ollypenrice

Blue bloating is a form of CA but might have varying origins. It might be that the scope's focal length in blue doesn't closely match its focal length in red and green. This would be because it wasn't sufficiently apochromatic. However, the blue bloat might come from a failure of the filters to block she shortest blue wavelengths as they approach the UV. Or bloat might arise from a combination of the two. I think the ED80 might be an awfully big scope to put on the guide scope carrier. You really do need a rigid attachment of the slave scope. This would be a lot better. https://www.firstlightoptics.com/misc/jtd-dual-rig-telescope-alignment-saddle.html I know it's expensive but our Cassady cost nearly twice that second hand! Olly

I don't think it's the way to go and don't know of anybody mounting dual rig scopes one on top of the the other. You need to keep the system as stiff as possible so side by side is best. (Otherwise you're using the lower tube rings to carry the upper scope with a huge moment working against the system. You also need to be able to align the scopes. The device of choice is the Cassady T-Gad but it's only available used. FLO offer a good alternative to the T-Gad. This worked perfectly from the off and never dropped a sub. It was working at 3.5 arcsecs per pixel, however. But... when we replaced the two Tak FSQ106Ns with a pair of TEC140s working at 0.9 and 1.1"PP respectively we find we sometimes get slight trailing on the 'slave' scope - ie the one not carrying the autoguider. There is plenty of internet experience of getting high resolution dual rigs to work but Peter Goodhew seems to have nailed it by using an active optics unit on the slave scope. Our dual TEC version of the rig in the picture is workable but it would be nice to elmiminate flexure all of the time. We still sometimes get a bit of it. Olly

Oh no, my hair's grown back! Olly

This is an argument I've often seen but I think it needs to be advanced with extreme care. In its simplest form it might say, 'We used to think that it would never be possible to fly but now we can go to the moon so, in the future, we will be able to... and at this point insert anything at all that takes your fancy. Teleport ourselves to the galaxies in the Hubble Deep Field? Travel backwards in time? (As Stephen Hawking said, 'Where are they?) So I'm wary of the argument. Firstly its premise is usually wrong. Plenty of people did believe, for example, that they would be able to fly (and they made it happen.) Plenty of people did believe that rockets could take us to the moon, and they were right and made it happen. But did this make them subscribers to the view that, in time, absolutely anything we care to dream up might become possible? Certainly not because they were scientists. We are not pre-industrial beings any more, our world view has changed, but as well as having more capabilities at our disposal we also have more constraints. We don't, for instance, believe in magic. We don't believe in witchcraft. We don't believe in alchemy. We don't believe in fortune telling or astrology. Whether or not we believe in miracles is not for this forum. So one might argue that our view of 'what might or might not be possible' has has been constrained by more that it has been liberated. Olly

It doesn't look very promising, does it?? 🤣 But perseverance pays off... Olly It was an amateur discovery (not far from where I live) so it's only fair that an amateur catch the best image! 👏 Olly

Nice. You see the patch of nebulosity right in the middle and extending up to the top of the image? I'm really curious about this because of its unusual colour. Yves found the same in his recent mosaic and I have it in my own capture of this region as well. It's not like the rest of the reds either side, being paler and maybe slightly lilac in colour. I wonder if there's a lot of OIII causing this. If not, what might it be? Olly

Thanks for drawing attention to this thread, which I hadn't seen. My remark about the term 'astro artist' being a loaded term was speciific to that thread and context. I think astrophotography involves a variable blend of art, craft and science. If I had to choose only one of those terms I'd go for 'craft' since it uses tools and materials (captured light) to make something not unconstrained, like art, but constrained by function like a chair, a piece of pottery or an item of clothing. So what is the 'function' of an astrophoto? It must resemble or provide information about the object it represents. That's the constraint not necessarily faced by artists. This argument isn't watertight. The IKO image challenge involves a kind of astrophotography which I don't do myself, ie narrowband colour mapping. As you know, the original idea behind colour mapping was scientific: different gasses mapped by different colours are presented as an exercise in information visualization. Very few amateur astronomers work in narrowband for this purpose so their use of colour is purely artistic and unconstrained. They have a free choice of colour. That's the difference between broadband and narrowband imaging and probably lies behind my own choice of broadband, albeit enhanced by narrowband. I don't want to move outside the constraints of natural colour in my imaging but this isn't a value judgement, it's just a personal preference. (By natural colour I mean the calibrated colour produced by RGB filters. It's a good approximation of what we would see with big enough eyes, as proven by daytime photography using this method.) When I add Ha to red and OIII to green/blue I have a copy of the original RGB open and try to stay as close to it as possible. The colours will certainly drift but reds remains red, greens green and blues blue. That's a constraint I like because it makes me more craftsman than artist. I'm uncomfortable with colour mapping because it strikes me (irrationally perhaps) as a game without rules.* It isn't for me but I'm glad others do it and enjoy it. Olly * Apparently even rugby has rules. You'd never have thought it! 😁

For a given aperture you have the same number of object photons whatever the focal length or F ratio. And yes, if you shorten the FL you make a smaller, brighter, less resolved image in the time - just as you do if you capture in bin 2 or bin 3 etc., or if you software bin the longer FL version. The camera lens notion that exposure time goes as the square of the F ratio is straightforward and true when the aperture is the variable, as it is with camera lenses. (You open or close the diaphragm.) It ceases to be meaningful and becomes very misleading when focal length is the variable. Changing the aperture and changing the focal length are very clearly not equivalent actions. Olly

First I saw the best OU4 I've ever seen. Then I read '47 hours.' Congratulations on both! 😁 Olly

