Tuesday, September 22, 2015

Creating Hubble Palette Images

I usually try to post here at the end of every month, but neglected to do so in August. I think the TS 107mm got more than just a little attention, it got a thorough workout: eleven hubble palette images and one HaOIIILRGB image since mid July, not to mention a few more that I yet to process. I have a few on the back burner that I'm saving for a rainy day.

The recent images from the 107mm scope can be seen here, starting with the Wizard nebula and ending with the Cave nebula. I have switched back to the Star71 for some wide field imaging.

I also got quite the workout with the stretch of clear weather. I had a severe case of astrophotographer fatigue, so I let August slip by without a new post. I did, however, get in a lot of practice on creating hubble palette images, and thought I would share a little bit of what I do to create these images here at Little Piney Observatory this month.

Hubble palette images are basically created by using data from three narrowband filters, SII (sulfur) is assigned to the red channel, Ha (hydrogen alpha) to the green, and OIII (oxygen) to the blue. Some people also call this SHO, substituting the SII (S), Ha (H), and OIII (O) in the channels represented as RGB, or red, green, and blue. Just simply putting the image together in this fashion will usually result in an image that is very green, since Ha is almost always very dominant in every image. The process I use to create the image is sometimes referred to as modified hubble palette, because it uses some selective color adjustments to manipulate the colors. However, this "modified" hubble palette is what creates what most are so used to seeing in the hubble images with the classic golds and blues.

To demonstrate, I'll use some data from an image I'm working on right now. I'm back to using the Star71 again to try to do some hubble palette images of some widefield targets. Right now I'm working on IC 1848, the Soul nebula.

I process each image before combining into a color image, being careful to try to keep star sizes as equal as possible in each image. Here are the three channels I will be working with.

SII - for the red channel

Ha - for the green channel

OIII - for the blue channel

As I mentioned earlier, a straight combine using no selective color adjustments tends to usually come out very green because of the dominant Ha channel. I put one together here to show the effect.


I don't particularly care for the green look myself. There are a lot of steps that go into creating the traditional look of the hubble palette images, but it's worth it in my opinion. I will go through the steps here and then show you a much easier and quicker way in the end.

Start out by taking this image with the straight combine of SII for red, Ha for green, and OIII for blue, and using Photoshop, make these Selective Color adjustments to bring out the classic gold colors seen in many hubble palette images.

First, align your color channels by using Levels and looking at your histogram to align the three channels.

After aligning your three channels, stay in Levels and select your red channel and set your black point to 4 and leave your white point at 255. (You will leave the white point at 255 on all channels)

Select your green channel and move the black point to 9 and set gamma (center slider) to 1.27

Select your blue channel and move black point to 8 and set gamma to 1.38

Now go to Selective Color, and with Method set to Relative, choose green from the dropdown menu and set Cyan to -100% and Magenta to -25%

Choose Yellow, Relative mode, and set Cyan to -100% and Magenta to 25%

Choose Cyan, Relative mode, and set Cyan to -1% and Yellow to -100%

Again with Cyan in Relative mode, set Magenta to -25% and Yellow to -100%

The final adjustment in Selective Color uses Absolute as the mode. Select the color Yellow and adjust Magenta to 15%, then select Blue and adjust Magenta to -20%.

Quite a few adjustments there huh? The easy way here is to buy Annie's Astro Actions from Anna Morris. This is a great action set for Photoshop for only $15, and it includes the Hubble Creation action as outlined above, plus a bunch of other very useful actions for processing your images.

Once I run this action, I get an image exactly like the one above, and then am prompted to adjust levels to even out my colors as necessary. Once doing that and clicking "OK" I have an image like this.


Now we have an image with the classic golds we usually see in this type of image, but there are a few more steps. If you do a search of hubble palette you will likely see a lot of these images that have pink/magenta colored stars. This is where I go back to PixInsight and use Pixel Math and type in this expression.

R/K: $T
    G:  m = min($T[0], $T[2]); iif(m>$T[1], m, $T)
    B: $T

symbols: m

Then just drag the triangle over to the image and the pink/magenta is magically gone from all the stars. The pink stars always gave me fits, and I'd like to thank David Ault for giving me this pixel math expression. It has been a tremendous help. Thanks David!! :)

After cleaning up the stars, I go back into Photoshop to work on the colors a little more. I also have Carboni's Photoshop actions and like to use his "Increase Star Color" action to get a little bit of star color here. Its tough to get good star color in narrowband images without taking some RGB data and overlaying that for star color, but this action does help. Be careful though not to over-do it.

Another action of his that I use a lot is his "Deep Space Noise Reduction." This noise reduction action masks off the brighter parts of the image so as to not lose any details, and focuses mainly on the background. It basically only affects the darker regions.

I also prefer more blue from my OIII compared to the greenish-blue hue seen here. To fix this, I use Selective Color and work with Cyan. Adjust the Cyan to -30%, Magenta to 30%, and Yellow to -100%. These adjustments vary from image to image so adjust these to taste. From this point I usually make some Vibrance and Saturation adjustments.

With this image I have only completed capturing my OIII data. I still want to double my SII data, and I need a lot more Ha. At this point I only have 6 Ha subs, but the Ha signal is strong, and I managed to produce this with the data I have. This is very close to what my final image of this will look like. It looks fine right now in this small image, but when seen full size right now the "warts" are easily seen. More data will get this cleared up.


One other thing that helps a lot on some images that are weak in SII and/or OIII, is a very mildly stretched hydrogen alpha image. I integrate that into the color image, then come back and use a fully processed Ha image as a luminance layer. Just saturate the color image to taste and then add the Ha/luminance for the detail and blend as usual. Images strong in SII and OIII do not need this done. They blend very well into the Ha. This area of the sky has strong SII and OIII components and did not need this.

I have really enjoyed shooting narrowband lately and putting together these hubble palette images. I always thought that I enjoyed shooting and processing LRGB more, but lately about all I have been doing is working with narrowband and hubble palette. Its very enjoyable to me and I hope that a few people find some of the information in this post useful for their own imaging endeavours.

So on this day of the Autumn Equinox, happy imaging everyone!


4 comments:

  1. Thank you for this nice writeup, I always wondered why my SHO images where dominated by green, this article really helped.

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    Replies
    1. Thanks! Glad you liked it! :) Sorry I just now saw this post. :/

      Richard

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