Padraic M

Absolutely Olly I agree, but you do still have some control over it, and processing at the linear stage is a relatively simple way to get started with HaRGB/LRGB/HaGB/(HaR)GB combinations etc. As you say, you are deciding how much you want to highlight the Ha content in your image at the expense of non-Ha red frequencies which is not a natural representation of the scene. You've given me food for thought on non-linear combination, work for another day! @Rodd the Red stack contains all of the Ha light as well as other colours - the standard red filters on the market filter in a continuous range of wavelengths both shorter and longer than Ha at 656nm. A narrowband Ha filter only allows through light at the Ha wavelength +/- half the bandwidth. It filters out all of the other red light that a standard red filter would have allowed through, but it doesn't include any light that is not transmitted by a standard red filter. So by selectively processing Ha and Red you are effectively emphasising that narrow band of Ha at 656nm by attenuating or even removing all of the other red wavelengths. All while (although not necessarily) leaving the green and blue colour levels untouched.

It might help your thought process to separate the original question into multiple questions: 1. how to register multiple channels from different cameras and scopes to a single reference frame and image scale 2. how to combine three OSC channels with one Ha channel to create a new three-channel RGB image which includes Ha to your desired recipe 3. as an advanced topic, and as detailed by @ollypenrice above, do you combine linear data only, or can you pre-stretch the channels to best advantage before combination? For question 1: I use Siril for stretching, but I don't use it for registration and integration. I use Astro Pixel Processor for that, and then import the linear integrated image into Siril. APP does all the hard bits relating to scaling and registering the images for you. It may be that Siril can do this too but I haven't tried it. To build a HaRGB integrated image in APP, you need to first separate the OSC linear image into separate Red, Green and Blue channels. On the Calibrate tab, select 'Split channels'. Then, integrate each channel separately to create Ha, R, G and B stacked images. You can register all lights to the same light frame at this stage if you like. Just make sure that the reference frame is integrated into its own channel, and then it is deselected but still marked as the reference frame so that it is not also integrated into each of the other stacks. (As an alternative, you can register and integrate them completely separately; then, reimport the integrated frames as Lights, register them together, save the registered frames and continue on as usual). Note: When you are registering light frames from different cameras or scopes in APP you need to set some specific parameters on the Register tab, or the registering and integration won't work. The most important one is to deselect 'Same camera and optics'. It is also advised to select 'Use dynamic distortion correction'. If the two pixel scales and fields of view are reasonably similar this is probably enough. You can crop the channel images using the Batch Crop tool to remove any stacking artefacts. But if the scales and the FOV are very different, the techniques required become similar to creating a mosaic. There are good tutorials on the APP website if you need to get into this level of detail. Also, having close to the same camera rotation angle on both setups is important, as even a small rotational difference can result in a lot of cropping around the edges. For question 2: There are many options for combining the four channels into a single image. In APP, after integrating each of the four channels, you can use the 'Combine RGB' tool on the Tools tab. You can play with different recipes or create custom ones here - try starting with HaRGB and playing with the sliders. Ha is usually added into the Red channel but you can experiment with how much of it you want to include, and also if you want to reduce the level of Red that comes from the OSC Red data. You can alternatively add the Ha data as a Lum channel; I find that adding it at much more than 20% washes out the OSC colour significantly. I do all of this is with linear data. When you have saved the integrated linear HaRGB or LRGB image, you can close out of APP and continue processing (stretching) in Siril as you would do with any RGB image. As the four channels are already registered, combining the channels can equally be done with Pixelmath in PI or Siril; or can be done with Photoshop or Gimp. I don't think the RGB composition script in Siril will help - afaik that is for combining RG and B channels from a mono camera and filters into a single RGB image. I'll leave question 3 to others with more experience!

That's a beautiful of piece of Victorian engineering art. I wouldn't like to have to tweak the polar alignment though!

A reprocessing of this image, using GraXpert gradient removal tool. Workflow is Stack all subs in APP -> linear stack into GraXpert -> linear output into Siril for stretching/median filter/stretching -> final touches in Gimp. I think this allowed more stretching for the same noise levels, and brings out more detail.

