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Jim-a

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Everything posted by Jim-a

  1. Spotted this blog post and thought I'd share it. Has some lovely photos from the Mars Reconnaissance Orbiter and some conjecture about an as-yet-not-fully-explained surface feature.
  2. Just spotted this and thought people might enjoy these two quick videos on the Lick Observatory's work on finding habitable planets.
  3. I think I'm eight in saying that Binocular Sky had the DPS1's in the recommended under £50 section until the price went up a bit and took them over the threshold. I have the DPS-1's and am happy with them - not really noticed an eye-relief issue, although I don't wear specs so its not such a big thing for me anyway.
  4. Lorne was right !! - I swapped two of the vanes and now everything is as it should be, haven't had chance to collimate it yet, but hopefully it will be easier now the secondary holder doesn't have a skew to it. Thanks for your help everyone
  5. Nevy - thankyou so much for taking the time to take those pics and post them - really useful. Lorne - Eureka!! I think you may have solved it - just couldn't get my head round it (defeated by a combination of rotational and reflection symmetry) - I'll give it a go tomorrow and let you know how it pans out.
  6. good thought Lorne, but unfortunately StuW is right - just turning them upside down would offset them - the holes are not 90 degrees apart
  7. While I was trying to sort out some double diffraction spikes I'm seeing, I took the spider vane assembly out of my lightbridge. I'm pretty sure that there is a problem with the vanes - and I'm pretty sure that it was probably an assembly error in the factory. I'd be grateful if you guys could double check that I'm not barking up the wrong tree. Here are a couple of pictures of the spider assembly that show the problem. and heres a sketch I did that shows the problem exaggerated and heres a sketch to show what I think it should look like The end effect of how the vanes are means that the secondary is skewed with respect to the focuser. Which would explain why I've never quite managed to line up all the collimation-ducks in a row (e.g. when the centre-spot was centred in the secondary, the primary mirror clips didn't look equidistant from the edges of the secondary). So question 1 is... am I talking rubbish... or is there something amiss? Assuming I'm onto something here, I reckon this must have been an assembly error in the factory - all four of the vanes are identical, but because of the way the bolt holes are positioned on the secondary holder two of the vanes will always be upside down. Heres a sketch of how the vanes are attached looking down from the top. My assumption is that either there should be two 'flavours' of vanes and that the factory fitted all the same ones, or the bolt holes to attach the vanes are in the wrong place. So question 2 is... can any Lightbridge owners have a look at their vanes and see if there are in fact two different types of vane, or if the way its bolted on is different from my sketch. Question 3 is... what can I do about it? Its not a new scope so whatever warranty was there would have run out long ago. I reckon the easiest option would be to find someone who can drill and tap new holes so that all the vanes can be fitted the same way up - anyone have any comments / alternative suggestions? Sorry to be long-winded and thanks in advance to anyone who can shine some light on this.
  8. Thanks everyone I've had a look this morning and on the face of it the spider vanes looked reasonably square... However, I did notice on closer inspection that the two vanes that run perpendicular to the focuser are very slightly bent (near the secondary holder) in a direction away from the focuser. I think a bit of fiddling with the tension on the vane screws may help. Im surprised that such a small bend would produce a second set of diffraction spikes that were offset at such a large angle (around 30 degrees or so), but I keep getting surprised by things with this astro-lark, so I'll tweak and see what happens
  9. Swinging the scope to Venus this morning I noticed a double set of diffraction spikes. Here's a back-of-the-envelope (literally) sketch. The second set are dimmer and so I guess its only looking at a really bright object that they become noticeable - but I'm scratching my head to work out why you'd ever see more than four. Any ideas? Ta
  10. Up until now I've only seen Jupiter late at night as it comes over the rooftops and with it low in the horizon the viewing hasn't been great. So wanting a better look, I set the alarm for stupid o'clock this morning and was out and set up by about 4.30. I had a really good session - I spent 20 minutes or so first on the Orion Nebula... my first reaction was wow! plenty of structure and gradations in the brightness - I could just about see the two 'arms' that arc back away from m43. Can't wait to take the scope to some dark skies and see it with better contrast. I then spent the rest session on Jupiter. Absolutely worth getting up to see it higher in the sky. I've read here how much it helps to spend a long time with something to tease the details out, and that was definitely the case. At first I could see 4 very obvious brownish bands. As I spent longer with it I could start seeing that the edges of the bands were uneven and 'swirly' (if thats a word!!) and started seeing variations in the colouring and shading of the bands. Spending around an hour on it I really noticed how much the seeing can change minute to minute. There were moments of really steady seeing - so another good reason to take enough time to really take it all in. One thing that did bother me a bit were the diffraction effects (not really spikes like on a star but more like smears). I think I might try making an aperture mask - hopefully that will take the spider out of the picture and as a bonus increase the f-ratio and reduce the atmospheric effect. All in all a good couple of hours and my first (but Im sure not my last) time packing up the scope in sunlight.
