25 February 2021 at 10:26 am #574900
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 🙂
Attachments:25 February 2021 at 12:32 pm #583886Robin LeadbeaterParticipant
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)
Robin25 February 2021 at 2:23 pm #583888Mr Jack MartinParticipant
Yes I did see something similar.
I use Atik 460 mono.
What camera are you using ?
Jack25 February 2021 at 7:06 pm #583892
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]25 February 2021 at 7:23 pm #583894
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
Kevin26 February 2021 at 11:05 pm #583897Robin LeadbeaterParticipant
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
Robin27 February 2021 at 1:41 pm #583898
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 🙂
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