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nova Cep 2014

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Robin Leadbeater
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 Nova Cep 2014 has been added to  the ARAS database.

http://www.astrosurf.com/aras/novae/Novae_Aras.html

The spectra can be found here

http://www.astrosurf.com/aras/Aras_DataBase/Novae/Nova-Cep-2014.htm

Contributions are invited

Robin

Nova Cep
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Jupe
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Robin,

I checked out your nova spectrum, nice job!

It's still too dim for me.

John

Latest Nova Cep
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Hi Robin,

I just checked out your latest Nova Cep spectrum.

Wow!

What is the absorption around 6900 Ang.?

John

Telluric line
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Hi John,

Yes novae spectra change very rapidly initially.

The absorption band at ~6900A is telluric from O2 and H2O in our atmosphere. There are few telluric bands which of course turn up in all spectra taken from the ground. You can see some of them identified in Christian buil's annoted Vega spectrum for example

 http://www.astrosurf.com/buil/us/vatlas/vatlas.htm

Cheers

Robin 

new Nova Cep spectrum 2104-03-23
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Just added another nova Cep spectrum for 20140323 to the ARAS database

 http://www.astrosurf.com/aras/Aras_DataBase/Novae/Nova-Cep-2014.htm

The H alpha line is still growing both in absolute and relative terms. V mag is ~12.5. No problems getting a good SNR spectrum with the ALPY 600 but unfortunately guiding on the overspill from the slit  is getting a bit tricky (now up to 3 sec exposures using a DSI Pro) and there does not seem to be any other alternative stars to use in the mirror slit guider field.  

Robin

 

Nova Cep
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Hi Robin,

Man, that's a perfect profile.

Am I correct in saying that Hyd. Beta has subsided and Hyd. Alpha has taken off?

John

Nova Cep 2014 evolution in absolute flux
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Hi John,

Here are the spectra plotted in absolute flux (using the V mag in the AAVSO database as the reference)

 

Cheers

Robin

Absolute is a tricky term
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If I could channel my own mentors for a second.... I'd caution you against using the word "absolute" to describe flux calibration.  Like voltage, all fluxes are measured realtive to something.  "Flux calibrated" is the better term to use.

flux calibration
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Hi John,

I dont understand this at all. The Volt is an absolute unit of measurement traceable to primary standards which makes it an absolute measurement to me.

The Y axis on my spectra is in erg/cm2/sec/A which to me is absolute flux, not relative. We could argue about the definition and accuracy of the standards used and the measurement uncertainties of course  

Cheers

Robin

process to calibrate absolute flux
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To clarify further - 

The spectra were "flux calibrated"  in relative terms, taking into account the instrument response and atmospheric extinction. 

The relative flux was then integrated over the Bessel V magnitude pass band and compared with the V magnitudes in the AAVSO database which can be expressed in absolute flux (erg/cm2/sec) based on the absolute flux calibrated standard Vega.   This is a function in ISIS

Here is an example of the term "absolute flux" being used in the way that I describe

 http://www.stsci.edu/institute/org/telescopes/Reports/isrs/scs8.rev.pdf

Cheers

Robin

Voltage
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Hi John,

You are correct, of course. While a volt is precisely definded, Voltage is a potential difference, not an absolute value. If you have two resistors of equal value in series with a battery, the voltage across either resistor will be halve that of the battery. Voltage is the potential difference.

Jeff HPO
Hopkins Phoenix Observatory

Earlier in my career, I used
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Earlier in my career, I used the word "absolute" in a public presentation to refer to flux and a senior collegue who I respect very much took me out to the figurative "wood shed" afterward to convince me to recant.  So my intention here (as was my senior collegue's) is to help.

Jeff covered how voltage is a difference.  In fact most units are actually relative in that they measure a difference.  A Volt is defined as a potential difference that yields a difference in energy of one Joule for one Coulumb of charge.  I always empasize with my physics students that they can't give me an answer in Volts without telling me what it is measured relative to.  The definition of Volts is not absolute becuase it does not include strictly a definition of "realtive to what."

