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Concerning rectification, I think it depends to what use you are going to put the spectrum. For your own personal use or in a specific study, you just need to do what is required for that application (Which as an extreme example could be all about as accurate as possible wavelength calibration and nothing needed concerning flux calibration if all you wanted to know was the RV) .
The problem comes where you might be uploading data to a general database where you do not know to what use the data is to be put. The rule I apply (which is the philosophy behind the BeSS database) is that you should do all reduction that the user cannot do (eg wavelength calibration, flux calibration by correcting for instrument response) but not steps that the user could do for themselves (eg heliocentric correction, rectification, telluric removal). making sure that the appropriate flags are set in the header.
In this specific case however, If I was uploading this to a database, I would rectify it. To upload it as is including the continuum would be misleading. The reason is that the continuum shape is completely artificial having been made to fit that of a typical star of similar spectral type. For all we know this might bear no resemblance to the actual spectrum. By rectifying it, we are not losing any data, since the continuum shape is not derived from the spectrum and the user should they wish, could add back the continuum shape using their own typical B star spectrum.
Concerning measurements on rectified spectra, EW measurement of absorption lines is no problem as they are measured relative to the continuum. Measurement of emission line strengths is problematic though as the actual flux in the line is usually the important parameter rather than the EW (as in general emission and continuum flux are not related) and measuring the relative strengths of these at different wavelengths (even in relative flux) requires a non rectified accurately flux calibrated spectrum.
Robin
Hi Kevin,
I don’t know 48 Per specifically but I think what you see is typical in some Be stars. The Be star spectrum is a combination of the normal main sequence B star (dominated by broad winged Balmer lines with He also) plus a Balmer line emission component from the disc, H alpha being the strongest but also sometimes in H beta if the emission is strong. If the ssytem is seen pole on as would seem to be the case here, then there is no doppler component from the rotation of the disc so the emission is a single narrow line which is narrower than the photospheric absorption line (because of the lower gas density in the disc). When the two are superimposed, the H alpha emission can dominate, hiding the whole absorption line including the wings, but where the emission is less as here in H beta we still see the photospheric absorption line with a small emission in the central core. (The same effect is also sometimes seen at H alpha in cases where the emission is particularly weak.)
Robin
Hi Kevin,
My guiding is also bad enough to get round this problem but there is a good example of this by Tim Lester who has a superbly stable setup running at a resolution R~9000 where in this case his good guiding and sharp star image worked against him.
http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=1240
Cheers
Robin
Hi Kevin,
Yes ISIS is not very good for comparing and examining spectra. I usually go to VSpec for this but I expect BASS also does this better than ISIS.
This peak in the instrument response can be a problem to correct for accurately for even using a A or B star. Here is the shape of my ALPY instrument response as generated using ISIS (ie including a correction for the lamp black body curve)
The key is to take care to remove the residual Balmer lines and not to over smooth when generating the instrument response. You can see the hump in the problem area in my instrument response here. It is important it is reproduced accurately in the instrument response. If it is flattened out too much when generating the instrument response it generates a hump in the corrected spectrum.
Cheers
Robin
Hi Kevin,
Just noticed on your posted spectrum plot that the wavelength scale is way off for some reason
Robin
Here are some brief tests I made on the precision of RV measurements with my ALPY
http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=618
Note though these measured the precision/repeatability not absolute accuracy of wavelength calibration
Robin
Hi Kevin,
Re-reading my post this morning it is a bit rambling so I thought I would summarise my thoughts.
The repeatability of the ALPY is good enough to measure a to wavelength precision of 1.5A given good technique (eg to detect relative changes in radial velocity in a given target) but an absolute accuracy of 1.5A is probably at the limit or perhaps beyond it without an external reference.
Even 1.5A precision can be challenging as many factors come into play at the sub pixel precision level. For example you mentioned the potential skewing of line profiles due to two or more stellar components. A similar effect can occur instrumentally if for example your star is slightly offset in the slit so the measurement is made on the downward slope of the star point spread function. The resulting shift in the centroid of the line can be significant at the sub pixel level. If high wavelength precision is specifically needed with a slit spectrograph then making sure the star image is significantly larger than the slit and even deliberately dithering the star across the slit can help. (fibre fed spectrographs have the advantage here as they scramble any gradients across the fibre aperture)
BTW did you take account of the intrinsic RV of the star (19.8km/s from SIMBAD) and the heliocentric correction which might account for some of the difference ?
