The Admiral

Where there's a will......

It's coming on, but as you say you are rather limited in data at the moment. It looks like it's a tough target though, and quite small. Ian

Perhaps on reflection you are right. I think I was confusing using different groups as tantamount to having separate stacks (they would need to be put into separate groups because they would need to be associated with the appropriate length darks) . Presumably that's not the case and that they are all essentially stacked together. Presumably you are also saying that if you produced different stacks on separate nights, that you would re-stack the lot together rather than combining the separate results? PixInsight does give one greater opportunity to finesse these things though, which of course is why it is quite complex. It still leaves me a bit confused about the DSS statement though. Ian

Whilst I was ruminating on the subject of stacking I thought I'd have another look at the DSS manual, and by chance came across a theory part that I hadn't seen before. No doubt others have, but here's the link anyway: http://deepskystacker.free.fr/english/theory.htm One of the details that caught my eye was the paragraph titled "Can I combine two (or more) resulting images?". Specifically this text, as I think it may have some relevance to Nige's question: " Absolutely, the square root rule applies with a small twist. When combining two images the SNR increases by 1.414 (square root of 2). If both images have the same SNR then this is the same as doing a single stack. That does not mean that the combination is giving the same image, just that the SNR will be the same. However if one stack contains more light frames than the other, the SNR of the two stacks will be different and the SNR of the combination will be lower than the SNR of a single stack containing all the light frames. Thus by combining the result of a 10x1 minute stack with a single 1 minute frame the SNR is roughly the same as the one obtained by combining 2 light frames. This is due to the fact that when combining two images the noise is additive and the best image is damaged by the worse image in the process." I'm still trying to get my head around this, but does it mean that if you stack, say, 100 x 30s subs and stack 10 x 60s subs, and then combine them, the resulting SNR will be dictated by the poor SNR of the 60s stack? If so, I think it means that the amount of time devoted to longer subs should be at least the same as to the shorter subs, in order for each to have the same SNR (or will it be the same???). But it seems a bit contradictory to me that adding more data should result in a poorer SNR though. I must be missing something (which, to be honest, wouldn't be surprising!). Also, I thought the noise would add in quadrature (i.e. the square root of the sum of the squares), not simply be additive, and the signals in the two images would be additive. So what would the resulting SNR be? Anyone got a clue? Ian

Good luck Ken, often a case of "more haste, less speed". I've been out in an all too rare clear sky trying to optimise the sensor-flattener spacing, but even by 20.30 the gear was covered in dew, so I don't know how long imaging would have been practicable. Given that they are forecasting a foggy morning, perhaps it's none too surprising. I managed to nudge a tripod leg trying to align on my second star, so had to repeat the first one again! Ian

Hang on! In an hour you'd get 40 x 90s or 80 x 45s subs. Ian

That's a thorny and often implied question, and I don't really understand the underpinning science. I always refer back to this thread: https://stargazerslounge.com/topic/245183-to-stack-or-not-to-stack-30-x-1s-1-x-30s/#comment-2664662 So far as I can tell, all things are equal, except for the read noise, which of course is higher the more subs you have, and the influence of sky-glow. Thus with a low noise camera, the more you can get away with. We actually need a bit of background light to get the sensor into the linear part of its response, so the question is, if you are just moving the response further up a linear response curve, what do you gain? Statistics presumably, but then you're taking more subs to counteract that. At the same time though, there seem to be a lot of folk who are definite that a longer exposure is better! I think that the longer your subs are, the deeper you'll see, but I may have that wrong. I may not be helping here!! I would just add that I'm in favour of doing dual duration imaging where the dynamic range is large, such as with M42, short to capture the bright bits, and long to capture the fainter bits. Ian

An alternative may be something like this : https://www.firstlightoptics.com/alt-azimuth/skywatcher-l-bracket-dovetail.html The only issue might be aligning on a star without a finder if the camera lens doesn't offer a very wide view. I guess if you are using a zoom, then finding your way around might be eased. I've not personally tried this though, so others may correct me. Ian

+1 to what Nige said. Vega and Capella are almost on opposite sides of the sky, so it's not giving the mount much of a chance to calibrate itself. If you think about it, the mount has a stored picture of the starry heavens, but it needs to work out whether there is any 'tilt', in any direction, to the hemisphere. If the alignment stars are opposite each other in the sky, it can really only do this in one plane. By including alignment stars which are roughly at right angles to each other, then the mount can work out any tilt in two orthogonal planes, which is necessary for accurate alignment. The mount does this by knowing the exact time and location, from which it can work out where stars should be, and compares them with what is actually the case. So, if you use alignment stars which are due north and south, it seems to me that because their change in altitude with time is relatively small, the errors in determining that tilt will be larger. So I tend to choose alignment stars not in those directions, if I remember . It's also why you shouldn't use Polaris as an alignment star because its position doesn't significantly change with time, so the mount can't get any useful alignment information from it. Ian

Welcome to this particular, rather niche, thread Yamez . You'll have seen my comments after your original post, but keep up the good work. Cheers, Ian

The secret is in the processing . That and stacking lots and lots of short exposure subs! Ian

