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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
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
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
These two spectra of C/2019 Y4 (ATLAS) were taken on 26 March (blue) just before the recent disruption of the comet and on 6 April (black) after. Both were taken with a LISA spectrograph on a C11 and are integrations of 130 and 90 mins respectively. Reports indicate that the event has reduced the brightness of the comet by about a magnitude and it has become more diffuse. This was apparent from the increased difficulty in guiding on the comet in the later spectrum. The later spectrum also suffers from greater noise beyond that expected just from the reduced exposure time. However, it appears that the event has not introduced any significant change in the spectral profile of the comet.
 26 March + April 6 2020.png)
Hi James,
I have heard nothing more from Richard Berry about availability of AIP4WIN. The stumbling block seems to be the attitude of the owner of Willmann-Bell Perry Remaklas. I have also used AstroImageJ and confirmed that it gives the same results as AIP4WIN but I find it a bit awkward to use just for photometry.
AIP4WIN does run successfully on Windows 10 if you follow the instructions and remove all previous versions first.
David
A low resolution spectrum taken last night with a C11 + LISA spectrograph confirmed it is likely to be a CV with a strong blue continuum and Balmer emission lines. Clouds prematurely terminated the run after 30 mins. Concurrent B and V band photometry showed variation of ~0.08 mag possibly caused by flickering in the accretion process. B-V colour index is 0.075. The concurrently recorded mean V band magnitude was used to flux calibrated the spectrum.
David
Although subjectively it seems to have been a poor year, probably influenced by the recent persistent clouds, the evidence shows otherwise. I am in South Oxfordshire in the UK.
I always calculate my statistics as the proportion of the nights when I was potentially able to observe (ie not away from home or otherwise engaged) that I did actually record useful observations, either photometry and/or spectroscopy. It is a reasonable assumption that if I had been available every night of the year, this is the proportion of nights when I could have observed.
Over the previous 14 years my annual average has been 42.1% of nights when I was able to observe. The std dev on this is 2.9% and the trend is absolutely flat – conditions have been very consistent over this period.
In 2019 the proportion of observable nights was 41.2%. Over the year there has been considerable variation. In Jan, Feb and Nov the monthly figures were all below 27%. In Jul, Aug and Sep they were >57%. However this variation is no worse that in previous years.
Best wishes for (more) clear skies,
David
Hi Nick,
This is based on a .lst file I was originally given, I think by Olivier Thizy when he was at Shelyak, containing emission lines which are present in the Ar/Ne calibration lamp which they supply.
To use it with ISIS you select file mode in the General tab, type the .lst file name in the box, and then on the Calibration tab you specify the wavelength of one of the lines in the calibration lamp spectrum in the “X coordinate of line at wavelength” box. I use the line at 5852.488 A which in my calibration lamp spectrum is the brightest line close to the middle of the spectrum. Then place the cursor over the centre of that line in the display of the lamp spectrum below and double click. ISIS then fills in the X coordinate of that line in the adjacent orange coloured box. When you then hit “Go” it should correctly identify the location of all the other lines in the .lst file and wavelength calibrate your spectrum.
If you still have problems email me off list and I will try to help further.
David
Hi Kevin,
In quiescence, during your first 3 spectra, SS Cyg has a relatively stable accretion disc which is being fed from the secondary star and is in turn transferring this hydrogen-rich material onto the surface of the white dwarf. This process produces the Balmer emission lines you see in the quiescent spectrum. When the star goes into outburst, the temperature of the disc increases elevating the blue end of the continuum, singly ionised helium HeII lines appear, and the opacity of the disc increases which absorbs some of the emerging light generating the absorption component of the spectral lines and reducing their emission component. This is a rather simplified description but conveys the general picture.
If you want to see how you can accurately flux calibrate your spectra using a V magnitude, have a look at my workshop slides in the spectroscopy section of the E&T website.
