robin_astro

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)

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

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

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.

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/exoplanet2/51peg.htm Cheers Robin

Give me a grant and I will build an instrument to improve the precision (The bigger the grant the greater the precision 😀 )

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 true in an anisotropic c universe either the dimensions of the instrument must depend on the orientation in a complex way (the camera sensor is orthogonal to the light beam) or the mean frequency of the photons (the clock) changed depending on the orientation of the instrument which seems unlikely given the random orientation of the atoms in space. Cheers Robin

I would say the relationships between wavelength, frequency and velocity and between the wavelength and the diffraction pattern are more universal as it is independent of the physical nature of the wave. (It is effectively just geometry)

It depends on the size of the effect of course but in principle as a thought experiment you could use a rigid stick so the two are guaranteed to be comoving. Perhaps though in a universe where the speed of light is anisotropic the length of the stick depends on the orientation 😉

Since the speed of light connects frequency and wavelength and diffraction depends on wavelength shouldn't anisotropic c move the position of the spectral lines from my spectrograph calibration lamp when pointing in different directions? Robin

Remember that you are not measuring a narrow well defined line 1 pixel wide. What you are trying to do with the Star Analyser is compare the distance between two poorly defined fuzzy blobs in the target and reference spectra (The zero order image and the image of the star at H alpha) They will be several pixels wide, dancing about due to the seeing and distorted by the change in focus and the aberrations caused by the converging beam configuration. See the images in the examples I gave above. Measuring this to better than 1% precision is a challenge. With a slit spectrograph the situation is much better as the slit (not the seeing, focus and other aberrations), define the spectral line which is reproduceable so you can measure the position of the (centroid) of the line to a fraction of a pixel. You can measure the wavelength to high precision by comparing with the spectrum of a calibration lamp with well defined lines at known wavelengths. Go for the SA100, not the SA200. It works better in the converging beam configuration, particularly with fast telescope like your f5 Newtonian (smaller dispersion angle, less field curvature, less chromatic coma.) The SA200 is for if you cannot mount the grating far enough away to get the dispersion you want eg if you have a close coupled filter wheel. See my website for more details http://www.threehillsobservatory.co.uk/astro/spectroscopy_16.htm The Star Analyser is not designed for making high precision observations. It is for learning the basics by observing objects with bold features qualitatively to reveal the astrophysics going on before possibly investing in much more expensive equipment. Observations like these I posted near the top of the thread for example If you experiment with one you will learn first hand the fundamentals of astronomical spectroscopy and understand the issues I am describing much better than me trying to explain them here. Cheers Robin

because of the limitations of accurate wavelength calibration with the slitless Star Analyser I would estimate the smallest doppler shift you could reliably detect with the Star Analyser is probably around 1% or 3000km/s based on the measurements of redshifts I have made using it Robin

The recession velocity of NGC7331 (800km/s = 17A at H alpha) is too probably low to measure with a Star Analyser in any case. For high precision measurements you need a slit spectrograph where you have fixed wavelength reference points from a calibration lamp For slitless spectrographs like the Star Analyser you need a point source but there are active galaxies (Seyferts, QSO) which appear point like and have high enough redshifts to be measured eg http://www.threehillsobservatory.co.uk/astro/spectra_3.htm http://www.threehillsobservatory.co.uk/astro/spectra_21.htm Instead of the small achromatic refractor I recommend using the C8-N which is the perfect choice for the Star Analyser 100 (and ALPY). You will need a mono astro camera though for faint objects like these. The DSLR will not be sensitive enough. In general colour camera are not a good choice for spectroscopy for many reasons Cheers Robin

If you start with the Star Analyser though you don't need to guide, just expose for as long as your mount can track for and align and stack multiple exposures

To get the best out of any slit spectrograph you are going to need a mount which can guide sufficiently well to keep the star on the slit for long exposures using the spectrograph guide camera. The telescope has to be reasonably well matched to the spectrograph too, for example the ALPY works best at ~f5 but the DADOS and LHIRES need an f10 scope

it really depends where your interests lie. The LHIRES III cannot do everything. It is good for high/medium resolution on bright objects but does not work so well at low resolution. The ALPY works better at low resolution and can go much fainter (and the spectra with my even lower resolution modified ALPY 200 are some of of the faintest recorded by an amateur) Cheers Robin