Yes, 20% overlap. The preview in the Framing tab looked fine.

I know that this is likely to be the start of a multi-year project, especially as for some reason the weather tends to be rubbish whenever Andromeda is in prime imaging position, this year being almost the typical example. But, as with all things that will take too long, the sooner you start the sooner it will be finished. I have two set-ups for imaging: - SW Esprit 80 & ASI1600MM Pro that I use for mono Hydrogen Alpha imaging in the city; this gives me 1.9"/pixel. - SW 150PDS & ASI553MC Pro that I use for OSC RGB imaging under dark skies. Scale is 1.1"/pixel. I had already captured some Ha as a 2x1 mosaic, giving all of Andromeda in frame. This weekend I had an unexpected clear and dark sky, with a little over an hour of Andromeda remaining before it set. An ideal time to start an RGB mosaic - NINA showed that I would need a 4x2 mosaic with the 533 to cover much the same sky as the 1600MM 2x1 Ha. Given that the weather is as it is, I decided to go for broke and get a small amount of data on each panel - 2x5 minutes on each of 8 panels to be exact. The good news is that the skies stayed clear for the 80+ minutes needed for that pass; the bad news is that there seems to be a gremlin either in NINA's mosaic planning or the sequencer; I haven't figured out the full coordinate map just yet, but there's a gap in the data between the frames in row 1 column 3 and row two column 3 if you know what I mean. I can always come back later, or in later years, and fill that data gap while capturing more data for the full FOV. In the meantime, APP did a really good job registering all 8 panels, and adding in the Ha mosaic, but it struggled with the data gap in the middle. Multi-band Blending (MBB) at 20%, Local Normalisation Correction (LNC) off gave the best results. Also the colour rendering is off slightly between the top half and lower half (stacked separately). So, a work in progress, but encouraging for continuing with the project. Now that I know the best settings for registration and integration, the next step is to stack all 8 panels in one go to see if I can get the colouration more even. Data to date: 4x2 OSC RGB 80 mins (2x5mins each panel) + 2hrs 40m Ha 2x1 (1hr + 1:40)

This is 2hrs of OSC RGB on M78. As an emission nebula it's ideal for OSC in dark skies, and an ideal size for my SW 150PDS/ASI533MC Pro combo. NINA capture, APP and Siril processing. The green channel is really clean, but both blue and red were quite noisy. Is this a feature of high humidity in the atmosphere? I excluded an extra 1 hour of lights as 99% humidity gradually became 100% cloud. Quality improved a little with less-but-better data and I could stretch more without highlighting the noise.

Had a similar experience myself last night --- spent a few hours between midnight and ~2am alternating between Jupiter through the C8 and touring widefield with the binocs. Similar estimated SQM here - just under 22, but last night I think was particularly clear. I could see the equatorial bands quite clearly, whereas I normally have to make myself believe that I can see them. I could see more of Andromeda's extremities than I normally see, and I definitely considered that I saw the Pleaides nebulosity. In fact, one problem I had was, with so many stars to be seen, it was getting difficult to make out some of the constellations. Woke up again around 5am, and Saturn was smiling in the window at me, but given that I'd only had a few hours' sleep at that stage, I decided not to get up. I'm setting up tonight for some AP, but while it's beautifully clear at the moment, the forecast isn't good for after midnight.

I'd agree with most of the posters here that an OAG is the best bet for both of the scopes you mention - both with mirrors, and especially the long f/l. HOWEVER, I'm writing this with gritted teeth, as in my own personal experience (I'm sure everyone is different!) an OAG is the single piece of astro kit that I have most repeatedly wanted to fling against the wall as I found it to be an absolute pig/beast/dog to set up. I'm trying really hard to pick words to describe the experience that won't get me blocked from the forum! I was using a C8 XLT which I finally gave up on as it's not really suitable for DSO AP, and the star shapes at the edge were horrible. Guiding was not great at all, but this is likely to be because of the poor optics out where the OAG was trying to operate. On the plus side, once you do eventually get the back focus correct and the focus correct and the prism depth correct and the sensor angle alignment correct, you can lock everything up and leave it indefinitely. As @ollypenrice said though, you can do that with a guide scope too! If I ever got to upgrade to an Edge HD I would bite the bullet and strap on an OAG.

