The Admiral

Well, by some very heavy cropping I managed to graunch it through, to get this: The centre of the nebula seems to be saturated. Ian

Hmm, I can't do anything with this image, I don't know where to start with the gradients. This is a straight auto dev on the fts. Wipe doesn't seem to do a good job. Ian

Ah, I found this image from ESA's Digitized Sky Survey 2, taken in blue, red and infra-red. I see it is only about 20" across, so very small. This image is 1° square. Ian

Forgive me, but I really must show my ignorance here . What is the Cats-Eye in these pictures? Is it the faint blue whisp in the 7-o'clock position from the blue 'star', is it the blue 'star' itself, or is the blue 'star' just the centre of it? I'm having a lot of trouble matching up published pictures of it with what I'm seeing in your images! Ian

I think this thread is worth a read: https://stargazerslounge.com/topic/245183-to-stack-or-not-to-stack-30-x-1s-1-x-30s/ I may be wrong but I get the impression that the 1 photon/minute example is a bit simplistic, because what you are stacking are images which are essentially sky background plus a little bit, and the signal to noise ratio for the wanted little bit of signal does improve with many subs. It is true, I think, that the longer the individual subs, the deeper you can go. Though perhaps that contradicts what I've just said . I still haven't got this noise thing fully under my belt! May be, what is a lot more important is the degree to which the wanted signal is above sky background. In other words, a dark sky produces the greatest gain. I look forward to the results of your experiment in this. May be it's just me Nige, but I prefer your first image to the second longer total exposure one, less noise. They are round the right way are they? The trouble is, slightly different processing in ST can make all the difference. Do you think the gains from all the extra exposure time is proving worth while? Incidentally, which of the 2 images have you uploaded? Ian

I haven't had that, but then I don't think I've loaded that many frames. I'm not aware of a limit, but if anything I wonder if it's a size limit. How much RAM do you have, though I'm not sure if this is critical to DSS ? I have similar problems with AS!2, though I know it's not the same beast. Come to think of it, is DSS 64-bit or 32? Sorry, not much help! Ian PS. Do you get any error windows or does it just stop? PPS. You could try using the Windows task manager to see how much memory is being used.

Well, I guess that you live at No 20! Not an ideal lighting arrangement is it, but it does at least appear to be directed at you rather than the sky, so at least you can get out of its direct rays. Baby or Elvis? Not until you mentioned it! Ian

Hi Ken, Well, that was fun, thanks for posting. I've got so rusty at using ST. For what it's worth, here's my scrappy attempt at processing using ST. Mind you, there is such a multiplicity of processing options, as I guess with any software, that one doesn't know just how much is 'real'. Ian

Nice one VSOP, and what a lucky capture, that meteor looks greenish. It would be interesting to see what more frames could do. What aperture were you using? Ian

I've been off-line for a while and all your image attempts are good for the soul. It's good to see progress being made, and I must get back into my stride; I haven't imaged for ages. Steve and Nige, your wide-field shots are tempting me in that direction too, but as Steve says, we get so few good nights and we have to choose. Nige, don't forget that the StarTools log file allows you to see what settings you made and when you made them, so in theory you could manually step through each setting in turn and see when the green spots appeared. I always wish that ST could do this automatically, or at least present you with a history on-screen so that you can click through in turn. Ian

Try a search for "130p-ds" in the "No EQ Challenge" thread, that'll narrow down your reading! But in any event, I'd say, "Go for it"! Ian

I think the 150p is going deeper Nige, but that 210mm short exposure shot looks little different to the long exposure ones to my casual eye. They are surprisingly good. Ian

That's a nice image Ken, with plenty of colour around the periphery. Considering your total exposure must be around an hour, and with an 80mm 'scope, perhaps your skies are darker than you thought! How much 'yuk' was visible before subtracting the background? Ian

Hmm, that's an interesting question Steve, needing a bit of thought! My gut feeling is that the answer is yes, but with a caveat. If the pictures are presented at the same size, then the cropped one would need to be enlarged more, and this would reverse the gains you've made by cropping. I guess the exception would be that if you've cropped to the point where the rotation is less than a single pixel, then it wouldn't make any difference how much it is enlarged. Then again, I imagine that would be fairly extreme cropping. I reserve the right to change my mind at any time Ian

You are right Steve, so long as you are referring the the same star. But, a star on the edge of the frame will appear nearer the centre when used with a shorter focal length. Another star, again on the edge of the frame, will still move by the same amount as your original star with the longer focal length. Another way of thinking about it is imagine a line passing through the centre of the frame. If the camera, or image, rotates by, say, 1 degree, that line will appear to rotate by the same amount, irrespective of the fl of the lens attached. Curry and a beer? The answer to most problems! Ian

Presumably these are just stacked and not corrected for field rotation. Ian PS. Sorry, I see I've not entered into the spirit of your post!

