ollypenrice

Assuming the calibration and debayering have been done in the right order, we have to wonder if the filter isn't killing the colour. However, I think you may have overdone the reduction in green because the little colour we can see in the Iris itself is magenta - which is what happens to blues when the green is too low. In reality the target is a very vivid blue, quite high in the cyans. Olly

There is a hint of colour here but no more than a hint. I'm not sure what I'm looking at. Could you comment on filters, OSC, etc? Olly

I have to endorse Steve's comments, not that they need any endorsing from me. I'd also highlight some issues regarding the C11 as a deep sky imaging instrument, especially with a DSLR or small pixel CMOS camera. It's very important to understand sampling rate, AKA your image scale in arcseconds per pixel. I'm sure you'll find this covered in Steve's books but there is no point in over-sampling, which means having too much focal length for the size of your pixels. Note that Steve uses a six inch refractor. So do I. I've also used a 14 inch catadioptric and found no real advantage over a smaller refractor now that pixels are small as well. However, the C11 makes a great planetary imaging scope because in fast-frame planetary imaging you can beat the seeing with ultra-short exposures. In deep sky imaging you can't. I do have a large SCT here and like it for visual, a 14 inch Meade (using a cable kindly made for me by Steve!) but I don't consider this scope useful for deep sky imaging. Olly

I don't think you'll beat the figures by eye. However, focus can change quickly with temperature. Olly

Did you pull down hard-edged halos there, or soft ones? If they were hard-edged you did a darned good job! Olly

I think those hard edged halos are from internal reflection. I know that some CMOS cameras have been prone to them but I don't know which ones. It might be worth Googling your camera model to find out. I find high haze and dew give soft halos. (Edit, I see Steve has identified your camera as one that's known for it.) The light house beam is from Deneb, as you suggest. I've had it as well! It is possible to mitigate this kind of thing by careful use of manual repairs in Ps but that is never more than a bodge. It was actually on Deneb that I came across the real way to solve the problem. A guest and I were working on a large North America mosaic and a panel corresponding to yours had the beam. However, the adjacent panel in which Deneb was in shot did not have it. Such beams are often created by out-of-shot stars so what you can try is a shot in which you've slewed as closely to Deneb as you can while still covering the part of your image with the beam. You don't need hours of data, just enough to layer in so as to tame the beam. This worked perfectly for us in the NAN mosaic and in repairing a horrible beam from Rigel in my Witch Head image. Olly

I guess my site means that I never need to use Background Neutralization. After DBE it's always neutral and star colour is good as well. I think it's better to get good flats than to use DBE against vignetting but it can be done. One problem is that we often frame the object (which is bright) in the middle of the image, meaning it's hard to give information to DBE about how the gradient increases towards the centre. (I think this is why Automatic Background Extraction is not so good. I find it often gives a dark ring around a central object like a large PN or galaxy because it has read the outer parts of the object as background and dimmed them when applied.) Olly

As Steve says, you can buy them but they end up being further and further from the lens as you screw them together. One of my robotic clients asked me to make a flat one to try on his camera lens and he finds it works better. Making them is dead easy. The flat lid of a freezer ice cream tub is ideal! Use stove or BBQ paint because it uses pigments rather than dyes and is less reflective. The compass-cutter comes from a graphics outlet. Oh, and cut out the outer circle first... 🤣 Olly

