Robin Leadbeater

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Viewing 18 posts - 1,141 through 1,158 (of 1,158 total)
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  • in reply to: Alt-Az mount for DSLR photometry? #577226
    Robin Leadbeater
    Participant

    Hi James,

     I would recommend checking out the tracking capability of the the  Alt Az mount you are considering.  This is much more demanding for Alt Az mounts as, unlike equatorial mounts where all that is needed is a single axis running at siderial rate, both axes have to run simultaneously at varying speeds under computer control. For low cost mounts you may find this rather than field rotation limits the useful exposure time.  I have a vague memory of  a low cost Alt Az mount (Celestron I think) which could be run in equatorial mode using a wedge which might be a better solution but this would depend on the mount drive software being able to cope with this configuration.

    Cheers

    Robin

    in reply to: BAA Spectroscopic Observations Database #577211
    Robin Leadbeater
    Participant

    A bit detailed for a forum discussion but ideally could the database fields be read from the fits header? That way the need to enter all the info for each spectrum, with possible transcribing errors could be avoided and any search could then be done based on header information (eg with this you could potentially search for say “all spectra of a particular object covering H alpha at a resolution > 5000 and an SNR > 100” )

    re the display of spectra, I wonder if any of the code used this VdS spectroscopy group database might be useful. I have contacts there if interested

    http://spektroskopie.fg-vds.de/index_e.htm

    (follow the link on the side to “database”)

    Robin

    in reply to: BAA Spectroscopic Observations Database #577209
    Robin Leadbeater
    Participant

    Since in principle it is possible to put any data into a fits file there is no fundamental reason why any spectrum profile could not use the fits format. The big advantage is that all the observation and reduction details can then be contained within the fits header provided apropriate keywords are included. The BeSS standard is a good starting point of course but perhaps the header could be generalised and extended to include further useful keywords/flags etc, for example to indicate things like the type of object, level of data reduction etc.   Provided the keywords required by BeSS were still present, compatibility with BeSS (or potentially any other database which used a subset of the included keywords) could be maintained (Note also that the BesS database system actually rewrites parts of the the header, ignoring some keywords and filling in others based on the information in the submitted header ie  the BeSS input standard header is slightly different from what is held in the database)  The “BeSS standard header” generated by ISIS software for example has already been modified in this way with added keywords for the observatory coordinates for example to make it suitable for the simple but more general  database (really just lists of spectra currently) already being developed  by ARAS. 

    Robin

    in reply to: Fireball Spectroscopy #577115
    Robin Leadbeater
    Participant

    Hmm, one of the images does not seem to want to be displayed for some reason – trying again

    in reply to: Fireball Spectroscopy #577114
    Robin Leadbeater
    Participant

    Nice catch Bill !

    I am wondering if your fireball spectrum is actually of the cooling afterglow after the remnant had passed on rather than the fireball at maximum, which would explain the change in the spectrum and the lack of OI/NI lines in the IR. If you look at the previous frames in the video where the main flaring took place, most of the fireball spectrum is saturated but the OI lines can be seen carrying  on straight through (see attached).   This would be analagous to this fireball spectrum published earlier this year (recorded in 2012) for example

     http://arxiv.org/ftp/arxiv/papers/1503/1503.06400.pdf

    figs 2 and 3

    Cheers

    Robin

    in reply to: Mizar & eps Auriga comparison! #577078
    Robin Leadbeater
    Participant

    Hi Steve, Andy,

    Provided you have good lines to work with it is fairly straightforward to measure radial velocities to a precision 1/10 of the resolution or better. (As I plan to show at the workshop in a couple of weeks) so an ALPY with say 10A resolution could measure a 1A shift or 50km/s at 6000A as a reasonable target.

