Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.


Recommended Posts

I've been imaging Thor's Helmet for the last couple of nights in Ha and O3. I notice that RGB data can be used to correct star colour. What exposure is recommended to capture the stars and how many should I take? It looks as if I've one more clear night for the time being.



Share this post

Link to post
Share on other sites

I'd be interested to hear what others do here too.  I have tried it a few times and only take short colour exposures for RGB stars i.e. 150 binned x 2.  However I don't consistently have success in adding them to the NB image.  Sometimes it works and sometimes it doesn't.  I register the images and then select and feather the stars on the RGB and then paste them onto the NB image and then use layers.  This is when it only sometimes works, as I have used blend mode lighten when it has worked.  If I don't use a blend mode then it doesn't work very well as the stars look artificial.  


  • Like 1

Share this post

Link to post
Share on other sites

Here is how would I do it, but do take into account that I've never done it :D

You need a good star mask.

For recommended exposure - go for one that will not saturate stars, even brightest ones. For number of exposures - same thing holds as always when imaging - as much as you can afford within your imaging budget.

Stack everything into corresponding stacks, make sure it's properly aligned to same reference frame.

Do your NB composure / channel mixing and stretch. Do the same with RGB - compose image, stretch and do color balance. Don't worry about star bloat in RGB or if you stretched to far (visible noise next to stars is not important), only thing that you need to worry is star color at this point - make it properly balanced and "rich" (to your liking).

Next steps I can't explain in terms of PS steps, but I'll list them in general - each processing software will have its own way of doing it.

Make a copy of NB image and split it into LAB components. Save L component of split (as mono image), or just keep it open - that is what we are going to need.

Take RGB image and do the same - split into LAB components, but keep A and B, discard L from this one.

Recompose RGB image from LAB by using L component saved from NB image and AB components from your RGB image.

Use star mask to blend this recomposed RGB in with original NB image.

Share this post

Link to post
Share on other sites

Similar to what I do Vlaiv, but I don't know of any Lab component in PS, only lab anything I know is lab colour, but I am sure that si completely different.


Share this post

Link to post
Share on other sites

Nope - tis the same thing... Photoshop can operate in LAB color space and split into Lightness, A (Magenta/Green), B (Blue/Yellow).

Should be under Edit > Convert to Profile > LAB Color (or something like that iirc - word of warning: do this on a finished flattened image, as it'll flatten it automatically as part of the conversion). Usually I do this on a copy of the final component images, to pick up the L layer, and then drop it on top layer back in my regular RGB composite and set layer blending to Luminance.

Edited by Marci
  • Like 1
  • Thanks 1

Share this post

Link to post
Share on other sites

Firstly I would only add star colour to a NB image which was aiming to approximate natural nebular colour in the first place, probably HOO.

Rather than try to 'fill in' the star colour on the HOO you could use Photoshop like this:

1) Process the RGB, the Ha and the OIII and align them to fit each other. In processing the RGB concentrate on the stars, keeping them small and colourful and don't worry about the nebula which will be coming from the NB layers.

2) Add Ha to red in blend mode lighten, making sure your Ha stars are smaller and fainter than your red stars. (Use multiple iterations of a star reduction routine like Noel's actions or whatever on the NB layers.) save three copies of this. Call this Ha to red.

3) Add OIII to green in blend mode lighten in one of the copies and call it OIII to green.

4) Add OIII to blue in blend mode lighten in another copy and call it OIII to blue.

5) Make a three-layer stack like this:

OIII to blue

OIII to green

Ha to red.

Now you can choose opacities for the top two layers which give you the most natural nebular colour. Your RGB star colour should be unaffected.  As you can see, this method does not work by replacing star colour so it gives a more natural look, yet the NB contribution to the nebulosity is the same.




  • Like 1

Share this post

Link to post
Share on other sites

Thanks for all the replies, that will give me something to think about. Unfortunately it looks as if it won't be clear enough tonight, so I can't take the RGB subs! As it's Thor's Helmet, I'm running out of time to image it as it's so low here. My image is HOO so I can try Ollie's method eventually, it looks a lot easier than removing stars in PI.

Share this post

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Recently Browsing   0 members

    No registered users viewing this page.

