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Background subtraction errors are also a common souce of instrument response problems in slitless spectroscopy and are worth looking out for. These tend to be most pronounced at the blue end where the instrument response drops off sharply so small zero errors have a large effect.
Robin
Yes flux calibration of meteor spectra, even in relative terms is going to be tough. The instrument response can be corrected for using a reference star and provided atmospheric conditions are stable, even the atmospheric extinction con be accounted for by taking reference stars at different elevations and applying a correction based on the elevation of the meteor (The usual simpler technique of chosing a reference close in elevation to the target to cancel atmospheric extinction is problematic here of course !) A big problem though is likely to be flat field effects which can be severe with these sorts of wide field systems. A normal flat field correction does not work with slitless spectrographs. The usual workround is to place the reference star at the same position in the field thus cancelling gross flat field errors but again this is tricky here, particularly if the spectrum is generated by integrating along the track. Measurements of reference stars at the apropriate locations in the field the night after the meteor observations could perhaps be used though. Measuring standard star spectra with the star positioned at different points in the field and comparing them would give an idea of the severity of the flat field problem.
Cheers
Robin
Looking for auroral lines could be particularly interesting as you could use higher resolution to enhance the contrast between any continuum spectrum background. (like imaging with narrowband filters) You would still have to watch out for auroral lines from our own atmosphere and subtract them of course.
Cheers
Robin
Hi Andy,
Hmm.. getting an even enough flat might take some arranging. I think you would need to go back to a flat light source in front of the telescope rather than rely on the internal flat lamp, to take into accout any vignetting before the spectrograph. You could perhaps test it though by taking a daylight sky spectrum which over the small field should give the same counts independent of position along the slit, once flat corrected.
Robin
With a long slit orientated appropriatelyand a suitable image scale you might be able to get all you need in one shot (Well 2, one exposed for the bright side) You would need a flat field illumination source which is nice and even in the slit direction so the subtractions work accurately.
Funnily I was just reading Paul Abel’s beginners article on Earthshine and musing about measuring that spectroscopically, perhaps mounting the spectrograph behind a camera lens to get a sensible image scale to match the slit. Much easier than Ashen Light though !
Robin
I have been runnng with my “ALPY 200” low resolution faint object spectrograph configuration again over the past few weeks and have been able to work my way down about a mag fainter to around 17.5.
http://www.threehillsobservatory.co.uk/astro/spectra_46.htm
I recorded the spectra of these two supernovae (asassn-15fa and asassn_15fz) a few weeks after discovery so they have evolved significantly compared the original spectra taken soon after they were discovered. The match with spectra of supernovae at a similar age is still clear though.
The supernova contribution is already much lower than the sky background which is significantly limiting the signal/noise. I am not sure if more aperture or darker skies will be needed to go much lower, probably a combination of both.
Robin
Hi Stan,
A couple of other safety tips.
Keep the front cap on the finder scope or make another filter for it from the offcuts of solar film
Be sure to put the filters on before aiming at the sun and remember to swing away from the sun before taking them off. (Seems obvious I know but I forgot once when packing up some time after an eclipse that the mount was still faithfully tracking the sun and within a couple of seconds of removing the filter the eyepeice cap had a nice neat pinhole melted through it! I keep it as reminder as to why you should never look at the sun through an unfiltered instrument.)
Robin
Hi Nick,
There is currently no centralised international database but the object specific database BeSS for professional and amateur spectra of Be stars makes an excellent model for such a general database.
http://basebe.obspm.fr/basebe/
This is a professionally funded high quality system with moderation of submitted data and a mentoring service for amateur contributors run by pros and amateurs. There were thoughts to expand it but it would need funding. I have been talking with AAVSO for many years about this (including advising several of the various working parties there have been over the years) and the people behind BeSS have offered the complete system to AAVSO to use but they appear to want to go their own way against the advice of other experienced amateurs in this area and I have currently given up on them.
In the meantime the most complete source of amateur spectroscopic data is the ARAS organisation who have been archiving spectra submitted to them on a range of objects
http://www.astrosurf.com/aras/
(although unofficial, ARAS is where most serious amateur spectroscopists can be found and they are currently the main force driving up standards in this area)
I plan to talk more about how ARAS is functioning at the upcoming VSS meeting using Nova Del 2013 as an example
Cheers
Robin
