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great_bear

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Everything posted by great_bear

  1. All good stuff above - and the definitive check as to whether you're compensating tilt error with rotation is to examine the positioning of the *shadow* of the secondary when viewed through the focuser. If your mirror is mounted centrally to the vanes (like most mass-produced scopes, and certainly the SkyWatcher ones) then offset of the secondary shadow should be in the direction of the primary like this: If on the other hand, that shadow is veering more to the top or bottom, like this: - then you've got a tilt/rotation mismatch. Hope that helps...
  2. Ultimately it's just a game of balancing the "push" of the hex screws against the "pull" of the centre-bolt. As you've (no doubt) discovered, you have to keep - shall we say - "re-asserting" the correct degree of twist of the secondary assembly in order to keep it looking circular as seen through the finder. If you simply tighten everything without doing this, the secondary can tend to do its own thing. It's a bit of a fight really - especially since the hex screws may have already ground slight dimples into the back of the mirror holder, thus influencing how it settles.
  3. Yes - that's how it's done. Whilst it can be argued that the whole assembly could be much better designed, removal is far less frequent than minor collimation tweaks so I guess it's a cost-cutting thing. I sometimes wonder if those who say that collimation is easy fully understand the subject. A full secondary collimation can be a very tricky exercise because the geometry isn't entirely straightforward and the tweaks interact. If the mirror is physically centred on the spider vanes (as it tends to be on modern scopes) then you need to offset the secondary towards the primary somewhat, which means it's not quite at 45 degrees and introduces some amount of cone error that you just have to live with. You also need to ensure that the secondary assembly doesn't get twisted relative to the focuser which is tricky since it does tend to shift as you tighten everything up - and of course, the simplistic mechanism means that all these adjustments will interact to some degree, so the process becomes iterative and time-consuming. Makes it all the more satisfying when it all comes together though! :-) - and of course subsequent adjustments are trivial in comparison.
  4. In contrast to the SkyWatcher, the Revelation 8" Dob has: Really nice high-quality Alt bearings with large tension knobs cleverly designed so that the tube just lifts straight on/off the base instantly You can fine-tune the scope balance if required Built in cooling fan with battery box Dual-speed 10:1 crayford focusser Large knurled collimation knobs on the mirror Better quality finder Decent (re-sellable) eyepieces with 2" extender tube
  5. I have a Mak 180 and the Revelation 200 Dob (which is better than the SkyWatcher one). The Dob is now my travel scope (seriously). The OTA fits into the roofbox on the car, and the dob base has no impact on space for me as I pack luggage bags both into and around it in the car boot. I bought it because: 1. It is considerably more stable in a breeze than a tripod-based scope - this is a big plus 2. It is considerably less hassle to set up and use than a motorised setup Il still use the Mak for those long urban nights binoviewing Saturn, Jupiter, or the moon etc. - but it is staying at home from now on. Life's too short for all the faff setting it up when visiting a dark sky site. The Dob feels like freedom - and the wide-field views are breathtaking!
  6. http://www.telescopehouse.com/cgi-bin/sh000001.pl?REFPAGE=http%3a%2f%2fwww%2etelescopehouse%2ecom%2f&WD=winged&PN=Orion_Winged_Rubber_Eyeguard_1_25__%2ehtml%23a07406#a07406 Sent from my SM-A300FU using Tapatalk
  7. ...I can't help but think the four photos on this guys ABS scope advert here <click>, somehow don't quite do justice to the size of the thing as seen here <click> Made me smile at least, on this very rainy day
  8. Yup - a sheet of white paper (with some Sellotaped-on string for easy retrieval) lightly wedged between secondary and primary. If rotated properly the secondary will present a white circle when peering into the drawtube, if not, it appears as an oval. It's also a good opportunity to check that the circle thus presented is positioned centrally relative to the drawtube.
  9. Apples and Pears really. I've moved away from Plossls / Orthos. I now settled on Long Eye Relief eyepieces (mainly Delos) for the following reasons: They are more comfortable to use than classic designsThey are *much* less affected by dirt and greaseThey are considerably less likely to get dirty in the first placeThey are easier to clean once they do get dirtyThe image projection feels more "cinematic"Now I've had the 4.5mm TMB Planetary II for about 6 weeks, I'd say the issues with it are the expected ones, namely: The dancing ghost reflection Stray light control issue (actually caused by the Smyth lens inner retaining ring)With regard to issue #1 above, I can live with this. With regard to issue #2, this definitely needs fixing. However, the problem is not the inner barrel baffling (as is often supposed), but the inside retaining ring for the Smyth lens. It has a matt black bevelled edge that is still quite reflective, machined at such an angle as to invite such reflections. It should be possible to fix it by attaching a circular, slightly conical piece of self-adhesive flocking paper (like an A4 paper hole-punch reinforcement ring, but black and in miniature) but it will take time to create the perfect shape and size to fit. Definitely do-able though - and will be well-worth the effort - on my to-do list...
