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philjdowns

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  1. Going for it tonight, first clear skies since before Christmas (with a little luck ?) All set-up, apart from my filters that still haven't turned up ? Using my home made Dew heater for the first time, though will have to spend some time finding the new focus point as to install the heater I've had to move my primary mirror ? Wrapped the exposed electronics in some pallet wrap to keep the dew out, now to wait for dark! Phil.
  2. Finally! Got an evening where it's worth setting up (apparently ?) so here's my completed heater installed, connected and running at 750mV (0.04W) to keep the primary a little above ambient as the temperature drops - fingers crossed. Not expecting to get much imaging done, for a start my NB filters still haven't arrived - two months overdue ? and as I've re-located my primary going to have to spend a while finding the correct focus point again. I've got an elasticated weather seal bag for the bottom of the OTA but again that still hasn't arrived so I'll be getting some air flow up the tube at the moment. As you can see, the back of the mirror is not exposed any more. The fly lead is the input 12v supply, and the power socket is across the restive load so I use this to set the output voltage. If I need to, this will be where a secondary mirror heater can be connected. Been so long since I've seen the sky, we'll see how it goes! Phil.
  3. @KevS Don't think were supposed to post links to other sites here, but if you search for it (using your favourite search engine ?) you'll find it quickly enough. Phil.
  4. Just a thought, anybody here donating unused computer cycles to the MilkyWay@home project, or any others? We could start our own team ? Phil.
  5. Quick update, I have run a test with the mirror assembly removed from the OTA, and in my workshop. Ran the heater at 3v for 30 mins, then switched off for 5 mins, then on at 250mV for 150 mins, logging the mirror temperature every 5 mins. Started at 16.1°, after 30 mins it was at 17.6°. A larger temperature increase than I wanted, but still safe. Setting the heater at almost the lowest setting (250mV) the temperature gradually fell over the next 2.5 hours to 16.5° So I now know that 3V is too high for the initial heat for my mirror, so the next test will be at 2.5V, and I'll up the the steady state voltage to 500mV and see how that goes. I see now that I've made this way over rated, but I guess it'll last so much longer! This test was obviously not done under dark skies or outside (and a good 12° warmer that the outside temperature) but I'm hoping that it'll all behave in a relative fashion, though I think I'll lose more heat when the mirror is under a dark sky. Looking forward(ish) to the next 3 hours watching my telescope mirror - I know how to have fun ? and with luck, the time I spend can benefit others in the same position! Phil.
  6. @KevS Thanks Kev, very interesting. though I'm not using fans to cool my mirror (it's kept in an unheated outhouse so it's always at ambient or very close) it's given me an idea of the radiant heat loss from my mirror. I'll be actually heating my mirror to about 1° above ambient then hopefully maintaining that temperature, my lowest setting for my heater grid (200mV) in theory equated to 0.00015W sop that's looking promising. My plan is to set-up and then set the heater to 3v? for 20 mins (I'll be checking the temperature all the way at first) I'm looking for a guide voltage that will give me a 1° increase in about 15-20 mins, then turn is down to 0.8v?? and see if it maintains the temperature over ambient. I'll have to see how things go as the temperature drops during the night. As it's cloudy for the foreseeable future, I'll be doing some of this early calibration in the outhouse area, though the mirror won't be seeing 'clear sky' so that'll skew any numbers I get. I'll keep you all posted how things get on. It's almost like I don't think this Astro Photography lark isn't complicated enough ? Phil.
  7. @Jkulin Hi, yes I agree that my heater is capable of outputting way more heat than I'll ever need, I just wanted to make sure that if it accidentally got connected at full voltage (12v) it wouldn't go up in flames. I'm going to start at about 0.8v (if I ever see the sky again ? ) so that's calculated at 0.05W total (0.001W/in²), I'm not sure how to work out the rate of heat loss from the mirror but it's likely higher than 0.001W/in², so I need to find the voltage that matches the heat loss and keeps the mirror above the dew-point. It's going to take a while to calibrate things I know, but I'm planning to restrict the controller to 8v (for now) which would give a maximum of 0.6W or 0.098W/in². Hoping to run at less than 5v, so when I have some usage stats I can restrict the controller further. I did have two fans attached but it made almost no difference, my location is on a hill by the south coast so I get warm sea air coming in and rising up the hill, cooling as it does. I get dew on everything in the garden almost every night. That's why I'm looking at heat and air-flow restrictions in my OTA. Very gentle heating and minimising the moist air entering the bottom of the OTA works in my head but time will tell. Hoping to get it all re-assembled this weekend, though no clear skies forecast for at least a week - the joy of astro in the UK! Thanks for your comments. Phil.
