Re: VV Cep evolution at low resolution
Posted: Thu Aug 10, 2017 8:24 pm
Hi Ernst,
Before we start discussion about what might be the case taken out of observations, I would like to step back and ask, how to observe. Everything counts.
My personal opinion and feeling is, that we may learn a lot, especially with this occultation. My concerns are about how to find a good calibration. This is not meant in a negative way, because we all may learn during the continuous observation cycle of the occultation which currently takes place. And the combination of all measures is important to gain a complete picture.
I tried first observations during OHP 2015 and also OHP 2017, where I tried to obtain a first spectrum with a high S/N taking a one hour exposure (co-added).
So far I need "more input" to compare the different observations over years and find hints to quantitatively explore the results. It may happen we do mistakes during observations. This is sometimes the case, if I'm getting tired during the night, finding defocussed stars doubled in my spectra. This was the case last weekend, when I tried to get a new spectrum at the beginning of the occultation, with the risk of my camera destroyed by rain. I hope not to find this every day.
That's why I wanted to continue with discussion especially focussed on how to calibrate my observations. This is not very easy. On one side there are not many well-known calibration standards. During my work at the Hoher List observatory (in the early 1990ies), we had discussions of how to use calibration stars of the Landolt sequence from CCD observations. I found this not very convenient, but painful in the final end. Air masses may not be very well corrected and even the distances of certain nearby galaxies observed have been measured faulty by factors of ten and needed to be corrected over time in literature. One particular reason is, these stars are distributed across equatorial longitude, but not equally distributed over latitude to correct for air mass for different observatories. The few known flux standards are also not very well distributed over the Northern hemisphere. A few of them, proposed by ESO later, are white dwarfs. More or less constant in flux, but less than the number of fingers of a hand, not equally distributed. Then, I was asking myself how to calibrate colors (or spectral bands) found in the open stellar cluster database. A few star clusters well measured, many of them providing large uncertainties in magnitudes and colors, however. Later I read a paper about the magnitude of alpha Lyr, which proposed a correction to 0.03 mag instead of the zero-point. I mean, what is a zero-point worth, if it shifts? Finally, I assume, nobody of us has ever taken a picture of an open stellar cluster to calibrate the local influence of atmosphere from air mass 1 to horizon. I did such photometric observations with my DSLR once with one single cluster, but hesitate to use it a a reference for my spectra, because the observation has been taken years ago.
On the other hand we observe a suspected faint blue star being companion of a red super-giant. Therfore, I would assume, we need a calibration especially in the blue. May Steve Shore do some comments on my findings, as I'm trying to learn every day...
So my conclusion is, there is not much to consider as "comparable" reference. I found alpha Lyra to be one of the most prominent stars with large S/N and known flux. This could be especially worthy with observations in the blue spectral range to study the companion.
Any more considerations or hints how to observe and calibrate?
Best regards,
Thilo
Before we start discussion about what might be the case taken out of observations, I would like to step back and ask, how to observe. Everything counts.
My personal opinion and feeling is, that we may learn a lot, especially with this occultation. My concerns are about how to find a good calibration. This is not meant in a negative way, because we all may learn during the continuous observation cycle of the occultation which currently takes place. And the combination of all measures is important to gain a complete picture.
I tried first observations during OHP 2015 and also OHP 2017, where I tried to obtain a first spectrum with a high S/N taking a one hour exposure (co-added).
So far I need "more input" to compare the different observations over years and find hints to quantitatively explore the results. It may happen we do mistakes during observations. This is sometimes the case, if I'm getting tired during the night, finding defocussed stars doubled in my spectra. This was the case last weekend, when I tried to get a new spectrum at the beginning of the occultation, with the risk of my camera destroyed by rain. I hope not to find this every day.
That's why I wanted to continue with discussion especially focussed on how to calibrate my observations. This is not very easy. On one side there are not many well-known calibration standards. During my work at the Hoher List observatory (in the early 1990ies), we had discussions of how to use calibration stars of the Landolt sequence from CCD observations. I found this not very convenient, but painful in the final end. Air masses may not be very well corrected and even the distances of certain nearby galaxies observed have been measured faulty by factors of ten and needed to be corrected over time in literature. One particular reason is, these stars are distributed across equatorial longitude, but not equally distributed over latitude to correct for air mass for different observatories. The few known flux standards are also not very well distributed over the Northern hemisphere. A few of them, proposed by ESO later, are white dwarfs. More or less constant in flux, but less than the number of fingers of a hand, not equally distributed. Then, I was asking myself how to calibrate colors (or spectral bands) found in the open stellar cluster database. A few star clusters well measured, many of them providing large uncertainties in magnitudes and colors, however. Later I read a paper about the magnitude of alpha Lyr, which proposed a correction to 0.03 mag instead of the zero-point. I mean, what is a zero-point worth, if it shifts? Finally, I assume, nobody of us has ever taken a picture of an open stellar cluster to calibrate the local influence of atmosphere from air mass 1 to horizon. I did such photometric observations with my DSLR once with one single cluster, but hesitate to use it a a reference for my spectra, because the observation has been taken years ago.
On the other hand we observe a suspected faint blue star being companion of a red super-giant. Therfore, I would assume, we need a calibration especially in the blue. May Steve Shore do some comments on my findings, as I'm trying to learn every day...
So my conclusion is, there is not much to consider as "comparable" reference. I found alpha Lyra to be one of the most prominent stars with large S/N and known flux. This could be especially worthy with observations in the blue spectral range to study the companion.
Any more considerations or hints how to observe and calibrate?
Best regards,
Thilo