  10. Well, the nice thing about an F13 scope on a motorised mount is that any respectable simple-design eyepiece will give you top-class results on the planets. For planetary use in such a scope I'd avoid TMBs / BSTs, and at F13 Meade 4000 Plossls will give results indistinguishable from Tele Vues at minimal cost. I'd take both the 9.7 and 12.4mm Series 4000 plossls over any one BST eyepiece for use in your scope (I've owned a Sky-Watcher 90mm before). But preferences vary of course... :-)
  11. Ah yes - where all my light pollution comes from
  12. That's not quite right. There are no "cheap clones" - this style of eyepieces all come from the original manufacturer. The dispute was over who had the right to sell, and under what branding. Since none of us are party to what was agreed in the contract in terms of minimum order size, penalties, redress, etc. etc., it's not our place to judge in absence of facts, but what we do know, is that additional focal lengths were added independently by the manufacturer. For the "inbetween" new lengths at the shorter end of the scale, these are likely to be fine (as these are neg-pos designs, 0.5mm - 1.0mm difference can probably be subtracted simply by lengthening the barrel, with no lens modifications and no adverse optical effects). Scaling the design above 9mm however is a little dubious... EdZ on cloudynights asserted that in resolution tests, the eyepieces scored very favourably. People's subjective experiences of these eyepieces however, vary considerably.
  13. You may find 8mm is pushing an F13 scope a little too far - it's certainly pushing past the edge of maximum resolution.
  14. When you say "offered" do you mean for free, or to buy - and if so, how much?
  15. Not by necessity - it's all down to the design of each one. The Nagler type 1s suffered from it, and as such it was an issue that was addressed and reduced in later versions. Unfortunately though it's not a published specification for eyepieces, so it comes down to knowledge exchange and personal evaluation.
  16. The 16mm is indeed lovely - the main problem with (any) 25mm Plössl is the excessive eye-relief, which they fixed in (some) Meade 4000 models by setting the lenses deeper into the barrel to aid eye placement.
  17. Sure - that following diagram is from www.telescope-optics.net, a free educational resource where a more technical description is provided at the bottom of the page. Light exiting an eyepiece can be thought of as thick "beams" (rather than simply "rays") of light, projecting (via the eye's own lens) onto the various parts of the image formed on your eye's retina. The thickness of those beams is the exit-pupil width measurement, and in an ideal world, when leaving the eyepiece, all those beams all cross over in exactly the same place. The distance of that position in front of the top lens is the "eye-relief" measurement. "Spherical aberration of the exit pupil" (a very common issue) is where beams representing the outer edge of the image cross over further away, or closer to (as in the above picture) beams in the mid-field angular range. The net result of this is that if you are too near or too far from the eyepiece, either the edge, or the mid-range of the image will lose illumination. When the mid-range loses illumination, an unstable kidney-bean-shaped shadow appears to swirl around the image. Finding the eye-position "sweet spot" to obtain stable illumination of the entire image, can be rather challenging. The situation is worse in daylight simply because the substantially-reduced size of the iris makes perfect eye-position critical or even impossible if spherical aberration is severe.
  18. The field curvature means the outer edge of field requires different focus than the middle. Younger eyes may not notice, but older eyes - if the problem is severe enough - may struggle to focus. I don't think that's the case here though, since the problem is probably still small enough to be masked by residual astigmatism. I've not been bothered by field curvature on Televue eyepieces, and my focussing abilities are as weak as a kitten these days... The exit pupil aberration means that the eyepiece is more likely to kidney-bean under daylight conditions.
  19. Yes - I don't think Tele Vue change country of origin once production had started - "what happens in Japan stays in Japan" :-)
  20. *clarification - the above statement applies to lenses *above* the field stop - Smyth lenses, Barlows etc. which lie below the field stop are another kettle of fish entirely...
  21. The trouble with Meade Series 4000 Plossls is they're so hard to pin down. Gone are the days when you could ascribe a certain meaningful vintage to them - today, the individual components are second-sourced, and so a unit produced on one day might be quite different to the construction of one produced the next. What I can tell you though, is that out of all the Series 4000 plossls that I have disassembled (which is quite a few of different generations, SP, LP, but not the infamous DS version) I have never come across a single one where the achromats are anything other than perfectly identical. I did however note with interest that the best-performing one of the lot was not the "Japan Smoothside", or the non-pseudo-masuyama Japan version, but a run-of-the-mill Chinese version. I find it disappointing that people assume any optic not made in Japan must be inferior - especially since Tele Vue eyepieces are "Made in Taiwan" anyway. Collecting Series 3000 Plossls is a little indulgence for me. Whilst they have plano exterior lenses, there must be *something* special going on (glass type?), since - for me - they are the best-performing Plossl I've tried so far. It doesn't matter that they don't have "blackened lens edges": A renowned eyepiece commentator on CloudyNights told me that when he dismantled an eyepiece a while back and manually blacked all the lens edges with paint, when he re-assembled it, he found it made no difference whatsoever. This should not surprise any optical designer - the only thing that unblackened edges should affect is the darkness of the area around the field-stop, and not the image within its bounds...
  22. That's right - the only eyepiece that corrects telescope coma by design is the Klee Pretoria.
  23. Is there something left that you'd like clarification on?
  24. From Wikipedia: "In the United States, the cost of defending against a patent infringement suit, as of 2004, is typically $1 million or more before trial, and $4 million or more for a complete defense, even if successful" For this reason, "protecting your property" is not the only goal for patents - a common reason for them is a pre-emptive strike to stop someone with bigger pockets from stopping you going about your business in future. This is called (I believe) a "defensive patent".
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