  • Similar Content

    • By Xiga
      Hi guys
      With the weather here in the UK being so bad recently, and with work commitments and other boring life stuff going on  I haven't done much imaging lately, but I did finally manage to get out and grab some OIII data a couple of weeks ago to go with my old Ha data of NGC 1499 from back in September. The Ha data was from first light for the newly modded D5300 and Baader Ha filter.
      We have an outdoor sports facility only a couple of hundred yards from our house, and annoyingly the ridiculously bright floodlights don't turn off until after 10pm on weekdays, so even though there was no moon to contend with, I had no choice but to wait until the lights were off before I could start shooting the OIII. I thought about shooting more Ha while I waited, but in the end I decided to shoot some short subs without any filter, just to use for RGB stars. I stupidly didn't use my IDAS-D1 filter, and the scope ended up pointing almost directly at a streetlight for all the RGB subs. The result was a stack that had a simply insane gradient running through it, and which made gradient reduction on the stack impossible, as it was changing so much between subs. In the end I had to run gradient reduction on each individual RGB sub before stacking, and then run it again afterwards! Thankfully I only shot 20, so it wasn't too laborious. In the end I had something which, despite having a really ugly background, did at least have useable stars.
      As for the OIII, boy was the signal weak with this one. I'm used to dealing with weak OIII signals on the D5300a, but this one really took the biscuit! Thankfully J-P Metsavainio's tone-mappng technique allows one to get the sledgehammer out for such cases, so I was able to stretch it far enough to get something out of it (even if it doesn't yield any fine structural detail for the OIII).
      So this is 23 x 8 min Ha, and 9 x 20 min OIII subs. Calibrated with Flats and Bias, and dithered aggressively. The stars are made up of 20 x 90s subs. Everything shot at ISO 200.
      The usual gear was used, HEQ5 Pro Mount, SW 80ED (with FF/FR), guided with PHD2 and a Finder-Guider and Legacy QHY5. Captured with SGP, pre-processed in APP, and processed in PS.
      I have to say, I really like having the stars in a separate layer in PS. So much so, this is how I'm now going to process all my images from now on. It makes things so much easier being able to adjust whatever I want and not have to worry about constantly protecting the stars.
      I'm not actually finished processing this one, but I thought I'd post it up for now anyway, and update it later. I still haven't ran any noise reduction on it yet, so I need to do that next, but hopefully the final version will not look much different to this. I did have some fun playing with the colour on this one.
      I have noticed today though, that it looks quite different on my work Dell monitor compared to my cheap Korean one from home. I think it looks a bit duller today, but I'm just not sure! What do you guys think, does it need more or less of something in particular? All comments welcome, I'm always looking to learn!
      Cheers all!

    • By steppenwolf
      The Crescent Nebula NGC 6888
      The Crescent Nebula rides on the Swan’s neck in Cygnus in a dense swathe of Milky Way stars - an ideal target for my first bi-colour image with my new Astrodon 3nm Ha and OIII filters and Esprit 150 telescope.
      The nebula was discovered by William Herschel on 15th September, 1792 and he described it as ‘A double star of the 8th magnitude with a faint south preceding milky ray joining to it.8’ long by 1.5’ broad’. This double star is not the prominent star with an apparent companion close to the heart of the nebula, rather, it is ADS13515 at the 2 o’clock position on the nebula’s bright periphery. The bright star off-centre of the nebula is particularly significant as this is the star that is powering the emissions from the surrounding gas cloud. This magnitude +7.5 star, HD192163, is of the Wolf-Rayet type and is also designated WR-136.
      Charles Wolf and Georges Rayet first described wolf-Rayet stars in 1867 following detection of their broad emission lines. The Wolf-Rayet stage applies to stars with an original mass in excess of 30 times our own Sun’s mass. This stage comes late in the star's evolution when a rapidly expanding shell of hot gas is powered outwards by the stellar wind only to collide with the much slower-moving gas clouds that were ejected thousands of years previously when the star entered its Red Giant phase. These forceful collisions produce a shock wave that generates an enormous amount of energy including wavelengths within the light spectrum, allowing us to observe them. This complex process displays as an arc of bright nebulosity that we identify as the Crescent Nebula. Long exposure images fill in this arc producing a crab-shell shaped nebulous region rich in Hydrogen Alpha and doubly ionised Oxygen emissions. WR-136 is fated to go supernova at some time in the future – watch this space!
      Image Stats
      Mount: Mesu 200
      Telescope: Sky-Watcher Esprit 150 ED Pro
      CCD Camera: QSI 683 WSG-8
      Sampling: 1.04”/pixel
      Guiding: OAG/LodeStar
      Filters: Astrodon 3nm Ha and 3nm OIII
      Exposure: 30 x 1800 sec Ha, 15 x 1800 sec OIII
      Date: 11/06/17 + 19/06/17 – much of which was under Lunar illumination
      Calibration: Bias, Darks & Flats
      Object Stats
      RA: 20° 12’ 04.6”
      Dec: 38° 30’ 46.0”
      Magnitude: +10.0
      Distance: 4700 light years
      The Crescent Nebula – NGC 6888

      Comparison of Ha and OIII data
      We imagers (well me anyway!) tend to think that Ha is the all-powerful emission line in nebulous objects but it is interesting to compare the Ha and OIII data for this structure as there is an enormous amount of OIII emission present in The Crescent Nebula.

    • By gnomus
      I lost astro dark a couple of weeks back.  I thought, therefore, that I would try capturing narrowband during nautical darkness on the basis that I may as well do something with all the gear.  I ended up with 6 hours of Ha and 6 Hours of OIII captured at my home observatory using my Esprit 120 and QSI 690 atop a Mesu 200 mount. Filters are Astrodon and exposures were 30 minutes each. Processing in PixInsight and Adobe Photoshop.
      I cropped the image slightly to eliminate a couple of brighter stars at the edge of the frame.  I'd be interested to hear what people think.

    • By gnomus
      We have been battling a bit over that last few nights trying to gather data on a detail of the Eastern Veil nebula - a section known as the Bat.  This is 20 minute Ha and OIII subs - 5 hours of Ha and 3 hours 40 mins of OIII.  Esprit 120; QSI 690; Mesu 200 - Astrodon 3nm filters.
      All comments gratefully received.

    • By gnomus
      This is the Pacman Nebula - 5 hours of Ha and 5 hours of OIII captured over 5 nights with my Esprit 120, QSI 690 and Astrodon filters.  Some of the OIII nights were a little indifferent and a couple of OIII frames were discarded but most were kept.  Combination was: Red - Ha; Green - 40% Ha plus 60% OIII; Blue - OIII.  Processing in Pi and PS. 

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.