A couple of lovely images there Nige. I'm amazed at how visible the Western Veil is with just 45 minutes of exposure. There again, a 6"aperture is more than I'm used to, almost twice the area. Strangely although I'm not a fan of diffraction spikes, somehow they seem right on M45. When I get around to imaging it (waiting for it to be well placed at a respectable hour), I might try using ST to add them. Ian

I can just about fit M31 in by using the APS diagonal, on a 715mm fl 'scope fitted with a 0.79x reducer (565mm effective fl). Ian

So I wonder if that means the cropped images needn't be the same size? If you can move one in relation to the other it follows that there will be borders where images will not overlap, much the same as if the images were different in size. Anyone know where the stipulation of the same size originated? Ian Edit. Ah, I see it was mentioned by a correspondent in the ST Forum thread. Worth checking out though, as his situation was quite extreme.

It's certainly brought out a lot more in both the images, but with the M33 I think it's at the expense of noise. Whether that can be tamed I'm not sure, probably. The thing about the plethora of these plug-ins is that I think, in theory at least*, one should be able to duplicate them using existing controls, so I'm a bit loathe to go out and buy yet more software. I guess it comes down to ease and speed though. Each to their own. Ian * Edit. Hmm, that may be more difficult than I thought, having had a play with your jpeg!

Don't we all! Consistency of forecasting has been woeful. Ian

That's great Steve, an order of magnitude better than your first process! Horse Head showing up nicely too. Ian

I've wondered what happens to diffraction spikes with Alt-Az imaging. Because the spikes are fixed in relation to the 'scope rather than the star field (in the same way that the image edges are too), then when DSS aligns the stars the spikes themselves won't be aligned. I can't say I've noticed much in the way of spikes with your previous images Nige, so I'd sort of assumed that if you are using Kappa-Sigma clipping they will be eradicated, like satellite trails are supposed to be. So I'm not too sure why the spikes are so visible here. Ian

I've been thinking a bit more about using ST to blend short and long exposure subs. Whilst DSS can be made to use a common master so that they will all be aligned, there is still the problem that if we crop the images in ST, as we have to to get rid of stacking artefacts, then unless the crop is exactly identical in both size and relationship to the star field, when we come to blend, I fear the stars won't be in the same positions. Obviously the situation is a lot easier with EQ imaging, but has anyone tried using ST in this way with images of Alt-Az origin? I guess it could be done so long as care is taken, down to the pixel level in cropping. Something to try when I next image, when my cold virus has run its course and the Moon ceases to impose. Ian

You've got every right to be pleased with yourself with that one, Nige. I think we are all pretty amazed at what we've achieved since we started this Alt-Az imaging lark, not known for producing anything of worth. Well, we've proved 'em wrong! Is that with combining in ST? I wonder if your diffraction spikes are because you are using a much shorter exposure time than you are used to, so the field rotation is less and they get less chance to 'smear out' during the stacking? Ian

That's a bit odd Nige, you'd expect the 10m of 10s to be noisier than 20m of 20s. Also, your second image doesn't seem to be as deep as the first, despite having longer time and longer subs! Is that just down to a difference in processing, or is there a chance that they could have got swapped? Nice result though. I also found this statement in the DSS manual: " If you select a reference frame that is not checked, the offsets will be computed from this reference frame even if the reference frame will not be stacked." So, all looks to be doable then. Ian

Thanks Nige. Ian

This morning I've been messing around with my M33 image again. This time I went back to manual develop in ST, and I didn't bin as I'd have to crop a decent amount for the final image. To be honest, binning 25% didn't seem to get me anywhere better. I think it's an improvement over the last attempt (https://stargazerslounge.com/topic/228101-the-no-eq-dso-challenge/?do=findComment&comment=3060403), and the colour balance seems better than my original (currently in my gallery), which was too blue. I think this is the best I'm likely to get to be honest. The reality is that it's not a bright object and there's a lot of noise, and it's not a large object in the frame. Finished off with a bit more noise reduction in Lightroom. Ian

Well done Nige, but I have to agree with Steve, far too late in the night/early in the morning for me to be out. I think the core is naturally 'messy', there's a lot going on there (see http://www.messier-objects.com/messier-42-orion-nebula/), and you've at least captured the characteristic green. It'll have to wait, but I do plan on having another go as I was only just setting out on this journey when I first did it. Then, I used 100 x 15s lights, but I think (a) the core may be a little blown, and (b) there is more to the nebular that I didn't capture, so this time around I'll try what you are doing and take subs with different exposures, though I'm still debating with myself what they should be. It seems to me that they can be combined in DSS, as I assume you did for your picture, or combined in ST. The only problem with our Alt-Az imaging is tracking and field rotation, so prior to stacking in ST they'll need to be aligned. If I understand correctly, as I've not tried this, you would take the output of, say, your 10 second stack and use it in the stack of the 20 second subs as a master frame (i.e. to which the 20s ones will be registered against), but presumably deselected so that it is not used as part of the stack of the 20s subs. Is that what you are doing? I'm not sure if DSS will allow you to use a non-selected image as a master though. More to learn . Looking forward to seeing what you get out. Ian