Nova Sgr 2015 No. 2 low-res spectrum, 18 Mar 2015 UT
The other morning I managed to image, take a spectrum and get a naked-eye view of this bright new nova. The spectrum is nothing to write home about but does show the strong P Cygni profiles. In the cropped image the nova is the bright star at centre.
Low-res spectrum taken last night shows significant changes from the one on the 18th, with the emission lines flattening out markedly. There may be a slight exaggeration of the effect in the attached montage because the spectrum I obtained was slightly smeared (not a tracking or focus issue, something with the grating which is a worry).
The nova was much brighter visually last night than when I saw it on the 18th, an easy naked eye.
Hi Ken - what I got last night were 'squiggles' for stars. Not focus or they would have been blobs. Not tracking because I did two sets of spectrum subs with lots of other ordinary images before, between & after and tracking was good. The squiggles were exactly the same size and shape in every spectrum frame regardless of exposure time, albeit a bit brighter in the longer set of subs.
Disconcerting really, I've had odd shapes generated before but nothing like this in elongation (the last thing you need!). Hope the grating's not breaking down and that it's some issue like a particular condensation on it or something...
Rob,
Hmmm don't know....
If the seeing was reasonably consistent in the other images, then somehow it could be related to the grating.
Dew? Finger print??
What I would try is a couple of exposures with different orientation of the grating to the horizontal DSLR frame - see it there's any difference in the zero image or spectrum....
Thanks for the offer Ken. Whatever the problem is, it lies in the grating unit. It could be a weird effect of some pattern of condensation that I haven't come across before (unlikely) but I'm more worried that something may have got between the protective glass plate covers and the blazed grating. It looks OK to the eye though but that probably doesn't mean much. The last spectrum was slightly less extended after I had very carefully & lightly cleaned the glass plate covers (for the first time ever!).
If it continues I think the best solution might be to get a new grating. I've had this one about 6 years so that's a pretty cheap annual rate for the tremendous enjoyment and interest I've had out of it! And it's not as if the unit has had an easy life. I'm not one to cosset and meticulously maintain gear (insert rough-as-guts smilie here)...
Rob,
If it would help I can test the grating for you. I have the equipment and could determine (hopefully) if there's a real problem.
Let me know.
ken
Thanks Ken but problem solved! I was right, it was the grating "unit" but not the grating thank goodness. Last night I got the same extended stars but then noticed that the little rolled-up light card tube I use as a dew protector had a loose corner that curled out on the inside of the tube. I rotated the tube 90-deg and I had good stars again. So a simple bit of glue will fix it, phew!
Just an update - the nova was naked-eye in Moon-free skies last night (12 Apr 2015) as it rises towards a third maximum. Here's a spectrum and an image I obtained. In the image the nova is the bright star with the diffraction spikes. At this zoom in the lens used, there are normally no spikes but last night I was experimenting with cotton crosshairs in front of the lens.
Some recent low-res spectra obtained on this nova. By the AAVSO light curve, it's rising to a fourth maximum, currently nudging towards 4.5. On 22 April it was a fairly easy naked eye again in dark skies.