  11. not sure if this has been posted elsewhere, but this link has a nice animated gif showing two shots of the comet taken 30 minutes apart which nicely show the movement. http://remanzacco.blogspot.co.uk/2012/09/new-comet-c2012-s1-ison.html?m=1
  12. We've got a cinder block built shed at the bottom of the garden - I built a cupboard under a workbench that the scope just fits into in its untrussed state - I built a little trolley type thing with castors so that its easy to pull it out from the cupboard and push it back in again. So far it seems pretty happy in there and no sneaky spiders have been able to find their way into the cupboard.
  13. I've weighed up the options - direct drive vs threaded rod drive as in Shane's pics above and I've decided to go with the threaded rod. Whilst I started wanting to build a direct drive, it just seems really difficult to find a motor that goes slowly enough (other than the EQ1 type drive, which I'm worried doesn't have enough power). I could buy a motor and then buy gears to reduce it down further, but decent worm gear type arrangements are quite expensive and would need pretty accurate metalwork to mount them on - so I've concluded its too much of a gamble when the rod drive is a relatively cheap, tried and tested mechanism. I've ordered this motor and this PWM controller which should do the trick. It will be a while before the snail mail from hong kong arrives, so in the meantime I'm going to brace the platform to reduce the wobble, and I'm going to rework it to lower the overall height as I think that will make observing a bit more comfortable.
  14. Shane - do you have some kind of mechanism to decouple the shaft from the motor when you manually rewind?
  15. Unfortunately the motor stopping last night wasn't because of a low battery. It looks like its fried (the electronics seem ok - but the motor itself is kaput). I'm wondering if it was ok at first as the initial part of the motion would be where the weight of the scope is helping - perhaps it died when it started having to push the scope uphill and it couldn't provide enough oomph. ( Not really sure what to do next - I could buy another similar eq drive motor, but it may end up going the same way, so I'd have to get to the bottom of 'why' it didn't work. I've looked and I cant find a replacement part for just the motor bit anywhere. So right now I think my options are.... - Buy another EQ style motor and reduce the size of the drive shaft to try and reduce the amount of torque required - I know that these drive units are used on other platforms, so they must work ok. If someone has one I'd be grateful if you could measure the drive shaft diameter for me. - Buy a low rpm motor + a further gear reducer (a worm drive or similar) to get something beefier that would be slow enough to direct drive - Abandon the direct drive and go for a threaded rod type arrangement - although with this I'm not sure what the best way to reset it at the end of the run is? I may load the scope up on the platform again and attempt to measure how much force is required to move the scope (I'm thinking of some heath-robinson style contraption involving ropes and a pair of kitchen scales), so that I can get an idea of how much of a problem this is. I knew it had been going too well so far!!
  16. So I've used it a bit more this evening. The motor stopped working half way through, but I'm hoping thats just the battery wearing down (I have no idea how long it had been in there). The wobble is the biggest thing that needs sorting out. It's so bad that you start nudging the dob, you think its actually moved on its alt/az bearings, but when you let it go it just springs back - so it must be easier to deform the platform than overcome the stiction on the dob itself. I'd maybe expect a bit of play in the arc that the platform is meant to move in, but this seems to be in every direction. Let operation 'eliminate wobble' commence
  17. Tom.. surely you mean thats a mighty fine looking platform under your old scope ;-) Hows the 16" doing?
  18. I had a tweak of the bearings today and got a much better fit, then while I was on a roll I fitted the adjustable feet and the eq1 motor. Eventually the motor will need hacking about a bit to put a remote on/off switch and speed control on the end of a long cable - but for testing purposes I'm just going to have to get on my hands and knees and fiddle with the controls under the platform. With the motor mounted, I really noticed how much the platform would just slide over the drive shaft with the slightest pressure. I think the solution will be to put some heat-shrink rubber tubing over the shaft.... however, because I'm impatient I racked my brains for a quick hack and decided to wrap a rubber band around the shaft and attach it with a cable tie at each end. Seems to work pretty well - the platform doesn't slip at all. Seeing that the moon is up I thought it would be a good idea to give it a spin while it was still reasonably light, so I levelled it - pointed it in a vaguely north direction and popped the scope on top. It adds quite a bit of height to the scope - at zenith the eyepiece is exactly at my eye level - any higher and it would be a tip-toes job!! I think if I were building one again I'd put far more effort into making it low profile. So the moment of truth...lined up on a moon crater with a 9mm eyepiece (so 168x mag)... I switched the motor on.... peered through the eyepiece... and.... the crater gently drifted out of view... but ... in the opposite direction from normal !!!!!!! - so I tweaked the motor speed and after a few visits to the bottom of the platform had things pretty steady. Went into the house for 10 minutes and the crater was still in the field of view. Result - I'm very chuffed!! There are still a few things to do - I noticed that the scope is much more wobbly on the platform than it normally is - even a breath of breeze was causing seasick-inducing views and the settle down time after moving the scope is really long (to the point where it would be pretty annoying when trying to find things). I suspect that the ply is moving around too much. Having only used 12mm ply, I'd always planned to add some bracing once the basic build was done, so hopefully that will help. I also need to give it a proper long test, put a bubble level and maybe a compass on it, build the remote cable, and put in a motor cut out for when it reaches the end of its travel... but for now I'm pretty pleased.