  8. Hi All, Well, no-one commented that I shouldn't so, I've mounted the lattice to a 1mm PVC sheet, and mounted the controller as well. Then fitted the unit into my primary mirror housing. (you can see where the original fan and connection points were) There will be a small plastic cover over the controller when it's all back together and tested. The added weight of this is only just over the weight of the removed fan so that's good. Now to fit the mirror, and re-install the housing into the OTA, re-collimate and find my new focus point as the mirror is 4.5mm closer to the front of the tube. I've got an elasticated dust cover that I'll try over the back end of the OTA when I use this, thinking it'll reduce any air currents up the tube from the heater and cut down on any moist air being drawn into the base of the tube, this should (in theory) reduce the dew on my secondary by using the heat radiated from the primary to keep the secondary just a tiny bit warmer. Might need a secondary heater though, but I still have loads of components left. All I need now are some clear skies - Please, it's been soooo long.... Clear Skies All! Phil.
  9. Information on my home built Dew heater posted here Forum | Backyard Astro Hope that works ?
  10. Hi All, After some re-thinking, finally built the first (well second really - won't mention the first one ?) operational version of my heater for my 8" Newtonian primary mirror. As you may have seen from earlier posts, I have serious dew problems! Soak testing here at 12.2v (0.912A - 11.4W) been running for a couple of hours and it's about as hot as I can stand on my fingers. Doubt very much if it'll ever be running this high for real, but I need to know it will not have any issues if accidently left on max output (though I doubt this much heat will do the mirror any favours!) Based on a standard ladder resistor configuration, using 52 x 680Ω resistors (rated at 0.5W but running at 0.22W max). Initially tried for a radial pattern but couldn't get a uniform distribution so decided to go for a grid layout. The overall thickness will be 4mm, 3mm for the resistors and 1mm for a insulating sheet back, the resistors will be touching the back of the mirror. Anybody else tried this? Phil.
  11. OK, so I've had a dig about and found enough components to make a flat plate heater for my 8" primary without buying anything - Thats a result! I've done a few tests and have the following data (hope this works, a copy/paste from Excel!): 10W seems the maximum around for an over-the-counter 8" mirror heater, what I have gets me to 12W, though I'm expecting to be using the lower (green) levels as I only have to warm the mirror to 1°(ish) above ambient and then match the heat loss of the mirror to keep it there. Does anybody have figures that they use - as in W/in² ? Or how many Watts they use to heat a Newtonian primary? I'm expecting to start the build this weekend if all goes to plan. Phil
  12. @petedevon Hi, yes I have a "SkyWatcher F4 Aplanatic Super Coma Corrector" it's optimised for f4 which matches my OTA. Getting started I thought I could get by without for a while but even before I had my imaging train fully completed, I was seeing some truly horrible coma which was also causing me problems focussing, and only the centre 1/5 of the image was even passable. So jumped straight for the Aplanatic. There are cheaper ones out there but you get what you pay for. Stars are now nice little points so it's doing it's job well, as far as I can tell at this stage. It was a bit of a pain getting the correct back focus (the distance from the camera side of the corrector and the imaging sensor) taking into account that you need to add approximately 1/3 the thickness of any optical elements in between (all my filters are 2mm so I needed 25.66mm) I've managed to get to 25.7mm so only 0.04mm out. I've only got an f4 and don't know what other issues you'd get if you used this on an f5, but there are f5 coma correctors out there. There doesn't seem to be many of us Newt people on here at the moment, but if anyone else has any thoughts please chip in. Phil.
  13. I've got an Orion Astrograph 200mm, just the one scope, almost got everything I'll need ? Got the primary out at the moment, building a dew heater.
  14. Having a quick stab at M33, just getting the hang of the set-up, not expecting much as my NB filters haven't even shipped yet! So mono with UV/IR cut, be happy if I can get it in focus at this stage, the dew seems to be keeping away at the moment - fingers crossed. Phil.
  15. Thanks, thought I might get away with it as my primary has a fan on the back blowing ambient air over the back of the mirror, so looks like I need to change tack. The dew heaters I have found are mostly the strap kind that fits round the outside of the tube, so that's going to be 1/2" from the edge of the mirror. Would it be better to warm the mirror directly? I'd have thought that providing a small amount of heat to the edge of a large circular mirror will only warm the outside edge, as the mirror will dissipate the heat quickly, leaving the centre 1/2ish still liable to condensation. Would it not be better to apply the heat uniformly over the entire back of the mirror so there is minimal thermal gradient across the mirror? Warming the secondary will be the next task! The front lens of my comma corrector is about level with the inside of the tube, would that need any action, it's not pointing at the sky but is open to the tube? Phil
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