[Robin Leadbeater]

Hi John,

The ripples come from the camera sensor. See my comment here

https://britastro.org/comment/4916#comment-4916

When you take a flat with a slit spectrograph, you are effectively taking a spectrum of the flat lamp. The ripples are from the wavelength response of the camera. These are also imprinted on the star spectrum of course so when you divide the star spectrum image by the flat they cancel out along with any other flat defects. 

The ripples are caused by interference in the light, between different layers in the sensor (an etalon effect)  and their spacing and severity varies from sensor to sensor. (Similar but much finer fringes are seen in high resolution spectra where they are caused by interference due to internal reflections in the sensor cover glass)

Cheers

Robin

[Robin Leadbeater]

The attached paper describes the mechanism which produces these ripples in CCD sensors (page7-11) The same effect is also seen in CMOS sensors.

Robin

[Robin Leadbeater]

Attached is my collection of QE ripples for various sensors

Robin

[Robin Leadbeater]

Hi Jack,

The two Na D lines are from the two different spin levels.  (3s  to 3p1/2 and 3p3/2 in absorption) There probably is Zeeman splitting of the lines as well but this will be very small in the solar magnetic field.

Cheers

Robin

[Robin Leadbeater]

As Andy says, BeSS standard practise is to leave the telluric lines in when submitting to the database.  When analysing spectra though you will want to remove them and it is quite possible for amateurs to do this.

For high resolution spectra where the individual lines are visible it is quite straightforward using a standard template for O2 and H2O lines which is scaled to fit the magnitude of the lines. Some software eg ISIS Visual spec have this function built in.

For low resolution spectra it is more difficult as the lines blend into broad bands. The same  technique can be used though by producing your own low resolution template. This is done by taking spectra of a hot star at two different air masses. If you divide one by the other, the result is the extra atmospheric absorption due to the additional atmosphere.  This can then be used as a template to remove the telluric bands. You can see a simple example of using a low resolution template in this poster paper

http://www.threehillsobservatory.co.uk/astro/Classifying_red_stars_using_a_Star_Analyser_VdS_poster.pdf

(This technique of measuring  stars at different air masses is used by professionals (and sometimes by amateurs) to correct for all atmospheric effects including extinction rather than trying to match the air mass of  reference star as is commonly used by amateurs. Correction for extinction (rather than just the telluric bands) using this technique though needs photometric skies so the atmospheric conditions do not change between the two measurements at different air masses.

Cheers

Robin

[Robin Leadbeater]

Note though that the instrument response for a slit spectrograph is only smooth provided a flat has been used otherwise it will have  ripples from the camera response.(A good reason for always using a flat).  Even then I find that my ALPY instrument response has a kink in it which I believe is quite common. (a combination of the flat lamp and atmospheric extinction curves) You can see it in my MILES star report here for example.

https://britastro.org/sites/default/files/attachments/ALPY600_MILES_tests_20161105.pdf

from this thread

https://britastro.org/node/8153

The acid test to see if your instrument response is correct is to rerun the reduction on the reference star using the instrument response. The result (except for any resolution differences) should perfectly match the reference spectrum (other than the telluric bands)

Cheers

Robin 

[Robin Leadbeater]

To mark the occasion I though I would take a spectrum.  It looks similar to others found in the literature from the 1980-90s

https://britastro.org/node/14188

Robin

[Robin Leadbeater]

So sad news. It was seeing what Maurice was doing in spectroscopy  over 20 years ago that got me and many others into the field. The supernova spectroscopy work I am doing is a direct result of his measurements then and the prophecy he made when president that one day amateurs would be able to confirm and classify supernovae spectroscopically, which he lived to  see fulfilled. He was an important champion of spectroscopy, encouraging me and others personally and helping to build the popularity it has today. He will be sadly missed by that community internationally.

Robin

[Robin Leadbeater]

I dug out my before and after grism alignment images (spectrum of the sky). The spectrum is horizontal as seen by the lines generated by dust specks on the slit. 

[Robin Leadbeater]

Hi Nick,

My ALPY (one from the first production batch) somehow came with the grism significantly skewed so I fixed it with Francois Cochard’s guidance. (Also I have a habit of taking everything apart to “see how it works” ! ) If it is only slightly skewed then I would not worry about it. The main reason for correcting the slant/smile is so that any lines in the sky background subtract correctly. In ISIS you select a strong lamp or sky line and it automatically measures the slant and curvature and corrects for it.  I expect Demetra does something similar.

Spectroscopically I “grew up” with ISIS so I know most of its quirks well and knowing Christian Buil, I had confidence that it would give good reliable results. I was down as a beta tester of Demetra but to be honest there was no pressure for me to change over, particularly as (at least at the time. I have not kept up with its development) it was not as comprehensive and flexible as ISIS so I never really got into it. Similarly with BASS which I have never used. 

