ollypenrice

They will be fine if the temperature is the same. To be honest you should try using and not using the darks when you stack because the true chip temperature for each sub can still vary. Olly

It's generally an imager's obsession, John. In the early days the trick was to get it at all and then trying to get it's colour drove us all to distraction! It turns out to be a brownish colour. We think. 😁 I've never tried for it visually but why not? Useful pointers from Jetstream. Thanks. Olly

A very well crafted mosaic. Olly

Super image, rightly recognized. Olly

Well, I do my stacking in AstroArt which has a pretty simple format. Sometimes I lie to it, so I'll put a set of flats into the 'Images' box and put a master bias into the 'darks for images' box and ask it to 'average combine' this lot into something I'm going to save as 'Master flat.' It works to my satisfaction because I know what the software is doing. And that's the key thing: understand the steps. I know you do. I think the best advice we can give to beginners is, 'Think it through.' Olly

Disagree with CCD imaging to the extent that you could, if you wished, faff about taking darks-for-flats but it really is a total waste of time since you can use a master bias as a dark for all your flats and find no difference whatever. This is not the case for CMOS flats which need 'correct' darks for flats. I take it one step further, but in doing so I 'fess up as a cheapskate. I use a luminance flat to calibrate all my stacks in whatever filter. Repeated comparisons have shown that this makes no difference whatever most of the time. On the very rare occasions when it does make a difference I'll shoot dedicated flats 'per filter.' Olly

You don't stack the bias frames with the images. You put them in the stacking software's appropriate box and the software will know what to do with them. With a Newt I would shoot my real flats in the dark. You don't want stray light getting into the bottom of the tube. Practice by day, though, certainly. Olly

Have a look at a single sub. You may see the same streaking top left to bottom right. If you do it's inherent to the sensor, I'd have thought. I'd try a large scale dither, at least of 12 pixels. You'll lose a bit around the edges but probably worth it. I don't have this problem but, if I did, I'd import the image into Photoshop, rotate it till the streaking was vertical or horizontal, then run Noel's Actions on it (Vertical or Horizontal Banding Reduction.) Noel's Actions is the old name but I'm old as well! It's here: https://www.prodigitalsoftware.com/AstronomyToolsActions.html Olly

It was a very useful one for an English teacher faced with students asking, 'But do you think the author really thought about all this stuff...?' Olly

YES!!! If there were an industry standard then one could make a case for the undercut but, since there isn't, and since we are also cursed with an assortment of conflicting clamping methods, they present more danger than security. A momentary snag of the kind you mention resulted in my 35mm Panoptic hitting the ground from the dizzy heights of a 20 inch Dob near the zenith. Olly

Welcome. Here's an astrophoto to go with your avatar! The problem you describe may well not be due to the mount's PE. This kind of behaviour isn't unknown and can arise from a number of things: - Cable snagging. - Loss of guidestar due to cloud or bad connection. - Incorrect calibration. Are you guiding at 0.5x sidereal? If not I'd try that first. Once the basic calibration routine has run its course have you tried running PHD's Guiding Assistant? It's very good. Just apply the recommendations it makes. - Backlash. If there is backlash in the gears and the mount is in perfect balance the guider may get into a vicious circle of opposite corrections causing the mount to oscillate across the free play. The usual fix, apart from adjusting the mesh, is to run the mount with the east side a little heavy so the payload/resistance will rest against one side of the gears. This works for backlash in RA but it isn't so easy to cure the problem in Dec. For this reason, and others, it's a good idea to post your guide trace, plus an image taken, and to know which axis is RA on your image and which is Dec. Basically ST4 guiding does work. Some prefer the extra features of EQ Mod but I'd be surprised if it would cure an inherent fault. I don't use it but others will come along and help on that. Best of luck, Olly

It's a sound idea to see what colour is passed by an Ha filter and, indeed, I've done that myself, but not by illuminating a white sheet with it. Good idea. My conclusion was that the Ha colour was rather more magenta than it usually appears in images. Another way to check is simply to look at the RGB, evenly weighted. Again that is usually more magenta than it will appear with Ha used to lighten the red channel alone. I also agree on salmon pink, usually the result of Ha going into luminance. As for endless tinkering, J.K. Galbraith said, 'It's usually around the seventeenth redraft that I introduce that element of spontaneity for which my writing has been praised...' 😁lly

