The Admiral

Well, a question worth asking, and the quick answer is anything's worth a try! A couple of points to note. Un-modded cameras vary in their red sensitivity, I can't comment specifically about the EOS 1000d, but there are plenty of folk on here who can. We tend to use 'scopes with a FL around 500mm and even so the Rosette's a tight fit. The Rosette is 1.3° in diameter, so I think you'd have problems imaging the whole object. As happy-kat suggests, you'd probably have more luck with your camera lens at 200mm. I'm not sure what you mean by 1:00-1:30. Do you mean 60-90s per sub, or an hour to 1-1/2 hours total? Presumably, if the former I guess you'll be using your EQ mount (off-topic for this thread ). But I'd certainly aim for a minimum of an hours total exposure, though I've revealed it with as little as 20m (see https://stargazerslounge.com/topic/261523-the-glorious-orion-and-rosette-nebulae/ though I've come on a bit since then!). Ian

Correct me if I'm wrong, but can't L be extracted from RGB? Otherwise it wouldn't be possible for Bayer arrays to work, would it? I'm not sure where the practise of doing a separate L channel exposure in astro imaging has come from. So why can't Ha just be used in place of R, why do you need to add an L channel replaced by Ha? I don't understand how that would add anything. This is an area where I know little, so I'm happy to be educated. Ian PS. There's a lot of detail in you Ha image already.

There seem to be a multitude of ways, just search on SGL. I've used a laptop screen to provide the uniform light before now, others recommend stretching a T-shirt over the dew shield and expose to a uniformly lit sky, but in the end I went down the route of using a home-constructed LED light which I've described on the forum (See https://stargazerslounge.com/topic/274057-flats-light-final-chapter/). It's worked out very well because I can slip it over the end of the 'scope without disturbing it, and so can run off flats any time during the session. Ian

Yeah, well you see, I'm only a beginner . This darned light pollution! Well here's what I got after a bit of stretching. This is the green channel, other channels are much the same, that is, a couple of bits of crap visible and vignetting at the extreme corners. Top more than the bottom, so I'm thinking there's a bit of focuser droop, which wouldn't surprise me as I get some elongation of stars towards one edge. Ian Edit. Oh hang on, you're referring to Ken's flat not mine. Ha Ha!

That certainly is a design flaw. It would have been good if ASI had offered to replace/upgrade earlier models. I'm pleased to say that I don't seem to have a great deal of problem with my camera; I think the sensor 'cleaning' cycle it invokes each time I switch off is doing a fair job. I'm surprised at the amount of vignetting you are getting Ken. Is that because the filters are only just large enough, or did you get that with your 'scope prior to use of the ASI? This is how mine turn out, which I'm very pleased with. Ian

It wouldn't surprise me if the dust is emphasised because of the narrowness of the passband, leading to interference effects. I guess that's the purpose of taking flats to correct for it! I had a solar 'scope once and it would have been interesting to have done narrow-band imaging of the night sky. I sold it though in order to acquire my current 'scope, and in any event I soon realised that I really needed a monochrome astro-camera for it rather than my mirrorless, which was more than I was prepared to invest at the time. Ian

Nice. I'm envious of your FoV! Ha does seem to give a 3-dimensional effect. Does Ha present particular problems in this regard? I'm thinking in terms of the emission spectrum of the source used. I use an led light, others laptop screens etc. and I wonder what they emit at 654nm. Ian

Thanks Nige. The variation between screens is a perennial problem when you want to put your conventional artwork onto the web, as you never know what the observer is seeing. One goes to great lengths to use a colour managed workflow, but once it's out in digi-space all control is lost. Ian

That's looking very promising Ken, plenty of detail in just a single sub., and you have the benefit of a FoV which gives good coverage. Looking forward to see the finished image. By the way, what version of Skysafari are you using, and does it use the iCloud to provide proper operation? There doesn't seem to be a decent comparison of versions. Ian

What's your target colour temperature? I set my monitor to 6500K, and I assume you can do the same with TVs, or do you use native. TVs generally are much brighter than you'd use as a monitor, so you will see all the stuff going on in the shadows I guess. Ian

I don't have Photoshop (but I wonder if the plug-in works with Lightroom). Then again, I think ST has one option for that, would need to check out the manual! Yeah, well!! Is it colour calibrated? Ian

Interesting Fabien, neat. But I think that most of my stars are saturated though. Ian

No, I think you are right, it's the sort of artefact I might expect, but I wouldn't normally push things that far to check! It's obvious in that crop, too linear to be anything else I think. I just kept cropping away until the odd colours in the corners disappeared after stretching. I can just about make something out in the original now that I know what I'm looking for. Ian

