Physics_dude

Fair enough, sounds like fibre is the way to go for high precision RV. My understanding of 3 main problems solved by fibre: - non-fibre telescope mounted spectrometers can bend/move slightly when rotated during star tracking, giving false redshift/blueshift/other problems. - non-fibre systems are more exposed to the elements, temp, vibrations, rain, wind etc that can give false redshifts. - small deviations from perfect alignment of a star and telescope line of sight can cause false redshift/blueshift for high precision RV measurements in non fibre systems. Fibre systems lose a lot of light on the air-fibre coupling, Christian's setup measured only ~5 photons / s per ccd pixel due to dim starlight, light losses and high ccd/nm density. It seems cross correlation computations can handle photon noise really well. That espresso spectrometer looks nice. I'm trying to find out how to download the fits data used by that paper (EDIT: ok got hold of some different espresso data, this should be fun)

Oh wow, I see how people are getting away with smaller scopes. I just learnt what an echelle spectrograph is! What a clever idea! I no longer need to waste 99% of the pixels on the array. This technology can improve the spectroscopic and velocimeter resolution by quite a bit! So that's how Christian did so well with his tiny 0.28m diameter scope, very clever.

[update] Did more calculations on photon statistics, actually the photon count matters more than I thought! Even with a 1 square metre collecting mirror! A 14 minute scan produces a photon error equal to the photon change on a pixel due to the spectra shifting across a sharp iodine peak from a 1.5m/s velocity change. Since there are multiple iodine peaks to measure the shift from, I suppose a ~5 minute scan with my proposed setup would be sufficient to resolve 1.5 m/s changes with a reasonably small error from photon statistics. Thanks rl for pointing this out! I definitely underestimated the photon statistical error. Ah yes, Christian Buil has very nice stuff! I browsed that page days ago but skipped over the error analysis because I didn't understand it. Now things make more sense. His telescope is much smaller than in my calculations and it's a 5.5 magnitude star, that's about 1000x less photons than in my calculations. This'll be a fun exercise to use my new photon statistics math to calculate Christians experimental errors. Oh nice graph robin_astro! Do you happen to have details on the measurements? Telescope size? Scan time? Pixel array? Wavelength calibration method? Trying to connect as much theory, data and experiment that I can. Learning so much stuff at the moment, thanks heaps guys! hmmm, "non radial pulsations", will have to look into that.

Not currently associated with any university. Trying to learn the astronomy terminology, science, technology and current knowledge with papers, forums, calculations, physics theory etc. Did some calculations on photon statistics, took a magnitude 1 star, 1 square metre collecting mirror, used a 2000x1 linear CCD detector with wavelength dependent efficiency and calculated the electron-hole generation rate per pixel to see how the signal compares to the noise: Looks like we have a total photon rate of 20 billions photons / s hitting the detector, and about 14 billion electrons / s produced by the CCD detector. For individual CCD pixels the electron rate is as low as ~4 million electrons per second at ~1000 nm wavelength. The read noise for a typical CCD is about ~10 electrons / read for each pixel and dark current is about 25 electrons / s per pixel. Seems the Poisson photon distribution of the signal dominates the other errors at these relatively high photon counts. A 10 minute scan gives a signal to noise ratio of ~35000 on the worst pixel, compared to ~11000 for a 1 minute scan. That's an impressive number of photons! Should definitely be possible to collect high signal, low noise data on the brighter stars. Sounds like the difficulties in a velocimeter measurements of bright stars are in stable, accurate measurements of the shifts, rather than the photon count. CCD vs two photodiodes around an absorption line vs other technology is a good question. Every velocimeter I'm reading about seems to be using CCD's. I suppose an iodine cell in front of a CCD guarantees a consistent, real time calibrated spectra, something I need. InGaAs arrays look pretty interesting, really good spectral response at higher wavelengths. Actually feeling more optimistic after seeing the high photon rates, signals are really high when measuring a bright star and a fair sized reflecting mirror. Looks like it would cost $$$ for proper equipment. My summary of velocimeter problems and solutions: - Thermal effects on spectrometer optics mounts: I2 cell should deal with slow thermal variations, Invar/zerodur material could be used for optics mounts to minimize thermal expansion. Air conditioner to minimize background temp fluctuations. - Thermal effects on CCD sensor: Could carefully control CCD temperature + use a CCD temp sensor and micro-controller for real time corrections to data - Other drifting effects of electronics: utilize optical chopper with calibrated light source to ensure all pixels have up-to-date calibration data on response, dark current, noise etc. - detector SNR: measure bright stars only and use a big mirror / lens area. - Vibrations: Make spectrometer + scope heavy and vibration isolated from ground. Shield from wind (observatory). - I2 cell aging / variations: Use really pure I2 gas, quality quartz container. Tightly control temperature and pressure of cell for stable calibration reference. - Tracking (slight star tracking error could cause a false redshift/blueshift? I think?) Will need to investigate this problem, may be important? Might need to orientate CCD perpendicular to star motion? Might need a really accurate and smooth tracking system? - 2 wings around a line problem: I think computational measurements of shifting spectra now allow for every peak and absorption line along a spectra to be measured statistically whether they are sharp or really broad? I don't think we have to throw most photons away anymore? I'm trying to find example software to understand this. Am I an idiot? Any major stuff I've got wrong or missed?