Robin

[Robin Leadbeater]

Hi Nick,

Are you saying the spectrum lines in the sky are slanted when the star spectrum is horizontal? It does not matter if it is not too far out as it can be corrected in the software (They will be slightly curved too)  Getting the star spectrum as close to horizontal as possible is more important.  The slanted lines mean the grooves in the grating are not exactly parallel to the slit. If it bothers you it can be adjusted in the ALPY core module. There are (two?) little Allen headed grub screws which if slackened off allow the grism to be rotated. Only slacken them off a tiny amount though otherwise the whole assembly can spring apart !. You can see the internal  construction on page 4 here

http://www.threehillsobservatory.co.uk/astro/ALPY200_VdS_BAAVSS_poster_2014.pdf

Cheers

Robin

 

[Robin Leadbeater]

Were there any recordings of the Newcastle Spring meeting ? (I know it was plagued by sound system problems but the content of the talks was very interesting) 

Robin

[Robin Leadbeater]

Hi john,

Yes the spectra of novae change dramatically with time, particularly during the first few hours or days. I have been adding my spectra to the BAA spectroscopy database which show how it has been changing. They show the same trends as you are seeing. See here for a summary plot

https://tinyurl.com/V392Per-BAASDB-2

The instrument response is difficult to get right because of the high air mass at low elevation. I am now using the nearby star HD29526 as the reference with the A0v Pickles spectrum. 

There were Fe emission line curtains in the spectrum but more recently they seem to have disappeared and other lines have appeared. See my posts  in the ARAS forum

http://www.spectro-aras.com/forum/viewtopic.php?f=5&t=2015&start=120#p10792

http://www.spectro-aras.com/forum/viewtopic.php?f=5&t=2015&start=130#p10805

The velocities in the H and He profiles are too high to be from a rotating accretion disc so the outer spikes in the  profiles must be from the ejected material. It is possible that the central spike is from the disc if it survived. (Narrow emission lines like this are a feature of dwarf novae in quiescence)  It is also quite possible though that all the features are from  the ejected material. Novae explosions show sigificant asymmetry of various tyes and there are various models which could fit what we are seeing. The profile shape is  similar to those seen in the novae reported here for example

https://www.aanda.org/articles/aa/pdf/2014/09/aa24274-14.pdf

See here on the ARAS forum for some discussions on possible models 

http://www.spectro-aras.com/forum/viewtopic.php?f=5&t=2015&start=130#p10794

http://www.spectro-aras.com/forum/viewtopic.php?f=5&t=2015&start=130#p10812

http://www.spectro-aras.com/forum/viewtopic.php?f=5&t=2015&start=140#p10840

I rather like the last one, the “double smoke ring” modelled by prof Steve Shore!

Cheers

Robin

Edited to correct a blizzard of typos !

[Robin Leadbeater]

The commentary by Prof Steve Shore to accompany amateur spectra taken of novae Del 2013 is here

http://www.astrosurf.com/aras/novae/Nova2013Del.html

No two novae behave exactly the same though !

Robin

[Robin Leadbeater]

Hi John,

Glad you found the source of your flexure. If screwed down tight I have found the ALPY to be remarkably stable. I could not detect any flexure on mine in tests moving at around the sky.

If you have not come across it before, Christian Buil  has some information on the effect of combining lamp and stellar lines which you might find interesting.

 http://www.astrosurf.com/buil/isis/guide_alpy/resume_calibration.htm    (method 3)

Though not  necessary for most applications, if you are looking for the highest accuracy using stellar spectra dont forget to take account of the earth’s motion and the radial velocity of the star which can add up to a few tens of km/s. The effects are small but can be an Angstrom or two,  detectable even with the ALPY.

Cheers

[Robin Leadbeater]

Hello Jack,

It is over years ago since I built my LHIRES but  just checked and I see I actually just moved both washers to the same side. Take care to tighten the screws evenly  and not to tighten too much as it brings the lens into direct contact with the metal holder.  I would only do it if you are seeing significant shifts in the lines backwards and forwards as the lens is rotated. Presumably the problem in my case was uneven thickness of the washers. I did it to prove what was going on. It worked so I left it rather than trying different washers. 

I suppose you could use the double line for calibration if you can identify the wavelengths. You could check Richard Walker’s Atlas to see if they are identified

Cheers

Robin

[Robin Leadbeater]

The host galaxy brightness from Vizier would be the integrated brightness over the whole galaxy rather than just the core. Perhaps this explains why your measured core brightness is not higher than the Vizier magnitude ? (At a rough estimate, I would expect the SN to have increased the brightness by ~0.5 mag)

Cheers

Robin

[Robin Leadbeater]

Hi David,

From the sharp H alpha emission line in the spectrum (from the host galaxy) the redshift = ~0.04 which corresponds to ~170MPc distance. The spectrum is clearly a type 1a. At this distance a type 1a SN would reach ~16.9 at maximum, not allowing for any redenning.  The ASAS-SN mag 17.2 does seem rather bright  for 9-12 days before maximum but ages estimated from the spectra can be notoriously unreliable so it could be closer to maximum than this

Robin

[Robin Leadbeater]

4200.67A  looks to be a blend in my lamp spectrum and H alpha has completely disappeared from my lamp now

Cheers

Robin 

[Robin Leadbeater]

Attached is the wavelength calibrated spectrum of my ALPY lamp used to produce the calibration fit above (1D fits file, you may need to change the extension to fit. for some reason the forum allows .fits file extension but not .fit)

Cheers

Robin

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