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

[Startrek]

Matt,
The main issue with the majority of light pollution filters like the L Pro used on broadband targets (reflection nebula , some planetary nebula and galaxies ) is that not only do they block certain elements of light pollution , they also block good signal from the object.
They can also affect Star colour amongst other things.
There have been many posts and discussions on CN etc… about light pollution filters for astrophotography , especially on broadband targets, and the general consensus is that your going to get better results just using a UV/ IR cut filter.Obviously some broadband targets also benefit from capturing Ha emission lines as well ( LRGBHa eg: M20 )
Depending on what sky conditions your imaging under total exposure time required can vary from target to target but at the end of the day , signal is king, the more the better.
Under my B8 City Suburban skies I shoot 30 sec subs max for my LRGB Mono filters with broadband targets and try to achieve an integration time of at least 12 to 15 hours minimum ( if the weather is kind , most of the time it’s not ) when I’m too lazy I just use my OSC 2600MC which has an internal UV/ IR cut window but never end up getting as cleaner image as my Mono rig.
Please get further advice and experiences from other folk as well
The L Pro is a great filter but folk have also captured and processed great broadband images without it under light polluted skies.

Clear Skies
Martin

[Matt S]

Martin
Thanks for this. I’ve ordered an IR/UV filter for my 294MC Pro and will try it out on a broadband target when weather and Moon permit, along with your module tips.
Clear skies,
Matt

[Startrek]

Matt,
Another important point to make is that using a UV/IR filter ( in essence a Luminance filter ) will require experimentation with both exposure length and gain setting as now your “opening up the flood gates” so as to speak in relation to signal (photon count) and unfortunately noise as well.
I’m not familiar with the 294MC but I’m sure that Cloudy Nights and other forums can give you guidance with respect to sub exposure lengths on broadband targets using your rig and just a UV/IR cut filter. Of course this is dependent also upon your Skyglow ( Bortle scale ) as well.
The last thing you want to do is overexpose your stars using longish subs which will just bleach them out. You could always shoot your stars separately with 15 to 20 sec subs and layer them in post processing later.
I’m sure you will find a sweet spot with using the UV/IR cut filter.
All part of the fun and challenge

Clear Skies
Martin

[Matt S]

Hi Martin

Thanks again. I tend to keep gain close to unity, at around 125, and have settled on 2min exposures as a default. My laptop isn’t the most powerful and I am occasionally blessed by air traffic causing me to lose frames. That said, I do acknowledge that I probably sound increase exposure when using a filter.

If I can pick your brains a touch more…? Barring the British monsoon, I’d like to image again this winter. The Cygnus Wall looks an attractive and suitable target for my focal length. I could use my L-eNhance filter, but I’ll end up with a red rendering. Would you recommend the IR/UV option instead and place my faith in Wipe’s wizardry?

Grateful for your musings!

Clear skies
Matt

[Startrek]

Hi Matt,
I assume you image from the suburbs and not a remote rural location ( say Bortle 6 to Bortle 8 as you mentioned about air traffic ) ??
If so , any emission nebula like the Cygnus Wall in a rich H II region is a bonafide Narrowband target. Imaging emission nebula in Broadband under light polluted skies is damn hard work trying to mitigate all the noise that comes with it and in some cases like B8 to B9 skies is generally a waist of time , that’s why narrowband filters were invented.Some folk opt do it as they don’t like Narrowband, but you need a load of hours depending on the brightness of the target ( +40 hours ) and short subs ( 30 sec )
Shooting this nebula with just a UV/ IR cut filter won’t pick up all the fine Ha detail and dust that surrounds this region.
I’d recommend to use your L enhance filter and maybe push 180 sec subs or even 300 sec subs if your in a B5 or B6 zone.
If you’re in a B7 or B8 zone leave your subs at 120 sec to control the noise floor.
I’d capture at least a minimum of 12 hours on this target , the more the better.
You could also capture RGB stars with your UV/ IR cut filter using 20 sec subs ( say 40 mins to an hours worth ) and combine them later( I have a procedure)
To create a Bi colour Narrowband ( Hubble palette type image ) with your OSC L enhance data you can you the Compose module. In this forum under User Notes , Guy has put together many ways to combine your Mono and or OSC data using Compose to create various colour channel blends.
Below is one I have used with some success years ago using my OSC 2600 MC and L Extreme filter to create a Bi Colour Narrowband palette. It’s definitely not as good as other Software programs like Pixinsight or APP or Photoshop but it’s simple to use. Unfortunately you don’t get the golden tanned hues or the bright blue hues of a stereotypical Hubble Palette with your OSC NB filters as the signal is restricted by the Bayer Matrix in your OSC camera eg: RGGB , 25:(25 25):25 . However with my new Mono 2600MM and dedicated SHO filters I can create a stereotypical Hubble palette using Startools Compose.

