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Ruud

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Everything posted by Ruud

  1. It's impossible if you consider how the exit pupil is formed.
  2. Above is how you can calculate the diameter of the eye lens needs to have given the apparent field of view (afov) and the eye relief.
  3. Sorry, I haven't yet bought anything from FLO's. That's because I live in the neighbourhood of Robtics.nl. Contact FLO, I'm sure they'll answer any question you might have.
  4. Hi Denys. The €49 TS 20 mm 70º is an Erfle. Erfles work better on slow telescopes. Your telescope is f/5, that's pretty fast. The 20 mm 70° will give you a true field of around 1.85° but the edge is likely to be noticeably less sharp than the centre. It's a pity that the 16 mm 82° Nirvana costs €105 at TS. It works well at f/5 and would give you a 1.75° true field with better edge performance. If you're in the UK you can get the Nirvana for less (£69=€76.60 at FLO). Have you checked what's available on the used market?
  5. Always check on Quickmap: https://quickmap.lroc.asu.edu/?extent=34.9607907,42.3315112,88.7221873,51.4307669&proj=10&layers=NrBsFYBoAZIRnpEoAsjYIHYFcA2vIBvAXwF1Siylw4oNEQBmOORxBaAOgA42d9K5QaSA Quickmap can zoom in incredibly deep. There are many boulders in this crater
  6. Agena has the starguiders for $60 https://agenaastro.com/catalogsearch/result/?q=starguider
  7. Ruud

