[Robin Leadbeater]

Hi Marc,

It is indeed possible to perform absolute flux calibration of spectra using photometric magnitude measurements as a reference (as opposed to just calibrating the spectra relative to the continuum). It  is a technique frequently used by amateurs these days to calibrate broad band (typically low resolution) spectra where an absolute flux calibration is required. The details of the method were developed on the ARAS forum. Tools were built into ISIS software and the procedure was formalised by BAA member here, David Boyd.

http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=897

and has been used in several of his (peer reviewed) papers in the BAA Journal.

It is not strictly valid to use the technique in the way Ernst has done here however as the spectral range covered is not the same as that of the V filter passband  The implicit assumption here is that the flux in the V band and in the continuum at H alpha are linearly correlated. This is not always the case and would need to be established, for example by performing simultaneous photometry in V and R bands. 

Alternatively more  conventional techniques for absolute flux calibration of spectra are also used by amateurs by recording spectra of  spectrophotometric standard stars, aided for example by the use of dual width “spectrophotometric” slits available for the Shelyak spectrographs which allow the total flux to be collected. This does need stable atmospheric conditions however during the observation. (analagous to all sky photometry). Again ISIS software has the tools to enable this.

http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=695

Cheers

Robin

[Robin Leadbeater]

Hi Matt,

Magnitude is a log scale so v(target)- v(comparison) is actually the ratio of the intensities of the two stars, which is independent of the actual count/intensity scaling in the image.

the magnitude of the comparsion star is itself a ratio (To a standard star with vmag =0 eg Vega)

Cheers

Robin

[Robin Leadbeater]

Hi Marc,

I assume BASS estimates the resolution in a similar way to ISIS by estimating the FWHM of the neon calibration lines, in which case it may not give a good estimate of the resolution on undersampled spectra like this.  (Not enough points in the line profile)

There is still something odd about the dispersion value. The native dispersion of the LHIRES 2400 at H alpha is ~12.8A/mm so for example giving 0.083A/pixel with my ATIK314 (6.45um pixels)  Your native dispersion should be around 0.06A/pixel so it looks like there is a further 3x binning going on somewhere. In any case I suspect you are losing some resolution compared with other LHIRES III 2400 spectra of the same object. Compare for example the cleanly resolved metal/telluric lines in similar spectra on the ARAS forum as here for example.

http://www.spectro-aras.com/forum/viewtopic.php?f=19&t=362

It seems that H alpha is likely to be well covered so I might take a look in the UV. With the potential for variation in size of both the H alpha disc and the M star. It could be important to establish the extent of the M star independent of the disc. It is a more difficult area to work in but I had some success in this area with the similar but much fainter eclipse of AZ Cas

http://www.threehillsobservatory.co.uk/astro/spectra_43.htm

There is a discussion involving Phil Bennett about this area here

http://www.spectro-aras.com/forum/viewtopic.php?f=19&t=1233

Cheers

Robin

[Robin Leadbeater]

Hi Marc,

I had not planned to specifically follow VV Cep but could contribute on an ad hoc basis. Are there any more details of the campaign? Is there any professional involvement? (Phil Bennett was mentioned as PI  in a post on the ARAS forum a couple of years ago

http://www.spectro-aras.com/forum/viewtopic.php?f=19&t=1219

but the link to  page given there and in the document you posted on the astronomical spectroscopy yahoo forum just leads to a blank page under construction and the rest of his pages on VV Cep linked from there are dead)

There also appears to be something odd about the dispersion figure on your graphic. At 0.33A/pixel your maximum resolving power  R would be at best 10000 at H alpha, not 12000 (limited by the Nyquist sample interval)  0.33A/pixel however is much too large a figure in any case for a LHIRES III and 2400l/mm grating with the pixel size of your DSLR unless you are binning.

