Jump to content

Banner.jpg.b89429c566825f6ab32bcafbada449c9.jpg

acey

Members
  • Posts

    3,681
  • Joined

  • Last visited

Posts posted by acey

  1. Indeed, it's all totally harmless fun for the number-crunchers among us. Good to know I have honourable cause for my indifferent LPR. And worth remembering that when people receive good advice they don't necessarily like what they're hearing...

    • Like 2
  2. From a quick look at the start of the members list ranked by reputation, I see that StuartJPP has a score of 3,508 for a content count of 1,738, giving a very impressive Like:Post Ratio of 2.02. That must be hard to top...

    Edit:

    Hang on, davefrance has 3244/1325 = 2.45 - is there possibly a 3 out there?

    Edit again:

    Congrats to astroavani who scores 1737/541 = 3.21.

    Can someone automate the search and let me get back to work please? ;)

    • Like 2
  3. 3 hours ago, Stu said:

    Silly game ?

    I found a contender (who will remain unnamed)  for the lowest with 0.001 LPR ??

    Perhaps "Like:Post Ratio" could be categorised logarithmically rather than linearly. For example:

    10: Super-particle (probably non-existent: an average of 10 likes for every post!)

    1: Higgs (impressive but rarely seen, and only likely to exist for a short time)

    0.1: WIMP (weakly interacting massive poster, pretty average)

    0.01: Neutrino (abundant but too often ignored)

    0.001: Dark matter (possibly ubiquitous and best left unnamed)

     

     

    • Like 4
  4. On 15/03/2016 at 12:51, cloudsweeper said:

    And has anyone got a like-to-post ratio of more than unity?

     

    Mine is well below, presently 1042 likes divided by 3578 posts to give a measly 0.29. Anyone else want to play?

    • Like 2
  5. For object data I use the info at these places:

    http://www.klima-luft.de/steinicke/index_e.htm

    http://cdsportal.u-strasbg.fr/

    I do like books, though, and for object descriptions (as opposed to astrophysical data) I have long used these:

    http://www.amazon.co.uk/Observing-Handbook-Catalogue-Deep-Sky-Objects/dp/0521625564

    http://www.amazon.co.uk/Sky-Atlas-2000-0-Companion-Descriptions/dp/0933346956/ref=sr_1_2?s=books&ie=UTF8&qid=1443447849&sr=1-2&keywords=sky+atlas+2000

    http://www.amazon.co.uk/Revised-General-Catalogue-Nonstellar-Astronomical/dp/0816504210/ref=sr_1_1?s=books&ie=UTF8&qid=1443447887&sr=1-1&keywords=revised+new+general+catalogue

    I find that for me, the most important data are the RA and Dec co-ords, the NGC description (if it has one), and the galaxy type (if it's a galaxy, which it usually is). I don't really bother about z-shift etc when I'm looking through the eyepiece, though it's nice to know which galaxy cluster/supercluster I'm looking at.

    For object appearance, I refer to the DSS images which are freely available online, or else in this book (which I use at the scope - I love it):

    http://skywatch.jp/ngc/

    • Like 3
  6. Don't know, but here's how to measure it. Unscrew and remove the eyepiece section of the finder. Aim the finder (which now has only its objective lens in place) at a distant light source (e.g. Moon, or Sun if you aim using the finder's shadow and take due precautions). Project a focused image of the light source onto a piece of card. Measure the distance from the finder objective lens to the card. That's the focal length.

    Alternatively use a nearby light source (e.g. light bulb). In that case, let d1 be the distance from light source to finder objective, and d2 be the distance from finder objective to focused image. Then the focal length is f = 1/(1/d1 + 1/d2). For distant light sources we can take d1 as infinity, in which case f = d2.

    • Like 1
    • Thanks 1
  7. To expand a little on my earlier post - stopping down the aperture of a refractor or reflector is a traditional way of reducing the effects of aberration and light scattering. It may be used for various reasons:

    1) If the objective lens or primary mirror is not properly figured overall, or has major imperfections, you can reduce to using a part of it that is of sufficiently good quality. In a refractor this means blocking off the outer ring of the objective, in a reflector it means using any part of the primary that is not obstructed by the secondary or spider (which would reduce light or cause diffraction).

    2) By reducing the aperture you effectively increase the focal ratio, so if there is coma in the objective or primary then the effect of this will be reduced.

    3) If the target object is very bright then scattered light in the telescope can obscure the view. Reducing the aperture reduces this effect and can sometimes give a clearer view.

    4) A small bonus in the case of a reflector is that there will be no diffraction spikes due to secondary/spider obstruction.

