ollypenrice

The colour filters, a red, two greens and a blue, are placed in a pattern known as a Bayer Matrix over the pixels on your chip. Each pixel, therefore, captures just one colour. However, the patterns vary from maker to maker so you need to instruct the software as to which pixel gives which colour. This is known as debayering and you have to find the correct pattern. Olly Edit, crossed with Freddie who's making the same point.

You have to beware of the F ratio myth when you bring focal reducers into the story. Binning would also speed up capture. You can hardware bin a mono CCD but you can software bin data from an OSC camera of any kind. The real purpose of the focal reducer is to open up the field of view and, in the case of the Meade-Celestron one, to flatten the field - which will be necessary on anything but a very small chip. Guiding does have its limits. An EQ6, working well under guiding, should deliver a guide RMS of 0.5 arcseconds or so. You need to double that to estimate the resolution your mount will support when the seeing allows. So you will probably be limited to about an arcsecond per pixel at the imaging camera. This calculator is helpful. http://www.12dstring.me.uk/fovcalc.php To be honest the non ACF Meades are not great for DS imaging and will add to your difficulties. I wouldn't let your decision be too heavily influenced by DS imaging with the scope. It will never be good at it. Olly

That's a splendid M31, very obviously. Great use of a fast system, despite the moon. You've also preserved a hell of a lot of detail into the core, which is always tricky. I think you have as much as any image I've seen. It's a great image. Once we regard this image as top notch (which I certainly do) we ask about reservations. I have two: As is often the case with OSC CMOS cameras I think it is red-biased. More blue signal would be nice. The other is that the sharpening does rather shout 'Pixinsight' to me. (Go on, shoot me down and tell me it was all Photoshop! 🤣) No but, seriously, I do feel I'm seeing 'wavelets' more than I'm seeing details in M31. If the image were not of the quality that it is I wouldn't mention either reservation. They are minor. Olly

Usually the guide scope-camera is working at a coarser pixel scale than the imaging camera so a 1 pixel dither at the guider will give an imaging dither of more than 1. An imaging dither of 12 pixels is often used to combat 'colour mottle' in DSLRs. If all you are trying to combat is small scale noise you don't need anything like that. Olly

'New scope on the near horizon' might be re-arranged into 'Horizon nearer in new scope...' 😄lly

I suppose 12 seconds might be long enough to contain enough thermal noise to prevent the master bias from correctly calibrating the flats. On one setup I was plagued by over-correcting flats. I tried everything I could think of including shorter and longer flat subs, bias as flat dark, dedicated flat darks, rigorous flushing between flats, etc etc etc. In the end I shot flats using a different capture program (AstroArt instead of Nebulosity) and, glory be, they worked (on lights captured in Nebulosity.) Well, they worked for a long time and then one fine morning they started over correcting again. Nightmare. I never did get any further with this so I'm not being much help. However, I'm pretty convinced that, in my case, the mismatch was arising from within in the capture program. Is there a 'capture mode' or 'download mode/speed' in your capture software which might have varied. The other candidate was the flushing process. Have you tried leaving a longer gap between flats? I don't remember the details but there was some talk of getting rid of the RBI (residual bulk image) when I was fighting this problem. I evolved a messy workaround in which I stacked with and without flats and averaged the two stacks. Olly

The problem remains that the moving mirror focus system is fundamentally unsuited to planetary imaging with its incredibly critical focus. Any adjustment shoots the planet off the chip. The electronic drawtube focuser is the ideal solution but a manual drawtube focuser also works well. Olly

I think John put his finger on it at the start when when he said, 'there are recommendations in there regarding the minimum aperture telescopes that are considered "useful" (whatever that means) for astronomy.' A lot of these books were rooted in amateur astronomers contributing to professional astronomy, very much a Patrick Moore thing. However, SPM was also a great advocate of the binocular. So if the 50mm binocular is worth having, why would the 50mm telescope not be? In a nutshell, if you are not doing science, but just enjoying the night, you can enjoy whatever you like. The only instrument I own, and in which I can see the whole Veil Nebula in one go, is a slightly less than 3 inch doublet. This TV Pronto cost a couple of hundred quid second hand. It can also show me the Rosette Nebula in full. I find that 'useful!' Olly

If using the moving mirror focuser it is vital to make the last adjustment a push of the mirror up the tube. If you do the reverse the mirror may take a while to drift back down the tube to catch up with the mechanism, so changing the focus as it does so. On most SCTs, I think, that means turning the knob anti-clockwise as the last operation. Check this for your instrument. It is certainly better, when imaging, to lock the mirror close to critical focus and to use a back-mounted conventional focuser. But... when the seeing is bad there is nothing you can do. Have you tried focusing on a star to see how consistently that behaves? Olly

That looks as if it should be fine. Your DBE-adjusted image shows that the gradient is well within the range DBE that can handle. You just need to refine the way you're using it. As it is you've got a pretty even background, as AKB's application shows. Since you're only attacking a colour gradient, now, I would suggest subtraction of the gradient model, not division. Sky gradients are additive, as Wim said earlier. Personally I would not place many markers (maybe 12?) and I'd keep them well away from the galaxy. (That's essential.) I find I rarely need to adjust the background sky colour after DBE but, if you do, you could try both SCNR green and Background Sky Neutralization to tweak it. The data will work! Olly

