ollypenrice

If you're thinking about taking up imaging, don't begin with a pre-conceived idea of a preferable mirror size. It really does not work like that. You can take good, bad or indifferent images with any size of mirror. Mirror size is about as important as the colour of the socks you wear during the capture. Olly

In truth I was making a rather arcane point about the fact that a colour channel extracted from a debayered OSC image is one which includes estimates for the pixel values of the channels other than those of that colour, for the pixels under the other Bayer filters. (That is a terrible sentence! I apologize for it.) Let's try it another way and be less garbled: The blue channel extracted from an OSC image has real, unadulterated data for one pixel in four, those under the blue filters of the array. The debayering algorithm has estimated the values for blue for the pixels under the green-green-red filtered pixels based on trends seen in the wider blue situation. The algorithm 'sees' a larger shape formed under the separated blue pixels and 'fills it in' for the green and red pixels. It's not 'pure' blue data, it's blue-plus-estimate. I think we would have to say that this estimation process is a source of noise in the sense that it is not pure signal. Not all debayering algorithms will give precisely the same estimation for the missing pixels. Now, does it matter? Quite possibly not, since the debayering routines are very good, but in AP we take multiple exposures to improve the accuracy of each individual pixel value so it goes against the grain to introduce a new potential source of error in the form of the debayering algorithm. The test would be to extract colour channels from 3 hours of OSC and compare them with one hour channels from individual colour filters. I haven't done that test but I think we can say that the OSC-derived channels, in equivalent cameras, can only hope to be as good as the mono channels. They cannot hope to be better.* Olly * Unless you think that the softer colour cut-off of the Bayer filters is preferable to the sharp colour distinction of mono RGB filters. My view is that that, if this were true, OSC images would have better colour than mono images. Personally I think the reverse.

An image with something to say... 👋lly

It's a good question and, since I wouldn't start to construct an image with a significant inequality between colour channels, I have no experience on which to draw. (Sorry if that answer sounds a bit 'holier than thou' but it's the truth. I don't recall trying this. I'll work on an image with, say, one sub short or maybe even two in a stack of twelve per channel but I don't think I've tried to fix a greater imbalance than that. ) I think, technically, that you can stretch the short channel to the same point as the others and will find that it's all there but will have far more noise. You could then noise reduce it and hope for the best. I'd be interested to see if @vlaiv agreed and suspect that he would, but I don't want to speak for him. There's also a bit more to it, sometimes, viz, 1) using your RGB-only as you would use short subs for controlling a very bright part of the image, overexposed in your luminance. Yes, you could shoot short luminance but you may already have what you need in the RGB itself. Apart from something with the dynamic range of M42 I often find this works very sweetly, but it does require full quality RGB and precludes binning the RGB in most cases. 2) Stars are often much better in RGB than LRGB. They are both smaller and more colourful but, again, they need to be from a full quality RGB layer rather than a 'fixed' one. Olly

Update: this post is ten years old. Telrads still going strong. Olly

I do DBE first, the only prior step being to edge crop any alignment artifacts. It works for me, though my dark site probably means that I never need Background Neutralization. I have the screen stretch (whatever those Spanish Inquisitors call it 😁) applied so I can see what its effects will be when I come to stretch the linear data. It seems odd to me to think of doing background neutralization first because, prior to DBE, I cannot be sure what is background sky and what isn't. Once I have a clear idea of what really is background sky I adjust its colour balance in Photoshop by having background markers telling me the R,G and B values in the background. Very small tweaks to the shadows colour balance get me to equality in the colour channels. I've always found BN hard work by comparison. Olly

Maybe the mask holding down the sky was too tight around the galaxies so that it held down the outer parts? It really does look to me as if something did. I prefer to do this stretch just above the background sky by using Curves and pinning the curve at and below the background level then lifting the curve just above that. I do this in Photoshop as a bottom layer then erase the top layer only where I want to keep the extra stretch - so I don't want it for the stars, for instance. Olly

PS It's the norm to need to add a counterweight to the front of an SCT OTA to obtain balance. You can buy the bits or just improvise something. Olly

You may have struggled with the background but, as it stands, it is even and of a good colour and brightness (not too dark.) However, I wonder if you might not have lifted it in post processing after having slightly black clipped the image. I say this because the faint outer parts of the galaxies are missing. This could be due to lack of integration time. You wouldn't expect to see the tidal tail in this exposure length but, still, I just wonder if you might not have clipped the outer faint stuff before raising the background. Maybe not, I'm just guessing. It's still a good Triplet, maybe a little colour-saturated for me but that's personal anyway. Olly

You can indeed have more luminance than colour, even a lot more, but it makes processing considerably more difficult. I only overdose on luminance when I have to, which is when searching for the ultra-faint stuff like tidal tails, IFN etc. I much prefer to have equivalent time per channel. Ha can be used as luminance but it's not what I want to do because it simply isn't true to the target. You will end up lighting blue parts of the target as if they were deep red, etc. Olly