    The obvious downside is that you are reducing the aperture, hence reducing the light grasp and resolution of the scope. In some cases the above points outweigh the downsides - a scope with larger aperture and better theoretical resolution may in practice give poorer views because of light scattering. Put a top quality 4" refractor alongside a basic 10" dob and you may well find that the 4" gives better planetary views than the 12". In practice I suspect there are few cases where the upsides outweigh the downsides, and in general you would anyway want a bigger hole (larger percentage of available aperture) than the one provided in the dustcap.

    The idea that larger apertures suffer more from atmospheric turbulence is an old myth, and has nothing to do with the present discussion.

    The presence of these holes on dust caps is, I think, largely a throw-back to earlier times, and I reckon most people never use them. Solar observing can be done with a filter (e.g. Baader solar film) across the whole aperture or the smaller aperture - the latter saves you money on solar film, and may give sufficient resolution for a satisfactory view. Full aperture will give higher resolution. In either case the film needs to be secured in place with tape so it doesn't blow off. If you ever do suffer the misfortune of solar film coming off while viewing the sun, your blink reflex will almost certainly save you from anything more than a nasty fright. Far more perfidious would be a dodgy filter which reduces visible wavelengths but is letting through infra-red and ultraviolet, damaging your eye without you noticing. I'm not aware of any dodgy filter film out there but you never know.

    In the case of a very bright target like the Moon or Jupiter, the small hole might give a better view if there is a lot of light scattering in the scope. Though in that case perhaps there might be a better way to deal with light scattering, and a neutral density filter ("moon filter") might give better results anyway.

    I suppose one other benefit of using the little hole would be that you won't get any dewing on your optics, but that wouldn't be sufficient reason for doing it. The reduced aperture means that the technique is completely useless on faint objects (i.e DSOs) - you would only ever consider using it on something bright, and only if it were going to offer any kind of advantage, which in most cases it won't.

    • Like 2
  8. It's for viewing with a reduced aperture, set off-centre because the secondary would block it if the hole were in the middle. Usually used for solar viewing (with safe solar filter over the hole). The other raised part is just to make it all look symmetrical.

    Stopping down the aperture can also be a way of reducing aberration and light scattering, though the hole in this case is very small and you may not get much of a view. No harm trying it on Jupiter, say, just for fun.

  9. Outside of physically moving to a remote area, are there any techniques you employ to deal with light pollution?

    No. The only thing that works is to get away from it or else choose targets that are very bright. Shielding your eye from surrounding glare will help a bit, and a light pollution filter might also help a bit if you happen to live in an area with old-style orange streetlights. I gave up back-garden visual years ago, and do all my viewing from a rural dark site.

    • Like 1
  10. It's as good as the person using it.

    Personally I've always managed fine with whatever optical finder has come free with the scope I've been using, but these things are a matter of individual taste and preference.

    I use the finder to aim the scope at a naked eye star, then use a low power eyepiece and a good map to get to the desired target. A telrad or gun-sight would do just as well for initial aiming. Important thing, IMHO, is the map.

  11. I picked up a used TV 32 Plossl for much the same reasons as Moonshane - something respectable for whoever doesn't get the 26 Nagler! It's pretty good but, yes, it's a Plossl.

    Olly

    What you pay for in a wide-field eyepiece is wide field. But FOV aside, is a Nagler really any better than a Plossl? Couple of interesting articles/threads comparing the two types:

    Eyepieces: Performance of Simple Vs Complex types - Shootout

    Telescope Reviews: Nagler vs Plossl - Round 1 (Indoors)

    Quote from first:

    "In almost every comparison I made, the simpler eyepieces delivered better absolute performance than the MUCH more expensive Naglers and the lone Panoptic in this test."

    Quote from second:

    "On-Axis Sharpness and Brightness - - Plossl Wins."

  12. Lots of scopes have 2" focusers, including some entry-level ones, and yes it's more flexible if you think you might want to use 2" EPs. But it's also a bigger hole that potentially lets in more stray light. I preferred the rack-and-pinion 1.25" focuser on my old Orion 8" to the low-profile 2" Crayford focuser on my 12" Flextube. You can't judge EPs and focusers simply by diameter, any more than you can judge a telescope by aperture alone.

  13. I used the 8-inch for 9 years and could have happily have gone longer as there were still so many things I hadn't seen with it, but figured if I was ever going to get into the heavy-duty league then I ought to do it before I was too old.

    My 12-inch is a flextube which is collapsible and therefore pretty easy to move around, but the base still has to be moved separately (and it's huge - only just fits through the rear door of my car onto the back seat).

    The 8-inch, tube and base together, was an easy one-hand lift.

