ollypenrice

They all need guiding, Wim. They can guide on the stars or they can guide on their own encoders but, if they do that, they need to work to a precise local model which involves the refractive index of the sky at varying altitude. If you want one more thing to go wrong, guide on your own encoders! Guiding on the stars is the elegant solution. Maybe a mount that you bought second hand 10 years ago? A mount that, in commercial use under regularly clear skies, has never ever dropped a single sub to guiding error? That would be my dream mount and I have it. The Mesu 200. There is no mount at any price, anywhere in the world, that I would rather have in my observatory. Olly

I don't think that problems will arise from the mount's ability to handle the weight. I think they'll arise from differential flexure in the mounting hardware. If every OTA had its own autoguider, directly attached to the OTA in use, then I think you'd be fine using one OTA (with its own guider) at once. As soon as you try to use one autoguider to guide more than one OTA you run into the flexure problem. My first dual rig involved 2 x Tak FSQ 106, each with Kodak 11 meg chip, imaging at (read this twice) 3.5 arcsecs per pixel. Mounting hardware was the now extinct Robin Cassady T-Gad alignment device, bought second hand but still at staggering expense. This rig worked perfectly from night one. It worked perfectly, in commercial use, for many years thereafter. This made me think dual rigs were a piece of cake. So we tried 2x TEC 140 with cameras working at about an arcsec per pixel. Uh-oh. We had created a monster. Last week I took the last vestiges of this infernal thing off the mount and breathed a sigh of relief at being re-united with my beautiful galaxy-busting TEC 140/Atik 460 combo, free from all the dual rig flexure issues which plagued the dual setup. None of this was the Mesu's fault. Conclusion? Don't create a monster. Olly

The earth rotates at a constant speed so we need to 'unwind' its rotation at a constant speed as well. Olly

Bill Bryson describes over-nighting in a UK bus shelter, having donned everything in his rucksack to ward off the cold. On waking, he asked an early morning gent if there was a café open and was directed to one. 'One thing, though,' said the gent. 'Before you go inside I might be inclined to take them pants off yer 'ead.' Olly

We get so many subs here that it would be bonkers to save them all. I save calibrated stacks. (Our internet means saving to the cloud would require a time machine.) However, I routinely add new material to old. 'Stacking stacks' is not, mathematically, the best way to go but I find it very effective, none the less. The arrival of the CMOS camera, with small pixels and a preference for more and shorter subs, means even more storage. If you have it, fine - but, as Carole says, save the relevant calibration files. I will continue to save calibrated stacks. Olly

Not the best, perhaps, but I remember watching a shadow transit of Jupiter from beneath a streetlight in my old home town. That was in a Genesis, a 4 inch F5. Olly

The limits are knowable from theory and cannot be wrong. When you say, 'Questar versus the rest,' what constitutes 'the rest?' Are the rest telescopes, or are they other items of consumer jewellery like Rolex watches - which struggle to compete with fifty quid Casios on accuracy? In my view, buy a Questar for what it is, if that's your thing. It's a slightly dated technical exercise and a very fine one. No surprises that the unscrupulous Meade ripped it off. But...the problem inherent in a small Mak is that it cannot beat the Dawes limit of resolution and yet it cannot give a wide field of view either. Personally I'd prefer a small scope that could do what small scopes can do and big ones can't, which is give a binocular field. Olly

For a long time the French magazine Astronomie presented its top selection of two or three images on full pages. You got the picture in a black frame bearing the object's name and the imager's and nothing else whatever. I thought this was excellent but, sadly, they went the way of the others. (Meaning graphic designers with ADHD scribble over everything!!! ) Like you, I was baffled to see the Iris itself, in your excellent image, glimmering faintly from beneath a text box. By the way, are you going to head west and haul in the Ghost at some time? Olly

Yes, as the roofs slowly collapse the chairs need to get lower and lower... lly

I don't know if my village counts as exotic but, if Dave tells me it does, I will pass this on to the Mayor since it has never previously been so described! I believe Dave may then find himself ennobled, here, as Chevalier d'Honneur d'Etoile St Cyrice. I'm not sure that my back patch qualifies as a 'garden' either!! lly

