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Rob Sellent

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Rob Sellent last won the day on November 14 2019

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About Rob Sellent

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    Star Forming

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    Spain
  1. I think a 3x Barlow would make a f/5 scope behave like a f/15. In effect, tripling magnification, reducing TFoV by a third and likewise the exit pupil. Depending on the Barlow you may need more out focus to accommodate for the narrower light cone being produced. A possible sollution is to unlock the Barlow-EP combo from the focuser and slowly bring it out and see if you can reach focus.
  2. In terms of visual reference, I tend to avoid images and focus instead on sketches from seasoned observers. Using them as a guide and understanding them as the best representation I'm likely to see myself. The linked video of Saturn probably presents a run of stacked images passed through some kind of software to tidy the image. With a power of 300x and an exit pupil of around 0.3mm it is not a handy guide for visual work. Nevertheless, on any reasonable night of seeing my 4" Vixen offered more visual information of Saturn this late summer than the video. However, no matter how much I worked with my Mak 127, I cannot remember ever seeing the Cassini Division. My Tal 100 achro would struggle and only on the best of best nights does the little TV76 reveal a hair line gap. Taking this into account, when observing planets with smaller apertures not only do the atmospheric conditions, one's experience and visual acuity play a significant role, but so to the quality of optics used. Finally, reading charitably the video and many sketches, I feel that astronomy is a very intellectual hobby. Many of the things we look at are barely visible or are whispering hints of something more just beyond our immediate grasp. Most people think we're nutcases. Sitting out in the cold and dark, observing barely nothing. I also think a lot of people find astronomy disturbing and even frightening. They either don't get it or simply don't want to. The reason we get exited about it and may exaggerate our visual experience is because we know what it is that we're looking at and it is that very knowledge which fuels our kick.
  3. In no manner has my own collection been as exhaustive or as profound as many of the other members'. My telescope history runs as: Tal 100 - my first scope purchased about twenty years ago. Built like a tank, good optics and well corrected f/10 achromatic. To this day, I still consider it a fine first time scope. Worth more than any asking price the Russian gem was donated to charity in 2014. TeleVue 76 - the little 3" is still arguably one of the most attractive scopes I've ever seen. Its primary function was for solar white-light but over the years I have grown to admire its night time capabilities: ease of use, swift cool down, compact, solidly built, with crisp, aberration free views. Lunt 60 B1200 - without doubt my most used scope. My astro-life changed with the Lunt. Now I could star gaze, sip on chilled cocktails and get a tan all at the same time. The revelation was more fundamental than the first time I viewed a faint fuzzy or Saturn at magnification. SkyWatcher 300p Skyliner - nice introduction to the universe 'Beyond Messier'. Decent aperture, quick set-up and good views. Sold in 2019. SkyWatcher Mak 127 - everything about this scope I dislike. Weighty, long cool down dew magnet, narrow field of view, sloppy focus, mediocre optics. Will donate when appropriate charity/club hits my radar. Celestron C8 - relatively light, compact and generally decent scope. However, it was sold on due to narrowish field, off axis aberrations, and uninspiring views. Ottiche Zen 10" Mirror Truss Dob - classic truss dob design, mirror from Italy. Solid, fluid, extremely compact offering outstanding views. If the Lunt is my most used scope this is probably my most travelled. It's been all over Spain and to France, Italy, Slovenia, Austria, Hungary, Slovakia, and the Czech Republic. Vixen FL 102s - absolutely stunning scope with incredible optics. The telescope staggered me at first light and hasn't let me down since.
  4. From my understanding it takes a good few millions of years for a large mass star to gradually die, less than a quarter of a second for its core to collapse, perhaps a couple of hours for that shockwave to reach the outer layers of the star, a few weeks to months to brighten as a Supernova, and then it just fades away with time. I recall observing the supernova in Messier 82 a few years back but can't remember it staying bright in the sky for long. With the 10" in action if I said it lasted a few weeks, I don't think I'd be that far out. Anyway, here's an amazing extract of the death of a star like Betelgeuse:
  5. Just to add to the complication (bear in mind I do not own either type, so just going on numbers)... Weight - + 1 for Ethos Eye relief - + 1 for ES92 On axis - both Contrast - I've always been a sucker for TV's field fo view contrast, so I'm guessing + 1 for Ethos Re-sale - if you bought secondhand and resold, you really wouldn't lose out either way. Focal Lengths - +1 for Ethos (more choice)
