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pete_l

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About pete_l

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    http://www.under-a-dark-sky.com

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    Benamaurel, Spain 37N
  1. An interesting lecture. The take-aways (for me, at least) are that with CMOS sensors there is much less that a manufacturer can add in terms of image quality, since so much is built in to the camera. (Though that doesn't necessarily tell you anything about component reliability or customer support) Second: nothing beats a dark sky (but we knew that, anyway) It looks like F/5 or F/6 scopes give very acceptable exposure times Exposure times of no more than a few minutes are all that's needed in urban environments. Unless you have a nice dark sky when longer will give better results.
  2. Easiest: The Moon. Any camera that is better than ones in phones (that can manually set exposure + focus and have lenses bigger than a pea) Hardest: A black hole - though that isn't very helpful For an amateur I'd reckon a low surface brightness nebula. Maybe the Cat's Eye, since there is a lot of contrast between the bright core and the dim surround. Like M42, it is very, very, difficult to get a decent image that shows both the bright centre and the gaseous surrounds. Cheapest: see easiest. Most expensive: Hubble deep field? For amateurs, there is no upper limit, apart from how much you're willing to spend. Though with the advent of robotic rigs that you can hire by the hour, it's cheaper to image a _single_ target with one of those than to buy a telescope specifically for it.
  3. I built something like this a few years ago. Before you start, make sure that the blocks aren't too wide. I used standard 40 x 20 x 20 blocks. With a small GEM on top, the blocks were too wide to allow a telescope to move unrestricted. There were positions when the OTA would bang into the corners of the blocks.
  4. Yes. An interesting question is whether the less distinct one could be photoshopped¹ to get it closer to the sharper one. [1] other image enhancement tools are available
  5. You could save weight (and money) by shortening the plate so that the camera end of the finder hangs out past the end of the plate. It might even be possible to arrange for the guider to be placed above the dew controller and mini-pc, thereby saving more area on the plate and more weight (and making balance easier).
  6. Is there any evidence that this is affecting the view / images? If it is nothing more than a cosmetic blemish, I'd just leave it be. There is a story that exposing the lens to UV will prevent further growth.
  7. Personally, I would say that the pixel scale (arc-sec per pixel) differences will not be noticeable. However, there will be an operational difference between an F/4 and an F/5 scope. The F/5 one will be easier to focus, since the light cone converges to focus over a greater distance, so the "in focus" zone will be greater. This would also affect how (or if) focus changes due to thermal expansion or primary mirror movement over the course of an imaging session.
  8. There are many "issues" regarding USB: cables, connectors, hubs and software drivers. Ans like a chain, the whole mess is only as good as the weakest link. If you are powering devices from their USB cable I would strongly advise watching this: Youtube video from Andreas Spiess In particular, pay attention to the voltage drops that can occur. Even when operating within the USB spec, you can get much less than 5 Volts going to your device. Personally, I would give each data intensive (i.e. video) USB link its own cable back to the PC, unless you are very confident in the abilities of any hub you might use. My preferred solution is the same as dr_ju_ju - have a computer at the mount. I have a small NUC / Brix PC on the mount and can feed USB3 directly in to that. My low speed devices (focus, mount, power) all get fanned-out from a small hub and the USB 2 guider cam also goes straight into the PC.
  9. My take on this is that if the energy the scope receives from a single star is spread out across many pixels, then each one will only receive a fraction of the total light. But when you read the data out of the sensor, you will get the full noise component from each pixel. So your signal / noise ratio will suffer. However, even with a CCD that is perfectly matched to your seeing: let's say your seeing is 2 arc-sec and your pixel size is such that each one "sees" 2 arc-sec of sky. Then is is incredibly rare that a single star will be exactly aligned with just one, single pixel throughout the time of a subframe. The atmosphere will still move that star around on your sensor and there is always the possibility of wind, minute amounts of flexture and temperature variations moving things around too. If your pixels are larger than your seeing, there is more chance a star will stay on a single pixel. But if that star is aligned across 2, 3 or 4 pixels just due to its position, then it's just like having that star oversampled. This is one of the reasons for dithering between subframes, to average out the errors. So in general I would reckon that whatever your design goals, most stars will always be seen by more than 1 pixel, but that when multiple subs are stacked you end up with a Gaussian (random) distribution that will give you round stars. And if they look bloated, just use a processing tool to make them smaller.