Cheers

Robin

[Robin Leadbeater]

Marc wrote:

 “Unfortunately until mid July we must undergo the annual period of the so called “gray nights” or twilight nights. Fortunately the beginning of August will give enough dark nights to register the start of the VVCep eclipse!”

With careful background subtraction VV Cep should be bright enough and far enough from the sun to follow at high resolution throughout  the summer from where you are.  For example I am further north than you (55N) and was able to follow eps Aur under much more extreme conditions.

http://www.threehillsobservatory.co.uk/astro/spectra_40.htm

Cheers

Robin

[Robin Leadbeater]

Hi Marc,

But the time on your spectrum graphic is 10:19 UT. Are you observing in daylight?

Cheers

Robin

[Robin Leadbeater]

Hi Marc,

I’ve just noticed the times on your VV Cep spectra. Where are you observing from? 

Robin

[Robin Leadbeater]

Hi Paul,

Here’s mine with the ALPY 200 from my reasonably dark rural site, from this poster paper

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

The comparison with the VLT was interesting. (note the high natural pollution from OH band emission in the IR) 

It has certainly become worse over the years in quantity and complexity though with a shift locally from LP sodium to a mix of HP sodium, metal halide and now  LED, particularly the ones high in blue.  Sky background subtraction does indeed work very well eg  as here where  the signal is much lower than the sky background (It still adds noise though and I think is ultimately limiting my sensitivity)

http://www.threehillsobservatory.co.uk/astro/spectra_46.htm

Cheers

Robin

[Robin Leadbeater]

Paul,

The relatively low level of the Na D line in your spectrum is interesting (The narrow band from LP sodium lighting still dominates in my spectrum) Is this not used in your area at all these days ?

Robin

[Robin Leadbeater]

Hi Matt and welcome to the BAA !  

Sorry I did not get back to you on the stargazer’s lounge thread about your latest spectrum of S617b.  This one is much less noisy and as you say now seems to match much better with what would be expected for a G0v star.  I am not sure why it looked so much bluer than its companion in your image though. (It is hotter (compared with the late G/early K companion) but not as much as the image colour difference might suggest)  Overlaying your latest spectrum of the b component over the spectrum of the a component or A comparison of B-V for the two stars should clarify the colour difference.

Precise spectral classification is a bit limited with the Star Analyser as the resolution is not really high enough (you really need around 10A resolution or higher to do this) It is not so bad at showing the difference between hot eg A stars and cool M stars but in the middle temperature range say mid F to mid K, you get hundreds of metal lines which merge at Star Analyser resolution. You can get an idea from the shape of the spectrum continuum but interstellar extinction can distort this so you have to be careful. Spectral classification is all about what lines appear and how strong they are so matching the details in the spectrum with the Pickles library can help decide what type as star is. I suggest overlaying the Pickles spectra on your spectra looking for matches in the features and general shape. 

Cheers

Robin

[Robin Leadbeater]

I was able to get just 10 mins on this object with the modified ALPY 200 before the clouds rolled in last night. This is a bright target for this setup though so it was enough to get a decent low resolution spectrum (R~130).  Here is my spectrum overlaid on the confirming spectrum from 14th May posted on TNS.

The Supernova identification program SNID confirms from my spectrum it is a type IIP near maximum light. There have been some significant changes in the spectrum over the past 3 days which is expected. 

 The blue shifted absorption component of the H alpha P Cygni profile is clear now in my spectrum and can be used to estimate the explosion velocity (~11600km/s) 

Cheers

Robin

[Robin Leadbeater]

There is also a lot of background information and amateur spectra at all resolutions on VV Cep posted on the ARAS forum over the past few years.

http://www.spectro-aras.com/forum/viewforum.php?f=19

Cheers

Robin

[Robin Leadbeater]

I was clouded out by the time it got dark enough/cleared the tree line so no spectrum from here. The confirming spectrum (type IIP) on TNS is fairly featureless with a blue continuum at the moment and a small hump at H alpha.

https://wis-tns.weizmann.ac.il/object/2017eaw

This should grow into a nice strong P Cygni line profile over the next couple of weeks though.

