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rl

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About rl

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  1. With due respect to Nikon (some of whose kit I also own) your lens may prove to be a slight disappointment as a scope even if you can find a short enough diagonal to make it work at all. I don't know this particular lens but I expect it's built to produce excellent colour correction and minimal distortion over a 44mm circle compromising absolute resolution on the process; the design tradeoffs with camera lenses usually give the best resolution when stopped down to about f/4 to f/8. Your lens has the additional design constraint of being a zoom..getting that right will mean the resolution probably is at its best over a limited range of focal lengths. Run fully open at f/2.8 the resolution probably won't get anywhere the Rayleigh limit since the camera pixel size will always be larger than the theoretical spot size wide open, and you will need excellent eyepieces to cope with so wide a cone angle without some horrible off-axis aberrations. Even TV only test to f/4. The usual advice using camera lenses for astro use is to stop them down 1 stop to improve the resolution ans star shapes in the corners. Astro telescopes are built to optimize resolution first, and trade off flatness of field Baader do a fairly comprehensive range of adapters; I don't know if they do a Nikon female to T-thread. They do a helical T-thread to 1.25" focuser which would open up the possibility of straight-through viewing. For low power-wide-field use you might be on to a winner capitalizing on the flat field assuming the eyepiece has an equally flat field. Regards, RL
  2. Fantastic shot which really brings out the Big Picture..somehow pixel peeping at the corners misses the point!
  3. Peter's advice is sound...get some visual experience first before hitting the steep learning curve that is astrophotography. If you insist on jumping in with astrophotography at the deep end, any of the the 80mm aperture apochromatic refractors on the market teamed up with a HEQ5 mount is a very popular tried-and-tested route into the subject. Possibly the most popular scope is the Skywatcher ED80 which has become virtually an institution with good reason. For AP the mount is at least as important as the scope....don't skimp here because it's unglamorous metal. Canon DSLR cameras have the best support for astronomical use. Universal advice is to read one of the excellent books available on AP before diving in. "Making every Photon Count" is a favourite. But...you need to budget for: telescope, mount, field flattener, camera, guidescope, guide camera, laptop, software.........and time to learn how to use it all. You may have some of the kit already but most people find there is little change out of £1000 by the time they have a setup that actually delivers results. There are cheaper ways in just to test the water, with a star adventurer mount and a DSLR with normal camera lens, but if you get the bug you won't want to stop there. The skywatcher ED80 or similar is a good visual scope but easily outperformed by a 6" or 8" Dobsonian when used visually. The real beauty of the ED80 is its versatility; even after you have got something larger for visual use they still make excellent widefield AP scopes, and are very portable. Many people keep them as a second scope.
  4. And try John's test with 32 and 52 Orionis! That will be worth a whole bunch of armchair theorizing...
  5. I'm not sure the Rayleigh is more relevant; it is certainly better defined but the choice of definition was failry arbitrary in putting the centre of one disc over the first dark ring. It has stood the test of time very well but but Rayleigh could just as easily have chosen the first light ring which would have included a definate gap. The Dawes is totally empirical and more subjective but applies to what people actually perceive at the eyepiece which is maybe what matters. The moons of Jupiter are nonstellar even in a humble 6" newtonian; compare them in the same scope with any 6th magnitude star and you will see a difference given enough magnification. All four are about 2 arcsec diameter from memory and the disc you see is noticeably bigger than an Airy disc for a stellar point source. People with scopes 12" and upwards have managed to image very rough details on Jupiter's moons that with a lot of imagination bear a dubious resemblance to blurred voyager photos .... but with small/ medium scopes all you see is an obviously enlarged disc with no real detail. Slightly off subject, one of the things that strikes me on this subject is the way that technology has not really improved on the basic physics (why should it, you may ask...); the same rules that worked 150 years ago have stood the test of time and are still used to judge modern scopes. Both Airy and Dawes used handmade long focus achromat scopes with simple crown/flint combinations, with no coatings. The results we get nowadays with ED glass and superb coatings are really just the same in terms of double star resolution. There are obviously other benefits of progress in terms of colour correction and contrast and scope length but from my own experience a well-made simple long focus achromat can deliver all the detail in a double star that can be seen with the best of modern ED triplets. Apart from the 5" I have a 4" cheapo skywatcher crown/flint achro amd a 6" uk made crown/flint/crown triplet with no coatings; both go down to the Dawes limit with ease on the rare occurance of a good night.