We need to be clear what the issues are. I don't see anything odd about the re-green gradient, red left, green right. We get those here even with virtually no LP. I often got them either side of a diagonal with my OSC CCD as well. DBE should easily fix that. (ABE won't do a good job on galaxies. It tends to put markers too close the galaxy and introduce dark/light rings around it as it compensates.) But why haven't the flats taken out the vignetting? Do they usually work properly? Olly

Flats with a CMOS camera need dedicated flat darks taken at the same gain and binning as the flats themselves. Could you clarify what the top and bottom images are in your post? I'm not sure what I'm looking at. Olly

It's not debayered. I don't know how to ask PI to debayer but presumably you've done it before. There must be a debayer instruction somewhere. Olly

If it doesn't work I'd just use a guidescope. An OAG with a small refractor strikes me as being a solution looking for a problem and creating one. But do you have a guiding problem if you don't refocus the guide cam? If you go for a mid-position compromise? Olly

'Get it dirty! Use it!!!' 'But what if I want to sell it?' 'You won't.' 🤣lly

1+ for polar alignment. You could try the DARV method. https://www.cloudynights.com/articles/cat/articles/darv-drift-alignment-by-robert-vice-r2760 Olly

I find Pixinsight inordinately complicated and would simply try stacking in a different program. It does look as if the stacking is introducing the problems including the stellar distortions. AstroArt is my choice for stacking and calibrating. The free trial version allows you to to stack but not save. Alternatively you could try DSS, which is free, or post your precise workflow on the PI forum to see if anyone can spot an error. Olly

I'm listening but not convinced. Once the L is added to RGB it ceases to be greyscale and becomes colour, yet we still see the improved level of finely resolved detail. My own explanation for why this happens is different. The L layer has, per unit time, about three times the signal of a single colour layer because it simultaneously captures R and G and B. Because it has three times the signal it goes deeper on the faint stuff and can take far more sharpening and contrast enhancement. (Sharpening is contrast enhancement but on smaller scales that what we consider contrast local enhancement.) I think that's why it is effective. I quite agree that a greyscale rendition of an image will allow us to discriminate more detail but the problem is that LRGB is not greyscale. It's RGB. The L is absorbed. If you want to be really crafty begin by making an L Ha-R G B image and save it. Now add OIII to the green channel and save this as OIII to green. Go back to the L Ha-R G B and add the OIII to blue, calling it OIII to blue. Now, in Ps Layers, you create 3 layers thus: OIII to green OIII to blue L Ha-RGB. This lets you adjust the green-blue balance of the OIII in real time while seeing the result. Olly

Have you tried a stack without darks? Olly

Are you stacking two stacks together or stacking all the individual subs in one fresh pass? I read your post as stacking the stacks. Olly Edit. Dither is an excellent weapon against walking noise.

A a general rule I find that the processing is made easier if you match exposures, so 3 hours of luminance for an hour each of colour. However, it is possible to shoot far more than that, say 6 or 9 hours of luminance for an hour per colour, but you may have to work on adding the L in iterations so as not to bleach out all the colour. As Wim says, the main place for Ha is in the red channel because it is red. It won't pick up any blue signal so it will dim the blue down if applied as luminance. The classic Ha as lum 'look' is pink nebulosity with big blue star halos. Not good. Once you've added it to the red you can try a bit as luminance but I limit it to about 15% opacity when I do use it that way. You can add OIII to both green and blue channels, too. Some people make an HaOIIIOIII image (called HOO) and then add RGB stars to it. This strikes me as an illogical approach. I make an RGB image with a gentle stretch to get the stars small and colourful and then lighten the red with Ha and the green and blue with OIII. It avoids the clunky artificiality of blending in the RGB stars and preseves any broadband blue in the RGB layer. This gives a better rounded colour space. Olly

Nice natural M31 with tight little stars. Good core, too, though I'd soften the very middle part with a bit of blur because it looks slightly stellar. If you go deep you can't fit in all of M31 at 530mm on a full frame chip. (I've tried!) It will need a mosaic. Olly

The stars are not points to my eye, Mike. They fan out to the right with soft extensions. The smaller ones show this best. This is enlarged and annotated to show the extensions I'm talking about. If I had to guess, I'd say it was not from guiding error but was optical in origin. This kind of artifact can arise simply from soft focus but it can also be explained by tilt or mis-collimation. Refractors can be out of collimation though we usually think of reflectors needing attention. The fact that you didn't get it on all images points either to focus having drifted for this image or to the angle of the scope affecting any tilt. Slack in the drawtube, for instance? I think your main concern is the diagonally aligned colour noise, though? Could the cooler have gone off for some reason? Maybe the chip temp is shown on the subs' file data? Olly

I think CMOS has changed OSC. Olly