ollypenrice

Nothing resembling an amateur refractor can, as you say, match this resolution. When/if lucky imaging comes to deep sky imaging, as it has come to lunar and planetary, then the refractor will lose out to the reflector. Although some lucky DS imaging is being done already it has some very considerable way to go. I'd be surprised if it didn't happen eventually, though. Superb image from a bargain scope. Bravo. Olly

You can use absolutely anything as a luminance layer, but what does that mean? It means that what you are using as luminance will define the brightnesses across the image. To take an absurd example, I could take a picture of St Paul's Cathedral with my DSLR and apply it as luminance over my RGB data of the Heart Nebula. The nebula would then be illuminated as if it were St Paul's cathedral. (I ought to try something like this by way of example but I wouldn't expect it to win any prizes! ) But my serious point is to identify what a luminance layer does when applied over a colour image. To quote Nik Szymanek, it gives that colour layer its light. The reason why we use a luminance filter to make a luminance layer is that it is truthful. This filter passes all colours of light and so gives an accurate illumination of the target. If we use, say, an Ha filter as luminance we are lighting the entire image in the light of deep red - which is less dishonest than lighting it in the light of St Paul's cathedral... but you see my point? What many of us prefer to do is to create an honest LRGB image, illuminated sincerely in its own light, and then enhance it with narrowband data. So we shoot a deep narrowband image, stretch it hard because the NB filter holds down the LP/skyglow, and use that to brighten the colour channel in which it appears. (So Ha brightens red, OIII brightens green and blue because OIII lies on the G/B border.) The highly stretched NB data is not used as luminance but in its own colour channel. If you use Ha or L-enhance data as luminance it will suppress the blue channel, Ha totally and L-enhance significantly. If the target has little or no blue reflection nebulosity you'll probably get a presentable or fixable result but it will never be a terribly honest one. Olly

If you do decide to search for a rare book a useful site (as you probably already know) is ABE Books. I mention it just in case you don't know it. Olly

Before reading your write-up I was instantly struck by the excellent colour separation in the stars as well as by the fact that they were well resolved. That suggests that your intentions were realized. OK, the stars could be smaller and tighter but that requires the seeing to be at its best. They are hardly shabby as they stand. When you said that you had variable seeing I found myself wondering what might happen if you made a stack of subs from the best seeing, irrespective of colour, and applied that as luminance, even only partially. Obviously you couldn't do that with a nebula but, on a globular, I wonder if you could get away with it? Olly

For comparison with Goran's image, this is mine from an F5 Tak FSQ and full frame CCD camera in 20 hours, from a site probably a tad darker than his. (SQM peaks at 22.) It simply isn't as good. As well as being fainter, the IFN shows much less localized structure. (The galaxies were enhanced by long focal length data, hence the Ha in the Cigar. The wider image is LRGB.) I was pleased with the image at the time because, back then, it was unusual to see colour in the recently-discovered IFN and finding it was our main objective. However, the greater aperture of the RASA and the high CMOS sensitivity have moved us forward. Olly

Nah, man up and get the drill out! A Sky and Telescope article years ago convincingly demonstrated that you need to bring air in from one side, just above the mirror, and blow it out of the other. Shoot that boundary layer clean across the garden! Olly

I would choose a RASA 8 because there are a couple on here which we know work. Simple as that. Olly

Sumptuous, Peter. Beautifully done. Olly

The RASA seems to be the instrument of choice, here! Olly

Yes, green control is another early and essential step. Olly

5.5 inch refractor. Olly

I have received a very expensive British-built Optimized Dall-Kirkham instrument here, on behalf of its owner, which was supposedly new but which both the owner and I considered to be a recycled return. It was in perfect functional condition but had cosmetic damage inconsistent with its being new. Olly

I think your priority needs to be gradient removal. I won't go anywhere at all with an image until I have a convincingly flat, even and colour-balanced background. In Photoshop-speak that means a background of 20/20/20 to 23/23/23 in RGB right across the image. If I don't have that, any step I take will be right up a gum tree and I won't take it. How to get there? I now use PI's Dynamic Background Extraction but it has rivals in AstroArt, APP and in Gradient Xterminator, along with others I know nothing about. But you must solve the gradient problem before you go any further. Olly

