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cloudsweeper

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cloudsweeper last won the day on March 26 2019

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About cloudsweeper

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    Brown Dwarf

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    Physics, maths, history (family, military, social), pre-modern architecture.
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    Merseyside, England
  1. With light pollution, and now a neighbour’s new enormous garden conservatory operated without blinds, I’m glad of the Moon, planets, clusters, and doubles to enjoy. And doubles don’t just have to be the popular, pretty, or fairly close ones. There is a much larger variety to be tracked down and savoured. For example….. 5.30pm, Sunday, ED80 Apo on Celestron GoTo mount. Jupiter low south – aligned on that. In Pegasus Three doubles in a span of about 0.5deg. See sketch. OΣΣ 241 – binary, matched, wide separation of 84”, very clear split at x20. Σ 2958 – binary, not matched, 3.9”. Had to go to x120 for a split. Companion at 1 o’clock, with AV. 52 Peg – with a separation of about 0.5deg, this was not feasible for splitting, but formed a nice, close trio of Pegasus doubles. In Andromeda Four doubles in a span of about 4deg. Pi And – quadruple, visible double, not matched, 38”. Easy split at x27, fainter companion at 7 o’clock. h 5451 – binary, not matched, 55”. Same FOV as above, very easy, fainter companion at 4 o’clock. OΣ 11 – binary, matched, 3.3’ (minutes). Extremely easy, x20, nicely matched, an up-down arrangement. AC 1 – binary, close match, 1.9”. This was the more eastern of a pair of similar brightness, x20, but I could not see it as a pair with any certainty, even at x192. The only “failure” of the evening. By 7.20, the Moon (waxing gibbous) had come from behind houses and was sitting lowish in the south. Very bright x80. Filter inserted. Plato and Apennine Mountains good. At x120, 160 – not too bright unfiltered, but a bit wobbly. With a bright Moon and the light just turned on in the Dobhouse, I finished with a look at M31, the Andromeda Galaxy. I admit to being just a little surprised at how easy it was to see, all things considered – a clear, fuzzy patch at x20, clearer still at x40. A good session of over two hours, very pleasing. Doug.
  2. It's a huge topic (eyepieces) - Plossls are fairly basic, but can give clean, contrasty images. One big reason for going for something a bit dearer is field of view. I'd recommend hunting round suppliers' sites to get a flavour of what's out there. As they get more expensive, they tend to have more glass elements and better "correction". The range of five eyepieces (EPs) would give you a good span of mags. Something around x100 would be a good addition. You could Barlow the 32mm, then have the Barlow for other uses as you advance in the hobby. (Or you could get a 15mm EP.) No point Barlowing the more powerful EPs - that would exceed the mag the 'scope is capable of. Celestron Omnis are nice basic EPs; also Revelation Plossls. Many members here like the Starguider series. And Celestron XCel LXs are very good, and not too dear. You'll no doubt discover that this great hobby can play havoc with your wallet! Again - have fun! Doug.
  3. Hi Gil - that's a great 'scope you've got! I had one, and regret selling it on. The mount is fine. I'd strongly recommend a dewshield for it, and eyepieces at either end of what you have. A 32mm (giving x47) would squeeze some more field of view out of it, and a 7mm planetary type would also be useful, giving x214. Have fun! Doug.
  4. I believe that the surface brightness of an extended object depends on exit pupil squared. Now a larger aperture usually (but not necessarily) means a smaller focal ratio, which in turn (for a given eyepiece focal length) produces a larger exit pupil. So - it is generally correct to say that the surface brightness of an extended object increases with aperture. (I hope.....) Contrast against the sky also has to be taken into account, plus there are other issues with increasing exit pupil. It's different for a point source, where dependence on aperture is straightforward. Doug.
  5. Same here Alan. The newish neighbour has put in a monstrous conservatory (no blinds) and an orange light down the garden to light his way to it. As if that wasn't enough, he has also erected three swinging lanterns. The glare from all this is awful. I mentioned it to him politely, and he desisted with the orange glare for a while, but now it's on again. Thoughtless, inconsiderate, ignorant. Doug. Also - the council will only act if the light trespass impacts on the house, not on my pastime!
  6. As interesting as Etendue, Neil? (Sorry!!) Doug.
  7. HET/Etendue Is an HET then essentially a fast telescope (large aperture and exit pupil) plus a WA eyepiece? For stars, brightness depends on aperture; for extended objects (and given EP and objective focal lengths) surface brightness also depends on aperture. What part does FOV play then? If it is increased, the integrated light throughput will also increase. But surely that would not make any difference for individual stars and EOs? Drat! Just when I thought I'd begun to understand a little bit about telescope physics! Doug.
  8. Might be getting out of my depth here, but I reckon the answer must be Yes, thinking purely in terms of taking a photograph with a faster setting. Doug.
  9. Yes, good point Iain! Under dark skies, a high exit pupil can render a brighter image (SB proportional to exit pupil squared, regardless of aperture). Doug.
  10. For a given focal ratio and eyepiece, the exit pupil will be the same, so the Surface Brightness will also be the same. For the same focal ratio but bigger aperture, the objective focal length must increase, i.e. mag must increase. And this means that the integrated brightness increases - the same SB over a larger image. Doug.
  11. Neil - for extended objects, Surface Brightness (brightness per unit area) depends on the Exit Pupil squared, and apparent magnitude is the integral over that area. The key point is that SB itself is not enough - magnification also comes into play. Now, as aperture increases, you get more mag for a given SB. At this point, some people believe that the resulting increased brightness is a perception issue, while others (myself included) think that it is because the SB is integrated over a larger area. I exchanged mails with Randy Culp over this matter, and he concurred. Think of it like this. A 10-LED torch is brighter than a 1-LED version, but they have the same SB. It is an interesting - and contentious - issue! (For a point source, things are different - brightness depends only on aperture.) Observation, equipment, theory - what a great pastime this is! Doug.
  12. Thanks Nikolay. No, I haven't studied Bailly yet - as you say, there is so much to see! I spent most of the time with Schickard, comparing it with the fine Plato. Others points I noted were that Schickard's walls are more worn, and because of its position, it is more noticeably elongated (foreshortened) than Plato. You will have seen that effect on Bailly too! Clear skies! Doug.
  13. My Bresser AR 102S (short focus) is a fine, light, grab'n'go OTA on the AZ4, giving very wide views and mag easily up to x200. AND....it has a very nice focuser. The Startravel focuser is nowhere near as smooth. Also - a frac would give crisper views. A Newt only has a big advantage when you get up 8 or 10 inches, and you have an 8" Dob anyway. Doug.
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