Anomolous absorption line?

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  • #574900
    Kevin Gurney
    Participant

    I have been capturing the HeI line at 6678A for Gamm Cas and have several results which look ‘normal’ (there is a dynamic of course…). All with Lhires 2400l/mm.

    On the 22nd however, I captured the profiles shown in the attachment: Gam cas itself, and a reference star (HD3360). Both have activity at the designated wavelength but, this time, there is also a ‘doublet’ just to the right of the main feature.

    I have never seen this before…. I was wondering if it might be ‘dust’ or something somewhere on the light path but the profile does look like a real absorption line. However, I do struggle to imagine the light path from slit to camera sensor, so I couldnt evalaute that very well.

    I also thought it might be some kind of atmospheric absorption, but I can’t see any O2 or H20 line near there? Looking at the online atomic line lists, There are some (truly!) metallic lines there, so I the thought it might be some more exotic, and transient, atmospheric absorption. Sheffield used to be heavily industrial and, while the bulk of it has gone, there are remnants of this left – who know what awful process they get up to occasionally at ForgeMasters down the road 🙂

    Any thoughts?

    Kevin

    #583886
    Robin Leadbeater
    Participant

    Hi Kevin,

    I suggest checking your flat image in this area. This double line effect looks typical of that produced by a dust donut. Take a profile across the flat at the location of the spectrum.  I was discussing with Jack Martin the other day who saw something similar (at a different location). It turned out to be a defect visible in  the flat which for some reason we don’t quite understand was not corrected by the flat (The defect was weaker in the flat than in the image)  

    Cheers

    Robin

    #583888
    Jack Martin
    Participant

    Kevin,

    Yes I did see something similar.

    I use Atik 460 mono.

    What camera are you using ?

    Jack

    #583892
    Kevin Gurney
    Participant

    Hi Robin

    Thanks for tip… I’ll have a look later.

    Its clear outside so running another session. Repeating the gam cas HeI…

    I’ll let you know if it spontaneously dispapears – or persists

    [BTW I am using a 460EX too]

    #583894
    Kevin Gurney
    Participant

    so looking at one sub just now – its gone!

    I looked back at the flats from last time and no obvious dust… I also did a ‘binning’ at the same level as the spectrum and no obvious defect in the resulting plot

    Kevin

    #583897
    Robin Leadbeater
    Participant

    Then I am stumped I am afraid. If it pops up again you could perhaps try moving the micrometer a touch to see if it moves with the spectrum and/or move the spectrum along the slit a bit to see if it is a localised defect

    Robin

    #583898
    Kevin Gurney
    Participant

    Well, it wasn’t in the Ha I took immediatley after…

    But good idea to try small shift next time (which might be a while as I have switched to Alpy for a bit 🙂

    Cheers

    Kevin

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