OK, no replies so far so I'll jump, provided you don't mind my being honest. If you do mind, stop reading now! This seems like an awfully complex process for a result which I don't think has hit the data's sweet spot, though I haven't seen the raw data so I can't be sure of this. However, I've been doing this for a long time and have a vague notion of what may lurk within. To my eye there is very little galaxy colour showing yet I would be surprised if it were not present in the data. (I could be quite wrong.) The trick is to find that colour without provoking an orgy of colour noise in the rest of the image but Photoshop has all you need for that. Similarly Photoshop has, I think, all the noise reduction and sharpening that you need provided you apply it selectively. There are two big patches of dodgy background sky, shaped like footprints, one around the brightest star and one below it in the middle of the trio. I'd want to know how those got there. They should not be hard to fix. But my main reaction is to feel that, the simpler the processing, the better. Olly

Whoa, step back in your processing of M31. What the histogram in Photoshop's Levels shows is massive black clipping of the data. The histogram peak is jammed up against the left hand side meaning the left hand half of the peak, containing all the faint data, has been discarded. Like every beginner I used to do this myself. Your earlier process was much healthier. See how you have gentle slopes both sides of the histogram pedestal? What you should do here is move the black point in on the left till it just meets the flat black data line. If you don't, you won't be exploiting the full range of brightnesses availabe to you. Then take a look at the sky brightness. If it's a bit too light you can bring it in a touch more but the natural sky is far from jet black. Olly

I agree that focus may be out, exacerbating the problem. Star control is a perennial problem. The closest I know to one-method solution is to stretch the image and then run it through (free) Starnet++ to de-star it. Next take the linear image (in RGB only if you use mono or just in one shot colour if not) and paste it on top of the starless one in Ps. Change the blend mode to Lighten and now all you'll see is the starless bottom layer. Slowly log stretch the top layer and the stars will gradually appear. Stop stretching when they are a size you like. Olly

Clean image of a nice target, well worth a look. Olly

A horizon is a good place to start with focus. Just dark-light. Olly

Very good going indeed for the time. It will certainly give more in post processing, too. The background is good on the right but both brighter and greener on the left. That's an easy fix in assorted gradient removal tools. What do you use for post processing? And then there are ways of getting more colour out of galaxies. Olly

Big Bang by Simon Singh. Olly

Temp 17.2C and cooler off isn't helping! Olly

I have owned an assortment of premium apos from 70 to 140mm and reflectors of 20", 14," 10", 8" and 4.5". I am also trying to answer the OP's question, not regurgitate the tired old refractor-reflector debate! And I think that he would be better served by a 12 inch reflector (make it 12, not 10) for the purpose he describes. As an imager I don't think refractors are better than reflectors, I simply think that they are usually easier. That's why I use them. Olly

That's fine, and perfectly correct, but, to be fair, it's not not what the OP was asking about. Olly

To be clear, I realize you're talking about visual observing and my reference to imaging is simply to say that I have never been able to replicate a visual cluster view in an image. If we were to show the OP the difference between a 6 inch and a 10 inch on a faint fuzzy I think we could do it. I suspect Obsession are guilty of having made a globular the benchmark but we'll forgive them for other reasons! 😁 Olly

It's very difficult to do that on stellar objects because the eye and the camera respond quite differently to stars. You can process an image of a nebula so that it gives a fairly accurate impression of the eyepiece view but clusters are notoriously difficult to image well. A classic case would be the Trapezium in M42. It is very easy to see four well separated stars at the eyepiece of even a very modest telescope but, as anyone who has imaged M42 will confirm, this is not so easy with a camera. Likewise the eyepiece sparkle of the Double Cluster is very hard to render in an image. The camera cannot easily control stellar size in the way the eye can, so closely placed stars run together. The great thing about a good apochromatic refractor is that it produces very pinpoint stars, the opposite of the way cameras behaves, which was my original point. I'd rather describe my experience of M13 in instruments I've used: 20 inch F4 Dobsonian. Instantly spectacular with very bright stars resolved to the core and a magnification allowing the cluster to fill a widefield, high power eyepiece. The mirror I had was not diffraction limited so the stars were not truly pinpoint but they were still well resolved. Darkish bakground. 14 inch LX200 SCT: Slightly less spectacular due to mildly reduced stellar brightness but tighter stars still resolved to the core. Background perhaps a tad lighter than in the Dob. I never had both at the same time so that's from memory. (Note: I also had a 10 inch SCT at one time. The 14 inch is much better on stars.) 5.5 inch TEC140 apo refractor: Slightly less spectacular again because of further reduced stellar brightness and an image optimized at lower magnification in order to preserve brightness. However the stars are the tightest, the background the darkest and the impression of sharpness to the core comparable with the others. Outlying stars look sharpest in this instrument to my eye. Trapezium not easy in the camera. This had a lot of fancy processing: https://www.astrobin.com/full/380941/0/ Olly