similar when the image has edge artefacts, but these have all been cropped.

Any idea what their origin could be and should I clip them them out?

[sharkmelley]

The spikes on the left of the histogram are where data values have been clipped at some stage in your processing.  So you end up with many pixels having the identical (clipped) value.

Mark

[tomato]

Thanks for the insight. The image has been cropped and a gradient tool applied but no stretching. Would gradient removal clip the data?

[sharkmelley]

11 hours ago, tomato said:

Would gradient removal clip the data?

I would not expect gradient removal to clip the data.  But it's easy to test for yourself if gradient removal is causing the problem!

[tomato]

Yes thanks, I’ll do some investigation. Something has changed as historically these peaks were never there in the histogram but they show up every time now.

[tomato]

Some simple stretching at various points in the initial workflow would indicate that the spikes are introduced by StarXterminator. However, they always appear to the left of the distribution curve and can be clipped without impacting on the main curve. Their removal doesn’t appear to have any impact visually on the image.

[ollypenrice]

'APP's combine channel tool, option HaRGB 1.'

I'm never keen on handing over a key stage of processing to a routine I don't understand. Does this APP routine give you any kind of preview? Is it adjustable? Or is it just some kind of generic guess? (Genuine question to which I don't know the answer.)

When I add Ha to red in Photoshop's Blend Mode Lighten, I can see in real time what it is doing to the red. Is it lightening the background? If so, I can bring in its black point or lower its curve low down. Is it doing anything at all? If not I can stretch it further with any custom curve, in situ, in real time, with a real time preview.

Tell me why I should ditch Photoshop! 

Olly