Goran, be warned that if this back order is going to be like the one for the newer, narrower OIII filter in 1.25 mounted, you may be dead and buried before it actually appears! About every six months I receive news of further delay from TS... Like you I think Goran's setup looks very convincing. For me it's the small, tight stars which distinguish it from the images from most other ultra fast systems. As you say, here we have 'fast and good.' The core also suggests that there is no problem from well depth, which is restored by the deep stack. Regarding the stars, while processing Yves Van den Broek's mega mosaic in Cygnus, I was struck by the tiny star sizes. His scope was an FSQ106. I have one myself (the earlier model) and I don't believe the small stars arise either from the optics or the focus. I can believe both might be a tad better than I achieve but the difference is well beyond that. It must be the camera, in my view. I'm not aware of this having been discussed on the forums but I haven't been looking. This may contribute to the success of Goran's scope-camera combination. Olly

I do live at a dark site, yes: we sometimes sometimes beat SQM22. We know from regular imaging that, with sufficient integration time, it's possible to mitigate LP. I'm heartily in favour of anything which helps, including EAA. Olly

I certainly agree on the immediacy of Ps. In my processing I rely heavily on layers to do what is done using masks in PI. I find it much easier to globally modify a lower layer then erase the top from only those places where I want the modification (and the extent to which I want it.) For me the PI/Ps debate hinges largely on the masks/layers alternative. Per used to say that PI gave a lower FWHM than other stacking routines but your preference seems to be more subjective. I suppose I ought to bite the bullet and do a comparison. I'll wait for a PI guru to turn up here and show me! Olly

It's not so much that F ratio doesn't matter but that it introduces confusion. If we compare two scopes of the same focal length, one F5 and one F10, then we can say for sure that the F5 is four times faster. Why can we say this? Because the area of the objective of the F5 must be four times bigger than the F10, so admitting four times as much light. But what happens if we take the F10 scope and put in a O.5 times reducer to make it F5. Does that make it four times faster? No, because the area of the objective has not been increased. There is no new light. All that happens is that you use the same amount of light to make a smaller, brighter picture of the object in the same time. You could also do this by binning 2x2 without the reducer. When daytime photographers use F ratio to mean aperture (a regrettable habit!) they are able to do so because their focal length is fixed and they are using an iris to increase or decrease the area of aperture they are using. They are not varying their focal length, they really are varying their aperture. However, when we use focal reducers we are not changing our F ratio by adding more light, we are changing it by reducing our focal length. This is not at all equivalent. When we look at a scope spec we're surely going to begin with focal length since that decides our field of view. Do we want a close up view of small targets or a widefield view of large ones? Once we've decided on an approximate FL, and only then, should we become interested in the F ratio. In fact, though, when the FL is fixed the F ratio and the aperture are describing the same thing, namely aperture, which his how daytime photographers think. Olly

You have it, John... Olly

Fortunately for me, as an astronomy provider, it certainly influences quite a few people's choice of destination!! 😁lly

Vegetables? 🤣lly

Move to a country where neighbours are not compulsory! 😁lly

The essence of astrophotography is the modifying of the linear view into the 'stretched' view. That's really what most image processing is about. There is no quick explanation of how to go about this because every image is different and, often, the best images combine different stretches. Olly

That makes perfect sense. I'll make my usual point that 'faster optics' and 'larger aperture' really mean the same thing but I take your point entirely. Olly

In what way do you find the PI results better, Steve? Olly

Have you considered welding yourself? I'd never done any at all but my pal and car mechanic said, essentially, MIG is so easy that even an idiot like you can do it. (I never knew what I saw in him as a pal but there you go... 🤣) But he was right. With a low-end Machine Mart welder I made a pier, then several other piers, then a chassis for a roll-off observatory, then a load of railings to protect the drops around our house, then a couple of roll-off roofs, more railings... and so it goes on. Just the other day I welded up a repair on one of the corner jacks of our caravan. Talk about a good investment. But my welding ain't pretty! I had a new guest years ago who said, 'Did you weld this pier up yourself, Olly?' I said that , yes, I did. Then I asked him what he did for a living and, after a short pause, he said... welder! After we'd both stopped laughing I think we enjoyed a beer together... 😁lly

Exactly. I've had several guests try to convert me to stacking-calibrating in PI. It goes like this: Look, here are our lights, our darks, our flats, so this goes here and that goes there and... click. Oh. Ah. Hang on, that wasn't quite right. Oh yes, I know, let's do this: there we go. Oh. Ah, no, that's not right. Hang on, lets try again... NO, let's not try-a-bloody-gain, let's use AstroArt!!! 🤣lly

If you fix the focal length the aperture plays exactly as F ratio says it plays. Of course the big reflector is quicker. Once you've got it working, of course. If you are strapped for good sky time, or simply want to keep on churning out quickie images the like of which we have seen endlessly already, then go for 'quick.' Once you want to start going for 'good' it all gets much more difficult. It depends what you want. It's perfectly obvious that one can find top quality images taken with every kind of serious scope. The devil is in the detail: exposure time, expertise in optical tuning, etc etc. Olly

I tend to think that decent filters are indeed parfocal. It's the optics which are not. Since I'm currently using good optics (Tak FSQ106N and TEC140) I find more or less full parfocality. However, that doesn't mean that the FWHM is consistent across the filters. You say your R gives O.5 and your B 3.2. The first question is, how low can you get your B by refocusing? The original difference can have several causes, viz - non parfocality of the filters. - non parfocality of the optics across the visible spectrum.* - imperfect blocking of the UV by the blue filter.* - camera's sensitivity varying with wavelength. - microlensing artefacts where there are microlenses on the chip. - spectral class of the star itself. The best test would be on a G2V close to the zenith. * These two will contribute to blue bloat which is really a separate issue from parfocality because, even in the best possible focus, the problem will persist. Olly

Agreed. Olly