I didn't address gradients in my first demo, I was focusing only on the stretch process, but Wim is quite right that they do need a fix. A gradient is simply a gradual drift in brightness across the image. If this drift is strongest in one colour (as it usually is) this will appear as a 'colour gradient.' Where they come from is anybody's guess. Even at one of the darkest sites in mainland Europe I still get them on 99% of the images I take. (I'd have said 'on all of them' but I recently shot some gradient free colour data!) Flats won't remove gradients caused by skyglow. As far as the camera is concerned any light going onto the chip is real signal - as indeed it is, even if you don't want it. Flats only remove the effects of uneven illumination caused by the optics, so vignetting and dust bunnies. Gradient removal is done in software by well engineered algorithms. Here are some pointers: Pixinsight's Dynamic Background Extraction. Stunningly effective. With the image still linear but with a temporary visualization of its stretched appearance on screen you place markers on what is genuinely background sky to tell the software that those places should be at the same brightness in every channel. It makes a map of the gradients it finds and you then subtract the map from the image. A nice by-product is that it also performs a fine colour calibration in the process. Don't use DBE to fix local artifacts like dust bunnies. It won't. Use a small number of markers to find the broad underlying gradient. Russ Croman's Gradient Xterminator. A reasonably priced Ps plug-in which is pretty good. There are videos online about how to get the best out of it. AstroArt has a gradient removal tool and is a top quality stacking and calibrating program, far better than DSS, for instance. Astro Pixel Processor is highly recommended by several friends and clients. I'm certain its gradient tools work because I just processed a 32 panel mosaic from individual subs made in APP. They were admirably flat. Core: just make a second stretch, almost as hard as the first, but concentrating on the core. Avoid getting that 'stellar' central part and put it as a layer under your present version and use a feathered eraser to remove the saturated bit on top. Olly

Have a look at my fairly detailed posts in this thread regarding black clipping. The image above is massively black clipped and, without any doubt whatever, has much more to give. Olly

Vlaiv, regarding CMOS and software binning, wouldn't this average out the residual noise of each pixel in the group of four? Olly

Agreed. However, the OP's signature lists two CCDs so the CMOS situation isn't relevant here. That's why I'm still to be convinced that the formula can work while ignoring the sampling rate. Olly

I'm afraid I'm not yet ready to believe this. As you're stating the formula it ignores sampling rate and surely this cannot be ignored? To give an extreme case, this formula would make no distinction between imaging in bin 1 and doing so in bin 5. This seems dubious. I cannot see what the F ratio, on its own, tells us. What we're really interested in is 'area of aperture per area of pixel.' This is what tells you how much light each pixel is getting. My objection applies not only to binning but to using larger pixels (which amounts to the same thing all being equal.) I absolutely agree that CMOS chips don't need long exposures and for the reasons you state. I remain unconvinced that we can ignore pixel size but I'm open to persuasion... Olly

You lucky souls! Have fun. Olly

SCNR green has always worked well for me. Did you try the slider at various settings? The method you used in Ps was OK and I got a similar result, maybe using a slightly different technique. In Ps I opened the histogram in All Channel View and moved the black point slider (per channel in Levels) to the right to align the top left of the blue peak with the green, as below. I don't know why the red historgram has this strange form. But the idea was to use the black point slider to get the top left of each peak aligned. I also brought the black point in in red as well. That's probably the biggest difference between yours and mine. Feeling the bright star was a bit green, still, I did a further tweak in colour balance adjusting the highlights. Marginal difference. I think we can expect the L-enhance filter to bring down the blue channel because OIII only passes the greenest blues and H-Beta is blue but very faint and is given out only by those gasses also shining in Ha. Broadband blue is blocked, if I have this right. Olly

Lovely. This is a great object. C and C? I'd say the greens have been pulled down a tad too far, causing the blues to turn slightly magenta. But as for the rest, spot on to my eye. Olly

Nice. I very rarely say this but, for me, the black point could come in a whisper but certainly not more than that. Olly

Good call from Mark and a very successful edit. Olly

Lovely image, very deep and yet totally unforced and natural. The colour, too, is unforced. Spot on. Olly

We all have similar tales to tell! What you do know is that with more data you'll get a really great result. For me the green needs attention since it's rather high both in the dark dust and the stellar cores. Not a difficult adjustment, though. Olly

I never do less than 15 minutes in NB but my skies are dark. I generally prefer 30 minutes. I'm not doubting your choice but what do you find with longer subs? Olly

An heroic choice of target from a light polluted site and an even more heroic result! This is a target which will guzzle all the time you can give it so you're doing brilliantly for four hours already. The combination of Astrodon filters and this lens is inspired because the one thing camera lenses can't do is hold down star sizes - but that's a thing Astrodon filters do to perfection. Great combination. Olly