  6. Well, I started looking at Zeta bootis around 1985. I used to look at the predicted separation in the pink BAA annual handbook and see how well I could separate it. If recall correctly the separation dropped from about 1.2 arcsec to 0.8 arcsec in this timeframe (1985-1995) and I was able to track the separation from cleanly split with a gap to figure-of-eight, to elliptical, to a single star as seen today. So I must have gone through both Rayleigh and Dawes limits at some point knowing the separation from the handbook. The Rayleigh limit for my scope is 1.06" and the Dawes 0.9" I've long since thrown away the BAA handbooks but someone might still have a set and can look up the exact separation and PA for each year but I remember being quite satisfied with the scope's performance at the time; 0.9 arcsec was detectable as a contact double with a waist where you could have got a rough position angle. If you're playing this game seriously it's important to have up-to-date information on the orbital details. Many older sources can be inaccurate. Theoretically the Rayleigh criterion is much better defined than the Dawes in that the centre of one Airy disc is placed over the middle of the first dark band of the second star. This still leads to some overlap between the discs but the waisting in the middle is clear and constant in shape. Dawes' limit is a lot more subjective and really amounts to the point where you can just be certain there is a waist in the middle of the combined discs. The wavelength of light is also critical; here the rods in your eye impose a natural filter at about 500-550 nanometres. It's important to appreciate that from the practical observational point of view neither Rayleigh or Dawes is a hard limit cast in stone where one side of the line the double is clear, the other it's invisible and all you see s a single star. There is a gradual coalescence of the Airy discs as the separation goes down and the exact point where an observer draws the line between split and unsplit is somewhat subjective; it's hard to say if the first Airy disc is exactly on the first dark band...even for Takahashi owners! A lot of experience and preferably averaging over multiple observations is required; you are working at the limits of the scope at maximum useful magnification where atmospherics wreak the maximum havoc. If you google zeta bootis there is quite a lot on it; many others have tried exactly what I did! It was a useful star because it's bright enough to be easily found and the two components have almost exactly the same brightness. It's too close now. RL
  7. Excellent article from John. I watched my 5" scope go through both the Rayleigh and Dawes limits. It was (still is) a f/10 doublet, with a 4.8mm Nagler MK1 eyepiece. RL
  8. I managed to follow zeta bootis (two equal brightness stars) down to below 1" with a 5" refractor during the 1980s- 1990s. Over a couple of years it coalesced from touching but distinct Airy discs, to an ellipse, to a circle. There were only a handful of nights in a season steady enough to be sure. This and Porrima are the only cases where I have seen stellar orbital motion myself. RL
  9. Vlaiv Many thanks, I had the method correct but was missing the factor of 2.355. RL
  10. Vlaiv I read your reply with some interest and was curious how you calculated the total FWHM from the individual components. Presumably you convert the scope aperture into resolving power using the Rayleigh formula (at 500nm?). Is there then some root-of-sum-of-squares calculation over 2 dimensions? I can't get the same numbers with a simple calculation. Regards, RL
  11. I'm looking at mine in the light of your experience and there are indeed a few marks on the black plastic case but not that would cause me any concern. I guess at 30 quid or whatever I paid for it some time ago, it comes in at the bottom end of astronomical expenditure as accessories go. It does a good job and has lasted a couple of years, but he bottom clips look like they might break easily given a good whack. .I've got both the telrad and the rigel; the telrad is sturdier but a lot bulkier. It too has the same sort of marks. Both are built down to a price and somebody has made a killing but I wish I had thought of something so simple myself! If Takahashi had made it there would be no change out of £200. Could you put up a few photos? Ultimately is it fit for purpose? RL
  12. Personally push-to every time. I've owned a couple of smaller ( read cheaper) go-to mounts (ioptron cube, celestron cg5) and I always spent more time frustrated with the recalcitrant go-to that didn't do its job properly trying to read a low-contrast display, than enjoying the session. Bigger ones are fine, I've got both an AZ-EQ6 and an EQ8 that work but they're not portable, certainly for grab-and-go. A decent purely mechanical mount, either alt-az or eq, with old school manual slow-motion drives, combined with a 50mm finder would be my choice. If you've got an equatorial, then a simple RA drive is ok and convenient , but the less there is to go wrong when you're out portable, the better in my book. But chaque'un a sont gout...... Regards, RL
  13. Brilliant shot as usual...I wish I needed to copyright mine!
  14. I have the same OO wonderful 8" f/4.5 and also own an OO VX6 f/6 1/10 wave (specially made, the usual choice is f/5 or f/8) and a skywatcher 150 PDS. The two 6" don't show quite as much as you might well expect but either shows a lot more visually than the proverbial 80mm ED doublet. They're too big for a star adventurer; I've got a Celestron alt/az which works ok but the OTA will collide with the tripod legs at some angles. I'm investigating an AZ4 but haven't found one yet. IMHO the 6" Newt OTA is vastly underrated; they often go for less than £100 secondhand and a good one well-collimated will outperform refractors up to 80- 100mm especially on DSOs. My VX6 does pretty well on double stars and planets once it has cooled down. It will realistically support magnification up to *200 on a halfway good night- my 12" won't do that much better. The contrast on the moon using a 4.7 ethos is surprisingly good; crater shadows really are black and the moon fills the FOV. A coma corrector is useful on the skywatcher. It cost me more than the scope.... RL
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