David
Hi Robin,
The level of the background sky in my LISA images is very flat in the Y direction, much as you have found. The flat lamp image as recorded is not flat, probably due to the different light path from the lamp into the spectrograph. So I think applying the vertical gain correction results in a better flat correction.
David
There is a preprint in arXiv this morning about this object at https://arxiv.org/abs/1909.05851
I am forwarding the following message on behalf of Josch Hambsch:
Dear BAA members,
The preliminary program of the EVS2019 is now online. You can find it under the Program tab on the website http://www.EVS2019.be. You are welcome having a look.
You are also still welcome for a presentation.
I look forward to welcome BAA members in Belgium.
Best regards,
Josch Hambsch
Organizer EVS2019
During 2018 I was able to obtain photometry and/or spectroscopy on 111 nights out of 261 when I was at home and available to observe. Assuming this proportion was representative of the whole year, this suggests observations would potentially have been possible on 42.5% of nights during the year (in South Oxfordshire). This is very close to my average over the past 14 years (42.1%) so for me 2018 was a very average year. The best months were May (61.5%) and August (60.0%), the worst March (22.7%) and December (21.7%). My records show 14,890 photometry measurements and 2,663 spectra recorded during the year. Although not nearly as noteworthy as Gary’s visual observation record, I see my photometry measurements passed the 400,000 mark during the year.
Best wishes for clear(er) skies during 2019.
David
The AAVSO maintains a list of observing campaigns at https://www.aavso.org/aavso-alert-notices-for-observing-campaigns-and-discoveries
A recent example was the request from Dr Moritz Guenther at MIT for observations of Herbig Ae/Be stars which he and colleagues were planning to observer with the X-ray satellite Chandra. https://www.aavso.org/aavso-alert-notice-658
These are practical opportunities for amateurs to make a useful contribution to the progress of science. Those of us who participated have received encouraging positive feedback from him.
David
LISA spectrum, 4500 sec integration, very similar to Robin’s above.
According to a recent email to the CBA group by Tonny Vanmunster reporting observation of superhumps, it is a probably an AM CVn type dwarf nova.
David
The best results will come from matching the focal ratio of the scope to that of the spectroscope, possibly using a focal reducer. In the case of the LISA that is F/5. Then it’s a matter of as much aperture as you can afford. With comets you need to be able to guide on the brightest part of the coma around the nucleus. It’s a bit of a catch 22 situation. Comets are often at their brightest when they are closest to the Earth but they will then be moving at their fastest. This will test your system’s ability to guide on a faint fast moving object. 46P was moving at about 8″/min last night which was not too bad. Even so, my system was struggling to keep in on the slit. You also ideally want a comet with a well condensed coma. 46P isn’t ideal in that respect. Although its total mag is around 9, the light is spread over an area greater than the Moon so it is very diffuse. Nevertheless the centre of 46P is till bright enough to guide on at the moment.
David
Here is a spectrum of comet 46P taken last night Dec 9.908 with a LISA spectrograph on a C11. Total exposure time was 3000 sec. The spectrum was integrated in the Y direction over 27 arcsec on the sky which is approximately 3x the FWHM of the bright central region of the spectrum. Sky subtraction regions were approx 3 arcmin away for the comet so although they will contain some light from the coma this is relatively faint at that distance and so should not significantly reduce the intensity of the central spectrum.
I have tentatively identified the most prominent lines using the table in Brown et al., Astronomical Journal, 112, 1197-1202 (1996). Although there is also an emission line from NH2 at 6300A, most references appear to attribute the very bright line at that wavelength to [O I].David
T Tauri stars are young dynamic objects experiencing accretion from a circumstellar disc. This is a stochastic process so changes over time are normal. An inverse P-Cygni profile is a signature of infalling material which is part of the accretion process. However accretion is not continuous and tends to happen irregularly so it is not really surprising if the inverse P-Cygni profiles come and go in the spectrum. It would be interesting to take further spectra periodically to see how they change. Changes in accretion may also manifest themselves as changes in the magnitude of the star as it brightens and then fades.
David