OK, the histogram on yours is slightly different to mine, partly because you are using fewer subs and partly because the poor subs will have introduced a little spurious faint signal into the stack. This signal would appear (I reckon) in the tiny flat line to the left of the histo peak which mine has and yours doesn't. Compare the raw histogram in yours and mine. Mine (your data but you know what I mean!) has a short flat line before the peak. Yours doesn't. While I've offered a theory as to why that might be, I can't be sure it's correct. This does change things in the stretching. When you get to this stage... ...your background is still too low to move into the Curves phase with the pinned background sky. You'll need to give it a further stretch in Levels to get the background higher, closer to 23. Now your histogram isn't going to behave as mine did so you may not be able to get it to 23 without saturating the cores. If so, settle for less but certainly not less than 20. When you went into the Curves phase the faint nebulosity had not been adequately separated from the background so you couldn't be sure what you were pinning, background or nebulosity. (You had realized this when you said, 'I noticed that my histogram is further to the left than yours at this point, not sure why that is. Setting the points on the curves is quite the fiddly task! ' Quite so, it was fiddly because the Levels stretch had not, on the new stack, opened up the histogram sufficiently. Give it another go in Levels. Or two. Or three! With one of my cameras I find the background sky stubbornly refuses to rise to the desired 23 so I settle for less but, when I see this coming in a stack, I go back to the beginning and give a pre-stretch to the lowest brightnesses using a curve like this: I lifted the lowest values here form 6 to 9 and then straightened the curve to the finish in hopes of lifting the background a tad to start with while leaving the rest linear. Then I'd go back to the Curves stretches as usual. Your histogram, since it rises so sharply from nothing, might be more manageable after a pre-stretch like this but I can't be sure. While this technique does help, I must say that it's less effective than I expected it to be when I first thought of it! 🤣 One thing we do see here, though, is that every image is different. There is no workflow that suits all - thank goodness. That would be dull. Olly PS The trick for placing the background marker on the Curve is to put the cursor on a bit of background and then Alt Click and the point goes onto the Curve in the right place.

I normally run lots of imaging tutorials with guests but we are, of course, 'Covided out' for now! Olly

Post processing is the interesting bit and begins with the stretch. Here's where to start. (This is in Photoshop.) We open the linear (unstretched) image and check it in Levels. There is a big gap on the left of the histogram where the thin dataline doesn't reach the left hand side. That means you're not using the full range of brightnesses available... ...so, move the black point slider (lower left) to the right almost to meet the start of the data line. Why 'almost?' Because it is vital not to clip into that line. It contains your precious faint data. I'll respectfully suggest that this was where Jamgood slipped up. The error is known as 'black clipping' and is very common among beginners. Now when we look at the image we see this: ...so we can very carefully bring in the black point to meet the data line like this: ...an d this gives us a nice healthy histogram: Now we are getting close to a reasonable background sky brightness. My favourite is 23 per channel but I'll settle for 21 or 22. To find out where we are we can use the eyedropper tool to put 4 colour sampler marks on the background. I've ringed those in red and ringed the place where Ps shows you the values. We notice that green is consistently low (which is actually unusual) so we can give it a lift either in Levels, Curves or the Colour Balance menu. I went for colour balance and didn't try to get it right up to the level of the others because green is 'the problem colour' in AP. That'll be a bit better and we can tweak it more carefully later. At this point there are different approaches. A good number of people would stretch again in Levels, which move the histogram peak further to the right, and then bring the black point in to the right as well to cut back to the desired background level of 23 or whatever. The trouble with that is that you'll be stretching the background sky and so increasing its noise. This will mean you'll need nasty noise reducing algorithms to make your background look shiny black. No no no!!! 😁 So I've chosen to stretch further in Curves rather than levels. Below, I have a placed a fixing point at the background level and a further fixing point below it, and then I've lifted the curve above that to stretch only the galaxy and stars. Notice that the curve rises steeply and then flattens to a straight line to the finish. This stops the core of the galaxies and the stellar cores from being over-stretched to saturation point. There is still a long way to go with this image but a careful stretch, always looking at the image and always thinking about what this particular image needs, is the way to proceed. Work in small steps and never black clip. When you've clipped it, it's gone for good. Olly