Robin Leadbeater

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  • in reply to: Nova Vul 2021 #584481
    Robin Leadbeater
    Participant

    Spectra from the past two nights. (The first predates the one in the ATel by 4 hours) I have marked the main lines but there are a few currently unidentified. Suggestions welcome

    in reply to: Nova Vul 2021 #584475
    Robin Leadbeater
    Participant

    A Low resolution (R~500) spectrum taken just now (20210716.915) shows strong H Balmer P Cygni profile emission lines with a velocity of ~1400km/s and other broad emission lines including He. (Looks like it is a Nova) 

    https://britastro.org/specdb/data_graph.php?obs_id=10094

    Continuing to take spectra.

    Robin

     

     

    in reply to: Mirror re-coating #584461
    Robin Leadbeater
    Participant

    Some more examples

    http://www.astrosurf.com/buil/forum18/_transmission_TELESCOPE.png

    From a thread entitled ” A la recherche des photons perdus”

    https://www.spectro-aras.com/forum/viewtopic.php?f=45&t=2277

    in reply to: Mirror re-coating #584460
    Robin Leadbeater
    Participant

    There have indeed been various discussions on the ARAS spectroscopy forum about some modern coatings (on mirrors, lenses and camera windows) which while very efficient in the visible, drop off rapidly in the UV, for example this plot for the GSO RC telescope which effectively cuts off steeply at 4000A

    https://www.spectro-aras.com/forum/viewtopic.php?f=45&t=2398&p=13202#p13470

    This has been a big issue for the UVEX spectrograph, summed up by  Christian Buil in the same thread

    “What always worries me is to see reflectivity of 99%, via dielectric treatments. These things are really our enemies in spectrography when we want to go to slightly extreme wavelengths. Besides that 99%, it is only for a narrow area of ​​the spectrum – it is misleading. Long live the coating with a single protective layer, even if we are at 96% reflection in the visible.”
    in reply to: Help for a newbie? #584428
    Robin Leadbeater
    Participant

    The effect of an off axis stop is an interesting one. The star image size will remain the same  (assuming seeing limited) so the maximum theoretical resolution is unchanged. The defocussing effect of field curvature should be less due to the increased depth of field with the higher focal ratio and the chromatic coma should also be less with the less steeply convergent beam. I did suggest this to someone recently though in a similar situation and they reported no obvious improvement.  Best approach I would say is to see what you can get on the existing setup and then try a stopped down aperture. (What spacing do you have currently?)

    in reply to: Help for a newbie? #584426
    Robin Leadbeater
    Participant

    If you are limited  in how far away you can mount the grating then the SA200 can give you the dispersion you need for useful resolution but it does not quite match the SA100 at double the distance. The field curvature is also more severe and you can see this in your spectrum as it widens towards the red end. Getting the best result is a compromise with what is a primitive setup but think of it as good inexpensive way to “cut your teeth” before upgrading to a “proper” spectrograph if the bug bites

    in reply to: Help for a newbie? #584424
    Robin Leadbeater
    Participant

    Sorry for any typos, I am on holiday and using a portable device. Normal service should be resumed next week.

    in reply to: Help for a newbie? #584422
    Robin Leadbeater
    Participant

    Yes it is standard practise to align and stack shorter exposures with slitless spectra (on the zero order image) as it avoids guiding, effectively using the same techniques as planetary imagers to select the sharpest images (but no further processing eg sharpening etc) modern CMOS also means the noise penalty is lower than with CCD 

    Cheers

    Robin

    in reply to: Help for a newbie? #584421
    Robin Leadbeater
    Participant

    As a rough guide to what setups work I developed a simple calculator which is hosted on the Rspec website. (The Rspec software there is good for beginners using the Star analyser but you will  outgrow it  as you progress to more research based spectroscopy)

    https://www.rspec-astro.com/calculator

    There is a tutorial on the Star Analyser on the site here

    https://www.britastro.org/downloads/15701

    and if you are interested in its development, I talk about it here

    https://britastro.org/video/11250/12234

    Cheers

    Robin

    in reply to: Help for a newbie? #584420
    Robin Leadbeater
    Participant

    You ar in the right place, I developed the Star Analyser

    P Cygni is an excellent test target  It shows nice clear emission lines of H and He, ideal to focus on (focus on the features in the spectrum, not on the zero order star image, noting that the field curvature, particularly with long spectra means that the focus will vary along the spectrum to some extent

    http://www.threehillsobservatory.co.uk/astro/spectroscopy_16.htm#SA200_filter_wheel

