robin_astro

Nice result. Next step where the stars have similar features (or you have a template) or are you are following the same star for binary orbits for example could be to use cross correlation  which gives potentially a much higher precision and is less sensitive to SNR. The technique is very powerful but  it only gives a differential result though unless one of your stars has a known RV. ISIS has a tool for this and the procedure is described here for example in the workshop tutorial "Observing with a LISA spectrograph" here

https://www.britastro.org/downloads/15701

where a 1 sigma precision  ~1/40 of the resolution was achieved with careful attention to detail

Cheers

Robin

On 06/02/2020 at 17:39, Dave Smith said:

 

I have seen somewhere a chart showing aperture of scope versus range of magnitudes that can be measured. Can anyone point me to this? 

 

A bit late to the party but this calculator can be used to estimate the SNR for a particular setup, conditions and exposure 

http://spiff.rit.edu/richmond/signal.shtml

Cheers

Robin

2 hours ago, Bajastro said:

Robin, which stars are the best for RV calibrations?
Last time, when I observed some H-alpha profiles in Be stars I used Capella spectra for wavelenght calibration. It isn't very good example, because this star is spectroscopic binary. No spectral lamp used.

Wavelength calibration based on lamps taken before and after the star spectra are usually good enough for amateur work but if you need higher absolute accuracy andthe spectrograph is stable enough to make it worthwhile, there are a number of catalogues of RV standard stars, for example this list of bright northern hemisphere targets on the ESO website

https://www.eso.org/sci/facilities/lasilla/instruments/harps/tools/observing_tools.html

but any non binary main sequence star and the RV figure given in SIMBAD should be good enough for most purposes. 

 For Be stars at high resolution around H alpha I would recommend rather than calibrating using star spectra, using a simple neon lamp to get the dispersion equation and then use the atmospheric water lines to trim out any offsets which can be done to high accuracy. Visual Spec, ISIS for example have nice tools to do this and the BeSS validation team use these lines to check the calibration when validating spectra for the database.

Cheers

Robin 

5 minutes ago, robin_astro said:

Small wavelength shifts can sometimes be seen in spectrograph lamps depending on the illumination of the slit

See here on Christan Buil's page an extreme example where the lamp is mounted in front of the telescope aperture and the line shifts depending on the position of the lamp  (about halfway down the page)

http://www.astrosurf.com/buil/isis/He_calibration/method.htm

Small wavelength shifts can sometimes be seen in spectrograph lamps depending on the illumination of the slit (In commercial instruments the light from the lamp is diffused either scattered off a separate surface in the case of the ALPY or by a translucent cover over the lamp in the case of the LHIRES to minimise this effect. ) These shifts are normally very small though (just a mainly an asymmetry in the shape of the line, rather than several pixels as here. Is the effect repeatable ?  ie are you sure it is the lamp orientation which is causing the shift and not some other movement in the instrument as you adjust the lamp. (The LHIRES was notorious for this if all the screws were not tight, the slightest touch could move the lines)

Note though that you cannot rely on the lamp for very high absolute wavelength calibration in any case as there are always some systematic differences between the lamp and the sky due to the different light paths. For high absolute accuracy RV measurements (rather than measuring relative changes eg in binary system) you calibrate out any systematic offsets using measurements of RV standard stars

Cheers

Robin

Nice result.  Did you allow for the change in the radial component of Earths orbital velocity between the dates ie make any heliocentric corrections?  

Cheers

Robin

Yes PNV means Possible NoVa to distinguish it from PN which means Planetary Nebula (and PSN which means Possible SuperNova)

eg on the CBAT TOCP

http://www.cbat.eps.harvard.edu/unconf/tocp.html

Cheers

Robin

29 minutes ago, robin_astro said:

You are confusing the strength of the gravitational field (which  depends only on the mass and is therefore unchanged by the collapse) with the force felt by an object in that field which is proportional to the mass and inversely proportional to the square of the distance. After the collapse the mass is concentrated into a single point so you can venture much closer to the centre of mass, compared to when it was a star,  where the force is much higher, until eventually at the event horizon it is high enough to prevent escape of the photon

Or if you want to think of it in terms of the warping of spacetime, the warping is only sufficient to prevent the photon escaping when at a distance from the singularity much smaller than the radius of the original star

12 hours ago, piff said:

the volume is the only thing that has changed. The mass remains the same. 