In which program do you stack? In AstroArt, which I use, you have to specify a maximum rotation angle so you opt for 180 degrees. There may be an equivalent in whatever you're using, perhaps a setting you hadn't noticed had changed? The stars are a bit odd and seem to have 'bites' out of each side. Do they look any different if you stack only pre- or post-flip subs? Most things would be the same either way but not something induced by tilt under gravity. Olly

Not yet. I suspect you might be right. We haven't seen him here for a while but Harel Boren was my dusty-object-imaging hero and he has always used fast optics, his own Boren-Simon Powernewt and then an OS Riccardi Honders. But I hesitate to hide behind my kit by way of excuse! The pandemic has obviously had an effect on my business and I won't be making any decisions till we have some idea of the long-term fallout or likelihood of repetitions, but I wouldn't hesitate simply to replicate your present system here. It would offer our guests high quality results very quickly, even more quickly than our dual Tak106 setup did. We must remember that a dual rig is worth only one F stop over a single, by definition. I'm glad you recognized what I meant by the 'backlit' dust. It's very impressive. Olly

The standard advice with an SCT would be to use an off axis guider. This is because 1) it 'sees' any movement of the mirror within the main tube when a separate guidescope, mounted on the main tube, does not. 2) You are imaging at a very long focal length and at very high (arguably excessively high) resolution in arcsconds per pixel so a short FL guidescope might not guide at the resolution you need. If using the reducer you need to bear in mind that an OAG will be fitted between the reducer and the camera, so adding to the distance between reducer and chip. This distance must be respected (check the spec of your reducer) and bear in mind that an OAG will add maybe 20mm or so to the kit between the two. If your chip distance is currently correct you'll need to lose the thickness of the OAG. In general the SCT reducers are not ultra-critical of chip distance, unlike most reducers, and when the chip distance is a little out the only result is that the true focal length will not be exactly what the reducer's reduction value says it is. Some people do manage to get wedge/fork mounted SCTs to perform well under autoguiding but the combination is not popular because it is very tricky to get right. A German equatorial is much easier, hence the overwhelming popularity of the design in imaging circles. A detail to bear in mind is that an autoduider cannot guide out polar misalignment. It can lock onto a guide star but a misaligned imaging rig will show field rotation over time around that star. You probably know the best news already: the best guiding software, PHD2, is free and straightforward to install and apply. Best of luck. Guiding is the life blood of deep sky imaging. Olly

You're very kind but I never succeed in getting the variety of colour in the dust that some manage to present, nor do I get such interesting illumination of the brighter dusty parts, a kind of backlit illumination. Olly

Yes, it is has more vignetting but less focus drift then the ED versions. Olly

I upgraded mine for about sixpence... lly

Again, flatteners for the FSQs? I only know of reducers. (My own FSQ106 is an early fluorite so I can't use the reducer.) Unlike the Baby Q it easily covers 35mm. A friend uses his with the 35x35 Kodak chip. Olly

Surely not for APSc? For more than that, yes. I know people using APSc with the Baby Q. And do you mean flattener or reducer? I had a reducer but not a flattener for mine. Olly

Yes, Lee's right. This is what I meant. You're working on a see-saw with green and magenta, if you like. Olly

Yes, they're a bit big and a little distorted, probably just because collimation needs a tweak. It should be easy on an SCT with spherical primary. They are slightly cometary with upward pointing tails. For some reason most SCTs used for DS imaging do produce rather large stars but they can be held down in processing. Here I tried to reduce their size and render them round using a couple of Photoshop tweaks. This is just from a screen job so it could be done much better on the TIFF. Olly

Or even Scilly ones in the case of the thread being blown off course. Olly

Get down to the gym!!! lly

I've come to regard this as an absolutely key early step. The background has to be right before you go any further and it needs watching with hawk-like intensity throughout the processing job! (The background can and should be a tad high to begin with, to be precise, but it must be colour balanced. You can clip it down a point at the end but if you clip it too soon it's gone for good. Olly

Open the image in Civilization (Also known as Photoshop) and, from the eyedropper tools, go for Colour Sampler. In the top menu set the sample to 3x3 or 5x5 average and place markers on the background sky. You're allowed four so spread them around. The values per channel appear in the Info box. Once I've made my LRGB image and know that the colour balance is at parity (RGB all nearly the same) I get rid of the samplers and just use Curves to read the background brightness. Open, Curves, put the cursor on a bit of background, Ctrl click and a point appears on the curve with a number telling you the current background level. I set the final level this way just so as not to have markers cluttering the image, but either will do. Olly

You're finding what I found, though I didn't buy two cameras in order to use one for lum and one for colour. I always expected that this would produce unbalanced data. In your shoes I'd shoot LRGB in the mono and OSC in the OSC and combine those. You could slew the mono exposures somewhat in favour of the lum and also use it to add NB on appropriate targets. In the end I went for dual monos as more productive but I think CMOS OSC seems better than CCD OSC and the pairing you have might still prove ideal, though not for adding equal timings of L to OSC to make LRGB. Olly