Beautifully done. The blue reflection nebula is very tricky to bring out but there you have it. The extra subs did their job. For me the main nebulosity was not so much over-colourful as rather magenta, which made it seem insistent. This could have been a by-product of the extra blue, I suppose. Olly

Thanks. I hadn't come across this as an established concept though I've long subscribed to the view that the universe has no need whatever to behave in a way which seems reasonable to me. In fact the whole charm of astronomy, cosmology and physics is that they throw our preconceptions out of the window. Didn't Einstein say that common sense was just the name given to that set of prejudices accumulated by age eighteen, or something like that? Olly

Cracking field. It looks good to me. Olly

I've never heard of the 'incredulity argument' as such but if I were to guess at what it might be I'd say it had something in common with Occam's razor and something alien to it. While the attraction of simplicity has served science well, the same cannot be said for the attraction of what seems reasonable to us. It seemed unreasonable for the earth to be in rapid motion and Galileo's greatest contribution (it seems to me) was to show that, in truth, it was perfectly reasonable. It seemed reasonable to assume that time and space were constants. It seemed axiomatic to physics that it be deterministic. God does not play at dice, said Einstein, to which Bohr gave the sublime reply, 'Stop telling God what to do.' I've never thought of this before but perhaps incredulity is the enemy of simplicity; does our sense of what is unreasonable prevent us from seeing what is actually quite simple? Relativity, for example, is not difficult to understand, when first you meet it. It's just difficult to accept. Historians and philosophers of science and evolutionary biologists seem to be converging on the notion that our sense of what is reasonable may, ultimately, put a cap on our comprehension. This because our sense of what is reasonable has been a tool essential to our survival - and our survival takes place in a local set of conditions, not a cosmological set. Doomed to die in ignorance. Aaaarrrgggh! Olly

Galileo certainly did. Throughout the dialogue on the world systems Salviati (speaking for Galileo) appeals to qualities like elegance and simplicity in urging heliocentrism over Ptolemy. A quick dip into the book produced this quotation regarding the length of the day in the heliocentric model: 'So you see how appropriately the extremely rapid motion of 24 hours is taken away from the universe, and the fixed stars (which are so many suns) enjoy perpetual rest like our sun. Notice also how elegant this first sketch is for explaining why such significant phenomena appear in the heavenly bodies.' In preferring to see a small Earth spinning than a vast universe, and in preferring the nearest star to have the same properties as the rest, Galileo is showing that the principle of Occam's razor informed his thinking. The Dialogue contains countless examples of the same thing. This is not the same as arguing that simplicity equates to proof, however, and I don't think Galileo says it does. Rather it is an instinct of his (and others) to feel that a simple explanation beats a complicated one when both satisfy the observations. This instinct has been a productive one in the history of science. Trust quantum theory to make an exception! Olly

I've always been troubled by an issue closely related to Johnjo's curiosity about Occam's razor. Galileo makes much of the beautiful simplicity of the heliocentric system in his dialogues - and yet the heliocentricity of De Revolutionibus is not simple at all. Indeed it isn't even heliocentric. Or it isn't, 'strictly speaking,' and there might be the rub. I always get the feeling that Galileo was writing about the Copernicanism of the Commentariolus before it all went pear-shaped (or failed to go ellipse-shaped) in De Revolutionibus. I don't think Galileo was particularly attentive to the details in other people's work and, although he said nature was written in the language of mathematics, he also thought like a physicist. Perhaps some blend of these two characteristics allowed him to see the movements of the Commentariolus through the tangled mesh of the gears making up De Revolutionibus. I like the point about the counting of epicycles being a poor indicator of simplicity because, with the single alteration brought by the ellipse, the Commentariolus can be restored. No equivalent operation could be performed upon Ptolemy's system. Perhaps Galileo simply 'felt' this. (Or perhaps he never bothered to read De Revolutionbus! I'm half serious. Did he leave an annotated copy or detailed commentary?) Olly

Great result. BTW You're in the Observing section here, rather than the Imaging. You should get more replies in Imaging. Olly

