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There has been some loss of flux on the blue edge too
I’ve made a quick plot of EW. It looks like the drop started in January and is continuing
Hi Hugh,
The high resolution H alpha shows that the red edge of the line has been steadily dropping over the year (most obvious during the summer in close season) but the blue component has stayed more or less constant which confims the drop in intensity you are seeing. I contacted Paul about it in August when I noticed the change over the summer but your post has reminded me that I did not get a reply so I had better try again. Something is obviously happening but I have no idea what !
Robin
It is a bit difficult to tell from the scaled stretched image posted but with the configuration (35mm fl lens, 500 l/mm grating, 11.5mm wide sensor, rescaled to 640 pixels, the linear dispersion will be ~10.3A/pixel in the posted image (small angle approximation) The weak lines at the right hand edge are therefore likely to be the second order image of the group at the left hand edge. With this information we can estimate the location of the zero order and produce a wavelength calibrated spectrum.The calculated calibration is also in agreement with the quoted approx 3600-6500A response
(The small angle approximation will not hold precisely at the dispersion angles from a 500l/mm grating but the method could be finessed using angular dispersion in place of linear dispersion to produce an accurate wavelength calibration)
Cheers
Robin
I don’t know if you can apply it here but sometimes if your zero order is outside the field and your spectrum is reasonably linear (low dispersion angle) you can estimate the location of the origin (zero order) using common features in the first and second order spectra. (The distance from the zero order to a feature in the 1st order ~ the distance between the same feature in the two orders)
Cheers
Robin
from Christian Buil
http://www.astrosurf.com/buil/comet/lovejoy/poster_lovejoy_20131126.png
Yep assuming that blue is on the right, that central band looks just like a molecular band with a steep band head on the red side tapering down towards the blue. Like C2 Swan bands in comets for example. (hydrocarbons from left over fuel?) If you are using coated lenses then the cut off will be around 3650A. Do you have a best guess wavelength calibrated spectrum to share?
Hi Andy,
With completely unfiltered CCD measurements (ie without cutting the IR) I vaguely remember something about CR (ie using R comparison star magnitudes) correlating better because of the extended response. Do you know if this is the case ?
Cheers
Robin
I see the press are at it again 🙁
Asteroid alert: NASA warning as kilometre long space rock set to skim Earth at 25,000mph
Here are the instructions given by Christian Buil for generating a file for use in file mode (method 3)
http://www.astrosurf.com/buil/isis/tutorial/calibration_us.htm
Robin
Though it does not mean I am right !. “Solar eclipses” appear to be a special case and should really be called transits or occultations
For me an eclipse involves observing the shadow in the light from a luminous body cast by one body on another body eg as observed from earth between Jupiter and its moons. The observer may be on the body which casts the shadow eg a lunar eclipse. A “solar eclipse” (total or partial) as observed on earth is an occultation or transit and is only an eclipse when observed from a location not on the earth eg from the moon or the ISS when even a “total eclipse” appears partial
Robin
EDIT: I suppose you could call observing the shadow of the moon racing across the landscape towards you or even noting the darkness of the ground at your feet during a “Total Solar Eclipse” observing an eclipse but for me the sight of the moon in front of the sun is definitely an occultation
I use the predefined mode with my ALPY but I use the file mode quite a bit with my LHIRES for cases where there is not a preconfigured setup. As well as the list of lines, the file contains the order of the fit you want and the approximate linear dispersion needed to approximately locate the lines so it is specific to a particular spectrograph and camera combination. You can generate your own file by doing a manual linear calibration first using the set (or subset) of the lines and noting the linear coefficient. (or you could try the linear coefficent from the fit generated using the predefined mode)This is then added to the list of line wavelengths along with the order of fit you want. (Note that it can fail if the dispersion is sufficiently non linear and the lines so closely spaced that the wrong line is picked. To avoid this, I suspect the predefined mode for the ALPY may allow for the non linearity of the instrument which the file mode does not allowing some more close spaced lines at the blue end to be used.) A typical file for the LHIRES looks like this for example
2
0.2461
6532.882
6598.953
6717.043
6752.834
where 2 is the order of the fit and 0.2461 is the linear dispersion (A/pixel)
(Ignore the doublespacing generated by the forum software)
Cheers
Robin
A related question – Are there any guider programs which will track moving objects relative to an offset guide star, allowing long exposures ? (I was issued with a challenge to take a spectrum of Borisov which I declined but it might be interesting to know how it might be done for future reference)
Robin
Hi Kevin,
There are a couple of people on the AAVSO forum looking at SS Cyg using Star Analysers with some success. I have been pointing them to yours (and others), spectra in the BAA database 🙂
Cheers
Robin
It does look a bit like that but no, it is just the result of normalising each row of a spectrum which is slightly tilted. Here is the whole “corrected” flat. The vertical gain correction algorithm only works correctly if the dispersion is exactly along the row direction.
This is a severe obvious case but some variations along the slit are much more subtle. For example these in a highly stretched LHIRES flat image from last night are down at the 1% level and are in the region where I take spectra. They are not anything to be too concerned about as an uncorrected flat should work ok to divide them out but not if they are tilted and I apply the vertical gain correction
Hi Andrew, David
I have some reservations. The artifact from the dust line is worrying (There is a technical name for these isn’t there?). They wont be a problem in the sky background subtraction as they will average out but if they occur where the spectrum is binned they could distort the spectrum or at least give the wrong total flux. In fact a conventional flat would work correctly in this region, correcting the flux for the loss of light but the gain corrected flat would give the wrong flux even if the dispersion was exactly horizontal
Cheers
Robin
Here is the comparison with and without vertical gain correction.(The step on the right hand edge is outside the corrected area)
It flattens the profile as expected but note the artifact where the dust on the slit was, caused by the dispersion direction not being exactly aligned with the rows (tilt). This is an extreme example of course but there must always be some small errors across the whole flat if there is any tilt. (Note when measuring these column profiles accurately any slant/smile should be removed otherwise this can distort the profile measured in a vertical slice)
The profiles were produced using a rather obscure French program called Teleauto which I found when I started doing some photometry. (I forget the name of the author now)
http://www.teleauto.org/indexEn.php
It is obsolete now (only handles 16 bit images and struggles to run properly on win7) but it has some cool unusual features, particularly for its day (~15 years ago)
Cheers
Robin
OK, for my setup the profile along the slit for sky flats is flat within ~+-2% but the internal lamp flat drops off by ~20% over the bottom third, presumably due to uneven lamp illumination, so unless this can be adjusted out somehow, it looks like the internal lamp flat would better match the true situation if the vertical gain tool is used. (The narrow dip on the left (bottom) edge is dirt on the slit)
Hi Andrew,
I think the algorithm just rescales each row the to the same mean level (on the assumption that the variation in mean from row to row is due to uneven illumination from the lamp or possibly variation in slit width) rather than averaging the columns so the pixel-pixel variation should still be there.
