[David Boyd]

Recent V-band data on the AAVSO website show strong modulation close to the orbital period of 0.05408978(5) d reported by John Thorstensen.

Peranso PDM analysis of 3 night’s data.

Spectra calibrated in absolute flux follow the changing brightness of the star with the Balmer lines evolving from initally pure absorption to now having emission cores.

David

[David Boyd]

Hi Robin,

As I just posted in reply to your question on the ARAS Forum, I think these lines are C II which are seen in He/N novae.

This spectrum was recorded last night 17th July with a LISA and calibrated in absolute flux using the concurrently measured V magnitude of 13.10.

David

[David Boyd]

Those following the class of this nova may be interested to know that the recent ATel 14728 has reported that it now looks more like a He/N nova than Fe II as was being reported by ATels up until June 16th. This change actually happened within 36 hours of its discovery on June 12.54 UT as this compilation of amateur spectra shows. Compare these with the spectrum of Nova Sgr 1991 in figure 2 of Williams, The Formation of Novae Spectra, AJ, 104, 725 (1992).

David

[David Boyd]

Further to Robin’s posting, the following plot shows the change in absolute flux of the nova over the last 4 days. These R~1000 spectra have been calibrated in absolute flux using concurrently recorded V magnitudes of 6.35, 8.38 and 9.77 respectively as described here.

David

[David Boyd]

The spectrum now looks very different from last night with strong and very wide Balmer emission lines.

David

[David Boyd]

Here is a low resolution R~1000 spectrum of the new bright object in Her. Taken with a LISA on a 0.28m SCT.

This has been instrument and atmospheric response corrected.

David

[David Boyd]

Hi Jeremy,

The BAA light curve shows it has been rising steadily for the past 2 weeks. 

David

[David Boyd]

This spectrum of Nova Cas 2021 was recorded with a LISA spectrograph on a C11 during a short break in generally overcast skies tonight. It shows strong emission lines of the hydrogen Balmer series and He I. There are P Cygni profiles extending to around -2500 km/s relative to the rest wavelength on all emission lines. The spectrum was calibrated in absolute flux using currently recorded B and V photometry which gave V = 7.77+-0.04 and B-V = 0.50.

David

[David Boyd]

Happy New Year!

My statistics for the year show it to have been an average year as far as observing is concerned. Because of the virus, apart from a few nights early in the year I was at home and available to observe every night if conditions permitted. My results show that I actually observed and obtained photometry on 40.1% of available nights (137 out of 342) with the best months being April, May and September (as usual) and the worst February and October. I also obtained spectroscopy on 114 nights.

Regards,
David

[David Boyd]

I found PV Cep at 16.72 in an Rc filter last night, 24th Dec. This is close to its brightness back in July before the recent deep fade.

There was no trace of the nebula in a stacked exposure of 29 mins although the sky was slightly milky and the moon was bright.

David

[David Boyd]

Hi Andrew,

Thanks, that will be useful. The CALSPEC files at STScI are Multi-Extension FITS files and can’t be read by ISIS. I had the same problem reading files in NGSL and also wrote a Python script to turn them into Basic FITS files.

The range of the commonly used BVRI Johnson-Cousins filter passbands is from about 3600 A to 9750A in case anyone wants to use the CALSPEC files to calibrate flux for the other filters.

The UVEX claims to cover the range from 3500 A to 10,000 A and the upper limit of the LISA IR version is also 10,000 A so the range you suggest looks good.

Probably the best place to put it on the BAA website would be in the Equipment & Techniques Section webpage on Spectroscopy.

If you ask David Arditti he can put it there.

David

[David Boyd]

This is the first of two spectra I was able to record last night, 26th Nov. Thick fog eventually ended observations. The H Balmer and He I emission lines have prominent P Cygni dips on their blue edges caused by absorption in the material expanding towards us from the nova explosion.

David

[David Boyd]

David,

There have been several articles in the CAP Journal (Communicating Astronomy with the Public) on this theme. Some of these may be useful.

https://www.capjournal.org/issues/15/15_05.pdf

https://www.capjournal.org/issues/17/17_36.pdf

https://www.capjournal.org/issues/20/20_35.pdf

https://www.capjournal.org/issues/21/21_36.pdf

https://www.capjournal.org/issues/23/23_20.pdf

David

[David Boyd]

John,

I calibrated your 3 spectra in the BAA database in absolute flux using V magnitudes interpolated at the times of your spectra from V magnitudes in the AAVSO photometric database. I have labelled the main emission lines which include the Balmer series of hydrogen recombination lines and collisionally excited lines of singly ionised iron Fe II. The latter have strengthened during the recent rebrightenings of the nova. There are also forbidden lines of oxygen [O I] formed in the lower density expanding nebula.

