Messier 13 : Great Hercules Cluster & Image Processing Guide
Messier 13 or M13, also called the Great Globular Cluster in Hercules or the Hercules Globular Cluster. Messier 13 was discovered by Edmond Halley in 1714, Messier 13 lies in constellation of Hercules.
The apparent magnitude of Messier 13 is around 5.8, so it will be barely visible with naked yes. Its diameter is about 23arcmins and it is easily viewable with small telescopes under dark skies. Messier 13 shot through insight observatory You can Image/Purchase High quality data set from link above.
Messier 13 stretches for around 145 light years and it composes thousands of stars. The estimated age o Messier 1 is around 11.6 Billion years and it contains around 300,000 stars which are tightly packed by gravitational pull, Messier 13 is a Class V globular cluster, it has an concentration of stars towards the center thus known as globular cluster.
According to Kenneth Glyn Jones, M13 is peculiar in containing one young blue star, Barnard No. 29, of spectral type B2 (Arp 1955). The membership of this star was confirmed by radial velocity measurement, and is strange for such an old cluster - Glyn Jones suspected that it may be a captured field star. However, it was found that this star is actually an evolved star, probably a genuine cluster member, which has burnt up its nuclear fuel and lost most of its outer layers, a so-called post-AGB (post asymptotic giant branch) star (Conlon 1993, Conlon et.al. 1994)
Processing Globular clusters
Processing Globular cluster requires different set of techniques as we are not dealing with any kind of nebulosity here. The main approach is towards the stars which are easily blown up or get over sharpened during the processing. We also have to make sure that during the capturing phase the the stars are not saturated, it is advised to take very short exposers when shooting globular clusters, otherwise you would not yield the stars colors and instead of that you would get silvery/white results. so it is always preferred to grab shorter subs to pull the gorgeous colors of globular cluster. Some people prefer doing RGB on such densely packed clusters but I always prefer doing full LRGB imaging on this as L delivers great luminosity strength on stars. This data was shot from 16" f3.7 Dream Astrograph Newtonian and 16803 CCD camera which has extremely great dynamic range which means images wont get saturated quickly, We shot Messier 13 with 600sec 1x1 and 300sec 1x1 sub - The 600sec had blown out the core so 300sec sub came to rescue
HDR Composition What is HDR? HDR stands for High Dynamic range and it is a way to combine two pictures with different exposers to deliver a result, HDR is a method that aims to add more “dynamic range” to photographs—where dynamic range is the ratio of light to dark in a photograph. For eg Above I used HDR Composition Process Icon to combine 2 images with separate exposers, The left one was 600 sec 1x1 sub and the right one was 300 sec 1x1 you can notice the core how it is saturated.
Now you should repeat these steps with Red Green blue subs- Make HDR and save the images.
Now it is time for combining the RGB images into one for that I used Channel combination in Pixinsight Place the Red, green, blue into their respective channels and press 'Apply Global' you will get RGB images combined but you would notice that the colors are not very balanced so to get a good color balance we need to go Photometric color calibration, I do have my own methods for color calibrating images but PCC works good sometimes and delivers fine results.
Photometric Color Calibration PCC is a very special tool for several reasons. Besides the quality of our implementation, what really makes PCC unique is the fact that it materializes our philosophy of color in deep-sky astrophotography, and primarily, our philosophy of image processing: astrophotography is documentary photography, where there is no room for arbitrary manipulations without the basic documentary criterion of preserving the nature of the objects represented. In deep-sky astrophotography, we understand color as a means to control the representation of information in the image. Following documentary criteria, such representation must be justified by properties of the objects photographed. This excludes, in our opinion, the classical concept of "natural color" based on the characteristics of the human vision, as applied to daylight scenes.
In the PCC tool, the default white reference is based on the average spectra of Sb, Sc and Sd galaxies. The average of these galaxies provides a source of the entire range of stellar spectral types and populations, so it can be considered as the best unbiased white reference, truly representative of the observed deep sky. Along with this default reference, PCC provides a rich set of pre-calculated white references, including spiral galaxies, elliptical galaxies, and most stellar spectral types. This allows you to select the most appropriate white reference in special cases, where you decide to maximize the information represented for some particular objects in the image. The possibility to choose one among a rich set of white references is also useful to understand the nature of the data, by analyzing color variations as a function of the properties of the represented objects. In this sense, PCC also materializes our vision of image processing as a creative and enjoyable activity: the how and the why are actually more important than the final product.(Pixinsight website)
How to use PCC
Open Photometric Color calibration
Open your RGB combined image
Open any of your FITs file which contain data of date, coordinates, pixel scale and focal length (Photometric color calibration is very picky you need to enter precise information otherwise it would fail)
Make sure you have downloaded the Gaia dr2 dr3 and APASS XPSD dataset files from the Pixinsight website . You do not need to download all 34 files but 10-10 from each are sufficient if you are using a normal aperture equipment.
Open your single fits file and press Acquire from image, it will automatically add data. Now selection of White reference, Since Messier 13 contains B2 type stars so the white reference would be BII but the results are not aesthetically so the White reference must be set to G2V type It delivers good balance of colors, however if you are a fan of accurate color representation then do one thing Copy the RGB image , Apply G2V to one and B2 to other and then blend them together to 50% so it might be good!
Here are the results which show Before and After applying the Photometric Color calibration. L-RGB Combination I always prefer combining the Luminance in Photoshop CC as I have more and more control while combining the images using luminosity mode. Open Photoshop - Your RGB image make sure your RGB matches with Luminance if not then use star-alignment process icon to match both of your images pixel by pixel, Open your Luminance and paste it on top and and change the blending mode to luminosity Try adjusting the opacity as Luminance would be too heavy on non linear RGB so there is also another way that is carefully stretch the RGB before combining the Luminance You can achieve that by either Arcsinh or Masked stretch in Pixinsight . Note Masked Stretch takes lot of time but it is quite good way to stretch your data so it is advise to make couple of small previews to make sure how it would run . After you stretch the image in Pixinsight Combine L into RGB by the method I described above , always make sure your stars are not getting saturated while combining, you have to be careful at this stage as it is the foundation of the image. You can apply bit of vibrance to the LRGB image as L outshines the RGB so to get it right , use vibrance tool in PS and apply around 15 or whatever your saturation taste is Now we have LRGB images prepared You can use the deconvolution in pixinsight to resolve the tiny stars or get better structure in stars, i applied only 1 iteration of Deconvolution as already the data was really good so did not needed any deconvolution process to be applied
In the end you can adjust the colors of the stars using camera raw in Photoshop or use color saturation in Pixinsight to boost saturation on the stars Regarding sharpening you could apply it a couple of careful iteration using unsharp mask it would be fine . I hope you liked the small walk through on how i usually process Globular cluster images if you liked it please follow me on my Instagram - @mr_deepsky