Robin

[Robin Leadbeater]

Hi Andrew, Andy

I come across this format from time to time and it would be nice to be able to convert multispec format spectra to single orders. Storing them in the database in multispec would probably not be a good idea unless they can be output as separate orders as none of the usual amateur fits reading programs can handle them as far as I know.

I have used a very long winded approach involving dismantling the multispec files and importing them into Excel but Otmar Stahl, the professional on the German VdS spectroscopy forum, kindly converted some to separate orders for me a few years back using an IRAF script I believe. You could ask on there or if you get no joy I can try contacting him and see if he would share the script.

Cheers

Robin

[Robin Leadbeater]

Hi Tony,

Except for a mismatch in the strength of the lines, probably due to a mismatch in resolution, the disparity at the blue end does not look that obvious to me from here. What does the fractional error look like if you divide one by the other and smooth the result?

I (and others) have also found getting a good flux calibration below 4000A tough. Most of the sources of error were probably been covered in this thread.

https://britastro.org/node/9700

and Chrisitan Buil’s site covers chromatic aberration, atmospheric absorption and the effect of position on the slit

http://www.astrosurf.com/buil/dispersion/atmo.htm

I suspect the best (though time consuming) solution when after highest accuracy in the continuum shape at the blue end is to use a spectrophotometric technique using a very wide photometric slit to determine the shape of the continuum and combine that with the resolution of a narrow slit spectrum eg

http://www.astrosurf.com/buil/calibration2/absolute_calibration_en.htm

David found using a wider slit gave better results, though the reason for the improvement was not clear in this case

https://britastro.org/node/9199

Cheers

Robin

[Robin Leadbeater]

Hi Tony,

These look really good, particularly below 4000A where i had trouble getting a perfect fit.  What reference star did you use to calculate the instrument response ? Did you use the same one for all the targets?

Cheers

Robin

[Robin Leadbeater]

Hi Peter,

You are getting there. The Hydrogen Balmer lines in Denebola in particular are nice and clear.

There’s no Methane in Sirius but the Hydrogen Balmer lines are visible.  The H2O telluric line marked should be at an absorption line. The most obvious telluric line is the broad O2 band at ~7620, off the edge of your plot but very clear in the image you posted on “Cloudy Nights” and “StargazersLounge” forums. The other humps and bumps are from the camera’s 3 colour filters. (Monochrome cameras are better in this respect for spectroscopy)

Cheers

Robin

[Robin Leadbeater]

Hi Paul,

Yes, The way I understand it is if you are using the AOD function in ISIS to calculate and correct for atmospheric extinction then the resulting instrument response  just takes into account the instrument  (ie not the “IR+extinction” commonly calculated using a nearby reference star.) You are therefore correcting your spectrum in two stages, instrument response and atmospheric extinction. (I understand this is the usual technique used by professionals who know their instrument response and therefore just correct for the atmosphere for that particular observation) You can see an example of the procedure here.

http://www.astrosurf.com/buil/atmosphere/annexe.htm

EDIT: linked from

http://www.astrosurf.com/buil/atmosphere/transmission.htm

Cheers

Robin

[Robin Leadbeater]

Hi Kevin,

I think the “type wrong” is Brian Skiffs comment against the reference. G8III and K0III are very close but  your K0III reference (and your own confirming spectrum of course !) is missing from Brian’s catalogue so it could be worth letting him know so he can add it.

Here’s another oddity currently under investigation (BD-1 2458 referenced  as G0v in the literature but clearly much hotter from a preliminary spectrum). Even visually the colour looks obviously wrong for the catalogued classification so it is not clear what is going on here.

https://stargazerslounge.com/topic/289163-s617-triple-star-work-to-do/

Robin

[Robin Leadbeater]

Hi Andrew,

email sent

Robin

[Author]

Posts