  6. Thank you all for such great replies. Its interesting to see what you guys are using and how your eyepiece set ups are in a good state of flux and evolution. The secondhand astronomy market being so poor in Spain and so tricky either to buy or sell decent used gear, I tend to drift towards the dictum, 'Buy once, cry once.' I also try to buy eyepieces that are recognised as working well without any aberration or distortion in any focal ratio scope be it an f/4, f/5, f/6 or what have you. I've found that scopes come and go, but eyepieces more often stay. Like Stu, I have a full set of BGOs. Optically they are outstanding and I used to use them all the time but now I'm older I find them a little tiring to use over extended periods. I also have 24mm and 19mm Panoptics, 14mm and 10mm Delos and Mark IV zooms for binoviewing. I don't have any comfortable, wide field of view, high power eyepieces. For many years I was quite content with just using a Barlow or BGO but perhaps for 2020, I'll keep a look out for a higher power Delos or DeLite and then I think I'll call it a day.
  7. To the question 'what is maximum useful/optimal magnification?' There seems to be a whole range of ideas.... 0.45mm x aperture in mm. 2mm x aperture in mm/50x per inch of aperture. 1mm x aperture in mm/25x per inch of aperture. focal ratio of scope x2 / focal length of scope (2mm exit pupil). focal ratio of scope x1 / focal length of scope (1mm exit pupil). Half focal ratio of scope / focal length of scope (0.5mm exit pupil). TeleVue recommends 2.5x mm of telescope aperture. Experienced observers going way beyond even these limits etc and so on and so fourth.... Depending on who you ask, a 100mm f/10, for example, will either have a maximum useful/optimal magnification of around 45-50x, 100x, 200x, 250x, or more. Plug in other factors like optical quality, collimation, seeing and atmospheric conditions, visual acuity, the type of object being viewed and these numbers may start to vary once again. In other words, all the detail through your scope will be seen at about (a, b, c, d, e) x per mm of aperture and that any magnification above will fail to reveal any more detail, although the detail might be easier to see. Clearly, as this thread is showing, such statements cannot be right. What magnification is useful is evidently variable. As such, all options should be explored when observing, or at least those which maximise the observer's experience.
  8. Just thinking outloud. But it might be worth placing the eyepiece's barrel a little way out of the focuser (cm by cm) and see if it comes to or near to focus. If so, it might be a 'problem' with the primary mirror's placement. If it is possible, raising the primary mirror just a small amount towards the secondary (mm by mm) might help. P.S: If that sounds a but mad, it might be an idea to hunt down a local astro-club and get some of the regulars to check over your system. For sure, it will be purely a mechanical problem which shouldn't take too long to fix. P.P.S: Looking at the photo, also make sure your secondary is at 90 degrees to your focuser and centered. If I'm reading the photo correctly it looks a tad skewed.
  9. I'm being dumb, but I don't get this, @vlaiv. I'm not disputing the math - I couldn't nor wouldn't. But I don't get how maximum useful magnification for a given eye is x0.45 the diameter of a scope's lens or mirror in mm. I apologise for calling you up. I've no doubt I've missed something but it's interesting what you've written and I'd like to understand it
  10. Aye, my own experience is just as @John says. Seeing conditions play a huge role in what is possible on a given night. Add to this the optical quality of the scope and eyepieces, cooling, collimation, the nature of one's own eyes, fatigue, and comfort and we've got quite a cocktail going on. There are also objects which can take magnification well (Saturn, Moon, Globs and some planetaries) and others which are not nearly so obliging (Mars, Galaxies, Sun, many nebulae). For general comfort viewing, I'm not a fan of pushing exit pupil beyond 0.6mm. The balance is to look for a crisp, sharp and contrasty image from which beyond you gain very little. Magnification, then, isn't so important in this case but rather the quality of visual experience. Regardless of the scope I'm using, I typical find myself working between around x50 - x200. Beyond this seems to only happen on exceptional nights of seeing.
  11. A shed like this might be handy to getting to dark sites and back...
  12. Avoiding the issue of budget for a moment, given your eyepiece case's current state what have been the deciding factors for you of choosing one eyepiece type over another? Do you have a particular favourite type of eyepiece you tend to use for Moon and planets and another type for DSOs or does a particular range of eyepieces dominate your eyepiece case? For example, a run of DeLites, XWs, or UWANs? Do you also enjoy using Barlows, Powermates or Zooms and if so, on what occassions? Finally, are there any eyepieces types on your wish list for 2020?
  13. @PHIL53recommendation is tip-top. It's great value and great fun. Very tricky to find a bad review.
  14. Aye, the predictions are certainly pointing in that direction. NASA published a report suggesting that the next 11 year cycle, namely, Cycle 25 "will be the weakest of the last 200 years. The maximum of this next cycle – measured in terms of sunspot number....could be 30 to 50% lower than the most recent one. The results show that the next cycle will start in 2020 and reach its maximum in 2025." Solar Cycle Science run by what appears to be two very competent astrophysicists also forecast a relatively weak cycle comparable to cycle 24 from which we're almost ending.
  15. Keep manual mounts and tripods, observing chairs and street light blockers outdoors in a brick shed. There's little chance of excessive damp or things such as slugs, spiders or gales. We might get the occasional creepy crawly in there but the local Tarentolas chomp them up pretty quickly. All optical gear, filters, maps and so on are kept indoors in one of the spare rooms.
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