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robin_astro

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About robin_astro

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    http://www.threehillsobservatory.co.uk

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  1. Meteor detection using Graves beacon

    Indeed. I saw the same effect when measuring the rotation of Saturn using reflected sunlight http://www.threehillsobservatory.co.uk/astro/spectra_28.htm (bottom of the page) Looking at the fig 3.4 and the equation below it in the paper you mentioned and simplifying it so the meteor is moving horizontally at constant velocity and making the distance between the transmitter and receiver large compared with the meteor altitude, I would expect delta f to be +2V at the far left, falling to ~V when overhead of the receiver, 0 when at the mid point between transmitter and receiver, ~-V when overhead of the transmitter and -2V at the far right. Robin
  2. Meteor detection using Graves beacon

    Wouldn't you need to know the meteor track relative to the transmitter and receiver to draw any conclusions about the speed of the meteor? The basic doppler shift equation is correct but this is just the component of the relative velocity between a transmitter and receiver in the direction of a line joining them (the radial velocity) so the observed shift depends on the relative positions of the transmitter and receiver and their direction of travel. Then for a scattered signal you have to add two doppler shift components. Consider first riding on the meteor. You would observe a doppler shift dependent on the component of the velocity in the direction of the transmitter. (Note that unless the meteor happens to be travelling directly towards the transmitter, this will not be the same as the speed of the meteor.) This doppler shifted signal is then effectively "retransmitted" as a scattered signal. We receive this signal on the ground, doppler shifted again according to the component of the velocity of the meteor in our direction. The total shift will be the sum of these two shifts. For example, a radar gun measures a doppler shift equal to 2x your speed when you are far away compared to how far the gun is from the midddle of the road. The measured doppler shift then drops as you approach the gun and becomes zero as you pass it. Robin
  3. Exoplanets Power

    While looking for these I also came across this simple but on the whole impressively accurate calculation of the predicted surface temperature of the rocky planets in the solar system. (It ignores any influence of the atmosphere so is wildly out for Venus and underestimates the temperature of Earth by 9 deg C) http://www.tfeb.org/fragments/2015/09/30/black-body-planet/#2015-09-30-black-body-planet-footnote-1-definition
  4. Exoplanets Power

    I expect there is a catalogue of these properties somewhere but as a starting point Wikipedia lists them in the physical properties for many exoplanets eg 55 Cancri e which has a calculated stellar flux 2590x Earth https://en.wikipedia.org/wiki/55_Cancri_e From an example of someone else trying to calculate the same thing https://physics.stackexchange.com/questions/237707/how-is-the-stellar-flux-for-exoplanets-calculated Also eg https://exoplanetarchive.ipac.caltech.edu/docs/poet_calculations.html One thing to watch out for is to avoid mixing up bolometric luminosity (ie the total energy emitted by a star at all wavelengths) and the luminosity measured in a specific wavelength range (eg from the absolute visual magnitude of the star) Robin
  5. Welcome to the spectroscopy board

    Once you have mastered using it with the small refractor, then the 300mm f5 Newtonian will allow you to go deeper using longer exposures, provided your skies are reasonably dark. You can find the official manuals for the Star Analyser on the manufacturer's website here http://www.patonhawksley.co.uk/resources.html and an on line calculator to optimise your setup http://www.patonhawksley.co.uk/calculator/ There are quite a few Star Analyser users on here so any other questions, just ask away :-) Cheers Robin
  6. Welcome to the spectroscopy board

