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Not just any M13, it's my first M13


Bob Andersson
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Hi folks,

Globular clusters have always seemed romantic objects to me. As many will remember at one point not too many years ago they were even embroiled in a controversy as it seemed that they were older than the best estimate, using other criteria, of the age of the Universe, a paradox which was resolved after the age of the Universe was more accurately calculated.

The Hercules Globular Cluster is well placed at the moment so I took the opportunity to grab 5 x 200 second subs at each of red, green and blue (Astrodon Tru-Balance filters) while waiting for NGC 7000 to rise. I wasn't expecting too much because even M13 is quite small on my camera's sensor at the prime focus of my TEC 140. Here's the full frame resized to a more forum friendly 1024 x 1024 pixels but you can click the image for a full sized version:

M13_Web.jpg
M13 - The Hercules Globular Cluster

I rather like this rendition precisely because it does cover such a wide field of view (about 2°) and so serves to emphasise what a jewel M13 is. M13 is described in detail in the Wiki entry here. With due apologies to experienced egg-sucking Grannies the headline features are: 145 light-years in diameter, it is composed of several hundred thousand stars and is about 25,100 light-years away from Earth.

For the extreme pixel peepers here's a central crop at 100%:

  • M13_CentralCrop.jpg
    M13 - The Hercules Globular Cluster

Not as good as this Hubble Space Telescope image, of course!

As globular clusters formed so early in the history of the universe they are poor in elements heavier than hydrogen and helium but it's nice to think that just maybe one or two stars with planetary systems might have got captured as M13 orbits the galaxy. What a spectacular place to live - or visit! :(

Bob.

Processing notes, executive summary: The 5 x 200 second R, G and B subs were all combined to produce a luminance image. That was combined with the separate R, G and B stacks to produce a colour version to be added back to the luminance version later. Those two images were then stretched and exported from PixInsight to Photoshop as 16 bit TIFFs. Photoshop's Shadows/Highlights tool was used to tweak the brightness near the centre of M13 and then the Topaz Labs InFocus tool was used to increase the micro-contrast. It's the second time I've used this tool on an astro image and it really does a fantastic job provided one takes care not to abuse it. A spot of noise reduction was applied and then the colour image was added as a "Color" layer. A vibrance clipping layer was applied to the colour layer and a few more cosmetic tweaks were applied.

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.

Wow, thanks for al the kind words folks. I'm almost embarrassed to admit that this image was one of the simplest to process I've yet done, not even needing a white balance correction after using PixInsight's dynamic background extraction tool on the initial colour version. I suppose I ought to grab a copy of that Hubble image, resize it to match my own (PixInsight's Star Alignment tool might do the job for me if I'm lucky) and then do a blink comparison to find out just how well, or how badly, Topaz Labs' InFocus tool recovered the detail from the luminance version but not today as I was blessed/cursed by the third clear night in a row last night and sleep deprivation is winning the battle! :(

Bob.

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Hi alpal,

Any brighter and, on my NEC PA301W, I see the noise from the sensor as I work in Photoshop. Do you think the difference could be a monitor thing?

Point taken about the exposure time. Unfortunately the next exposure for which I have calibration frames is 1,000 seconds and that would have saturated the sensor at the core of M13 - maybe I should get around to collecting some 500 second calibration frames! :(

Bob.

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Hi folks,

Sleep deprivation is still with me but I couldn't resist trying to find out whether the micro-contrast adjustment in Topaz Labs' InFocus Photoshop plug-in was creating false detail or not. My first issue was trying to match some features as the Hubble image is only about 2.5 arcminutes across. I got there in the end and by dint of downsizing the whole Hubble image and upsizing a tiny 95 pixel square crop from the centre of my own image and applying a very extreme curve I got this result:

  • MyCoreAccuracy.gif

Very rough and ready manual image registration - I'll leave you to work out which image is from Hubble! :)

But I do take comfort from the fact that there is a reasonable correlation (might have been slightly better if I wasn't so knackered that I took my crop from the published JPEG rather than the original Photoshop PSD). On this evidence that InFocus is not inventing (a lot of) false detail that plug-in, together with their DeNoise plug-in which I find indispensable, is very definitely a permanent part of my image processing tool-kit from now on. :(

Bob.

Edited by Bob Andersson
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Hey, very nice, Bob. I found this just after posting my own from our TEC140 - but I cheated and added a bit of 14 inch luminance data to the core. Your colour is much redder than mine which strikes me as being more likely to be right in view of the astrophysics of globulars.

Good stuff,

Olly

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Hi alpal,

Any brighter and, on my NEC PA301W, I see the noise from the sensor as I work in Photoshop. Do you think the difference could be a monitor thing?

Point taken about the exposure time. Unfortunately the next exposure for which I have calibration frames is 1,000 seconds and that would have saturated the sensor at the core of M13 - maybe I should get around to collecting some 500 second calibration frames! :(

Bob.

I have a Viewsonic 27" monitor model VX2753MH-LED

& the background of your pic is coal black.

Maybe you need to take 2 sets of subframes?

Short ones for the core & longer ones to get the background.