Cheers
Robin
Hi Kevin,
A nice result. the Ca H, K lines are showing up nicely in the violet. If you mean the broad hump around 3950A, I have seen something similar on occasion and I suspect it may be an instrument response problem. The raw instrument response from flat corrected spectra produced by the ALPY peaks sharply in this region, for my instrument at least and it is in the area where the Balmer lines are crowding together so it is difficult to fit the response curve accurately around this wavelength. ISIS attempts to mitigate this to some extent by internally applying an assumed black body spectrum for the flat lamp which tends to flatten the measured instrument response but I dont know if Demetra does anything similar. Have you tried running your reference star spectrum back through Demetra using the calculated instrument response to double check that it ends up looking like the reference?
Cheers
Robin
If it is a radial velocity effect, the wavelength error should be proportional to the wavelength ie the error should be greater at H alpha than at H gamma say. If as you say the error is roughly constant in wavelength and given that Demetra is in beta testing, I think I would double checking against a reduction done using ISIS for example in case Demetra is incorrectly measuring the relative positions of lamp and star spectrum for some reason.
Small absolute calibration offsets between internal lamp and the sky are not uncommon though, due to slight differences in the geometry of the light paths and we are talking a fraction of a pixel here so even if the error persists, it may not be anything connected with the star. (Are you seeing it consistently on any other stars?) Measurements of the solar spectrum (eg the daylight sky) can be used to quantify these sorts of instrumental offsets.
Another thing to check is instrument stability. I measured my ALPY to be very stable but changes in temperature and orientation can produce small shifts. Did you measure the lamp spectrum at the same time as the star with the telescope still aimed in that direction? Are lamp spectra reproduceable before and after the observation
Cheers
Robin
Also the ratio of H alpha/H beta emission in nebulae for example is useful for measuring the amount of interstellar extinction eg as described here
https://web.williams.edu/Astronomy/research/PN/nebulae/exercise2.php
In fact there are a number of interesting measurements you can make based on measuring the relative intensity of various lines in planetary nebulae using just a low resolution spectrograph, as described in Francois Teyssier’s low resolution spectroscopy observers guide
http://www.astronomie-amateur.fr/Documents%20Spectro/SpectroscopieBasseResolution_En.pdf
and detailed on his website here
http://www.astronomie-amateur.fr/feuilles/Spectroscopie/NGC2392.html
Robin
The H alpha emission line profile is very useful for indicating the presence of and tracking the movement of material in the neighborhood of the star such as outflowing winds, eg in P Cygni, accretion inflows eg in T Tauri, rotating discs eg in Be stars, explosions eg in novae and type II supernovae, transfer of material between stars eg in symbiotics. It is not the only useful line though. eg the narrower metal lines are better if you want to measure a star’s radial velocity and some systems don’t have hydrogen eg WR stars where the winds can be tracked using eg C III, IV lines (as in the current WR140 colliding wind campaign) or type 1a supernovae where the Si absorption line is the key one to look for. In low temperature systems you might look for low excitation lines like the 7699A K line in eps Aur which I used to track the density and velocity of the material in the cool eclipsing disc.
Robin
It all looks pretty clear to me reading “From the President” in the latest copy of Journal. It is obvious that it is just an electronic pin board for members to put up what they like, when they like ,no more, no less. The simple search facility is perhaps a bonus but in no way can this be considered a database of observations and clearly was never intended to be one. (That would be a very different beast, more along the lines of the VSS or the new spectroscopy databases). As the president says in the Journal concerning content posted on a member’s page:- “Don’t forget however that you should also send your observations to the appropriate section director to ensure they are properly logged”
Robin
Hi Tony,
Yes ISIS can generate a BeSS compliant fits spectrum. You can test your spectra at the BeSS website without actually submitting it to see if it passes. Submitted spectra are then examined by a human for quality before being included. Note that you have to sign up and supply details of your setup before submitting spectra.
