The Admiral

You must have been about the only one with clear skies last night. Looking good for tonight down south. You're facing a challenging target here Ken, and your dogged persistence is paying off I'd say. The dark lanes look rather clearer on my tablet screen than they do on my calibrated monitor Ian

Ken, didn't you say that you'd realised that you could use your StarSense? Are you using it now and how have you found it? Ian

If you look at the subs you can see that they don't necessarily move in the same direction. I.e. the image wanders. And plus there is field rotation. And I think I read somewhere that dither just makes the rejection algorithm more effective, so I'm guessing that so long as different pixels are exposed that it must help, even if not truly random. Ian

Have no fear Steve, if you stick with DSS I don't think there is the option Ian

Yes, or flat and flat-darks. But just not using darks may not be sufficient. As I say, you'd need to stack with a rejection algorithm, and possibly also a bad pixel map in order to replicate the other function of darks, to remove other defects, though I'm not clear on this. Ian

I absolutely agree Steve, but I get the feeling that one needs to replace them with something else. I'm coming to the conclusion that no-one (or at least very few) really know the answer to this. A more recent thread is also worth a read https://stargazerslounge.com/topic/276824-dithering-vs-darks-dslr-how-many-darks-anyway/ It seems the accepted approach is to use the bias as darks and use a rejection mode (e.g. kappa-sigma clipping) for stacking. A bad pixel map is also referred to. More reading is needed I think. Ian

The 'darks' debate has been around for some time. If you are interested, here's a thread that might set you thinking https://stargazerslounge.com/topic/223975-dslr-imaging-do-you-really-need-dark-frames/?page=1 Ian

Indeed Ken. I seems to me that the tracking ability of our mount (or perhaps, slight lack of ability) means that we may be doing the equivalent of dither (is it random?), so all we should need is a defect map and a clipping algorithm in the stacking. I'm none too sure of the exact procedure though. I think that now I have AA I could be in a position to create a defect map. I know Olly doesn't recommend darks for DSLRs, but rather uses the dither/reject approach. Does your cooled sensor need darks? Ian

Hang on, I need to get my head around this . When you take darks, flats and biasses, they will always be matched to the sensor, pixel by pixel. Field rotation doesn't come into it. If the remaining noise in each pixel is truly random it shouldn't matter which pixels are stacked together, so this shouldn't be affected by field rotation. After all, with EQ mounts it is common practise to 'dither' and it doesn't affect the background noise. As I see it, the problem is that there will be some hot pixels which are not random noise, and as they are stacked with other rotated frames they will leave a trail of hot pixels. Unless they are first removed by using the dark frames. Or, to use kappa-sigma clipping in the stacking. Grief, I can't keep up with all the posts during the time it's taken me to write this! Please forgive any overlap. Ian

Nice one Ken. Good to see the complete loop of dust and nice tight stars. The Trapezium is 'just' resolvable I think, though not as clear as with your SCT. It's hard to get a handle on the difference in scale. I'm led to understand that the bright area around the Trapezium should be greenish when in true colour, an unusual colour to find, from the oxygen luminescence. This I think is more evident in your SCT image than the Esprit one. Ian

Me to, but in my case I couldn't bare to spend so many hours on one target . Ian

You raise all the vexing questions that I guess we've all been through, though we may still haven't come to any conclusions. The issue of image noise and ISO are very pertinent here. I don't know if you've read Craig Stark's article " Profiling the Long-Exposure Performance of a Canon DSLR ", but if not I think it's worth doing so. It seems that manufacturers get up to all sorts of tricks in order to give decent conventional images, which play against our use for astrophotography. Every image is affected by dark current and read noise, which manifest themselves, if you like, as a feint background glow. By taking bias frames (for the read noise) and dark frames this 'glow' can be subtracted out. The problem is, both the read noise and dark current have a random component of noise which can't be subtracted out. The best one can do is to take multiple frames and stack them, so that the random noise averages out to a smaller and smaller component. It seems that what Canon does is reduce the effect of the dark current (i.e. 'glow') before it is written to raw, but what it can't do is reduce the noise from the dark current, which continues to grow as the ISO is increased. So all is not what it seems. I must admit from that perspective I'm debating the worth of doing darks, but that said, I'm not sure to what extent it corrects for things like fixed pattern noise. You say that the darks you take are completely black. Is that after severe stretching? +1 for getting a spare battery. I'm sure we've all be caught out by that one and a spare is essential. Don't forget also that in the very cold nights their capacity will be reduced, so keep your spare warm before use. Ian