  10. That's the thing about clear nights, especially inland. It does get very, very, cold. -5° is pretty routine where I am and -12° is not unknown. The altitude helps with that, too. The good thing is that those cold, clear, nights have very dry air and give some fantastic seeing.
  11. I feel that a lot of this is due to authors having to write the article that the editor wants. If those editors are, themselves, stuck in the past and have mis-remembered scenes in their heads about endless numbers of clear nights, dark skies and Hubble-like views, then that is the sort of article they will commission. It could also be that those editors know their advertisers will sell hundreds more "junior" telescopes than multi-thousand £ ones with buttons and batteries. Especially when those little scopes on their wobbly tripods look like "proper" telescopes. Even if they are only used once and then left in a cupboard until they are tossed out. I wonder how many telescopes bought after Stargazing Live were never used again after the first disappointing experiences?
  12. I think that one major reason why such small telescopes were suggested had more to do with price than usability. In the 1960s optical gear in the UK was very, very, expensive. Most telescopes would have been hand-made and therefore of dubious optical quality. They also didn't have the modern glasses, design tools or eyepieces we take for granted. Even today, prices increase very quickly with aperture: it must have been even worse 50+ years ago. I reckon that is why the recommendation for binoculars started. More due to cost than providing decent views (and it was only eyeballing, photography was virtually non-existent in the amateur sphere). Even so, they would start at more than twice the weekly pay of most people (roughly £10 a week in 1963, before tax). So £30-£50 for a pair of 10x50s was a luxury item - think £1,000 now! Even army surplus was pricey. There's an interesting article about binoculars here
  13. Increasing the focal length of a telescope has the effect of darkening the background. If the reason a star cannot be seen is because of sky brightness, it may not be necessary to increase the aperture at all. Just the focal length. Obv. if the star is so faint it is below the level your eye can detect, then the only way to increase its brightness is with more aperture.
  14. Yes, I like these. Not just for the form factor but for the capabilities too. I have one NUC6CAYH running W10 a Gigabyte BACE and a Gigabyte BXBT. Each has 8GB and an SSD (240GB is more than enough). The great thing about the Gigabyte boxes is that they support Windows 7. Although these don't have screamingly fast processors, that is unnecessary for my use: rig control, PHD2, image previews, electronic viewfinder/widefield and a little bit of basic stacking, afterwards. All done across a remote desktop link. I did try using an ACEPC with inbuilt eMMC, but I was not happy with its performance. To paraphrase Clarkson if it went any slower, it would travel backwards in time. Maybe the current generation is better?
  15. Sequence Generator is essentially a tool for automating the manual operations you have to perform with the tools you are using at present. As such it takes away the "hands on" aspect of image collection. Some might like that, others prefer to feel they have participated in that part of the hobby. I have used SG Pro in the past, but I feel it lacks a workflow, the property of a design that guides a user logically from one step to the next. I also concluded that it is great for when everything works as it should, but that it lacks the ability to deal with exceptional or unforeseen circumstances. Things that (in my case, at least) seem to be not uncommon . It also requires you to know, before you start, just how long you want your exposures to be. Something that a user can experiment with by taking test images and adjusting parameters with more "interactive" tools. So if you have a rig that you know is reliable and will do exactly what you ask it to, then SG Pro is useful. It is also great if you want to get some sleep and have the confidence to let your telescope and mount get on with it, unsupervised. But it lacks flexibility.
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