    New hobby

    Hi Monty, and congrats on your new telescope
  8. You might also like this: http://carolrpt.com/astroguidev9complete.pdf A Simple Guide to Backyard Astronomy Using Binoculars or a Small Telescope Assembled by Carol Beigel Discusses 'How to Find Things in the Sky, Charts, Books, Gizmos, Binoculars for Astronomy, Telescopes for Backyard Astronomy, Star Charts, List of 93 best objects in the Sky with charts, Good Binocular Objects, Charts to Get Oriented in the Sky and the Messier Objects'
  9. I can see non-illuminated crosshairs against my sky, so I don't have illuminated finders. The one I like best is a SkyWatcher 6x30 7.5° RACI. My 8x50 5.4° GSO RACI shows more stars but too many stars is actually confusing. The 7.5° view and the more easily recognisable star field is why I prefer the 6x30. I also have a red dot finder as Robindonne above. You can turn up the dot till it's very bright and I rather like it for daytime use. You have to be behind the thing while looking through it, which is easy when the telescope points horizontally but uncomfortable when it points up.
  10. Good idea - ask before buying. Zoom binoculars promise a lot, but they are crap really. There's another type of binoculars we warn against: "auto focus" or "focus free" binoculars. Those are no more than binoculars without a focus mechanism. Focus is fixed to one distance and if you want to study something at a different distance you're out of luck (a swan's nest that's 15 m away for example).
  11. By the way, Mr Niall, you can easily make a finder yourself for the C70 from a ballpoint pen.
  12. I had a C70. It worked best at the lowest magnification I tried: 23x with a 32mm GSO Plössl. This was about the only magnification that my minimak handled with any grace. I blame the erecting prism. I kept the Plössl and gave the C70 to a shop 'selling' free second hand stuff. I still had the original 25...75x eyepiece and all else: table tripod, box etc. I put a note on it saying that I thought it would probably make some kid happy and not to look at the Sun with it. You could invest in a 32mm GSO Plössl if you want to. It's a decent eyepiece and I still use mine on my new 70mm telescope.
  13. Buying things makes me very happy. I won't be going on vacation so I bought an eBike a few weeks back with an extra battery to have a greater range. I am delighted with the thing. If you have a bit of money do spend some to discover the world. If the sky is your world spend some on that. Use your mind, spend you money well and nothing but good will come of it. Work and save for later but don't forget that you also live today. Today I discovered a weird bicycle path going right through a farmer's fields. There were no ditches to separate the animals from the cyclists so I had cows, calves and cow pats all around and on my path. Hundreds of each. It was quite an experience. The bike was expensive but makes me a bit happier every time I use it.
  14. Gee, Louis, that looks awful! It's difficult to call that a ring of fire, it's more like a rainbow disk! @Don Pensack Google kind of fails me when I search for “chromatic aberration of the exit pupil” (with quotation marks to avoid partial matches). I find two categories of references: The first is about chromatic aberration of a Galilean telescope using non-achromatic lenses. That’s not what we are talking about. (It’s odd, btw, that these links refer to exit pupils of Galilean telescopes. Galilean eyepieces do not form an exit pupil in the sense of a region behind the eye lens through which all rays pass). The second category involves discussions on cloudynights.com. The links referred to are associated with fat and hideous brown rings inside the exit pupil. That is not what I’m addressing in my post here. For a quick impression have a look at this search result: link Another thing I am not talking about is chromatic aberration in single positive rather than negative-positive eyepieces. Most of us already know that when an eyepiece has chromatic aberration, it tends to get worse toward the edge of the field, so the stop shows it more clearly than any other part of the field. I briefly considered asking a mod to change the title of my post, but really the well defined coloured ring close to the field edge of neg-pos eyepieces is what we call a ‘ring of fire’. This may even be the case in most places. I’d like to refer you to this thread, in particular the second post: And yes, this ring does not always need to be cyan or blue. It really depends on what kind of chromatic aberration the Smyth lens needs to correct for. And of course, the less chromatic aberration the Smyth lens has to correct for, the thinner the ring will be.
  15. A Barlow is a negative achromatic lens that can be inserted before an eyepiece. It elongates the light cones from the telescope’s primary mirror, effectively increasing the focal length of the telescope. With a Barlow lens in place the magnification of any eyepiece used in that telescope increases. Ideally, a Barlow affects magnification only and will not introduce any aberrations. Smyth Lens Charles Piazzi Smyth, 1819-1900, was an Italian-born astronomer and Astronomer Royal for Scotland from 1846 to 1888. He devised a new application for the negative achromat, using it as a field lens to correct for field curvature in an otherwise well corrected lens. Smyth’s idea can de used for other corrections as well, and over the years Smyth lenses have been developed to correct for all sorts of undesired side effects in eyepiece designs. This has led to significant improvements of eyepiece performance. Many modern wide field eyepiece designs involve a negative lens group, followed by a positive lens group which forms the final image. Collectively these negative groups are called Smyth lenses and invariably their purpose is to correct for aberrations. Unlike a Barlow, a Smyth lens is not an optional element: it is an inherent part of the eyepiece design. Removing it would result in a poorly performing eyepiece. Eyepieces that have a Smyth lens are also called negative-positive eyepieces. Light from the objective enters the eyepiece through the negative Smyth lens and an image forms at the field stop which in these eyepieces is internally located between the negative and positive groups. The eye looks at the image through the positive lens group, the last lens of which is the eye lens. Ring of Fire In eyepieces, lateral chromatic aberration is caused by light of different wavelengths being focused at different positions. This happens in the positive lens group of for instance the Delos eyepiece. If used without Smyth lens, the positive group in the Delos would cause the image in cyan-blue wavelengths to be magnified slightly more than in other wavelengths. This would cause some nasty lateral colour, adding a red-cyan fringe to all high contrast edges. Of course the Delos is a near-perfect eyepiece. A Smyth lens corrects for the positive group’s lateral colour. It achieves this by projecting a slightly smaller image in cyan-blue light than in colours of other wavelengths: This works well across the field, except for rays at the very periphery. There cyan-blue light can pass while the other colours are being blocked: As a consequence of the compensating chromatic aberration of the Smyth lens in the Delos eyepiece, a star that drifts out of the field briefly turns cyan-blue at the edge of the view before it disappears. Also, during daytime observations or when the Moon fills the view, a hair-thin cyan-blue ring can be seen at the edge of the field. This is the Ring of Fire. It's actually quite pretty when a bright star drifts out of the field with a tiny explosion of blue light. Thanks for reading!
  16. 14 is very close to 12.5! To fill the gap between 12.5 and 24 I'd choose √(12.5×24) = 17.3mm or something close to that. From 24mm to 17.3mm is 38% more magnification, and from 17.3mm to 12.5mm is again 38% more signification. So 17.3 splits the gap of in equal magnification steps. The 16mm Nirvana is an amazing eyepiece: 82° and tremendous value. Flo has it for £69.
  17. Excellent! 65° eyepieces and an exit pupil that isn't too wide. A much better choice.
  18. Would you mind porro prisms? They are heavier and bulkier, but you tend to get better quality for the same money, especially above 42 mm aperture. 10x50s should have a field of view of about 6.5°. You may have to settle for average edge sharpness and colour fringing. Transmission should definitely be above 80%. For roof prisms, phase coating makes a difference: it prevents loss of contrast and saturation. Find a shop where you can try the impressive Nikon Monarch 7 8x42. At £360 they are good value and may be a significant improvement over you currents binoculars, but do they fit your budget? Have a look here: https://www.allbinos.com/binoculars_reviews.html Also check out tests, reviews, impressions and discussions on bird watching sites. What are deal breakers for you? For me they'd be poor transmission and contrast, more than average false colour and less than average edge performance. Here are a few few places to visit: https://www.birdwatching-bliss.com/binoculars-for-bird-watching.html https://www.birdforum.net/reviews/ http://www.birdwatching.com/optics.html https://www.bestbinocularsreviews.com/by-use.php https://www.audubon.org/gear/binocular-guide http://www.binocularsky.com
  19. It's exceptional of course. A beauty!
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