Iris Nebula - star colour with L-Pro

[Matt S]

Hello fellow StarToolers

It’s been a while since I last posted: the northern hemisphere summer nights and a lengthy diagnosis of sensor frost have thwarted my ambitions in 2024..,

Attached is a recent image of the Iris Nebula. It’s 4:50 hrs integration, using an auto guided 9.25” SCT reduced to F6.3, with an ASI294MC Pro camera with an L-Pro filter. Lights/darks were 120secs long at 125 gain and at -5degC. Flats and Dark flats were also taken, but at 3sec exposure and 10 gain (to help with the 294’s foibles).

I’m broadly happy with the image, but my expectations are modest! Guiding varied between .5 and 1 arc second, using my EQ6R-Pro on a night with variable high level cloud and a breeze.

I am unhappy however with the star colour, especially the little orange blighters in the centre of the image. Can anyone suggest what might be the cause? I did use a star mask to set the white balance. Given I used an L-Pro filter, should I have loaded the stacked image into StarTools (v1.8) via the Compose module?

Grateful for any tips, insights and advice. Despite my lack of StarTools competence, I am still loving how it works and how quickly it can produce a decent image through a basic workflow.

Clear skies
Matt

[Startrek]

Hi Matt,
I’m from the southern hemisphere but I believe the Iris nebula is a broadband target ( reflection nebula ) and therefore best to image it without a filter or just a luminance or UV/ IR cut filter under minimal moon glare or preferably no moon around. Your L Pro filter is best used on emission nebula and better still use the L Ultimate or Antlia ALPT under heavier LP skies. I’ve used the Antlia ALPT under B8 skies with 2600MC and it’s brilliant on emission nebula.
Also for broadband targets using a OSC camera there’s no need to use Compose. Compose is more suited for Mono with separate filtered data sets or OSC with dualband or Triband filters where your using separate channels to create a narrowband blend , Bi colour or Hubble palette etc…

Some useful tools in Startools to colour correct annoying star colours and star halos are as follows -

Colour Module
You set full mask or specific Star mask
“Highlight Repair”can help to colour correct abnormal star colour but works best with Narrowband data sets
Increase from 6 pixels to see the effect or repair


Shrink Module
Star Colour correction using Shrink Module
Create a good Star Mask and ensure all star cores and halos that require colour correction ( including diffraction spikes ) are sampled or green. Grow mask as required

Open Shrink
Adjust iterations to 0
Change Mode to Tighten
Color Taming increase to between 4 and 10 pixels as required.
Note: Be careful not to increase Color Taming too much as this can cause Star cores to bleach out colour or “white out”. Start off with 2 pixels , then 4 pixels and so on until Star colour is reasonably uniform in the core and halo area.
Halo Extend increase to 1 or 2 pixels
Un glow Kernel leave off
Abnormal Stars core and halo colour ( and diffraction spikes ) should morph or blend into a uniform colour closest to adjoining colour of star core.

Filter Module
Filter module can minimise unsightly and abnormal colour around halos (medium to large stars)
Create a good Star Mask , stars green or sampled
Select “Fringe Killer” and 3x3 Sampling method and adjust mask fuzz accordingly
Click on the stars halo and/or main core area, after each click colour will change closer to star colour.

Looking at your image I’m sure Shrink with a good mask will tone down those orange stars to a more pleasing colour tone. Be careful not to push it too far as the stars will eventually bleach out with no colour. It does take practice with a good mask.

Hope the above is helpful

Clear Skies
Martin

PS: There are probably other ways to deal with those stars like layering etc …. however I’ve had good results with the above tools with both Broadband and Narrowband data sets using my ZWO 2600MC and ZWO 2600MM cameras

[Matt S]

Hi Martin

Thank you for taking the trouble to pen a comprehensive reply: I’m most grateful.

Your comments re the use of the L-Pro are interesting, as it is marketed as being suitable for reflection nebula as well as other broadband targets. That’s why I purchased it, anyhow. Are you suggesting that it’s best for galaxies and simply use an IR/UV cut for reflection nebulae? I’m interested to know more…

I’m most grateful for the tips regarding the StarTools modules, which I will try soonest.

Thanks again and clear skies!
Matt