    You can go much, much more precise than this though if you use an off telescope fibre fed spectrograph and correlation techniques as is done for exoplanets. eg Buil measured to 50m/s (yes metres!) using a 0.6A resolution echelle spectrograph which I think works out at   ~1/500 of the resolution

    Cheers

    Robin

    in reply to: Polaris #577030
    Robin Leadbeater
    Participant

    Just checked Cartes du Ciel (software similar to Stellarium). It reports for Polaris as of today

    Apparent RA: 02h52m45.576s DE:+89°19’29.12″
    Mean of the date RA: 02h51m08.460s DE:+89°19’50.32″
    Astrometric J2000 RA: 02h31m52.656s DE:+89°15’50.60″

    Robin

    in reply to: Symbiotic ag Peg! #577007
    Robin Leadbeater
    Participant

    As it happens – Francois Teyssier has just updated his reference star finder for low resolution spectroscopy

    http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=1227

    It finds the potential reference stars nearest in altitude to your target and now will even plot a trajectory so you can see how the relative altitude changes with time

    It now includes all the MILES A and B stars (which have professionally measured spectra) as well as a long list of bright main sequence A and B stars with low interstellar extinction and published spectral classification. These should have spectra similar the  generic (eg Pickles) spectra for the particular spectral type but they do not have published spectra so you need to be a little careful using these, perhaps at some point comparing them with a MILES standard for example to confirm they are typical of their type.

    I think measuring a few MILES stars of different spectral types is good practice if you are starting out or testing new equipment as any deviations between the measurement and  the database version can be seen and followed up to improve one’s technique. There are some examples here where I did this using the Star Analyser and ALPY  

    http://www.threehillsobservatory.co.uk/astro/spectroscopy_21.htm

    I find getting the blue end below ~4000A perfect particularly difficult for a number of reasons:-

    the sensitivity of the instrument drops off sharply 

    any errors due to atmospheric extinction are greater

    any problems from selective sampling of particular wavelengths by the slit due to for example atmospheric dispersion or chromatic aberrations in the optics are greater

     it is difficult getting enough signal from the (halogen) flat lamp to keep the noise level down

     the increased crowding of the Balmer lines towards the Balmer jump makes it more difficult to get a perfect match between measured and reference spectra.

    Cheers

    Robin

    in reply to: More meteor spectroscopy #576974
    Robin Leadbeater
    Participant

    This document showing how a range of slitless (Star Analyser) spectra were corrected for instrument response using a MILES standard A0v star might also be of interest

    http://www.threehillsobservatory.co.uk/astro/SA100_miles_instrument_response.pdf

    from this page

    http://www.threehillsobservatory.co.uk/astro/spectroscopy_21.htm

    The results are pretty good, though some small errors can be seen at the blue end due to atmospheric extinction which was only partially corrected for. Using matched reference stars at the same elevation as each target star rather than a common reference would have improved this. 

    Robin

    in reply to: More meteor spectroscopy #576973
    Robin Leadbeater
    Participant

    Hi Bill,

    You wrote..

    > In the basic library with Visual Spec the spectrum of Vega, for example has a sharp cut off then some oscillating components. 

    There may be something wrong here.  The 5A/pixel Pickles A0v spectrum in the Vspec library (file a0v.dat) shows a smooth decline in the continuum at the blue end as the Balmer lines merge towards the Balmer jump and a relatively flat continuum beyond that.

     You can see it here for example where I use it to calculate the instrument response of my star analyser setup

    http://www.threehillsobservatory.co.uk/astro/Spectroscopy_BAA_VSS_workshop_2008.pdf

    in slide 38

    Are you using a different spectrum at very high resolution perhaps ?

    Note that correcting just using the CCD (or photdiode) response will not give a good result as the response of the grating (and the atmosphere) is also significant here

    Robin

    in reply to: More meteor spectroscopy #576972
    Robin Leadbeater
    Participant

    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

    in reply to: More meteor spectroscopy #576970
    Robin Leadbeater
    Participant

    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

    in reply to: Ashen Light and Spectroscopy #576964
    Robin Leadbeater
    Participant

    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

    in reply to: Ashen Light and Spectroscopy #576962
    Robin Leadbeater
    Participant

    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

    in reply to: Ashen Light and Spectroscopy #576960
    Robin Leadbeater
    Participant

    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

    in reply to: spectrum of sn2014AS (was PSN J14005449+4058596) #576859
    Robin Leadbeater
    Participant

    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

    in reply to: filters #576796
    Robin Leadbeater
    Participant

     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

    in reply to: spectrum of sn2014AS (was PSN J14005449+4058596) #576550
    Robin Leadbeater
    Participant

    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

     

Viewing 18 posts - 1,141 through 1,158 (of 1,158 total)