There is the same issue with flux.  It is slightly different because we can pretty much all agree that zero flux is no energy.  However the problem is in practice because your detector never measures zero.  (Absolute zero is in fact a theoretical and not an experimental construct.) We can calibrate it to zero by measuring a bias and a dark current.  But there is uncertainty in that and as a result uncertainty in our definition of zero.   Therefore strictly speaking the zero point for our flux scale is relative to the bias and dark we measured.

There is an additional constraint with the dector because it does not measure ergs/m2/AA.  Once we have calibrated it to zero (with the uncertainty there) we calibrate it to a known source.  The difference between our defined zero and the known source gives us the relative scale to convert counts to ergs and then we measure the size of our aperture and detector pixels to get per m2 and per AA.  The issue here is that if you use one known source and someone else uses another known source there will be no absolute agreement because you calibrated relative to different sources.

Yes, this means there is really no "absolute" measurement.  That is a truth that haunts all science.  All measurements are at their core relative to something.  Which is why it is better to refer to a measurement as "calibrated" and then list what it is calibrated relative to.  That avoids any arguement about the infallible absolute.

The comparison with volts is
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The comparison with volts is irrelevant here (incidentally you are  confusing potential difference and the definition of the volt)

Leaving  that to one side if we talk about flux we can (ignoring such esoteric considerations  such as vaccuum energy negative energy and dark energy and the like) we can certainly talk about energy and flux in absolute terms (We may have systematic uncertainties in our measurement such as zero errors etc but this does not mean we are not measuring flux in absolute terms, just that we have an uncertainty in our measurement. )

Here I  am using the term absolute flux to distinguish it from relative flux where the spectrum is normalised to some value in the spectrum as is done for the spectra submitted to the ARAS database both of which are  "flux calibrated" by your definition and in the literature. 

You may wish to dance on the heads of pins here but I think anyone actually studing the spectra I generated would be clear as to what is meant.  Perhaps of of more value (and interest) would be a disccusion about exactly what is happening here in astrophysics terms

Robin 

Challenging ones seniors
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BTW seniority and correctness do not always go hand in hand. It is just that much of the time we are not in a position to question our seniors.  Being one myself now I have no such qualms -  You knew you were right then as I am now. You should have challenged him :-)

Robin

I can't vouche enitrely for
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I can't vouche enitrely for now, but 20 years ago the term "absolute flux" would have never been allowed in the literature to refer to a calibrated spectrum.  It gets us into trouble when we're so sure of our ability to determine absolute.  I assure you there are many other pros that would raise an eyebrow and just assume.  Its a term you should avoid.

Comming form a different area
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Comming form a different area of Physics I have similarly many times raised eyebrows at what professional astronomers would claim to be correct. Astronomy is  indeed at times a strange branch of science.  

When in Rome, however----How then as "an astronomer" should I  distinguish between a spectrum in measured in absolute flux in physical units  from one measured in relative flux.  Both are termed "flux calibrated" as far as I can see from the literature. The difference is of practical physical importance of course as the two tell quite a different tale in this case. 

Robin

In any case I dont believe I
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In any case I dont believe I would have much problem building a decent case for  this more usual definition of "absolute" and "relative" in this context, even within astronomy, for example this reference for the reduction of Gemini spectra 

 http://www.gemini.edu/?q=node/10147

where it states:

 "The msreduce task can be used in two ways. One can extract individual spectra without carrying out telluric corrections, if the fl_standard flag is "no". This produces a wavelength calibrated raw spectrum. Or one can extract the object and calibration spectra together and produce a wavelength calibrated and flux calibrated output spectrum, if the fl_standardflag is "yes". The flux calibration can be either absolute or relative depending upon the options chosen."

Going back to the original subject.... Many amateur spectroscopists remark on the huge increase seen in a nova H alpha line with time. This is becase they are usually looking at normalised spectra plotted in relative flux. Once one plots in absolute flux (as per my usage) the story is somewhat different with, except for the intial phase, the H alpha flux (in absolute units rather than measured as Equivalent Width for example) staying relatively constant for some time.

Cheers

Robin

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