David

[David Boyd]

Here is a spectrum of Nova Cas 2020 recorded with a LISA spectrograph on a C11 SCT and calibrated in absolute flux using a concurrently measured magnitude of V = 13.395. Using PlotSpectra, the integrated absolute flux of the H-alpha and H-beta lines are 3.53E-12 and 2.52E-13 erg/cm2/sec respectively. These results are consistent with the values 3.54E-12 and 2.54E-13 reported in ATel 13905. The apparent FWHM of the H-alpha line is 508 km/s and there is evidence of weak P Cyg absorption on the blue side of H-alpha extending to -800 km/s relative to the peak of emission. There are also emission lines of Fe II 5018, Fe II 5169, [O I] 5577 and an unidentified emission line at ~6160 A.

David

[David Boyd]

Hello Keith,

Welcome to the exciting (and challenging) world of spectroscopy! You are clearly well-equipped. There are some useful resources on the BAA Equipment and Techniques section webpages at https://britastro.org/node/19378. Francois’ book is also a good resource and a good guide for a beginner to get up to speed.

There is an active online discussion group on spectroscopy mainly based in the US but including some of us in the UK which is coordinated by Woody Sims in Phoenix. Several of us now have LISA spectrographs. I will ask Woody to make contact with you. 

It looks from the spectra you have posted that you have some problems with wavelength calibration as the Balmer lines in your spectra do not always lie at the correct wavelengths. I have been using Christian Buil’s ISIS software to process my spectra for a long time but have just started looking at Demetra. It has a good facility for checking how well it is locating the lines in your calibration spectrum so it is worth spending some time working with that. You should be aiming for an rms value of around 0.1 with a LISA. Make sure your LISA is well focused and that you are not saturating the lines in the calibration image. 

So try to master wavelength calibration first then move on to response correction. This involves taking the spectrum of a reference star whose spectrum you can find in a library and using that to generate a smooth response profile which will enable you to correct the spectrum of another star for atmospheric and instrumental losses. The reference star should be close in airmass (=altitude) to your intended target star so the atmospheric correction will be the same. You can find spectra of possible reference stars in the MILES library (these are actual measured spectra) or in the Pickles library (these are synthetic spectra of the main spectral types). There is a useful spreadsheet which will give you a list of possible MILES reference stars close to a specific location in the sky at a specific time here

http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=941

Try to pick stars which are main sequence stars (spectral class V) and have spectral types B or A as these have spectra with a smooth continuum which makes it easier to generate a smooth response profile in Demetra.

There is also a lot of information available on the ARAS spectroscopy forum at http://www.spectro-aras.com/forum/index.php and that is also a good place to ask questions as many active spectroscopists use it.

That’s probably enough information to keep you busy for now. It’s good to have you in the amateur spectroscopy community. There is much useful work to be done.

Best wishes,

David

[David Boyd]

Thanks Andrew, these are useful references.

When Christian Buil wrote ISIS he recognised that once you subtracted the bias or offset signal, the remaining dark current in CCD cameras scaled linearly with exposure time. A dark frame taken with the longest exposure time could therefore be scaled to any other exposure time required. Hot pixels behave differently and are dealt with separately in ISIS. Exposures in spectroscopy can vary from 10 sec on a bright reference star to 600 sec or more on a faint target, so a set of darks at every exposure time with sufficient images per set to reduce statistical noise could potentially take hours to record. The approach taken in ISIS was a pragmatic solution. With temperature regulated cameras such a dark frame could have a relatively long useful lifetime.

With CMOS cameras having low readout noise per exposure and nonlinear dark frames, this approach may need to change.

David

[David Boyd]

This could be an issue in spectroscopy as the widely used ISIS software uses the scaled darks technique. 

Do you have a reference for the evidence of non linear amp glow?

David

[David Boyd]

I use AIP4WIN for aperture photometry. My pragmatic approach in trying to measuring SNe is to choose an aperture for the SN itself which extends out to where the SN light distribution blends into the background but no further. Then a small gap of a couple of pixels followed by an annulus which includes at least 10 times as many pixels as contained in the central aperture. This will inevitably include light from the surrounding galaxy but the distribution of this light is likely to be a gradual slope as you move away from the galaxy core. The annulus will record an average of this background intensity which should be reasonably representative of the galaxy background under the SN itself. You can try varying the various radii involved to see how stable the result is. As I say this is a pragmatic solution and not perfect but in my experience it seems to give a result consistent with other reports.

David

[David Boyd]

This spectrum of type 1a supernova SN2020hvf was taken on 11 May with a LISA spectrograph (R~1100) on a C11 and has been calibrated in absolute flux using a V magnitude of V=12.67 measured concurrently with the spectrum using another scope. The spectrum has been smoothed with a Gaussian factor 2.5.
The blue-shifted location at 6118A of the prominent Si II 6355A absorption feature indicates an expansion velocity of ~11,200 km/s.
David

[Author]

Posts