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Everything posted by JamesF

  1. I've used zip ties before now, but I've known other people use weights attached to magnets. James
  2. I did wonder about that when I looked back at this photo Fortunately we have a different tablecloth now. James
  3. A good point. I'm clearly out of practice with all this OSC stuff :) James
  4. I seem to recall that when I did mine I set the OTA up on a tabletop and then went around the front end, trying to align my eye with the optical axis, about a metre or so in front of the OTA. If you're not even close, I think you see reflections of the mirrors drifting off towards the edges of the OTA and you need to adjust the collimation to bring them into line with the optical axis. I feel certain I also used a collimation cap for some purpose or other, but it was such a long time ago I'm afraid the reason escapes me for the moment. Things are so bad these days that if I can remember what I did last week I count it as a bonus, to be honest :) James
  5. My Mak has similarly gone "gold" in places. In fact I think the corrector cell is partly gold and partly black where the end cap was in place but it doesn't completely cover the anodised area. That particular OTA used to spend a lot of time outdoors on sunny days for white light solar viewing and imaging and I've always assumed that was what caused it. James
  6. I'm assuming you're using the standard finder shoe on the Mak and that is offset to one side of the OTA to mount the Evoguide? I think I'd be tempted to add a bit more weight to the other side of the OTA to balance it out, to be honest. Or you can probably add the weight elsewhere if it would be more convenient. The alternative is probably to add a second shoe to the OTA in line with the axis so the weight of the guider is balanced when placed in it. James
  7. The filter is an optical element of the imaging train. Light refracts through it. And at the relatively steep angles of incidence occurring at small f-ratios perhaps that results in focusing/bloating issues as well as light loss due to the greater amount of material through which the light has to travel? James
  8. I assume that's partly what the seal is there for. If you screw up the back until the seal is nipped in place but not completely squashed, it will deform a little further during collimation, but hopefully stay in place. I guess if you start with the "pull" collimation screws as far out as feels safe and then refit the back you'll probably only ever be tightening the back against the seal rather than loosening it, if you see what I mean. James
  9. You can blow some of that dust off the mirror now, too James
  10. The seal wasn't a ring when I took my 127 Mak apart either. I just put the same seal back when I reassembled it. Seems to be fine :) I can't recall how I kept it in place during reassembly though :( In my case I think it was the black mounting ring at the bottom of the picture that screwed inside the main tube and the backplate was fixed to that with the "pull" collimation bolts. If yours is wobbly I'd guess that either the collimation bolts have worked loose or the mounting ring needs tightening. Assuming it is constructed the same way as mine of course, which is not necessarily a given. James
  11. I printed a PLA mounting for the anemometer and wind vane of my weather station that has been outside since December and it's still looking fine. It is a bit "over-engineered" perhaps, but I really wasn't sure how strong it needed to be. It's not solid: 30% infill, as far as I recall. James
  12. I am still using (as a wifi gateway in the blobservatory) an old Acer Aspire One (1.6GHz 32-bit Atom CPU, 1.5GB RAM) running an Ubuntu variant called Lubuntu. It's a lighter weight build with a smaller footprint and lower CPU requirements than the standard releases, but it works just fine. I suspect the last release or so have become a bit more porky, but there are other lightweight distributions as well. They're very handy for squeezing a bit more performance out of older hardware. James
  13. I don't know. It just doesn't feel right as an explanation to me. I'm not sure if DSS stacks and then demosaics, or demosaics and then stacks. My immediate thought is that it would be better to stack the raw frames as generating the full colour image acts to "smooth out" the data, but I'm not certain. I don't get the bit about SNR either. I'd have to work through the maths to be certain. Perhaps there's a dependency on the stacking method used. I don't doubt that Mark's arcsinh stretch works, mind. I've used that myself and been happy with the results. The way he implemented it in PS is clever too. I'm not sure I'd have thought of that myself. James
  14. I can see no possible response to this other than "wibble". James
  15. No idea, to be honest. But if it is then my first guess would be that the green photosites are more sensitive or have a wider bandpass than the red or blue ones. James
