Stretching for star removal.

[ollypenrice]

Here are two different stretches of the same Rosette linear data in 3nm Ha.

The lower one was stretched conventionally in Levels by moving the mid point slider to the left in a series of iterations and adjusting the black point each time.

The upper one was stretched only twice, using a very aggressive stretch followed by a slightly less aggressive one as shown here:

I don't know which rendition you prefer but anyone wanting to enhance an LRGB or OSC image by blending this Ha into red would do well to use the top one.  Now the big drawback of the aggressive stretch is that it gives large, bright stars. In this case it isn't much of an issue because the 3nm Astrodon filter gives tiny stars at capture, but you'd be likely to struggle if stretching luminance data this way. For this reason I tended to use the aggressive stretch only on narrowband data though I would often have liked it in luminance for its feisty contrasts.

Enter StarXterminator (the first star removal software to work convincingly and often for me.) Since you can remove and replace the stars with softly stretched ones, the case against the hard stretch breaks down. You can replace the stars with any stretch you like, including a log stretch if you prefer. This is the hard stretch re-starred with tiny stars but you have total control to do as you please.

Star removal is a game changer and invites us to re-think our workflow right from the start.

Processing here has been pushed to extremes to illustrate my points. These are not images I'd publish as they stand.

Olly