Omg, incredible story rl! Had no idea a single line could get that accuracy, I was considering spectrometer technology because I assumed you needed to average multiple lines to measure small shifts. That answers some questions I had. Also, wasn't sure if repeated long term measurements would be low enough noise for my purposes, sounds like it could actually work. I never considered that imperfect diy mirrors could be used for a spectrometer, that's good to know. I'm all about absolute accuracy. Haven't factored in tidal bulges from the moon yet (+ Sun & Jupiter tides), hard to model exactly but should be easy to measure. Barycentric motion of earth was simple with de405 ephemerides from nasa. Wgs84 geoid and sidereal time for calculating our observing station motion as it rotates around earths axis. Arbitrary precision numerical vector calculations make the math for 3D relative motion redshifts straightforward, just tell the computer the star data, observer data, date/time and let the computer calculate and display all the predictions. This is getting exciting, might have an idea that could actually work out.

I've read that using techniques like temperature controlled Iodine cells in spectrometers allows for +/- 1.5 m/s velocimeter resolution. My understanding is this is equivalent to observing a 0.00003 Angstrom shift at 6000A wavelength. Or in other words, measuring a spectrum shift 1/1000th of a pixel on a spectrometer CCD array. Apparently we can do even better by removing the atmosphere in spectrometers and making the whole system from non thermally expanding materials. Wow, this seems incredible! My Question: Can we actually measure a stars speed to 1.5 m/s precision. Is there some weird solar effect like flares / rotation / expansion/contraction of atmosphere that would stop us from measuring this accurately? To ask my question in a slightly different way: I've done a 3D simulation of an observatory on Earth observing Alpha Centauri redshift. Due to the spin and tilted orbit of the earth, we can see fluctuations in the redshift. I've drawn tiny error bars of +/-1.5 m/s (or +/- 0.00003 Angstrom shift) to show the size of the velocimeter measurement error. The error bars are absolutely tiny compared to the the effect produced by the spin of the Earth! Would we actually be able to reproduce the theoretical curve really accurately with an expensive velocimeter? Image 1. Redshift curve Image 2. Zoomed in Redshift curve - You can just make out the minuscule error bars. Another way of asking this question is, consider an ideal scenario. No spin or orbit of the Earth and the star is moving radially away from us and also isn't spinning. Would a single velocimeter measurement tell us the star's speed to within about 1.5 m/s? Would something like rhythmic expansion and contraction of a stars atmosphere cause a false measurement? Thanks for any help, John