    16 inch is a big aperture for a slitless converging beam setup though unless the seeing is exceptional. The resolution is at best defined by the size of the star image relative to the length of the spectrum. The problem is as you increase the length of the spectrum the aberrations of the simple setup get worse and if you try to use a lower focal ratio to keep the star image small, the aberrations also get worse due to the steeply converging beam so these setups work best on modest apertures. give it a go though and I can help steer you to get the best out of  the setup

    don’t worry too much about software at this stage, concentrate on getting a clear image. It is a low resolution “broad brush” type of device and most of the interesting features the Star Analyser are immediately visible. This is an example of the sort of features you should be able to pick up. 

    https://britastro.org/observations/observation.php?id=20201216_234948_8cabda965bfe692f

    Cheers

    Robin

    in reply to: Nova in Hercules #584369
    Robin Leadbeater
    Participant

    A nice collection of spectra building up here in the Spectroscopy Database showing the evolution 

    The shift in the H alpha line from predominantly blue to red while still centred on the rest wavelength is curious. I wonder what that means physically

    Cheers

    Robin

    in reply to: Nova in Hercules #584353
    Robin Leadbeater
    Participant

    Prof Steve Shore comments on the evolution of Nova Del 2013 based on amateur spectra here. Not simple though !

    http://www.astrosurf.com/aras/novae/Nova2013Del-1.html

    Cheers

    robin

    in reply to: Nova in Hercules #584349
    Robin Leadbeater
    Participant

    Very broad P Cygni H and He by the looks. Almost overlapping with the high velocity. Almost a supernova like spectrum

    in reply to: Nova in Hercules #584340
    Robin Leadbeater
    Participant

    Intriguingly a low resolution Star Analyser spectrum by Rob Kaufman in Australia earlier today failed to show any significant emission

    https://hosting.photobucket.com/images/ww271/Rob_Kau/TCP_J18573095_1653396_spectrum_12_June_2021_text.jpg

    After clear blue skies all afternoon the clouds have rolled in here but no doubt we will have better spectra tonight

    in reply to: Solar rotation from Doppler measurements #584332
    Robin Leadbeater
    Participant

    While watching the recent partial eclipse in largely cloudy skies I wondered if it might be possible to follow an eclipse even in cloudy conditions in spectra of the sky. The shape of the spectrum line profiles should alter slightly as different parts of the rotating solar disc are covered. (Similar to the Rossiter-McLaughlin effect see in the spectrum of eclipsing binaries)  Does anyone know if this type of observation has ever been attempted ?

    Cheers

    Robin

    in reply to: 2021 June 10 partial eclipse livestream #584315
    Robin Leadbeater
    Participant

    Against all odds there is actually intermittent sun here 

    in reply to: Spectrum of Nova Cas 2021 #584278
    Robin Leadbeater
    Participant

    Here are 6 spectra around that date from the BAA spectroscopy database (From David Boyd (2), Kevin Gurney, Hugh Allen, John Coffin and me)

    https://britastro.org/specdb/data_graph.php?obs_id=9120%2C9031%2C9159%2C9035%2C9118%2C9039&multi=yes&mix_flux=yes&plot=Plot+up+to+20+Spectra

    They all line up in wavelength precisely

    Cheers

    Robin

    in reply to: Spectrum of Nova Cas 2021 #584277
    Robin Leadbeater
    Participant

    In fact I see further down the thread Hugh actually wavelength calibrated  Mike’s prism spectrum to produce a graph of the spectrum with a linear wavelength axis.

    https://britastro.org/comment/10463#comment-10463

    You can see there that the wavelengths then agree in Mike’s Hugh’s and my spectra (and indeed with those by David Boyd and John Coffin in the same thread)

    in reply to: Spectrum of Nova Cas 2021 #584276
    Robin Leadbeater
    Participant

    I just checked mine and Hugh’s spectra from 23rd March in the Nova Cas 2021 thread. The graphed spectra are calibrated in wavelength with a linear X axis and the Balmer (and other line) wavelengths are correct and consistent between the two spectra

    https://britastro.org/comment/10272#comment-10272

    https://britastro.org/comment/10277#comment-10277

    Mike’s spectrum image is not calibrated in wavelength.

    https://britastro.org/comment/10242#comment-10242

    Prism spectra are very non linear with the short wavelength end significantly stretched and the longer wavelengths compressed

    in reply to: Spectrum of Nova Cas 2021 #584275
    Robin Leadbeater
    Participant

    All spectrographs are non linear to at least some degree and have to be individually calibrated in wavelength. Mike’s spectrograph is fundamentally different to mine for example. His uses a prism and the dispersion is due to refraction whereas mine (and Hugh’s) uses a diffraction grating and the dispersion is due to diffraction. As a result the relationship between wavelength and distance along the spectrum are completely different and in fact run in opposite directions !  (with diffraction the short wavelengths are least deflected whereas in refraction the short wavelengths are refracted the most)

    Cheers

    Robin

Viewing 20 posts - 341 through 360 (of 1,188 total)