Here’s how to use Compose with your OSC L enhance data set.
***Compose OSC Bi Colour Images for Emission and some Planetary Nebula using L enhance or L Extreme filter

Open Startools and use “Compose” to load data set

Select Luminance Colour - L+Synthetic L from R (2xG ) B, R ( GB) (GB) ( Bi Colour from OSC DSLR )

Load your finished stacked OSC data set into each channel ….
R / S11
Green / Ha
Blue /Oiii

Leave Luminance file None
Color Channel Interpolation On
Channel sliders are set to default or 1hr 3600sec ( no need to adjust )

When loaded press Keep , then choose Linear

Start your normal workflow in Synthetic Luminance or Mono until you hit the Colour module

Colour Module
Style Scientific ( Colour Constancy )
Saturation Amount 180% to 250%
Bright Saturation + 2.0 to taste
Dark Saturation + 2.0 to taste
LRGB Emulation Try both RGB Ratio CIELab Luminance Retention and Straight CIELab Luminance Retention
Bias Slider Mode - Sliders Reduce Color Bias
Red Bias 2.0 to 3.0
Green Bias 3.00 to 4.00
Blue Bias 1.20 to 1.60

The above settings are only a guide. Green bias must be reduced and Blue bias increased to obtain a good Bi Color image. Adjust as necessary.

The above should result in a good Bi Colour image when using one of the first 4 four SHO Options in the “Matrix” at the bottom right

Generally for HOO , HHO, OOH options in the Matrix , reduce the Red bias, increase the Green bias and leave the Blue bias similar to SHO options.

For Interpolate and RGB options in the Matrix , again adjust the Red bias and Green bias to taste. Normally the Blue bias can remain between 1.20 to 1.60 ***

Hopefully you can produce a OSC Bi colour image rather than a “red blob of ink”

Also here’s how to use and combine RGB stars into your Narrowband image ……….,

Add RGB stars to Mono SHO , HOO or OSC Bi Colour image using “ Layer Module”

Process NB image as normal and make sure you keep a record of Crop module parameters ( save image as tiff file )
Process RGB stars image as normal and use same Crop parameters as NB image ( save image as tiff file )

I think Startools keeps a record of Cropping for Layer in Background and Foreground windows

Also use same Gain and Binning for NB image and RGB image.

Load NB image in Background Bg
Load RGB stars image in Foreground Fg
Create Star Mask in Foreground Fg image for all stars you want to include in NB image
Set Layer Mode to Color of Fg
Adjust Blend , Mask Fuzz and Filter Kernel Radius to smoothen the Star Color to taste.
Once satisfied with final image save as jpeg to nominated location.

Note: Ensure you capture at least 30 mins each of R, G and B data for Mono or 40min to an hour of OSC using UV/IR cut filter only. ( generally 20 sec subs for Bortle 8 and 60 sec subs for Bortle 3 location )

Hope all the above is helpful
For more info regarding Startools features refer to the User Notes section from Guy which has excellent information on each module. That’s where I found most of my solutions together with experimentation. Oh and loads of reading too.

NB: I’ll attach one of my OSC Bi colour narrowband images of the Prawn nebula from 3 years ago using my 2600MC and L Extreme filter ( 6” Newt ) and my recent 2600MM Mono SHO narrowband image of IC 4628 Prawn Nebula ( 8” Newt ) both using Compose . You can tell the difference immediately , the OSC is a good bi colour image but the Mono with dedicated filters is the gold standard in Astrophotography. My 4 year OSC journey was fantastic though after using an old DSLR for 3 years before that , but the Mono is another level , so much better.

Clear Skies
Martin

[Matt S]

Martin

Honestly, I don’t know what to say other than to thank you for your incredibly helpful and comprehensive reply. I do use Guy’s notes but it’s useful to have someone else’s take to fill in any gaps. On that note, I would pay good money for Ivo to record a series of tutorials, not least because my laptop is slow which doesn’t lend to much experimentation.

Apparently I live under B4 skies, but my local sources of LP (eg neighbour’s security lights) interfere somewhat. I should be able to push the exposures further though. Currently, I am for 6hrs integration per image but rarely achieve it. More would be greater, but I often go months without imaging anything due to the UK’s glorious weather and work commitments. This explains my choice of an OSC over a mono camera: do what I can, when I can.

I imaged the Cocoon Nebula with my L-eNhance filter not so long ago, but my normal workflow resulted in a rather uninspired red blob: with your notes I’ll reattack using the Compose Bi-Colour approach and see what I learn.

Thank you once again and clear skies!
Matt

[Startrek]

Hi Matt,
Glad to offer assistance as it’s not an easy hobby
Hope you benefit from the above info
With B4 skies at least you don’t have to wrestle with heavy LP, that’s a big plus !!
Good luck with your journey

Clear Skies
Martin