I agree with SilverAstro, they don't look like rotation, more like just the normal effect I see on subs as a result of the mount moving. But the fact remains that field rotation is not visible, when I'd have thought it would have shown up. I don't really know how kappa-sigma clipping works, but will DSS just ignore those stars that are badly trailed? Ian

Ha ha, yes! And though I may sound convincing, I may not be right . But in this case, having thought about it again, I don't think you'd end up with needing to crop as much by rotating the camera as if you just let it run without a break. Indeed, I suppose if that wasn't the case then de-rotators wouldn't work. Yup, we're back to them! Ian

Steve, the trouble is that rotation is rotation, and it won't be affected by the focal length of the lens/telescope. This is not the same as cropping which, although simulating a longer FL, would allow the most affected parts of the image to be removed. Ian

My emphasis. Why rotate in software, why not rotate the camera at the beginning of each set so that you get back to roughly the same starting position for each set? Presumably DSS would stack them all as one set? You may need to do flats for each position if the field uniformity is not axially symmetric, in which case I guess you'd need to stack each set separately. Ian

Well by my reckoning at an altitude of 60° and due South, it would take about 20s for a 0.1° rotation, which is what I use as a basis for acceptable limits. So for a 1 minute sub you are talking of a 0.3° field rotation, and I don't know what that would be in terms of pixels. I can't see any rotation in jimbo's image, so perhaps my criterion is a little too strict? What do others feel? I can't help thinking that if DSS was morphing the streaked stars into one rounded one, then we'd see a noticeable increase in star size towards the edges. And I don't see why it should because it's not as though the 'streak direction' is random, they'd all point in the same direction. Interesting! Ian

Unlike a finely tuned EQ mount, there are two main problems with the Alt-Az mount: field rotation and tracking performance. In order to minimize the manifestation of field rotation on one's images one needs to use short exposures, only a few tens of seconds, so a total exposure of 4 hours or more will produce hundreds of subs. Star streaking will be evident in a percentage of these subs, the more as the exposure time is increased (even if that time is acceptable from a field rotation point of view), and DSS will reject them, so the integrated exposure will be less than one might hope for. I think it's also true that the overall tracking performance of Alt-Az mounts is somewhat below that of the correctly adjusted EQ mount, so if one did embark on a long exposure session it would have to be done in a number of shorter bites with mount realignment in between. I'm not saying that all this couldn't be done, but the reason I'm using Alt-Az imaging is that I have to set up the gear each time I use it so it needs to be simple and quick to set up (and I can't see the Pole star from my observing position). As Steve alluded to, this is because here in the UK the weather seems so unpredictable that often the decision to image is a last minute one. Personally, I'm not striving for salon-quality images, but just to reveal the glory of what I can only see as a grey smudge when viewed by eye. That is not to say of course, that I don't want the best output that I can achieve within the constraints imposed by this style of imaging. But one has to be realistic about the law of diminishing returns. Don't forget also that much of the UK is blighted by light pollution, so that is yet another issue we have to contend with. Ian

Trouble is, a 4 hour or 9 hour stint using an Alt-Az mount does create other problems! Ian

Steve, I may be mistaken but I think I've read that kappa-sigma clipping is used in association with dither. Whether the meanderings of an Alt-Az mount and/or field rotation is equivalent to dither I don't know, but it'll be interesting to see what effect it has. To be honest, I don't think I know exactly what kappa-sigma clipping actually does. I thought that using darks also has the effect of removing things like amp glow. Deary me, it's so long since I've done any astrophotography I'm pretty rusty on all of this Looking back I see that I generally have used k-s clipping when stacking, but I haven't done a comparative study between using darks and normal stacking, and no darks and k-s clipping. Ian

I've no experience of using that, but I think I'd be concerned about mud coming through. The bark mulch idea would be cleaner I should think. Ian