[Robin Leadbeater]

It is interesting to estimate the net difference in flux between Marc’s and Jack’s setup. (A  C11 with a 35um slit compared with a C14 with a 23um slit.)   at 3 arcsec the slit throughput is 70% and 40% respectively but because of the higher collecting area of the C14 the difference in flux is reduced. The net effect is the total flux after the slit should be quite similar (Jack’s is ~5 % lower) Jack’s resolution will be ~50% higher though because of the narrower slit. 

A good estimate of the actual resolution can be made from the FWHM of the internal calibration lamp lines.

Robin

[Robin Leadbeater]

Hi Marc,

I am not sure where the throughput being proportional to the square of the slit width comes from. There are trade offs between slit width, SNR and resolution but the change in throughput with slit width depends on various factors. Ignoring diffraction effects, for an extended source the increase would be proportional to the slit width but with a stellar source it depends on the FWHM of the star image relative to the slit width. A good guide is to aim for a slit width equal to the star image FWHM. With a slit wider than that you are gaining little in throughput but losing resolution. If the slit is set narrower you will increasingly lose more throughput but you may prefer to take the resulting increase in resolution if you still have enough signal to still give the required SNR.

Regardless of the camera or gain, shorter exposures means fewer photons and thus higher shot noise (which dominates at high SNR). There is no free lunch here

Robin

[Robin Leadbeater]

There is a nice calculator for this on the CAOS website

https://spectroscopy.wordpress.com/2009/05/22/slitpinhole-flux-calculator/#more-525

which I believe is incorporated in to Ken Harrison’s modified version of Christian Buil’s simspec, the spectrograph design spreadsheet.  Plugging in the values for my setup (C11 at f10 and 3 arcsec seeing) it calculates 50% throughput at 23um, 70% throughput at 35um and 85% throughput at 50um slit width.  I normally run a 35um slit.  

Note that other variables such as guiding accuracy and position of the star on the slit can be as, if not more important though.  The definitive test as to how much light you are using is to measure the flux of the spectrum of a star with and without the slit. (or in the narrow and wide parts of a photometric slit of you have one)

Cheers

Robin

[Robin Leadbeater]

Impressive RV precision !  There are various interesting physical properties that can be measured from spectra like this. Francois Teyssier has a worked example on his website

http://www.astronomie-amateur.fr/feuilles/Spectroscopie/NGC2392.html

Robin

[Robin Leadbeater]

Hi Paul,

There are some examples of its use with ALPY here which might be useful

http://www.astrosurf.com/buil/redshift/demo.htm

EDIT:

Also described in the ISIS change log under v5.1.0

http://www.astrosurf.com/buil/isis/new/release.html

Cheers

Robin

[Robin Leadbeater]

I think my limited work with the ALPY suggested a few tens of km precision but high RV precision using low resolution instruments is certainly possible though for example as  David demonstrated at a BAA workshop using a LISA a couple of years back with if I remember, an impressive 5km/s 1 sigma ?

Robin

[Robin Leadbeater]

Hi Paul,

a 200km/s error with the ALPY does seem unexpectedly large (roughly a pixel or so). Like you I find the ALPY very stable and I would expect to be able to measure the position of a strong narrow line in an extended object like a PN to better than say 1/5 pixel.  In stellar RV work one source of significant error is the position of the target on the slit. (if the star is too small it “rattles” in the slit and even if overspilling the slit, slight offsets in guide position alter the shape of the line profile and move the centroid.) I would not expect these issues with PN though as they are effectively extended objects. Since the error  is variable we can also rule out an offset in the calibration lamp spectrum. PN are expanding so show different RV depending on the area examined but I understand these are only a few tens of km/s at most.

I dont know the quality of the wavelength measurement on Plotspectra. Have you tried measuring the lines in ISIS?  Also since the ALPY calibration is so stable and you are comparing the PN using  lines at the same wavelength, you could try just overlaying the raw uncalibrated profiles of the two PN and look for any shift directly to eliminate any potential errors in wavelength calibration. 

Cheers

Robin 

[Robin Leadbeater]

Hi Paul,

It is a bit tricky to tell as the final version is a combination of the two on line documents.  “Analysis and interpretation of Astronomical spectra” and “Practical Aspects of Astronomical Spectroscopy”  Also the on line copies I have are probably not the last versions that Richard published (mine are v8.6 and v2.0 respectively). Perhaps Marc has a better idea of what has been added/changed significantly. Alternatively a comparison of the list of contents might show up any obvious differences.  

Cheers

Robin

[Robin Leadbeater]

Just as clarification, CUP gave me complementary copies of these two books in return for my views on the market for amateur astronomy books in general and on spectroscopy in particular which they asked me about this some time ago.

I have long been a fan of Richard Walker’s Atlas in its free on line format for many years and I was pleased to find that CUP did a good job transferring it to print.

I do have some reservations about the accompanying “Spectroscopy for Amateur Astronomers” though which is based on two other on line documents previously  published by Richard. It is a useful reference book with a good list of references for further reading but the practical parts are somewhat limited to what Richard has worked on in preparation of his atlas and may have benefited from wider input from other experienced amateurs . For example the  use of flats and difficulties of flux calibration he mentions are areas which, while tricky subjects, have largely been successfully tackled  by other amateurs. I would have preferred these areas and other areas to have been addressed head on in more detail rather than just  put in the “too difficult for amateurs” category.This is my personal view of course which I did convey to CUP when they originally asked for my comments on an early draft.

Robin 

[Robin Leadbeater]

No C2 bands or emission lines then ?

Robin

[Robin Leadbeater]

A small correction:- Umberto used an ALPY 600 but Paolo used a LHIRES with 150l/mm grating giving a similar resolution.

Robin

[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

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