Jump to content

Banner.jpg.b89429c566825f6ab32bcafbada449c9.jpg

acey

Members
  • Posts

    3,681
  • Joined

  • Last visited

Reputation

1,267 Excellent

2 Followers

Contact Methods

  • Website URL
    http://crumey.online

Profile Information

  • Gender
    Male
  • Location
    55° North

Recent Profile Visitors

The recent visitors block is disabled and is not being shown to other users.

  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...
  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?
  3. 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)
  4. Mine is well below, presently 1042 likes divided by 3578 posts to give a measly 0.29. Anyone else want to play?
  5. It's empirically verified that the speed of light in a vacuum is a universal constant, independent of reference frame. There is no analogous principle for sound waves. Hence one cannot attempt to reframe special relativity using sound, or any other form of energy transfer apart from radiation.
  6. 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/
  7. I have the 2-volume atlas, have used it a lot, and have never felt the need to buy the additional guide volume. The atlases have an index of objects (so you know what's plotted), and object data can be found in many online sources. Buy the guide if you fancy it, but don't feel you have to.
  8. 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.
  9. A true dark site is not "pitch black". Once your eyes are fully dark adapted the sky is markedly bright with stars, Milky Way and natural airglow. Moving around without any artificial light is easy. A lamp is only needed for seeing small objects, reading etc. Foreground objects (trees etc) look truly black against the bright sky. In a telescope at high power, the sky background looks truly black (you can't see the eyepiece field stop). After viewing for some time, when you look up at the sky again it's dazzling (you need to shield your eye from it while looking through the eyepiece). At a light polluted site it's very different. The eye adapts to the ambient light level (dictated by the pervasive glow of streetlights etc, even if not directly visible). Under those conditions a clear sky can look "pitch black", but only because the eye can't adapt fully. So apparent blackness is not a good test of sky quality, it only tests dark adaptation. The test is limiting magnitude. If you can see stars down to 4 mag then you should manage a few bright DSOs in a telescope (e.g. M31, M42, M57, M13, M81/82). If you can see to 5 mag then you'll see many more, and the Milky Way may be visible. If you can see to 6 mag then the MW will be clearly visible and you'll be able to see all the Messiers above your horizon with a 100mm scope (or smaller), and all (or nearly all) above-horizon NGCs with a 12". If you can see stars fainter than 6 mag then you have very good eyesight. Light pollution is of three types. One is direct glare from steeetlights etc. You need to shield yourself from that, e.g. by choosing your viewing spot, putting up barriers etc. Two is ground light reflected off walls etc; you can't see lights directly but your garden is indirectly lit up. Shield yourself by putting a hood over your head at the eyepiece and give your eye time to adapt. Three is skyglow caused by ground light reflecting off water vapour in the air. This limits the faintest stars you can see, and there's nothing you can do about it (unless you find that a "light pollution filter" works for you). "Nebula" filters (OIII, UHC etc) are effective on emission nebulae (e.g. M42, M1) but have no effect on other types of objects, i.e. clusters, galaxies (note for pedants: there can be some slight effect on a handful of large galaxies at a dark site, e.g. M33). At a light polluted site the easiest DSO types are open clusters, bright globular clusters (e.g. M13), bright planetary nebulae (e.g. M57, Eskimo, Cat's Eye). Diffuse nebulae (emission or reflection) are generally more difficult, though with a few bright exceptions (e.g. M42), and galaxies are generally very difficult, again with a few bright exceptions. The reason for all this is that light pollution hurts the limiting surface brightness of a scope more than limiting stellar magnitude. For example see Figure 18 of this paper: http://arxiv.org/pdf/1405.4209v1.pdf A telescope can't improve the surface brightness of a target, and most galaxies are of about the same surface brightness as the Milky Way. So if you want to see galaxies well, you need to be able to see the Milky Way with the naked eye.
×
×
  • 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.