    If you're trying to make the most of a half-hour break in the clouds then smaller is definitely better.

    Another good point raised by Paul is height. With the 8-inch I could sit on a camping stool and find the eyepiece always at a convenient height for viewing. Larger aperture means a longer scope and a bigger variation in eyepiece height: with the 12-inch I have to use an adjustable stool (some people use ironing stools; there are special astro chairs that are easier to adjust but cost lots).

    With the 8-inch I saw all the Messiers and Caldwells above my horizon and hundreds of NGCs. For planetary views I find the 12-inch no better than the 8-inch, because the latter had less issues with internal reflections causing stray light and loss of contrast. I'll deal with that by flocking when (if) I can be bothered. Suffice to say I don't think of it as a dedicated planet scope (or of myself as a dedicated planet observer).

    I look at faint galaxies from dark sites (Abells, Arps, Hicksons etc). The 12-inch is great for that, and worth the effort of getting it there. But if you want to see Saturn's rings or M51 there are easier ways that won't give you back-ache or a hernia.

    • Like 1
  14. All the planets out to Saturn are bright and easily visible with the naked eye. Even a very small telescope will show Saturn's rings, the phases of Venus, and the moons of Jupiter - Galileo saw all of these with a 2-inch telescope. Any modern telescope will show you the same things in better detail, and you don't need huge aperture, you need good resolution and high contrast. A top-notch 4-inch refractor will outperform a low-cost 12-inch dob on planets. Also with a dob you have to keep pushing to hold the planet in view, which can be annoying at high power. So a dob isn't the perfect planet scope - though it is a very good "all round" scope if you also want to look at deep-sky objects (galaxies, nebulae, clusters).

    With those, it all depends how dark your sky is. If you can see the Milky Way very clearly with the naked eye then 50mm binoculars will show you all the Messiers; a 12-inch scope will show all (or nearly all) the NGCs above your horizon - thousands of galaxies.

    If you view from a typical suburban sky then you'll see a lot less, and might struggle even to find Messiers with a 12-inch.

    Planetary views aren't seriously affected by light pollution - it's heat rising from buildings that causes more of a problem.

    I think dobs are great and ideal as a first scope (and second, third, fourth...), but I'd suggest 8-inch as a good starting point. Don't just look at the cost, look at the size and weight. And think about what you want to look at. If it's just planets then a 100mm apochromatic refractor might serve you better.

    I currently use a 12-inch dob but started with an 8-inch and don't regret it. When I first got the 8-inch I thought it was huge.

    • Like 2
  15. How far from the city does one need to be in order to escape skyglow?

    No simple answer. As you move away from a city, the visible light dome reaches lower into the sky. Question then is how low you need it to go: if you've got a good dark sky overhead then that may be all you need. Equally, if you go to a site where all the light pollution is to the north, then even if the light dome reaches near the zenith, it would still give you the south to look at. Also of course depends how big the city is. But for a reasonable size city I'd be wanting to get maybe ten miles away - assuming it would take me to rural or semi-rural land. Suburban sprawl complicates things, and necessitates going further.

    It's always a trade-off between the effort of getting to a darker site, and how much more you see when you get there. I drive about twenty miles to a dark site because it's so much better than when I used to only go ten. I could go thirty or forty and see a little bit more - but I want to spend my time observing, not driving. Good enough is better than perfect.

    In the meantime your UHC will certainly prove a good investment.

  16. I think you did pretty well to see M13 and M57 at all from west London. A nebula filter (OIII or UHC) would definitely improve the view of M57.

    Light pollution has two aspects, the direct light you get from nearby sources, and the overall skyglow caused by light scattering off moisture (also dust etc.) high in the air. You really notice the latter when you get to a dark site well away from a large town and see the light dome over the town.

    Skyglow is lessened if the air is very dry - but in UK that doesn't happen very often. And I would assume that polluted air scatters more light than cleaner air, though I don't know the extent of the difference.

    Definitions of "dark sky" are subjective, but if it means dark enough to see the Milky Way then you aren't going to get that anywhere in the London area. There will certainly be areas with reduced light pollution from direct sources, but unless those areas are many miles across they're not going to have any less skyglow.

    Still, from where you are you can see some DSOs - and planetary/lunar views can be just as good from a city as from anywhere else. You also have the advantage that at least in London you've still got some real night-time at this time of the year. In the more northern parts of UK, our "dark" skies don't get properly dark at all right now.

    Personally I'd think twice about seeking any kind of "dark" site in the London area. It probably won't be dark enough to make astronomy any better, but it might be dark enough for the sort of terrestrial activities you're better off avoiding.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.