I was thinking something along those lines too as I was replying, but not in such a clear way! I don't think that there is any way of precisely colour balancing, it's not as though one can introduce a grey card! There would have to be some assumption made about what constitutes a neutral reference. Ian

Thanks Ken. Just to be clear, when you say "..in your first image" are you referring to the one I posted yesterday, or to the first of the two images I posted today? I'm coming round to preferring the first one I posted today, at least at the moment I'm not convinced about a stacking artefact in the lower right, I can't say it's obvious to me. Pixel values in the 'black' areas in the lower right appear to be very similar to others in the image, at least within the variation. Also, in say this example image, there is a trace of nebula in that region. I don't use either of those but I suspect that there is a feature in AstroArt. I've already trialled PI so wouldn't be able to try that again. Ian

Thanks Nige. Colour is such a personal thing I find. With images in conventional photography it's generally very apparent when colours are a bit 'off', as we're all used to seeing them for real. Not so with astro though, particularly when there is so many images using narrow band and false colour. Looking on t'internet reveals such a variety, and more often than not the fact that H-alpha was part of the blend is only in the small print. For what it's worth, I didn't use the colour module in ST or make any colour temperature changes or hue shifts in LR, so I guess it's as native as it can be, barring any default colour balancing that ST does I suppose. Interesting that both you and Fabien have commented about it though. Ian Edit. I could do this with it! Not sure which I prefer, or is the more valid.

Here's my result for the Rosette nebula which I imaged on 4th January and 19th January (exactly one year after my first attempt at imaging this object). I collected a total of 360 x 30s subs, from which I managed to stack 213 in AstroArt. I used 60 flats and a generic bias from 200 frames. Processing was performed in StarTools and final polishing in Lightroom. I didn't use the ST colour module for this as it seemed to produce gregarious colours; I generally find I don't use it for targets which give me a decent signal, but for weaker objects I find it brings colour to an otherwise fairly monochrome image. The FoV of my 'scope only just covers the Rosette, so I needed to avoid having to crop too much from the frame, especially as this turned out to be long imaging session where field rotation was going to be an issue. I stumbled on a ST workflow where I would do an initial wide crop of the frame, trying to avoid the stacking artefacts, do a 'wipe' with 'temporary develop' switched on, and then if I was getting poor results I'd cancel the 'wipe', crop off a little more, then repeat the exercise as many times as needed to get a decent 'wipe' result. Why I hadn't thought of doing this before goodness knows The usual equipment: Fuji X-T1 through an Altair 102mm f/7 Super ED with a TS Photoline 2" 0.79x reducer/flattener. Mounted on a Nexstar 6/8SE Alt-Az mount. Hope you like it. Ian

It's a bit more than just a crescent though, isn't it? It looks like chromatic aberration because there is blue on one side of the stars and red on the opposite sides. This was similar to my case, except that it was green and yellow (just!). And I did use the ST colour module so the colours could well have changed. I suppose it could be an atmospheric diffraction effect we're seeing, as the targets never get that high in the sky, and there could be ice crystals in the upper thin haze of cloud. On the other hand, I do wonder what aberrations a Bayer matrix might give (though my Fuji has a non-Bayer filter array, though presumably just as susceptible but with a different result). This is all waffle of course as I don't really have an answer! I would need to go through the ST log files to see what differences there were in processing, though I do know that in the final one I used a 50% bin rather than a 35% one, and the effect isn't visible. I can't remember now what software you use to process Fabien. Ian

Thanks Ken, and it's worth a diversion on that link too to look at the information on the Subaru telescope. Quite an outstanding engineering marvel. Ian

Well, here's another go at processing the Christmas Tree/Cone nebulae. Over processed as usual in an effort to squeeze out as much from this fairly feint object as I can, so much so that it's looking rather messy. You guys with modified cameras should be able to do a lot better. This time I binned 25% rather than 35%. Not sure if that was the cause of the 'greenish' stars, but whatever, I don't think it is so evident this time. I think I'll leave it at that now I rather like this picture of it: https://apod.nasa.gov/apod/ap130806.html Ian

An even greater proportion in the Fuji X-Trans! Ian

Having another look at this I noticed a very straight edge to the nebula about half way down on the LHS, and I was beginning to think it was an artefact that I'd encountered using the deconvolution tool. BUT, having just Googled some images it appears to be the case! Quite remarkable that the nebula edge should appear so straight, naturally. Ian

No, they're in the original TIFs. When I re-process I'll keep an eye out. The target was drifting a bit so I had to re-align a couple of times, so I wonder if some slight optical distortion is putting the stars in slightly different positions relative to each as they drift, so that they don't align perfectly. Anyway, we'll see. Ian