Yes it is possible for amateurs to detect galaxy rotation but unless you have access to a large telescope, only on a few of the brightest galaxies. Some examples http://www.astrosurf.com/buil/forum/ngc7331_poster.png http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=1682 http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=2232 http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=2420 http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=2618 Robin

This is probably beyond the range of amateur equipment because of the trade off between resolution and limiting magnitude. ie you can either measure high velocities of faint objects like this high redshift QSO with my modified ALPY 200 (resolution ~45A) or low velocities of bright objects eg the pulsations of Deneb at ~0.3A resolution using a LHIRES III

This was done using a LHIRES slit spectrograph at ~0.5A resolution https://britastro.org/observations/observation.php?id=20200710_225300_18b30a0a785a7ab3 Cheers Robin

There are also some nice examples on here using the 3D printed Lowspec spectrograph at high resolution eg

You need the right kit for the magnitude of the velocity you are measuring. The Star Analyser can measure the high velocities (redshifts) of some galaxies eg http://www.threehillsobservatory.co.uk/astro/spectra_21.htm and the expansion of supernovae (bottom of page) http://www.threehillsobservatory.co.uk/astro/spectra_6.htm but to measure orbital velocities you need a slit spectrograph with higher resolution (with good technique and a stable slit spectrograph a velocity precision is ~1/10 of the resolution is relatively straightforward eg at a resolution of 5A ~20km/s and at 0.5A resolution 2km/s) examples are:- David Boyd "observing with a Lisa spectrograph" measuring binary star radial velocities to ~6km/s, slide 43 on https://www.britastro.org/downloads/15701 My measurement of velocities to 1km/s precision in the dusty eclipsing disc during the eclipse of epsilon Aurigae using a LHIRES spectrograph https://britastro.org/node/19640 and velocities due to pulsations in Deneb to 0.5km/s in my talk "pushing the limits of commercial spectrographs" using a LHIRES spectrograph https://britastro.org/node/19378 and measuring exoplanets to a few metres/second precision using a stable high resolution fibre fed echelle spectrograph as here by Christian Buil http://www.astrosurf.com/buil/exoplanet2/51peg.htm Cheers Robin

Unfortunately most of these gratings tend to be very inefficient. (Spectroscopy spreads out the light very thinly so we need all the efficiency we can for astro spectroscopy.) A simple test is to look through the grating. You want as much light as possible to be in one of the first order spectra. Any light in the zero order (the light that goes straight through) or any other order is wasted. This is what the SA100 looks like (from the user manual)

If you do decide to go this way, I would not worry about the wedge prism, at least to start with. It does not make a big difference to the resolution with the SA100 but makes wavelength calibration more difficult. You can see an example of the difference in tip #3 of Christian Buil's useful tips using the Star Analyser here http://www.astrosurf.com/buil/staranalyser3/userguide.htm Cheers Robin

This is only part of the problem (The focal plane is curved so you can only be in focus at one wavelength. You will also find with achromatic refractors that you will not get perfect focus at all wavelengths, particularly at the blue end because of chromatism in the telescope optics. This appears as a fishtail like shape at the blue end). The main problem is the converging beam. Because the beam converges leaving the telescope you get what is called chromatic coma which means you can never get a perfectly focused spectrum image even using a wedge prism which as you have seen can actually make the problem worse. This gets worse at higher diffraction angles (more lines/mm) and at lower telescope focal ratios (The beam converges at a steeper angle.) This is the reason the Star Analyser has 100l/mm and is not recommended for low focal ratio telescopes. For a full analysis of the converging beam setup, see Christian Buil's website here http://www.astrosurf.com/buil/us/spe1/spectro1.htm Cheers Robin

If you want to try using your 500l/mm grating I can suggest mounting it in front of a DSLR lens and using it to produce spectra of bright stars using the method on my website. (The high dispersion grating works well in this case giving a sharp spectrum as the beam is very parallel, having come from the distant stars) http://www.threehillsobservatory.co.uk/astro/spectroscopy_11.htm About 20-40mm focal length lens would be about right, not the 200mm shown there as that is for the 100l/mm Star Analyser. Note though that most cheap gratings are not very efficient so the spectrum may be too faint. (Unlike more expensive blazed gratings most of the light ends up in the zero and other orders) Cheers Robin