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About JoeP

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    Kenilworth, Warwickshire
  1. This diagram shows plenty of eyepiece designs that start after the focal plane: I'll test my Vixen NPL, and if that won't work, I can just use a cheap plossl. If I go down that route, it would probably be easiest just to glue the mirror in place, and glue the sensor into a 2"-1.25" adapter, and make marks on each so I know how to match them together. Having said that, when you do the calculations the sensor doesn't need to be that far back from the mirror - only a couple of cm, so it might be possible to integrate that into the eyepiece itself.
  2. I've just had another idea based on that diagram - what if instead of a hole there was a tiny mirror? That way, the sensor could be relatively far back, allowing the light rays to spread out again, and I can still have an eyepiece to look through. Something like this: Or if I can't get a small enough mirror, this:
  3. Good point, I hadn't thought about that. LDRs tend to have a VERY slow response time - when I was doing some tests, it would take about 2 minutes for the resistance to reach a stable value after turning the lights off. It seemed pretty sensitive - a fairly dim LED would cause the voltage output from my circuit to increase my an easily measurable amount even from 1m away with the LED facing away from the sensor. I'm not sure about the QE, but if I add in a small capacitor it should smooth out any wobbles caused by the Poisson effect.
  4. I suppose it depends how small I can get the sensor's FOV - if it covers 30', then you will always have enough stars to give you a significant offset. I could, perhaps, use the data to estimate the star density and magnitude distribution in the region, then use the standard deviation or something like that to predict the sky brightness. I'm sure I'll think of something. What do you mean by 2 eyepieces? So their interchangeable, or do you mean that I should use binoviewers? I don't have any binoviewers, so that could be a problem. I've measured the LDR, and I reckon the actual sensor bit is only 2mm*2mm. Can I just use the formula @vlaiv gave in his first response: tan(sky radius)=sensor radius/focal length? That would give (for a scope with FL = 1200mm) 2.9 arcminutes. Would this not be small enough? This finder chart shows a VS in Cassiopeia (which is usually jam packed with stars), with FOV 9 arcmins and magnitude limit of 14. If you shrunk that down by 3 times, you would only have one additional star in the sensor. Also, if the star being measured is, say, magnitude 9, then the 14th magnitude star will only have a light intensity of a 1/100th of that of the M9 star, due to the logarithmic nature of the magnitude scale. All I would have to do is subtract the sky brightness (or maybe not, as it could be cancelled out by the calibration stars). If I positioned the LDR before the eyepiece, then it would form an obstruction, and to measure a star I would just have to position the telescope such that the star is hidden by the obstruction. Would this work?
  5. Thanks for the advice. Precision pinholes look pretty expensive, so I would either have to make something or try a different method. One option I was considering was to allow other stars to be recorded by the sensor, but take a series of measurements as the stars cross the sensor, so that some clever software can estimate the magnitude of each star somehow. As you say, this is definitely more of an ambitious electronics project than an astronomy project, and if I were to submit my results to AAVSO, then I would do it as a visual estimate and only if I agreed with the results. The temperature dependency of LDRs shouldn't be too much of an issue, as I'll be measuring the calibration stars from the same FOV as the estimated star. The wavelength dependency could cause some problems, though perhaps not much more than a visual estimate. I could use a red filter to ensure my measurement are consistent I suppose, if it turns out to be a problem. Thanks again for the advice, Joe
  6. I'm working on an electronics project that estimates the magnitude of a (variable) star using a light dependent resistor. The idea is that a microcontroller with an ADC measures the voltage across the LDR (which forms a voltage divider with a 1M resistor), and you use a few known stars to calibrate it. Then you can press a button and it will interpolate (perhaps against a curve) the magnitude of the star. The electronics side of things is going well, but I'm struggling to work out how to connect the LDR to the eyepiece. It would need to be fairly focused so that only the star I'm measuring is included, but also needs to somehow allow me to work out where it's pointing. My initial idea was to use a small convex lens, projecting the star onto the LDR (which is a little circle or diameter ~5mm). However, I was wondering whether there was a better way to do it, perhaps before the eyepiece. I have a cheap 3D printer, so I can make custom parts if I need to. At the moment it's more of a concept, and I'm still unsure as to whether the LDR will be reliable enough at such low light intensities. Any thoughts/questions/suggestions appreciated.
  7. @FLO , there seems to be a slight problem with the clear outside app, showing temperatures of -60 in a couple of days:
  8. I'll give it a go tonight, should be interesting to observe.
  9. Under the Locations tab, tap and hold your location, then tap edit location and you should be able to set it as your home location.
  10. Thanks No-one else at school is doing the astronomy GCSE, I'm doing that in my own time.
  11. Perhaps the heritage flextube 130p? Good aperture, compact and not too expensive.
  12. You can't do that! I would, but I would probably never use them. And if I did, I would stop using my 'grab n go' scope. Maybe one day I'll get some nice bins like those....
  13. I've managed 13 since halfway through January, due to the fact that my GCSE coursework means I have to be out as often as possible to take variable star measurements. I've definitely noticed a huge number of clear skies, at least compared to last year.
  14. There are, in my mind, two reasons for liking a particular object: because it looks pretty, or because it's interesting/amazing when you consider what you're actually looking at. In terms of looks, there are few galaxies which look anything beyond a smudge in my scope, yet they are amazing when you consider their distance. Nebulae often have very interesting appearances, and are also incredible when you consider their 3D aspect. Planets are very pretty, but aren't there most of the time. Clusters also look good, but at the end of the day they're just a bunch of stars. Variable stars have become a more recent interest of mine, which are fascinating to track, yet they don't ever look special. If I could only ever observe one thing again, it would have to be nebulae for the variety of appearances.
  15. In the day, point your telescope at something far away, like the corner of a rooftop, or a church steeple. Make sure whatever you're using is right in the middle of the eyepiece. Then look through the finder scope, and carefully use the adjustment screws to get the point you're using in the center of the finder. Double check that the point is still in the eyepiece, then make sure the scope doesn't get knocked. Get a star map of the object you want to see. Planets are difficult at the moment, I would recommend M42, M31, the double cluster in Perseus, and pleiades, all visible now. Google them and you should find a rough star chart. The other alternative would be to use stellarium Use an app on your phone/stellarium to identify the constellations on the star map in the night sky. Work out the direction (N, E, S, W) then spend a good 10 minutes recognizing constellations. They should be a perfect fit to what you see on the map, it's easy to get constellations wrong. Now work out where the object you are looking for is in relation to the constellations. I find the best thing to do is make a geometrical pattern in your mind, usually an equilateral triangle, an isosceles triangle or a right angle triangle, but others prefer other methods. Having said that, your scope will have a wide field of view, so you should be able to spot it from a little way off. Looking through the finder scope, point towards where the object should be. Get that point in the middle of the finder scope. Then move on to the telescope eyepiece. Start with the lowest possible magnification (highest value eyepiece in mm, although don't worry about it if you've only got one eyepiece), then once you've centered the object you can move on to a higher magnification. Focus the view. There will be a focuser knob, turn this until all the stars are as small and focused as possible. Hopefully you will be able to see something cool! One point worth mentioning - know what to expect. You have a small telescope, so don't expect anything brilliant. You can get a good idea by searching google images for 'M42 nebula (or another object) sketch', then finding the most unimpressive of the results and looking at it from the other end of the room. That said, you'll still be able to see some amazing things.
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