The amateur Be star monitoring program was originally set up for high resolution observations to study the shape of the H alpha line profile. (The LHIRES III spectrograph was designed around this requirement as at the time there was no commercial instrument capable of sufficient resolution). If you are measuring targets which are too faint for high resolution or are rarely observed or if you pick up a significant change in any target (eg in EW or from absorption to emission or vice versa) even at low resolution, this would definitely be of interest and should be reported on ARAS/spectro-l forums and submitted to BeSS. The companion website ARAS BeAm details which stars currently need observations.
http://arasbeam.free.fr/?lang=en
see in particular “why we observe Be stars” in the side-bar
Cheers
Robin
Hi Jack,
Take a number of long exposure darks, at least as long as your longest exposure and with the camera at the same temperature. (I normally try to take 20, using cloudy nights)
Median combine them and subtract the master bias (offset) (ISIS for example can do all this for you) The result is the thermal contribution from the camera, which can be scaled to correct any length exposure
This page describes the master images needed for ISIS (In French but Google translates well enough)
http://www.astrosurf.com/buil/isis/guide_lhires/master/calib.htm
and this page (in English) describes how to generate generate them in ISIS
http://www.astrosurf.com/buil/isis/guide_lhires/tuto1_en.htm
see section 3 – spectral calibration
Cheers
Robin
Hi Kevin,
Here is the reference I think Andy is referring to.
https://britastro.org/node/8153
A MILES A star is used as a calibration reference and then to verify the quality of my observations and data reduction, other MILES stars were measured based on the calibration from the reference star and compared with what they should look like.
I use PHD 2 to guide both on the slit or offset using a field star. The later version had some features added compared with the original PHD to help with spectroscopy guiding, particularly the ability to guide on a specific XY position which can be saved and recalled and to nudge the guide position a fraction of a pixel at a time. It is not perfect when guiding on the hamburger shaped split star image though as it has a tendency to hunt. Others swear by AstroArt which has a special algorithm for guiding on the slit and is being improved in other areas specifically for this application.
http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=771
To find my targets (some of which are extremely faint when I am using the ALPY 200 – down to mag 17) I use a webcam mounted on the guidescope plus guider image to check and update if neccesary the alignment on nearby bright stars (using an EQ6 with eqmod and Cartes du Ciel) then zone in on the required field using the guider image. Astrometry. net usually solves the ALPY guider field for me if necessary but usually comparing the guider image with the projected DSS image in CdC gets me there. For me though spectroscopy was the driver that pushed me to a simple permanent setup. The setup and teardown time was just too time consuming.
http://www.threehillsobservatory.co.uk/astro/observatory/observatory.htm
Cheers
Robin
OK I now understand what ISIS is doing when it calculates the coefficients. It adds 1 to all the pixel positions before calculating the fit. You can see this in the attached trivial example where I manually entered a series of wavelengths and pixel positions where the wavelength = pixel ^2 before making a 2nd order fit. It only gave the expected coefficients when I subtracted 1 from the pixel positions.
I still have no idea why though! (For information VSpec calculates the coefficients conventionally)
Cheers
Robin
OK… The coefficients generated by ISIS using the automatic line finding calibration systems and displayed in the GO window (as used in the predefined and file calibration modes) work correctly if entered into the dispersion function accessible from the profile tab to give the correct calibration so at least ISIS is internally consistent. It is still not clear why using these coefficients in Excel for example does not produce the right results though. Perhaps the primary fit that ISIS does is connected with this. The px values in that fit do not appear to be pixel values so perhaps some resampling is done in this preliminary calibration and the final calibration coefficients are calculated relative to this generic fit? (pure speculation on my part though)
Hi David,
This anomaly does not seem to be limited to the file mode. I just checked John’s coefficients and the resulting fit posted above and they do not seem to agree either (The residuals are around 3-4 Angstrom)
I’ve no idea what is going on either but the coefficients in the GO window do not seem to be the correct ones to give a good calibration fit so presumably would not work if transferred manually to the dispersion window.
I also went back to v5.4.1 and this does not transfer the coefficients to the dispersion window automatically when using the ALPY 600 balmer line mode either
All very strange
Robin
Correction. I just tested this and ISIS (v5.7) does not appear to transfer the calibration fit coefficients to the dispersion window. Perhaps this is a bug introduced in a later version. You can still calibrate other spectra using the Balmer lines in the calibration image as explained at the end of the tutorial part 1 but if you want to use the predefined coefficients described in part 2, I think you would need to transfer them manually.
BTW the translation in some parts is not so good because Christian added some sections later which he translated
Cheers
Robin