  16. I agree that it's the same phenomenon, but I got to the point where I wasn't happy with the simple explanation. It didn't seem to account for what happens in a way that felt sufficiently "thorough". Factoring in the behaviour of the water due to the rotation of the planet for me provides a greater depth of understanding. In fact I might suggest that now I understand it (at some level), whereas before yesterday I would merely be repeating what I had been told was the reason. James
  17. Thanks for that. In fact, double thanks, as doing some of these courses seems like an ideal way to spend some time whilst I don't have much work. James
  18. Is that a Lancaster Uni course? My son is hoping for an offer from them to study Natural Sciences, if Cambridge won't have him. James
  19. Ignoring the tidal bulge on the side of the Earth opposite the Moon and the effects of the Sun etc. for the moment, we all know it's mostly the Moon's gravity attracting the water in the oceans, right? Well, hmmm. I was lying in bed the other night contemplating life, the universe and everything when it struck me that I wasn't really sure that's a particularly good explanation. (Look, just don't ask me to explain what goes on in my head, ok? I'm not entirely sure it's all under my control in the first place.) My thoughts followed thusly: The Moon's gravity exerts a force on an ocean facing it, but it's nowhere near the force exerted on the water by the Earth, so the Moon can't actually move the water towards it. It just means that the net force on the water in the direction of the Earth is reduced. It won't be dragged "up" towards the Moon because of the Moon's gravitational pull therefore. It will stay where it is. So what actually causes the "bulge" of water that occurs? Is it that due to the spinning of the Earth the water actually wants to obey Newton's first law and disappear off into space all the time, but is prevented from doing so by the Earth's gravity, though not prevented quite so much when the Moon counters some of the Earth's gravitational attraction thereby allowing it to move further from the centre of the Earth before Earth's gravity "overcomes" it? Perhaps that's not a very good explanation either. Assuming it's basically the correct idea perhaps a clearer explanation would be to show how all the forces involved are acting. So it wouldn't be the Moon's gravity attracting the water, but that force changing the equilibrium of all of the others? It leads me to another question as well: Does a given mass of water weigh the same at sea level at all points on the Earth, or does it vary depending on the position of the Moon? Is there some other explanation? Or should I just be taking more Valium? James
  20. What are peoples' feelings about the oldest version of MacOS likely to be in use these days? I believe oacapture etc. should still run on 10.6, but maintaining a build environment for that is becoming increasingly challenging. Moving forward a few years would probably make life simpler if people just aren't using the older versions any more because hardware has been "retired". For instance my MacBook is seven years old and won't run anything earlier than 10.9 and not even all versions of that release. I'm actually expecting it to "fall off" Apple's list of supported systems fairly soon. James
  21. That looks quite interesting as a way to try out some stuff I've been playing with. At the moment I can't find a UK source at a delivered price that is close to £40 though James
  22. There are some comments in that video that bother me... The first is one about the green histogram being further to the right because there are twice as many green photosites on his colour sensor than there are red or blue. I'm not sure I follow the logic of that. Surely the green histogram would shift to the right if the green photosites counted more photons, or if more than one green pixel value were added into the demosaic process without any scaling of the values used (which I don't believe will happen). I'd guess that the reason is actually that the green photosites are just more sensitive and count more photons? The second is the moving of the black point in the individual channels before the stretch is done. If the point of this particular stretch is to preserve the colour balance, then surely moving the black points beforehand defeats that goal because in doing so the colour balance is changed? James
  23. My son goes kayaking with school every Tuesday, sometimes in the sea, but often on the rivers in Devon. I think the Exe and the Dart are particular favourites. He wasn't able to go this week because the rivers were too low due to lack of rain! I think this morning may have corrected the situation however :D James
  24. Thanks for that. I often wonder myself if I'm interpreting the output of lsusb correctly James
  25. Nicely recycled. We hate most single-use plastics :) The "Celebrations" lids certainly seem to have a bigger lip -- I have a few of these that I've recycled for storing parts for projects and so on. James
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