I try to avoid forming an opinion on the authenticity of images until I've shot the object myself. I've never tried this one but my curiosity is piqued. An exotic PN in Ha and OIII in less than five hours at a conventional F ratio has to be a tempting prospect... Occasionally, cheating is blantant. There was an image showing a faint galactic extension on Astrobin, purportedly taken in less than 10 hours with a DSLR. I couldn't find it in 20 hours of luminance, which means with far more light than captured through an OSC colour matrix. I have no interest in 'exposing' this obvious fraud but I noticed, some time later, that the image had been deleted. In my opinion there are no significant techniques for faint data stretching which aren't in the public domain. Those that are known can be applied with more or less skill and success and the quality of the data depends upon a number of things, sky quality being a big one. In the end there is no magic involved. In order to separate the faintest nebulosity from the background there has to be a different pixel value between the two in the linear data. Effective stretching simply means identifying and exaggerating that difference. On occasions when I'm struggling to find faint signal which I think might be there I will simply sample the linear ADU values around the suspected nebulosity and compare them with 'empty' background sky. If there is no difference between a good sample of both regions then the data contain no trace of the nebulosity and that is emphatically that. I found this while chasing the outer glow of M31. A region I'd once seen in a reputable image wasn't present in my deep stack of 15 minute CCD subs. I tried a second run with 30 minutes and there it was. (Lots of arguments will tell you why this can't be true but you'll understand why I'm not convinced by them...) As for software noise reduction, it makes no difference beyond a cosmetic one. It does not allow us to stretch harder, it simply allows us to stretch as usual and get less grain or speckle. The first thing to know is what's in the data. NR won't put it there. I do use NR on most images but only very lightly and in very carefully selected places. The best I've tried, by far, is Russell Croman's NoiseXtermeinator which uses AI. My advice would be, Don't worry about other people's data, attend to your own. One of the best amateur imagers in the world had a piece of paper taped to his PC screen with the massage, 'Make your own picture.' Olly

That's quite a question to ask on here because SGL has writers who write for either one or the other... I'm an occasional writer for Astronomy Now so of course I'm not biased! What I can say is that AN is a pleasure to write for. Many people believe that equipment reviews are little more than 'advertorial' material but I know for sure, since I've written equipment reviews for AN, that the magazine put me under no pressure whatever to modify my reports. If there is a selection pressure it may be this: a writer will be asked if he or she would like to review the new RubberWobble Mk 4 mount imported by Consistently Naff Imports and manufactured in La-La-Land. Given that writing such reports involves enormous disruption to the life of the observatory, the writer is likely to say, 'No thanks.' But offered a nice Takahashi refractor they are more than likely to say, 'Yes please.' That might be an explanation of why reviews seem generally favourable. My SGL friend, whom I won't name because he can speak for himself, has said the same for Sky at Night, for which he writes. Olly

That's great and all you need is more of the same to make an absolute winner. Everything's in place. Olly

Excellent and something new. Great stuff. Olly

Masterly work. Damn, that's a good image. Olly

In Never at Rest, Richard Westfall's definitive biography of Newton, there is little doubt that Hook gave Newton some conceptual pointers from which his theory of gravitation grew. Newton vehemntly denied this and Hook vehemently insisted upon it. Another interesting Hook snippet concerns Newton's famous line, 'If I have seen further, it is because I have stood upon the shoulders of giants.' Often seen as an uncharacteristic bit of modesty and generosity on Newton's part, it was almost certainly a swipe at Hook who was dwarfish and deformed. In other words Newton was saying that he had certainly not stood upon the shoulders of Robert Hook. History isn't inclined to believe him, I don't think. Olly

The focal length of the ST102 is 500mm and that of the Dob is 1270mm. That's an enormous difference. I used to have a TV Genesis F5, also with 500mm FL, and absolutely loved using it with widefield 2 inch EPs. The ST102 will have much more distortion at the edge of field but, still, you're getting a binocular field in a telescope, really, and there's so much to enjoy. Olly

