RBA Premium Astrophotography
Posted: March 18th, 2011
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GOALS AND PRESENTATION
It would be easy to produce a "standard looking" image with this data. Truly minimal processing would be required, but in this case and at this time, for me that wouldn't be nearly as fun neither challenging.
The goals when producing this image were very precise. Just a glance at the image should reveal what these goals are, and I am somewhat satisfied with the results - meaning the goals were achieved to an extent. Clearly the main goal was to reveal any subtle and faint data - the data that typically sits right above the noise. A secondary goal was to do this while containing galaxy brightness and at the same time, being able to bring out small scale details in such galaxies.
To enhance details in the galaxies, a dynamic range compression process was applied to reduce brightness in the larger galaxies, as well as wavelets-based HDR enhancements. Preserving background illumination was accomplished by very careful gradient reduction and further non-selective histogram adjustments.
There may be some people who find all the "dust" in the background distracting, or the small details in the galaxies somewhat the result of processing and not a "natural" depiction of the field, but ...In order to appreciate this image you need to understand the goals set for it, then conclude whether those goals were met or not, rather than whether you would have aimed for different goals.
DETAILS IN THE DUST
In order to better see the "dusty background" I have also prepared a monochrome image that you can see here:
Click on the image for a larger version.It may be surprising to see an image with all this dusty appearance in this direction, out of the galactic plane. To be honest, that is not for me to judge, but this is what came out of my data, and the processing involved was as careful as possible. Data capture wasn't perfect but all the data was taken from the same location, during the same hours on three different nights, and on nights I was very discriminative as to whether I should capture data for this project or not: all nights displayed an SQM reading of magnitude 21.7 or higher at the zenith at some point during the session, and were all consistently above 21.5 at any time.
Although most of the dust above the background you see in this image is likely from our own galaxy, if you look closely, on top of NGC 4435 (the "eyes"), you may see a thick faint tail moving towards 10-11 o'clock, and that is most likely pulled from that galaxy, not a foreground cloud. The "1" arrow in the image below points to this area. There's also some even fainter strikes visible (barely) going from M86 towards NGC 4435. The lines to the right of the "2" in the image show where these strikes are happening. You will need to go back to the larger version of the above image to better discern them (the green lines in the image below are covering the most visible strikes).
There's also some intergalactic "fluff" - though very diffuse in the image - from M87 to NGC 4461 and NGC 4473. And of course, the well-known interaction between IC 3481 and IC 3483 is clearly visible towards the bottom of the image:
Other than that, I personally cannot tell whether other faint signal is intergalactic, it belongs to our Milky Way or, suffice to say, might be an enhanced artifact during capture or processing.
The hardest part in processing this image was the gradient removal process. It's not that the gradients were complicated or severe. In fact, they were very smooth and subtle, thanks to the dark skies of the DARC Observatory, and if I hadn't gone after the fainter signal, it would have been quite easy to deal with it, but there was a great mix of very light gradients from so many frames, and it took me a lot of trials and analysis until I was convinced I had built a background model that would mainly subtract gradient signal and nothing else (and nothing more). Although it's quite possible that not ALL of the faint but visible data is 100% accurate, I believe most of it is. Here's some of the data and processing involved to "flatten" the master luminance data.
Below you can see the master luminance after being cropped (to remove "bad" edges due to misalignment between frames) and with nothing else done to it but a non-linear stretch tailored to reveal the gradients in the image:
And here you can see a non-linear stretch version of the final background model applied to the image:
As you can see the background model is very smooth and gradual even after a strong stretch - obviously when the background model was subtracted, it was in linear form and visually it looked like a completely dark image.PROCESSING
The processing of the data did not include at any time any curves transformation, DDP nor selective or masked histogram stretch. All data stretching was done by unmasked and non-selective non-linear histogram adjustments.
More specifically, after the gradient removal, a rather standard process was done to the luminance/lightness data, that mainly included:
- Slight deconvolution, with masks at three levels: local, global and external. The purpose of the local and global masks was to avoid the Gibbs effect, and the external (lightness-based) mask was used to avoid applying deconvolution to areas low in SNR, and increase the deconvolution effect as the SNR improved.
- A first non-linear unmasked histogram stretch.
- Masked ACDNR (noise reduction). The lightness-based mask here served the opposite effect of the external mask used for the deconvolution: applying the noise reduction only on areas low in SNR and gradually reduce the effect as the SNR improved.
- A second non-linear unmasked histogram stretch. This is because after the noise reduction, our histogram has been altered, and so it allows for a new adjustment.
- Masked HDRWT. This is the process that reduced brightness in the larger galaxies and also revealed some of the data and details in them. The lightness-based mask here is also helping avoid ringing.
- A third non-linear unmasked histogram stretch. Again, we readjust the histogram because after the HDRWT process we have a different scenario that allows a new adjustment.
- Slight masked sharpening using wavelets. The lightness-based mask here kept noise from being sharpened.