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Aww man! You've just knocked the wind out of my sails! I'm bang in the middle of my first ever astropic and it's M13 - and I can tell you it's not going to be a scratch on that! Sulk!

Bloomin lovely pic though :(

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...Maybe you need to take 2 sets of subframes?

Short ones for the core & longer ones to get the background.

What background? I can't recall an image of the area which shows the IFN although I imagine if you go deep enough you can find it. Or is it the brightness distribution of the stars that I've got wrong? As a matter of aesthetics I do aim for a coal black background unless there is actually something worth showing as Olber's paradox was resolved many years ago.

That was a joke, BTW, but it would be helpful if you could point to images which didn't have a coal black background but which actually showed meaningful variation in their background, as opposed to light pollution induced gradients, so I can judge how my processing might be improved. Thanks. :(

Bob.

Edited by Bob Andersson
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Hi alpal,

No worries and thanks for the link. My last post was made at about four-thirty in the morning after a night of imaging so my lack of sleep probably showed through. Even so I only managed three hours of sleep before the pigeons started cooing under my window this morning. They don't know what danger they were in! :(

I think part of the difference is that I use PixInsight's Dynamic Background Extraction tool. It needs care, and I'm still learning, as it can take out wanted signal if you let it. With this image the signal removed was quite low with no sharp gradients and, of course, that meant that during later processing in Photoshop when I adjusted the black clipping level to remove the last vestiges of sensor noise, and it definitely was sensor noise, that left me with the coal black background. It's actually a look I quite favour provided it isn't achieved at the expense of useful data.

Terry has a lovely shot there and I really shouldn't pass comment without him being in on the conversation but given my own effort was a quick capture (5 x 200 seconds for each of R, G and :) I think the greater depth, which isn't that huge IMHO, of his shot is down to his total exposure time of 9.5 hours (320 minutes with the QHY9M and 250 minutes with the modified 500D) and, as you so rightly flagged earlier, his use of both 5 and 10 minute subs because of the dynamic range of the subject.

Food for thought and feedback I'll definitely bear in mind the next time I return to M13, not to mention my first attempt at the anagrammatic M31 later in the year which should be an even bigger challenge so far as dynamic range is concerned.

Bob.

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On background sky, I go for a value of around 23 in Photoshop. On my gallery site this tends to get clipped back a little further so mine looks darker there than on my monitor in Adobe RGB 16 Bit. I don't have a hard and fast rule about RGB parity in the background. Sometimes a couple of points higher in the blue suits the picture, sometimes not. Contrast with the object's colour plays hard in governing the 'look' of the background. I used to set the sky to about 15 but no longer like that look.

In order to pull any faint fuzzies out of the murk I'll do a very hard stretch and then a gentler one, both till they give a background of 23 (by clipping the hard stretch more.) The hard stretch has both the fuzzies and the noise. I use the soft stretch for the final image but retain just the hard-stretch fuzzies from a Layer in Ps.

Multi exposure times? I rarely find they work. Obvious exception is M42. Ironically my M15 subs from the F7 TEC were 5 minutes but those for the brightest part of the core were from the F6.8 ODK at 10 minutes. I find that the stretch is the key to dynamic range. I just do a softer one for the brightest parts and layer it.

What I would say, though, is that star colour benefits from some short subs.

I must repeat my thanks to you, Bob, for inspiring my own re-process of the colour balance. You had it right first time.

Olly

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Hi alpal,

No worries and thanks for the link. My last post was made at about four-thirty in the morning after a night of imaging so my lack of sleep probably showed through. Even so I only managed three hours of sleep before the pigeons started cooing under my window this morning. They don't know what danger they were in! :(

I think part of the difference is that I use PixInsight's Dynamic Background Extraction tool. It needs care, and I'm still learning, as it can take out wanted signal if you let it. With this image the signal removed was quite low with no sharp gradients and, of course, that meant that during later processing in Photoshop when I adjusted the black clipping level to remove the last vestiges of sensor noise, and it definitely was sensor noise, that left me with the coal black background. It's actually a look I quite favour provided it isn't achieved at the expense of useful data.

Terry has a lovely shot there and I really shouldn't pass comment without him being in on the conversation but given my own effort was a quick capture (5 x 200 seconds for each of R, G and :) I think the greater depth, which isn't that huge IMHO, of his shot is down to his total exposure time of 9.5 hours (320 minutes with the QHY9M and 250 minutes with the modified 500D) and, as you so rightly flagged earlier, his use of both 5 and 10 minute subs because of the dynamic range of the subject.

Food for thought and feedback I'll definitely bear in mind the next time I return to M13, not to mention my first attempt at the anagrammatic M31 later in the year which should be an even bigger challenge so far as dynamic range is concerned.

Bob.

No worries Bob,

Look - I had so many problems trying to process globulars.

Just look at the lousy pics on my flickr photos.

I used to think always of trying to get a pic of the target

but now I realise that the background is just as important.

I think the idea is to go for as longer subs as you can without

the light pollution drowning out the image.

As for the core - shorter subs & then painting it in

on another layer.

Your core is excellent.

cheers

Alpal

Edited by alpal
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