The 533 is a newer model than the 183. Based on the specs alone, (https://astronomy-imaging-camera.com/product/asi533mm-pro/ and https://astronomy-imaging-camera.com/product/asi183mc-pro-color/) the 533 wins out on pretty much all aspects. Better QE, read noise, 14-bit ADC vs 12-bit, much bigger full-well capacity all make the 533 a more sensitive camera with a higher dynamic range. For the small price difference I think the 533 is a better bet. I understand that the 183 suffered somewhat from amp glow, and the 533 does not. This is not a massive issue because calibration will remove this anyway and you will need to calibrate both cameras, but the less adjustment that is required, the better. The other qualitative difference that could sway you is the size/shape of the sensor. The 183 has a rectangular sensor with more pixels. The 533 is square. Some folks don't like that but it's down to preference. Play around with Stellarium or telescopius.com to see the different image frames captured with each camera paired with your WO scope. I have the 533MC and am very happy with it. If I have dark enough skies, I can sometimes get away without calibrating at all.

Yes, you can combine different image sets from different sessions on different rigs. Uncheck the 'same camera and optics' option and it will automatically rescale the images. There's also a 'distortion correction' option that addresses differences in curvature etc. between the two rigs. It's very smart. Here's a test shot showing how the smaller FOV red, green and blue channels are registered in the centre (given that one of them was the reference frame), with the larger FOV Ha frame rotated and overlaid and added to the red channel. The size of the central RGB square is approx 3000x3000 pixels which is the sensor size of the 533MC so that suggests that all subs are resized to match the reference frame.

Thanks @Ouroboros. This was the first time doing this process and there is lots to learn. I'm surprised how much difference there is in the FOV given the pixel scale difference and the bigger sensor in the 1600MM. I'd like this to be a regular feature as I have the Esprit/1600MM setup in Dublin using narrowband to beat the light pollution, and the 150PDS/533MC is in Kerry under really dark skies, where OSC works well. I have both systems working reasonably well at this stage, so the thoughts of swapping the cameras from one to the other would fill me with dread! I'll need to do some more experimentation and see what works best.

I managed to capture some nice Ha data of the Crescent Nebula over two clear, if moon-lit, nights in the last two weeks. City skies, which Ha tolerates well. I also had some RGB data from last year. The starfield was a bit psychedelic. iirc, it was also from two full-moon nights, but in otherwise very dark skies. I've processed both sets together, using the two integrated stacks from APP, separated the RGB channels, and added Ha to the red channel as a new HaRGB image. Stretching and tweaking in Siril, with final adjustments in Gimp. The Ha data is SW Esprit 80/ASI1600MM Pro, and the RGB is SW 150PDS/ASI533MC Pro, so very different fields of view and completely different rotations. After a bit of trial and error, I managed to get APP to register the four channels perfectly, cropped and combined. Siril photometric colour correction and some judicious stretching. It's still got a lot of stars, but my experience with star reduction/removal hasn't been great so far. Might leave that for another day. Siril registration really is great. I think the key to getting it to work was increasing the number of stars for Star Analysis to 1000, and using one of the RGB channels as reference for registration. Capture times were: Ha 37x 600s (6h10m) RGB 70x 300s (5h50m) Total of 12 hours. Full Ha frame below just for reference.

Play around with the calculators on this website: https://astronomy.tools/calculators/dust_reflection_calculator Those spots are reasonably well defined, so not very far from the sensor. Definitely not something on the t-shirt. Based on your camera pixel size, and the focal length and aperture of your scope, the calculator quotes a distance of 2.7mm from the sensor. Does that sound like there's dust on the Optolong??? Anyway, rather than trying to clean your filter (you risk scratching it, and you will just get more dust on it anyway), getting proper flats will calibrate out any dust or dirt anywhere in the imaging train. Maybe just a quick blow from an air blower would also help.

Great shot, and well done! I don't use Deconv much, as if the image has any level of noise tends to will emphasise it. Something to come back to later when you have very smooth images.