ok thanks, I'm new to this stuff and learning a lot the last few days. My understanding of current methods: Any star with a clearly measurable spectrum can have it's spectral class deduced and mass / gravity approximated by fitting to a model. It might be inaccurate or wrong in certain situations, but meh it's an easy technique for an approximate answer. - close binaries: We can use parallax + angular separation to get orbital distance and redshift to get orbital speed and hence period. From Kepler's law we then deduce the mass and then calculate gravity. - Close lonely stars: Parallax + angular movement to deduce proper motion & tangential speed. Measurement of redshift to get velocity redshift + gavity redshift. hmmm, we can nearly solve this to get gravity field at surface. Unfortunately there's many radial velocity/gravity combinations that would give same redshift. dammit, so close! So astronomy scientists currently can't solve this case unless fitting to spectral models? - more distant lonely stars: We know redshift, spectrum profile and intensity. Sounds tricky to accurately determine it's distance? Tricky to separate velocity redshift from gravity redshift? Fitting spectral models are the only way? Any mistakes / major techniques I've missed?

Binaries makes sense, we have lots of measurements like orbit period, angular separation, angular size, differing redshifts etc. How about a random, lonely star in the sky?

How exactly can we measure the gravitational field of stars? I know they get redshifted, but how do we tell velocity redshift from gravity redshift?

So observing angular changes is the only way?

I know that we can measure transverse speeds by combining parallax and proper motion measurements. Are there other ways to measure transverse speeds? How do you tell the transverse speeds of far away stars? Could distant stars be moving at fairly large angles to our line of sight without us realising?

Nice! Another interesting link. 463 m/s spin speed at equator, ~320m/s in France, and the radial velocity due to Earth's spin for the Tau Bootis star (DEC 17,27',24") as observed from France would be +/-305 m/s. I'm actually working on a fun mini project to theoretically calculate the exact redshift shift for any latitude/longitude/altitude + UTC time/date + any star DEC/RA & speed. I'm looking for data to test how the model compares to the real world. I've attached a screenshot to show the graph and input code. One sub goal is to understand what equipment could measure this radial velocity change. So sounds like I'd need to detect a 0.001 Angstrom shift (50m/s resolution on velocimeter) to clearly detect the Earth's spin in France when measuring the Tau Bootis redshift. I think?

Thanks for the post, I had actually discovered this very website earlier and was reading it for hours and then saw your post lol. This is exactly the kind of stuff I'm looking for. On this page of the website it appears Christian has measured the "final tau Bootis phase diagram" with about +/- 100m/s error bars, and the corresponding redshift resolution is ~0.002 Angstroms. So this is about the redshift detection limit of what skilled individuals can do? If I understand this correctly, Earths rotation is ~460m/s at the equator, so we'd be looking to see a change of ~2x400m/s = 800m/s over one night (ignoring orbital motion). So we'd expect a redshift change of a close star about the same (about 0.16 A peak to peak over the night) than what the final tau bootis made (0.17 A peak to peak or 850m/s change). I'd like to measure a cleaner curve than the Tau Bootis curve, so looks like I'll need to find better redshift measuring equipment, if it exists? <0.001 A resolution (<50m/s velocimeter spectrometer). Ideally I'd want a 10m/s velocimeter to measure stars relative speed, but I doubt I could buy/make one.... Have I made any silly mistakes in my reasoning?

Hi, I'm thinking of getting into astronomy, I'm a science nerd hehe. I have three questions: 1. Has anyone ever measured the spectrum of a star and noticed it's redshift change slightly over hours /days/years due to earth's spin and orbit? 2. What equipment would you recommend to measure 0.1 0.001 A resolution spectrums? (Say $10,000 US budget probably can't afford it) 3. I've heard iodine gas cells can be used to create reference marks in a spectrum. Are there other good techniques? I'm trying to figure out how to track a star through the night and see the subtle changes in redshift due to the earth, I think that'd be fun.