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Everything posted by brianb

  1. The flattener simply alters the curvature of the focal plane ... by an amount dependent on the distance between the flattener and the focal plane. The curvature that requires to be corrected is essentially only a function of the focal length, in the case of a doublet or triplet objective with the elements touching or almost so. In other words, aperture and focal ratio are irrelevant, except in so far as they affect the focal length. Providing, of course, that the diameter of the flattener is such that, when mounted at the correct distance from the focal plane to correct the curvature, it is la
  2. But the odd thing is that - in terms of naked eye limiting magnitude - you hardly notice; certainly not 1.5 mags loss (half the aperture) by age 60. 0.5 mags I'd believe ... but even then I think it's at least as likely to be a brighter sky due to increased light pollution. I do find I need brighter light to read comfortably than I used to, but I don't think there's been much change in my perception at the dim limits of vision.
  3. Such as? Actually the King James Bible does a pretty poetic job of the creation ... but it's a rotten piece of physics, even by the standards current 400 years ago.
  4. It was hard to focus even at low power last night ... blame the very unsettled air ...
  5. You can't see gamma rays ... and until the universe has expanded & cooled quite a bit, almost all the radiant energy is gamma & hard X ray. If you were there, you'd be instantly boiled away, without even a concrete wall to leave a shadow on like the victims of the gamma flash caused by the Hiroshima bomb.
  6. Actually they do, when the kinetic energy is converted to heat the sudden vapourisation is effectively the same as an explosion. The key here is that the size of the final crater is scores of times bigger than the size of the impacting body. Think of it that way & it's not surprising that the crater is a good approximation to circular, unless the impact angle is very shallow indeed.
  7. Yes. A bigger scope makes objects bigger because you can use more magnification, but most DSOs are just too faint for the eye to see much colour, whatever scope you're using. A 12" / 300mm Dob is HUGE. Get a 8" / 200mm, use it for a year or so then decide what you want to do in terms of going bigger (and accepting that you will be able to see smaller, fainter DSOs but will not see Hubble type detail) or smaller for learning imaging.
  8. For imaging, x5 with a f/5 scope is very reasonable, given good seeing. For visual, IMO you don't need a barlow at all.
  9. The 5" Mak is a better planetary scope than any 4" refractor will be. Which makes the smaller refractor more useful for what the Mak does badly: low power wide field work ... If you're happy to get rid of the Mak, then a good quality 4" f/7 apo would be a good instrument to have. But it will be less "grab and go" than either Mak or f/6 80mm frac.
  10. For the highest useful power (without a barlow): take four fifths of the focal ratio of the scope. Your 130P is f/5 so that's 4/5*5 = 4mm. With such a short focal length you will need a long eye relief design. The standard Skywatcher 10mm is inadequate with a fast scope: spherical and chromatic aberration on axis, and fairly severe off axis aberrations will be apparent. A good 10mm will have a sharper image (when the scope is properly collimated & cooled and the seeing is steady), things will remain sharp out to the field stop, the field of view may be wider and the eye relief may be longe
  11. The prog has been on before ... entertaining enough in its way; the presenter is an eccentric in the best tradition of British amateur scientists. Another prog which has been on before is the extended March edition of "The Sky at Night" which is being repeated on BBC4 next Sunday evening, in fact the whole evening is space / astronomy themed - something to do with commemorating 50 years of manned space flight, I think. If the sky happens to be clear, there's always the video recorder, or the BBC iPlayer service.
  12. But the OP has a 5" Mak ... given that, there's a lot to be said for a high quality short focus 70-80mm scope for rich field views ... the difference in bulk / weight / field of view between a 80mm f/6 scope and a 100mm f/7 scope is considerable. If it was my money - and given the rest of the kit that the OP lists - my first choice OTA would be the Equinox 80 PRO FPL-83 or the Meade series 5000 ED 80mm APO triplet. (If I had the idea of doing imaging as well then I'd definitely prefer the triplet.) The AZ4 will have plenty of reserve to handle it; loading a mount up to its limit is in my exper
  13. Jovian & Saturnian moons always seem to show up better if you turn up the sharpening. What's impressive about your images is that the satellite has been rendered more visible without sharpening the planet enough to cut yourself shaving on ... not a fault that your image has, the planet & rings look just about perfect to me. I'll be trying Registax v6 next time the seeing cooperates ... Saturn was just a fuzzy blob last night, it was hard to focus at x108 ...
  14. Olly has described this just about perfectly. Increasing gamma is a useful way of pulling up "shadows" without burning out "highlights" ... Just remains to add that level stretching can have a very ugly effect on the background if your flat fielding isn't done properly!
  15. Possibly a slow moving satellite. I got a "fright" yesterday evening - there was a mag 3 "star" in the approx position of the recurrent nova T CrB; on inspection with binoculars it was moving very slowly & faded out over a period of a minute or so. Undoubtedly one of the thousands of pieces of "junk" which now litter earth orbit.
  16. But you won't get a wider field of view, this is limited by the 1.25" hardware attached to the scope. A 2" EP will be heavier, will likely cost more and the extra length of the adapter may result in difficulty getting to focus. Fact of the matter is, the Mak is not a good design for a rich field scope ... suggest you get a cheap short focus refractor or giant binoculars to use instead of your Mak when you really need a wide view. The Mak is great for planets etc.
  17. Looks very good - the rings are really bright; opposition effect I suppose.
  18. I think you'll find the MN 190 is very close to the limit with an EQ6 ... the FLT 110 is a cracking little scope, the correction is much better than the Megrez which is only a doublet, but you still need a flattener with a large imaging chip.
  19. Indeed - but the ejecta cloud nevertheless has a distribution which can be used to show which direction the impactor came from & approximately at what angle.See for instance the ray pattern around Proclus in this image 2011 Feb 15, 0111 UT.
  20. Yeah, last night looked fairly transparent (in the gaps between showers), unfortunately I was unable to use it as I had an appalling migraine from the afternoon hail/thunder shower.
  21. Seems reasonable ... I've managed to image 3 of Uranus's moons with DMK21 & 11" (that's going down to mag 15) with only a few seconds exposure. To get a reasonable S/N you'd just need lots of long exposures, plus proper calibration with bias & dark frames. But whether it's worth it or not is a different matter - fighting equipment that wasn't really designed for the job when optimised kit is easily available is a good way to get discouraged.
  22. Not with modern coatings, you should get 20 years + from the coatings, in any case recoating is a nuisance rather than a major expense. The more expensive scopes (irrespective of type) tend to be better built than the cheaper ones and will last longer ... apart from electronics, the weak parts are wearing out of threads in the bits where they get bolted together etc. But in practise even cheap scopes tend to last until you drop them.
  23. But the focal ratio of the primary of a C14 is f/2, the tube length of a 14" f/2 Schmidt would be only 56" which is, I think, quite reasonable (the same as a f/4 Newt). And there's very little point in making a Schmidt camera with a slow focal ratio - even in huge sizes, f/2.5 is typical.
  24. Ummm ... the basic issues with the "classic" Schmidt design are (1) tube length = twice the focal length, (2) curved focal plane, (3) inaccessible focal plane. There is a variety of two-mirror plus corrector plate designs which are apalanatic, anastigmatic, flat-fielded and free from distortion - one variant (Baker-Nunn) has an accessible focal plane, a tube length less than half of the focal length and works acceptably at f/2. Whether one really needs an f/2 system is dubious, given atmospheric sky glow at even the darkest sites and the sensitivity of modern CCDs; but Baker-Nunn cameras of a
  25. They're fiddly & in my experience have a tendency to help the eyepiece fall from its socket (thud!)
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