StarTools

I'm wondering if it would be practical to extract SII from the output of two other filters?

I had been using a stock D5300 with an Optolong CLS-CCD filter and refractor for nebula work, and am in the midst of making some changes that I hope will help me do better with my less-than-ideal skies. Currently using APP + Startools

The D5300 has just been modded, and I am waiting (and waiting...) on delivery of an Optolong L-Extreme filter. This should give me fairly contrasty OIII + Ha.

But what about SII?

It occurs to me that my CLS-CCD filter is capturing SII along with Ha and miscellaneous reds. And I can easily split red channels out with APP (and, I assume, Compose). What if I log some time-on-target with each filter? Let's assume for now that I'll be able to get the images all registered against each other.

Would it be practical to use Layers to somehow subtract the L-Extreme's red Ha channel from the CLS-CCD's wider red, leaving a red channel that was mostly SII? That would give me a narrow OIII, narrow Ha, and something mostly SII, wouldn't it?

At 50,000ft, this seems like a really interesting idea, but I'm having trouble wrapping my head around the practicalities of workflow timing and the whole-image effect of that subtraction. Seems like the nebulosity would likely come out as desired, but what about the stars?

Thoughts?

Thanks!

Indeed, subtracting the Ha signal from the red channel of the CLS filtered data, might yield a somewhat useful residual "other-reddish-wavelengths" dataset (I wouldn't say it is "mostly" SII however; a CLS filter passes much more than just Ha and SII).

Combined with StarTools' ability to process luminance and color separately (yet simultaneously), you can experiment with this quite easily.

Just load your CLS data as the red channel in the Compose module, keep the result (should yield a mono representation of the CLS' red channel). No need to turn on Tracking.

In the Layer module, open the Ha data in the foreground (with the extracted red channel of the CLS data still in the background), and subtract it.

To avoid "holes" in the stars, do this;
"Copy" the result into the buffer. "Swap" to swap foreground and background (now you should have the Ha in the background, CLS red in the foreground)

Turn off Layer Mode Subtract (e.g. just set it to Blend).
"Paste" the buffer into the background,
Now set Brightness Mask Mode to "Where foreground is dark, use background". Set Brightness Mask Power to something high.

Now you should have an new dataset that (roughly) contains everything but green, blue and Ha. (or a poor man's S-II )
Save it.

Launch the Compose module once more. Open your new dataset as red, Ha as green, O-III as blue. Set the exposure time for red to 0, as it doesn't make much sense any more - we just want it for the coloring; detail will still be wholly provided by Ha + O-III.

Process as normal and see what happens in the Color module!

Let me know how you go - it'd be a pretty cool experiment.

Thanks, Ivo

When the new filter (and clear skies!) arrive, I'll try this out!

- Bob