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Found 2 results

  1. Ten or twelve years before this evening, I was shocked on a winter night that I could plainly see the tremors of turbulence in my old Nikon 10x50 binocs while I watched the Moon. I had never noticed such a boiling of the air ever since, up until today. A clear sky after days and days of clouds and mist prompted me to take my super-fast-set-up 80mm achro (on an AZ-4mount) to have a quick look at the Moon. The first look through the 6x30 finder surprised me a lot: I could once again see the large-scale waviness that seems to make the whole Moon wobble, plus the smaller-scale turbulence that makes the edge of the Moon serrated, sharper, serrated in slightly different manner, then sharper again, then a bit serrated, in rapid succession. Noticing the focus was a bit smooth, I turned the objective a half-turn or so; turbulence became a little less disturbing and ugly, but still visible, even the smaller wavelets. Temperature was 1° or 2°, wind very still, tranparency and dryness excellent, making the probability of rain 0%. The usual conditions that give great transparency in winter, together with the horrible air agitation counterpart. Normally I start moongazing at 150x with my achro, lowering the power by steps till seeing allows it, but this time I gave up observing altogether because 24x in the main scope was already too agitated. I thought 10x was the lowest power at which I would ever see turbulence, 6x is a shocker. What is the lowest power at which you saw turbulence?
  2. Hi all. I'm hoping for some advice on a subject which I know has had a lot of air time, but which I'm still wrestling with. Firsty, I know I get dew on my secondary mirror and it has wrecked many a DSO photgraphy session. I've fitted a home made heater, and this seems to fix it. It's insulated with neoprene, and runs at a very low wattage (about 0.4W) , so hopefully not too much turbulence. Next step is to do the same to the guide scope, which also mists up after about an hour, and then maybe the primary with a very low wattage. The main puzzle for me is this: Some folk say the mirrors are huge heat reserves, and take ages to cool down, during which time they cause turbulence. But other folk say the mirrors drop in temperature rapidly and go below the dew point, and then suffer condensation. I suspect theres some truth in both statements, but this doesnt simplify how to deal with it. Probably the main consideration is that the primary is much less ikely to suffer dew than the secondary, and so they have different problems and solutions. It would be easy to say that turbulence is in the eye of the beholder, but for sure when I'm focussing - using the camera and Live View with high screen mag - there appears to be turbulence aplenty even after an hour of cooling and before putting the secondary heater on. Of course this could be atmospheric, rather than tube currents. Rather than go through all my (muddy) thinking, can I just ask some direct questions please which relate to all this. 1. Is the small amount of heat on my secondary likely to cause any significant turbulence? 2. Is the turbulence that I see on the screen likely to be caused by the OTA? 3. I havent actually seen dew on the primary - but I've read that low grade condensation can affect the reflection. Is heating of the primary maybe just unneccesary? 4. If I heat the guidescope with a belt at about 2.5W, might this adversely affect the guiding property? 5. If I use an exhaust fan to cool the primary, is there any prospect that the air current will: a. increase dew on the primary and/or secondary by drawing in more damp air? b. reduce dew on the primary and/or secondary by evaporation? I'd be really grateful for any help with this. My next step is to wire up some kind of controller, and it would be useful to know what I'm likely to need to run before doing this. Many thanks
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