You are confusing the strength of the gravitational field (which  depends only on the mass and is therefore unchanged by the collapse) with the force felt by an object in that field which is proportional to the mass and inversely proportional to the square of the distance. After the collapse the mass is concentrated into a single point so you can venture much closer to the centre of mass, compared to when it was a star,  where the force is much higher, until eventually at the event horizon it is high enough to prevent escape of the photon

16 hours ago, piff said:

 Why and how is the black hole's gravity so much stronger after the core collapse, that even light cannot escape, when its mass is no greater than it was before when light could escape it perfectly easily?

For a collapsed star the diameter of the event horizon is much smaller than the diameter of the original star so any photon emitted at the same location as the surface of the star was would have no difficulty escaping even after the collapse.  After collapse, as you get closer to the black hole  the force of gravity increases as the inverse square of the distance to the point where at the event horizon the photon cannot escape

Don't forget to add your Be star spectra to the BeSS database

http://basebe.obspm.fr/basebe/

and consider adding other targets to the BAA database

https://britastro.org/specdb/data.php

Cheers

Robin

Nice resolution!  Did you need to refocus for the different wavelengths ?

Cheers

Robin

Taken last night using the ALPY 600

Spectrograph guider image

Processed spectrum here 

https://britastro.org/comment/7756#comment-7756

Cheers

Robin

12 minutes ago, robin_astro said:

That is what the AAVSO data is suggesting. One caveat though is that the last two points were taken by the same observer who's name does not appear in earlier data so I would be interested in any corroborating evidence.

The long term trends for V,H and J do indicate though that The H,J IR bands have similarly been unaffected by previous dimmings suggesting we may well just be seeing a similar pulsation driven dimming though deeper than normal. 

1 hour ago, PhotoGav said:

That's a fascinating observation, Robin. I will be using that! So, essentially it is just in certain wavelengths (the visible ones) that dimming has occurred.

That is what the AAVSO data is suggesting. One caveat though is that the last two points were taken by the same observer who's name does not appear in earlier data so I would be interested in any corroborating evidence. There is some suggestion from I band spectrophotometry done using a Star Analyser that the drop in brightness reduces into the IR

https://www.cloudynights.com/topic/645652-betelgeuse-is-faint-for-it/?p=9904083

(part of a long thread with lots of other discussions and speculation on the subject)

  H,J band brightnesses are not  easy measurements to make and amateurs with the capability of measuring in the IR are rare (they use photelectric photometry, essentially a single pixel camera with an IR sensitive photodiode) 

https://www.aavso.org/infrared-photoelectric-photometry-program

AFAIK there was only one commercial instrument for work in the IR and that appears to have been discontinued

https://www.optecinc.com/astronomy/catalog/ssp/ssp4.htm

Note also that is possible that Betelgeuse may not actually be dimming at all when all wavelengths are considered as most of the radiation is in the IR  and a recent measurements in H,J bands in the AAVSO database appear to show no significant change.

Betelgeuse is one of the MILES standard stars  which was recorded in 2000/2001 when the brightness was more typical so you could compare with that (the non deredened version eg in ISIS, filtered to match the Star Analyser resolution.)  My spectrum with the ALPY 600 on 30th December 2019 shows only  very subtle changes in the visible spectrum compared to then

http://www.spectro-aras.com/forum/viewtopic.php?f=38&t=2433#p13514

and there has been  no further change up to 9th January. (The later spectrum is with the modified ALPY  at R~130 so the typical Star Analyser resolution)

http://www.spectro-aras.com/forum/viewtopic.php?f=38&t=2433&start=20#p13584

The spectra are in the BAA database

Cheers

Robin

9 minutes ago, robin_astro said:

 You can measure the velocity of the explosion from the blue shift in the material coming in our direction (about 8000km/s at this point)