    Hi, I am the chap who developed the Star Analyser back in 2004. Looking at your list of equipment I would start with one of your small refractors, (the one with the lowest chromatic aberration) and the mono camera with largest sensor size. This will give the sharpest spectra and will be the easiest to process the spectra. A good reference for this sort of setup is Jim Ferreira's website. http://www.lafterhall.com/spectroscopy.html (Look for the results using the Star Analyser with an 80mm APO and a DMK41) Cheers Robin www.threehillsobservatory.co.uk edit: added link
  7. Hi John, If you are already experienced at using the Star Analyser and are using a high dispersion (low A/pixel) then it helps with resolution a bit. The improvement is pretty marginal with the SA100 (about a 20% improvement in resolution typically) but more useful with the SA200 where it helps to keep the focus constant across the spectrum with the higher diffraction angle. Other variable factors such as seeing, focus have a much greater effect though. One downside is that it makes the wavelength calibration non linear There is a comparison on Christian Buil's site here (tip 3) http://www.astrosurf.com/buil/staranalyser3/userguide.htm Cheers Robin
  8. An early evening target for more northerly observers? Probably the brightest supernova in the sky currently but few images or observations up to now
  9. sn2017gxq was discovered in ngc 4964 by the Gaia satellite on 17th September but not announced until 28th. I classified it as type 1a using spectroscopy on 30th. Details and the spectrum are on the Transient Name Server website https://wis-tns.weizmann.ac.il/object/2017gxq It is currently close to maximum at about mag 14 so is probably the brightest supernova in the sky at the moment but there are no observations or images currently in David Bishop's famous "bright supernovae" website. http://www.rochesterastronomy.org/snimages/ Robin
  10. Yes it is optically the same as eypiece projection. It will not work using a 500 l/mm grating without the eyepiece and camera lenses though. The key is that the eyepiece makes the beam parallel through the grating which is essential if you are using a grating with more than ~200l/mm. Here is a similar arrangement by Christian Buil using camera lenses. The LORIS spectrograph http://www.astrosurf.com/buil/us/loris/loris.htm Robin
  11. For a simple slitless setup, I always fancied trying something like the RS spectrograph, which is similar in concept to the junk box spectrograph. http://www.astroshop.eu/rigel-systems-spektrograf-rs-spectroscope/p,51408 At this price it is silly but it could be done quite cheaply if you already have one of those big wide field eyepieces and a DSLR. It is a fully collimated design and the camera is angled so you can use a high dispersion grating (I think this one uses 500 l/mm) and get good resolution. The only possible drawback with some DSLR eg cannon is that the front ring rotates as you focus. Robin
  12. 11 billion years ago.....

    Hi Steve, The total exposure was 8x600s but the key features in the spectrum are visible in a single 600s exposure. Yes it is a slit spectrograph which isolates a narrow (23um) strip of sky including the target, described here http://www.threehillsobservatory.co.uk/astro/spectroscopy_20.htm You still have to isolate the wanted star from any others which might be trapped in the slit and remove the spectrum of the sky background which includes light pollution and natural air glow. I use Christian Buil's ISIS software to do all the data processing http://www.astrosurf.com/buil/isis/isis_en.htm You can see the raw spectrum image with and without the sky background subtracted below. (The slit can be seen on the left and the QSO is the bottom of the three spectra) This was just a bit of fun really. The main use of this setup is to confirm and classify supernovae down to ~mag 17. This is my latest classification https://wis-tns.weizmann.ac.il/object/2017gjn Cheers Robin
  13. Yep that's amateur spectroscopy pioneer Maurice Gavin. He's on here from time to time. He made a lot of innovative unusual designs and observations and is one of the people who helped me when I picked up spectroscopy. Yes you can use a Barlow to collimate the beam from the telescope ahead of a prism (in this case an Amici prism which keeps everything in line) or grating and then use a camera lens to focus the spectrum. Similar to the junk box spectrograph but using a concave barlow lens as a collimator instead of an eyepiece. http://www.threehillsobservatory.co.uk/astro/spectroscopy_19.htm The Barlow does not produce a secondary image ahead of the grating though so you cannot add a slit with this design. Cheers Robin
  14. If fact I see the paper I just referenced here does this https://arxiv.org/pdf/1707.05873.pdf (fig 2) so I am in good company Cheers Robin
  15. You cannot form a decent spectrum at all with a prism in the converging beam, the aberrations are just too severe. You need a parallel beam and the camera at an angle. You can also angle the camera with a grating to minimise the effect, or use a shallow angle wedge prism to deflect the beam back on axis but once you go beyond ~200l/mm various other aberrations rear their head and you really need collimator to give a parallel beam and an objective to refocus on the detector to get a decent spectrum.
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