This was E E Barnard's great question. As the stunning visual observer that he was, he didn't know the answer. When he became a pioneer in astrophotography he finally concluded that there were dark nebulae which were dark because they obscured what lay behind them. This is conclusively the modern view. I think we confirm this every time we stretch data on dark nebulae and see the background brighten as the dark nebula does not. https://www.amazon.fr/Immortal-Fire-Within-Emerson-Barnard/dp/0521444896 Olly Edit: The book is back in print so you don't have to pay silly prices for it.

I haven't tried this but I think it ought to be fairly easy. I'd take the unfiltered OSC image and process it only for the stars. Don't worry about what the rest of it looks like, just get accurately colourful stars. Paste this on top of your present image, use Noel to select brighter stars, expand and feather that selection, select inverse and delete the rest. It might look OK like that but, if not, try switching the blend mode to Colour. While the stars are a top layer you can adjust saturation and, in blend mode Colour, use blur as well. In Blend mode Normal blur will blur the stars but not in blend mode colour. It has to be possible. Olly

I think your image is well processed. Firstly nothing shouts or pokes you in the eye. The background is neutral and the black point well judged. (This is tricky on the NAN because there are patches of dust which are much blacker than the background sky. Stars are small and tight, noise is low and there is no visible noise reduction. There's no star colour but that's the filter's drawback. I wonder if a short unfiltered run might let you get hold of some? Other than that the only suggestion I can make would be to try to give it a bit more punch in the form of stronger local contrasts. Since you use Photoshop (wise man!) do you have Pro Digital Astronomy Tools, formerly known as Noel's Actions? They are excellent. 'Local COntrast Enhance' would probably work on this. Normally I use it as a layer and don't select the contrast enhances stars when flattening. Olly

That's a very interesting problem to have, an observation which might, quite reasonably, encourage you to throw a brick in my direction! But I say it because, having made huge numbers of mosaics with up to 35 panels, I've never had it. I've had plenty of others, but not that one, yet I see straight away what you're talking about. Just for background info, I'd have a look at Rogelio Bernal Andreo's image of a similar field just to see if you spot anything comparable. Did you combine the linear panels then stretch? It's nice to do so and sometimes it works, but what to do when it doesn't? This is what I would do: partially stretch all the individual panels till the background sky value is the same in all of them and the histo peak similarly located. While we might want to end up with a background brightness of 23 in Ps units, we might settle for 15 at this stage. Next, look at the star count in the dark panels and their neighbours., especially where they overlap. What happens if you pin the background in the dark panels and stretch only above that? Can you get the star count to resemble that of the neighbours in the overlap regions? If not, you'll have to re-shoot them, I think. Personally I like attacking these problems in Ps because I can put one panel next to its neighbour and say, 'That one needs to mesh with this one, so what do I need to do?' Olly

Right, so you can pin the issues on the scope, I'd say. It might be worth giving this info to Celestron and/or the dealer. Olly

Mid thirties C are an annual occurrence here in SE France, with this year seeing the occasional 40. Observatory internal temperatures are a good bit higher than that. The only items to suffer have been a couple of the UPS units which back up all the robotic scopes. These benefit from having a small fan running next to them. Three oil-spaced TEC140s have been fine along with all other parts of seven imaging rigs. Olly

It might be worth finding a flaring star, imaging it, rotating the camera and imaging it again, just in case the flare is produced outside the telescope. Does the flare follow the rotation or not? Olly

The first one is better and lassoing the core is not the way to do it. You've produced an artificial, star-like bright spot in the middle which certainly doesn't exist. I don't think you need anything fancy with these data but, if you want to get clever, look into 'high dynamic range layer masking.' Check this out: https://www.astropix.com/html/processing/laymask.html You don't always need two or more different sub lengths, you can just do two different stretches, one hard and one gentle, and blend them as per the tutorial. Olly