 

You can also tell the type of supernova from the spectrum  (Core collapse of a massive star or explosion of a white dwarf which exceeds critical mass) 

Here is another spectrum of a type II about 1 month after it exploded (A very early spectrum of mine using a simple diffraction grating mounted directly in front of the camera. ) You can measure the velocity of the explosion from the blue shift in the material coming in our direction (about 8000km/s at this point)

http://www.threehillsobservatory.co.uk/astro/spectra_6.htm

22 hours ago, lukeEdfarley said:

Robin i looked at your work and it looked absolutely fascinating. i really liked the experiment which you ran looking at the crab nebula, at the frequency of pulses from the pulsar in the centre! More on topic, how would the spectra vary with time would the light to begin with be very low wavelength as the debris gets further away the photons get less energetic? how does it work?

Supernova spectroscopy is a particular interest of mine. Most supernovae are very faint even at maximum so I am one of only a couple of amateurs who confirm and classify supernovae spectroscopically using a spectorgraph I specially modified for the job.  

http://www.threehillsobservatory.co.uk/astro/spectroscopy_20.htm

The spectrum changes quite rapidly in the first few days/months, mainly due to the hot material cooling (shifting from blue to red over all) and becoming more diffuse (giving a spectrum with strong emission lines like a nebula) . You also see different lines appearing as the new elements produced decay.  Most SN are too faint to be followed very long spectroscopically by amateurs but it is possible with the brightest ones. Here is one (a type II like Betelgeuse will become) which I followed for a year  from mag 13 to mag 18 (from from a BAA presentation I gave)

Cheers

Robin

5 hours ago, verreli said:

thereafter there will be no detail to see so there's no point looking at it 

There would still be much of interest from a spectroscopic point of view though and something that bright would give professionals a big headache.  Even with my equipment I am not sure how I would cope. I would probably have to resort to defocusing and sampling the defocused image using the spectrograph slit as I did for Vega with this simple setup here for example

http://www.threehillsobservatory.co.uk/astro/spectroscopy_18.htm

(Off topic in this sub forum though)

7 hours ago, lukeEdfarley said:

 

the object you describe is very small but very bright so very intense, when this is reflected onto the secondary mirror will surely this intensity will go up could this pose a problem?

Not likely to be a problem for the secondary as the light (and heat) is unfocussed at that point  and being a mirror, most of the light and heat is reflected. But once focused then  a source as bright as the moon concentrated from a telescope of signifcant aperture onto a few pixels (or on the retina) would  be something to avoid

1 hour ago, lukeEdfarley said:

whoop my first ever post! anyways yes i heard the same thing...   the dimming is not unusual its the speed of the dimming they're getting excited about a bit of reading has told me that when it finally explodes it will be about half as bright as a full moon... (idk whether that prediction has any truth) however none the less rather spectacular me thinks. at the end of the day it's quite a long way away and considering intensity diminishes with the square doubt it will be blinding. that might be me underestimating supernovas though :)) it may be nice though a moon filter! haha.

Supernovae put out a lot of light though (Around a billion or so times more luminous than the sun at visible wavelengths)  so yes, around the brightness of the full moon by  the time it gets here.  Something with the apparent size of a star in the sky producing as much light as the full moon will be painfully bright.

48 minutes ago, Ouroboros said:

Does anyone here have any idea how large its apparent diameter might grow in the years following the explosion? 

SN 1987A (another type II but a very different progenitor) was ~0.1-0.2 arcsec apparent diameter after 3.5 years according to this Hubble image

https://hubblesite.org/image/20

which given the ~300x difference in distance would suggest the Betelgeuse remnant would be perhaps Jupiter size in the same timescale?.  SN 1987A was made more interesting because is also lit up a ring of previously shed circumstellar material but I am not sure if there is much of that from Betelgeuse. (SN 1987A was a blue supergiant)

That will be mainly high energy (gamma to UV) though initially.  We have to wait for it to cool a bit and for the energy from the decay of newly formed unstable elements to be generated to see the maximum in the visible light.