That's a very laudable approach Ken, but with the frequency of usable nights we get I think I'd worry I'd end up with a pile of unfinished targets! Ian

In my case, it's just that East and South are the only directions not affected by town lights. That, and that targetting towards the East allows me to capture objects in the evening as they rise, and before they reach too high an altitude. Ian

Yes I know what you mean Ken. I did try the colour module in a quick run-through in ST and the stars took on a lot of colour, but the nebulosity didn't respond well. May be with a bit more finessing it could come good. I've yet to discover how I'd do that in AA! I took 180 x 30s subs, most stacked by AA, about 150 in DSS, if I remember correctly. Ian

Is this the image where you've tried to reduce the star field Nige? I think that somehow you need fewer of them rather than just less intensity. I'm not sure how you'd do that though, unless you can construct a mask to include only the smaller/dimmer ones and then remove those stars. Haven't a clue about doing that though, I'm not good with masks! Ian

Well, I guess it's time to add my contribution . A bit of an experiment this. First off, I haven't had a go at M45 before, so this is a first. Knowing that we are dealing with a lot of bright stars here, I thought I'd use ISO400 rather than my usual ISO1600 to improve my DR, and I retained the 30s exposure to maximise the photon count to get the nebulosity. The usual flats, lights, darks and bias. Also, because it's a bit of a pain using DSS with my Fuji RAFs as I first have to convert all the files to DNG, I've bought a copy of AstroArt. This will happily accept my RAFs, but of course it is a new application so I'm still feeling my way (the manual rather assumes that you know what you are doing!). I started off with AA, but I got some peculiar artefacts around the bright stars. OK, now try with DSS. That turned out OK, but I normally use Kappa-Sigma on my lights in DSS so I thought I'd try the equivalent in AA again. Hey presto, that got rid of the artefacts . So I've now got a direct route to stacking my Fuji files. Great . I processed the O/Ps of DSS and AA in StarTools. Then I thought I'd have a go in AA, and to be honest, pretty comparable to ST. But then again, the target probably isn't the most challenging. I'm sure that there is a lot of improvement here, but still, it's a start. All three O/Ps needed polishing in Lightroom. This is the result using AA, but as I say, they all look much the same. Looking at the numbers that DSS throws up, the maximum score ~400, which is way less that I'd normally get with my reducer/flattener, so perhaps the conditions weren't that conducive to good imaging. Or perhaps I was a bit more cr** this time! I'll have another go on Monday/Tuesday, Met Office permitting. An interesting (dare I say, "cool"?) feature of AA is that you can look at a 3-D image of the star field: From that I'm guessing that a lot of the stars are saturated (hence the flat tops), so I'd need rather less exposure, but then either I I'd have to take an awful lot of short exposure subs, or do two sets with different exposures. Hmm Ian

I think the nebula has come out pretty well, but I agree with Ken that the mass of stars are a bit of a distraction. You could do that trick to separate out the stars and then blend them back in at less than 100%. Ian

That's a nice wide-field rendition there Ken. Alnitak is probably a lost cause so far as burning out I should think, it is so much brighter than the nebulae. Ian

Nope, you've still got me confused! What do you mean by 'tracking mode', and how can you set up tracking without using the handset? Ian

Good grief! I wouldn't entertain that with my 4 megs download speed, never mind upload Ian

Very seldom is my body willing when it comes to very late nights. That's what is nice about winter time, you can get a decent number of subs under your belt and come in at a reasonable hour! Ian

Well it was reasonable here until about 9 tonight, when some fragments of cloud started to come in. I managed to get an hour and a half on M45. Good to get out again after so long. Happy-kat, you have me confused! Surely you don't use Polaris to slign your Alt-Az? Ian

Great to hear Paul and good luck for 2017! Ian

Hi Nige, I've been reading some articles which seem to explain the issues rather well. They are by Craig Stark and can be found on his website: http://www.stark-labs.com/craig/articles/articles.html The relevant ones (Parts 1-5) are from 2009, "Signal to Noise". Ian