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digital_davem

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About digital_davem

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    Star Forming

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    South east London
  1. Re: True FOV v Apparent FOV again

    Hi digital_davem,

    Out here in the Arabian desert, one can feel a bit astronomically isolated! I was out camping in a beautiful area of rocky and sandy desert last weekend. Amazing because recent rain had led to a brief sprouting of grass and the Bedouin had brought their herds of camels into the area to graze. I set up my small refractor on top of the huge dune we were camping on only to see clouds beginning to obscure the view!! Out here, clear skies are the norm, (but you do have to be careful when the wind whips up the sand and dust). I did get some nice views of Jupiter at least. A (photographic) friend queried when I explained that I was changing to an eyepiece with wider AFOV. I trotted out the stock answer about the difference between TFOV and AFOV, but quickly ran into difficulty when he persisted in asking why!! French philosophers used to say if you keep asking why about six times (or ten... can't remember!) you always run into God!!!

    I loved your questioning thread on this topic a couple of years back... By the end, I think you were theorizing that a telescope objective optic (lens or mirror) may produce a maximum illuminated circle. That the real difference between EPs with different AFOVs was how much of this maximum illuminated circle they could usefully use? With better quality (more expensive) optics and size, their field stop could be set wider giving the wider fields of view? Did you ever get to the bottom of this? Could you summarise your current understanding of the 'real' difference between TFOV and AFOV for us relative newcomers who still struggle!

    Thanks again

  2. Given the amount of slop, i wont be locking them at all. Still don't understand why there so much movement. I don't think k the gears in this thing would be much use in a clock or watch....?
  3. When the locking clamps are unlocked, is the worm drive doing anything - it feels like it is bypassed.
  4. Makes sense. So the movement is in worm drive rather than the main bearings?
  5. Hi I'm not sure what to do with the counterweight on an alt az (it's kind of obvious on an EQ) so I screwed the bar into the top mount and put the weight in the middle. It does help - now I can nudge with the clamps unlocked reasonable smoothly and it doesn't drift under the scope's weight. It's very usable like this. I still don't really understand why such a huge mount isn't rock steady, though. There should be zero movement with the clamps locked. Given that a simple cheap camera ball head is completely steady within its weight limit it doesn't say a lot for SW's quality control. The clamps are effectively useless as they don't clamp!
  6. Very nice, looks exactly like my Prinz 660 before I refurbished it. They can easily be upgraded to use standard 1.25" eyepieces by swapping the visual back. Someone should preserve this classic...
  7. Hi Knighty I wish I could be so sanguine as you, but my experience has been very disappointing. Example: observing Jupiter. I line up the planet in my finderscope and lock down the mount. Jupiter in smack in the middle of the eyepiece view. I then adjust the focuser - the resulting movement is enough take the planet entirely out of the field of view of the eyepiece. Or when "nudging" with the clamps unlocked I position the planet right in the centre of the view, then let go of the eyepiece and it jerks to some other position. It can end up anywhere in the field of view, usually right at the edge necessitating further adjustments. Observing then can only get done in brief interludes between adjustments. I do agree that the slack doesn't seem to affect use of the slow motion controls so much, once the scope has moved and settled at the lowest point of the slop. However, the lack of tightness with the clamps unlocked makes it difficult to track by nudging. And when you clamp the locks tight, there really shouldn't be any movement at all from a 3Kg scope on a mount rated for 2x 13kg. My manfrotto camera tripod heads are perfectly rigid when clamped and they are one tenth the weight and dimensions. It's just simple engineering. I remember seeing posts on here that say the skytee can have the play adjusted out by tightening internal bolts and I have seen a step by step guide for the left-right axis but not the up down. Can anyone help out with this?
  8. Hmmm... the only reason I bought mine at all was because of the problems I had with previous mounts and the assurance that the skytee would fit the bill and would be "rock solid"....
  9. Hi I got my Skytee just over a month ago and have had a small problem with movement of its axes. Basically, there is about 1mm of play in both the main axes with everything clamped down as tight as can be. This shows up without a scope mounted: just gripping the "T" and twisting back and forth with my hands reveals a sudden movement. I've sent videos to the supplier and obtained permission to attempt to tighten it up without risk to the warranty. However, how to do it? I've seen talk of grub screws and lock nuts and the like but it would be handy if anyone has a step by step guide, particularly on how to adjust the up/down axis which is the one that is most annoying. Using the right tools is a concern as well, that warranty amnesty might go out the window if I'm belting it with a club hammer :-) This link goes to a video showing the problem: video Any help would be appreciated. Cheers Dave
  10. Had a brief outing in the garden last night before it clouded over. Seems to be quite a lot of movement in both axes of the Skytee. Even with everything clamped up tight, if I grab the eye piece and wiggle it, the objective end wobbles up and down about an inch! Not smooth movement but simple slop. I know the Skytee has a bit of a reputation of being quirky but is this just an adjustment issue or should I be thinking of returning it for replacement?
  11. Just to stay off topic for a bit longer... A black hole event horizon marks the point where gravity is so fierce, light would have to exceed the speed of light in order to achieve the escape velocity necessary to get away from the black hole. As light can't go faster, it is the cause of not being able to see what it is going on in that particular region. The expansion of the universe at a rate greater than the speed of light is another cause of not being able to see what it is going on in a particular region. In both cases the seeing problem is a limitation produced by the light speed barrier but the regions are not the same thing. One is a hidden region because escape velocity is higher than light, the other is a hidden region because it is moving away from us faster than light can come towards us. The qualities of each region are very different, the only commonality is that we can't probe those regions using light. So, no, not being able to probe those regions is frustrating but not evidence that we live in a black hole.
  12. Thanks. That makes sense in that it explains why I prefer the crispness and "look" of views at 100x compared to 150x with scope. I'll try some double stars to see if the higher mag works better.
  13. Yes, a refractor. According to much published wisdom aperture in inches x 50 is the max magnification you should use. My scope ought to be good at 150x by the reckoning but I prefer it at 100x, just looks better, if smaller. I suppose these formulae don't really take into account seeing, transparency, light pollution, thickness of the atmosphere. Perhaps if I were pointing the scope at the zenith from the Atacama desert 150x would be a piece of cake. Perhaps not so easy in south London, especially to the north where on a good night the sky is a yellow/orange glow...
  14. Don't knock £70 telescopes! My 76mm f/16.4 classic refractor cost £27! My 130p reflector with eq2 and tripod cost £45. :-)
  15. When I look at Jupiter through my 76mm f=1250 f/16.5 achro I see a very small circle with a line across the northern hemisphere and a line across the southern hemisphere, no more detail (the moons are pinprick sharp). This is at 100x with a 12.5mm plossl. If i use my xcel LX 7mm for 178x, I see a larger, blurrier, dimmer Jupiter with the same detail. F16 protects me from chromatic aberration unlike your fast scope but not sure that huge magnification with a small aperture is the way to go. 100x should be OK through a 3" refractor but Jupiter won't be huge at 100x, certainly not a centimetre across. A 4mm eyepiece is tricky to see through. My scope was originally fitted with a 0.965" 4mm ortho and I could barely see anything through it!
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