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robin_astro

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About robin_astro

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  1. Only provided you preserve the precision in the result. For example averaging a series of 16 x 8 bit images to produce an 8 bit precision result is not the same as summing them and expressing the result to 12 bit precision
  2. Yes the dimensions are assumed to remain constant under rotation but the mean frequency of the photons is based on transitions in a large population of randomly oriented atoms so is not orientation dependent. These kinds of arguments could equally be applied to almost any observation, even those of flat earthers. . In the absence of evidence, only Ocam's razor can save us from these. Relativity explained anomalous observations. I would argue unless there is evidence to the contrary there is "nothing to see here". Cheers Robin
  3. True and since my apparatus is made up of solid components the inter-atomic forces which define its dimensions would have to be anisotropic in an anisotropic speed of light universe. This was not the argument put forward in the original article though which claimed that in a relativistic universe it was impossible to measure the speed of light independently in both directions. I claim that my apparatus can do this without needing to resort to a round trip or two observers with "synchronised" clocks.
  4. Most people normally run the Star Analyser without guiding, taking short exposures and stacking them but the distance from grating to sensor is not that critical so this might allow you sufficient leaway to mount the grating before the OAG say and you can still guide on a field star zero order if you want. You can use my calculator hosted on the RSpec website to explore the range. https://www.rspec-astro.com/calculator/ (Closer gives a more concentrated but lower resolution spectrum for fainter objects while a larger distance gives more resolution up to a point for brighter objects.
  5. Hi Steve, 50mm spacing is a fine starting point for your sensor There are a few obvious issues 1. Yes you have severe chromatic aberration which is varying the focus along the spectrum and causing the fishtail effect. Fast achromatic refractors are not good for spectroscopy. The Newtonian will be much better. 2. It looks like you have some sort of Bayer pattern in the image. Does your software think it is a colour image ? 3. Your first two spectra may be over exposed. Err on the underexposed side to start with which makes it easier to see and focus on the features
  6. Indeed. This is conceptually similar to my suggestion here. I am still to hear a convincing argument that this cannot demonstrate that the speed of light is isotropic to any given degree of precision. (My tests have already demonstrated this to 10^4) Robin
  7. This was with a LHIRES III telescope mounted slit spectrograph at ~0.4A resolution. It is a notoriously unstable instrument so I superimposed precise wavelength markers on the star spectrum with a calibration lamp. I talk about it briefly here at 18:36 min https://britastro.org/video/13862/14769 Ultimately though slit spectrographs are not so good for precise radial velocity measurement where you are trying to measure the centroids of lines accuracy because the shape of the line can change significantly depending on where exactly you place the star relative to the central line of th
  8. Yep it gives you a double hit. You use all the photons and cross correlation over a wide wavelength range gives you incredible precision (again helped by the choice of star, G/K/M dwarf stars have a lot of lines)
  9. I think the current state of the art is the ESPRESSO spectrograph on the VLT which was specified for 10cm/s precision and recently got down to 30cm/s when measuring Proxima Centauri-b and were able to detect activity due to star spots https://arxiv.org/abs/2005.12114 Robin
  10. For bright targets I suspect you will find the systematic errors are much larger than the photon statistics. In Christian's case he believes they are fibre noise and spectrograph thermal stability. In the professional case ISTR is the stability of the star which makes sub m/s precision difficult
  11. As an example here are some measurement I made of the variations in the radial velocity of red supergiant Deneb due to non radial pulsations which are several km/s and as far as is known are chaotic in nature, compared with the much more stable main sequence star Vega.
  12. The answer is yes it is done to this precision by professionals for measuring the wobble due to exoplanets. As you say you do have to make corrections for many factors and chose your star as the stability of the star puts a lower limit on the measurement. (Fortunately In the search for planets capable of supporting life the stars are likely to be stable, like our sun for life to evolve). You might be interested in this measurement by Christian Buil which describes in detail how it can be done even by an amateur to a 1 sigma precision of 5m/s http://www.astrosurf.com/buil/exopl
  13. Give me a grant and I will build an instrument to improve the precision (The bigger the grant the greater the precision )
  14. Actually I have already done it when testing the stability of my ALPY spectrograph. (To a precision of better than 1 in 10^4) The spectrograph consists of a lamp filled with excited neon atoms, a transmission diffraction grating and a camera recording the spectrum, all mounted rigidly in a line. I pointed it in a number of different directions including east and west for example. There was no detectable movement in the position of the centroid of spectral lines on the camera sensor as measured from the counts in in each pixel. ie the measured wavelength was constant . For this to be tru
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