In focus, decent tracking... so all it needs is more. You'll be astounded by the improvement that comes with stacking many subs but some research on processing would be well worthwhile. Olly

I stack in AstroArt which, like most stacking routines, has a hot pixel filter. I find it very effective. Because I've been using dual rigs for ages I haven't been dithering and, honestly, I can't say I've missed it. Being lazy, I tend to think that a bit of polar misalignment (within reason!) doesn't do any harm. What does intrigue me is why my 'noisy' old Atik 11000 produces a far cleaner background sky than my 'low noise' Atik 460 with Sony chip. But it does, believe me. Olly

When we set up the original dual rig (2 x Tak106/Atik 11000) on a Mesu using guidescope and Cassady T-GAD tilt-pan adjuster it just worked from day one and never missed a beat, literally never. Not a single sub lost out of thousands. This was at 3.5"PP. The exercise was a total success. With the present dual TEC, working a little either side of 1"PP (Atik 460 and Moravian 8300) we sometimes see minor trailing on one side but it mostly works fine. The fact that the focusers are both good may have a lot to do with it. One of the TECs has a TEC focuser and one has a Feathertouch. My system is rather old-school and manual so labour is saved by the dual rig. Someone running fully automated systems might find two mounts easier than a dual rig. This ignores the cost! (It's always best to ignore the cost in this game...) Olly

I don't think the problem is reducing the core. It's extracting the contrasts from it which will find new structure. Just dimming the whole core region will leave the contrasts as they are and introduce the over-controlled, unnatural look. In my version I was obsessed by tracing spiral and other structure as far into the core as I could, which I succeeded in doing by means of pure thuggery, and found my core details were similar to RBA's, which was reassuring. However, there is no way that I can see of going about this without moving away from the natural 'look' of your version, which is its charm. I don't think you can have both forced core details and the look you've achieved here. I'd leave well alone. Great job on the background. Given the need to reveal the very faint outer glow I don't think it's way too dark. You won't like this but panels above and below would perfect this excellent M31 by letting it sit comfortably, in its entirety, in the frame. (Runs for cover!!!) Olly

Focusers. On a Dob they remain horizontal so they really don't have much work to do. You also adjust them by hand on a fairly regular basis when you swap observers or swap eyepieces, and making a small adjustment isn't an issue. (All of this is very different for imagers, for whom focusers are a big deal.) The Moonlite is a very nice focuser for Dob users, though it has a lot wrong with it for imaging on an equatorial mount where it can be in a vertical position trying to hold up a heavy imaging camera. But is it worth spending Moonlite money on a focuser for a Dob? I spent 15 years with a 20 inch F4 and never felt that it was. I just used a basic chinese affair to move the eyepiece in and out. My advice would be, 'Spend the money on the glass.' The mechanical side has to be tip top for imaging but it's the glass you look through when observing. Olly

I prefer the first as well. It's a very natural looking* image and the first is more relaxed. There are just a couple of things I'd look at if it were mine: the background sky is not a neutral dark grey but, without measuring it, my hunch is that green is too low. I'd also reduce its colour saturation (just the background, not the rest.) Also the stellar blues are rather magenta which would square with the greens being a bit low. However, I admire the hell out of this rendition. Olly * Good question to which there might be many answers but here are mine. 1) Many imagers have old-school film renditions in the backs of their minds when they look at digital images. These old images have a look which many of us think of as 'natural' though that is not a particularly rational use of the term. Cameras and film are not works of nature but, still, they influence our perception of 'natural-looking' pictures. 2) When you spend a lot of time processing images you develop an eye for the effects of particular processing inputs. Noise reduction, sharpening, colour saturation, star reduction, local contrast enhancement, stretching, setting of black and white point etc etc. You know what these actions produce if taken just a tad too far so a 'natural-looking' image is one in which they haven't been taken too far. They've been used to just below the point at which their signature becomes visible. 3) The imager's intentions and style: one person might want to extract every last scrap of information from an image through intensive processing. This is rather like trying to make a highly informative diagram and is like graphic design. Another might be trying to keep the processing invisible and accept that some details cannot be rendered without introducing a diagram-like look. Both are valid activities but the second aims to look 'natural.'