- Very light masked morphological transform. This process reduced overall presence of the largest stars, bringing them back to their form as they were prior to the last histogram adjustments. A star-based mask is necessary with every morphological transformation process, otherwise we would be applying the morphological changes to structures that shouldn't be affected by it.
- A last histogram adjustment.
- A last masked Laplacian sharpening.
In between some of these processes, I did integrate scaled before/after images via PixelMath at some stages during the processing, to apply a process only so slightly...[Hide image details]DATEFeb 22,27, March 9, 29, and 30, 2011
PHOTOExposure: L: 62 x 10', RGB 15x5' each
Total: 14.08 hours
Focal: 385mm, f/3.6EQUIPMENTImaging Scope: FSQ 106 EDX w/Reducer
Guide Camera: StarShoot Autoguider
Miles driven: 1200 (1930 km)SITE & CONDITIONSDARC Observatory
Transparency: Very Good
:: 15 Comments
CommentsKeerthi Kiran (Contact, Page), April 21st, 2011, 21:47A truly remarkable project. Also it is very generous of you to share all the details about the image. It is definitely one of the best Astrophotos I have ever seen. I loved all the subtle details in the bigger version of the image.
APOD would be honored to post this photo on their site.
S. K. LAHIRI (Contact, Page), April 22nd, 2011, 0:52Incredibly beautiful astrophoto crafted by lights that left the Virgo cluster some 48 million light-years back and finally captured & home-delivered to our desktops!! Thanks for the great job.
Ginge (Contact, Page), April 22nd, 2011, 1:28Another great shot! Thanks for explaining your procedure in such detail aswell. I'm very curious about your extensive use of masks of different sizes and how you arrive at settings for generation of these. It is what you've described in the tutorial on your blog, right? I'll definately give your technique a try next time.
Dan Schroeder (Contact, Page), April 22nd, 2011, 7:15Bravo! Processing aside, the composition is perfect and the level of detail is most impressive.
Robert Gillette (Contact, Page), April 22nd, 2011, 7:24A superb image of the Virgo cluster's heart. Would very much like to know exposure data and scope/lens used with, I presume, the ST11000.
Derek Kuhl (Contact, Page), April 22nd, 2011, 8:23Another well deserved APOD! You clearly have established yourself as the Dean of widefield astrophotography. Your projects are so demanding and require so much expertise I am continually astonished. Your work has inspired me to emulate your efforts because I feel your images more beautifully expose the glories of the heavens than the traditional narrow field views. I further appreciate your sharing of your processing nuances with us. Not only does it help those that are learning, it also reveals the skill and complexity of the work presented. I hope that our admiration of your work will, in a small way, encourage you to continue your efforts not only in capturing more images but also advancing the technology of wide field image capture.
Noel Carboni (Contact, Page), April 22nd, 2011, 9:46Many astroimages are limited by our atmosphere. This one seems limited only by the interstellar dust. I'm awestruck. Great job Rogelio!
RBA (Contact, Page), April 22nd, 2011, 19:40Thanks everyone for the overwhelming comments!
Ginge, it would take a while to explain all the masking techniques in this comment. They're all based either on the luminance/lightness of the image, or through wavelet transformations.
Robert, for the details, click on the "Show image details" link, right above all these comments.
Noel, it's an honor to see you stopping by, even more so after reading your words!
Again, thanks everyone!
John E. Pool (Contact, Page), April 23rd, 2011, 3:35Using your full ~27MB Virgo Cluster image, I gamma enhanced until dark areas were significant. I downloaded a 60x60 arcmin DSS2 IR image and enhanced it. Faint dark patches above NGC4371 corespond with your image. I think you've captured local galactic cold dust clouds. Excellent work; extremely good resolution.
Andreas Elleringmann (Contact, Page), April 23rd, 2011, 11:13GREAT picture and explanations. I've to confess I've never seen these details. Staying in Paris /France for holidays, enjoying a glas of red wine and seeing this picture is really an event. THX!
Flavius Gligor (Contact, Page), May 2nd, 2011, 12:57Awesome picture! Shows that one can get Hubble-like results with a small scope. :) You just need to get creative.
Juan (Contact, Page), October 20th, 2011, 5:59Incredible
Juan de Dios (Contact, Page), December 30th, 2011, 3:09Hola Rogelio! despues de buscar por toda la red una imagen del cúmulo de Virgo para realizar un pequeño artículo en mi blog didáctico de astronomía, la mejor es la tuya con diferencia , me gustaría poder utilizarla; ¿puedo?
Chris (Contact, Page), February 18th, 2012, 18:26Amazing work Rogelio! I like your approach to processing this very interesting part of the sky, and I would agree that you did meet your objectives. Having a home observatory, I would aspire to travel less kilometres than you did! Well done.
Based on your demonstrations and discussions in Brisbane (Oz) last year, I have just purchased and installed Pix Insight and am looking forward to using it.
Guillermo Abramson (Contact, Page), December 5th, 2012, 18:32Rogelio, sos un genio.
Lukas (Contact, Page), April 29th, 2014, 10:20Bravo! Processing aside, the composition is perfect and the level of detail is most impressive.
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