  19. Made reasonable progress yesterday. I cut some stainless steel strips from an old cooker hood we had lying around and epoxied them to the north and south segments. Have to say my cutting is pretty rubbish, but once the epoxy is fully cured then I'll take a file to the edges and make sure they are smoothed out a bit so they don't catch on anything. To drive the platform i'm going to use an eq1 motor drive. I figured that a 10mm drive shaft would be about right and seeing as the bearings I bought have a 10mm hole in them I can use them to support the shaft. The bearings sit in two notched pieces of wood and are held securely by some brackets I made from a spare stainless steel strip (they don't look great - see my comment early about my lack of metalworking ability - but they seem to do the job). The shaft is a 10mm bolt. The connection for the motor (which has a 6mm connector) was made by filing down the head of a 6mm bolt so that it would fit snugly inside a 10mm threaded connector and then filling the gap with epoxy and hoping that it would set straight (which it did near enough). Thats then just screwed onto the 10mm shaft. I'll need to hacksaw the thread off and then file a flat section for the connector screw to engage with, but hopefully that will do me for a drive. Mounting everything on the base board was pretty tricky. I did the south section first then balanced the top board on the south rollers with the north end propped up on some bits of wood so I could mark out where the north bearing and drive shaft should go. I think if I was doing it again I would first screw the top and bottom boards together with some pieces of wood to separate them so that they were exactly parallel. That way I'd have been able to fit all of the rollers so that contacted the segments much more easily. So... the first moment of truth... I can move the platform around and it stays in contact with all of the bearings and the movement feels smooth and 'equatorial' (ie it moves roughly how I imagined it would). I don't think I have all the bearings exactly square on though - some of them are contacting on the edge, but have a fraction of a mm or so clearance on the other edge. I'm not sure how much this will matter - I guess it may cause there to be more wear over time? I'm going to have a fiddle to see if I can improve things. Using the spirit level app on my phone I measured the amount of 'safe' movement as 11.5 degrees... so this should give me 40 mins or so of continuous viewing between resets - which I'd be really happy with. So next steps will be to see if I can improve the bearing contact and then put the scope on it to check it takes the weight / doesn't fall over etc - after that I'll put some end stops on to limit the movement to the 'safe' zone and fit the motor.
  20. Was woken up by a sound outside this morning at about 5ish. Peeked out through the curtains to look, but there was Orion in a clear sky (which I haven't seen for ages)... forgot all about noise and excitedly went and grabbed binoculars... didn't go back to sleep. Anyone else have stories of astro-bug related behaviour?
  21. Gutted I didn't see it. Happy for everyone who did though.... must have been awesome (in the true sense of the word)
  22. Good idea Shane - I have an eq1 drive that came with a scope I borrowed from a friend before I bought the lightbridge. Hadn't thought to experiment with that - at least I can try before I buy, though like you say, I suspect that the scope would have to be really well balanced for it to have enough oomph - but watch this space
  23. The bearings arrived today from these guys. I went with 10x30x9's - no science to the choice really, but with a pack of 10 for around £6 I have enough for spares (I may double then up on the north bearing - havent decided yet). This evening I started the base board fixings for the south segment... my plan is to get that done then fit the north bearings and then get the scope on it (first real moment of truth!!!) and check it out for smoothness / balance etc before diving into the motor stuff - that way if it doesn't work then I still have the option to move things around, tweak the design etc. My original plan was to direct drive the north segment, but I'm struggling to find a motor + gear arrangement that would move slowly enough - if anyone has any suggestions then that would be great... otherwise I may just have to revert to a threaded rod type arrangement, but then will have to think more carefully about how to reset it at the end of its run.
  24. thanks for the advice guys. I guess I better put a O-III filter on the (getting ever longer) wishlist
  25. +1 for the Olympus DPS1's about £60ish - also check out http://binocularsky.com/ for some excellent advice on choosing.
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