This comet is currently (and easily!) the brightest comet in our skies at the moment. It is nearing naked-eye brightness and is really well placed for southern observers. I've obtained a few rough low-res spectra of it recently as it came within range of my gear and moved out of busy Milky Way fields, by far the best result a spectrum I took last night, 12 Dec 2014 (attached).
It would be interesting to see some high-res spectra taken with a spectroscope & slit to see if there are any narrow metallic emissions and to characterise the large coma. It has a long faint ion tail too but getting a spectrum across that might be too difficult!
that looks like fun, rob.
I have a Star analyzer, i was going to put in the DMK51 nose piece and try for the comet again using the EON 80mm. But how have you mounted it into the Canon? A picture if you could.... 8)
Cheers Al - your wish is my command! It's mounted in an old lens cap. I cut a small piece of thread off a bodgy eyepiece and glued that in a hole cut in the cap - that way I can use any filter with a 1.25" thread. I now wrap a short tube of black cardboard around the grating as a dew cap.
The photo is an old one showing previous Canon camera and it used to be mounted on an oversize tube ring around a homemade short fl refractor which was hand-guided with slo-mo knob on an EQ1. Now I'm lazy, long live the Vixen Polarie!!
ooh.. found out that I can mount the start analyzer close to the 450d using a t-adapter tube. The adapter is the front section from a orion starshoot guider i don't use anyway.. . Now for the hard part... rgds, alan
Good stuff Al! First, you need to stack on the comet head, not the star. Your spectrum as posted is smeared, and at an angle. The AVI has gif dithering and is not really that useful. I'd have a go if you could get original frames to me (send a pm if you don't still have my email address). In the meantime I screen captured several frames from the AVI and produced the attached. We can do better!!
Is there a similar grating with more lines same mounting , not to much more in price??
or it a slit type ( cost?)...
rgds, alan
Star Analyser has released the SA200, double the lines per inch. But Robin or Ken would be much better placed to answer your question, hope they chime in!
Should the spectra be always horizontal to the camera chip?
As, the comet is moving at usually an angle to stars, should the SA100 be rotated to make the spectra align at 90 degrees to movement angle?
or, perhaps aligned so that it appears ahead of the moving object?
I definitely need longer , exposure seconds, but will that make spectra banding more visible? I am expecting that only broader features are achievable.
If you want to make the spectrum longer you can increase the spacing of the grating from the camera sensor. (There's no need to go to a higher dispersion grating) In this case though the coma looks quite star like and it looks like the limitation is the contrast of the spectrum against the sky background so making the spectrum longer will only make things worse. (The main features in comet spectra tend to be broad molecular bands rather than sharp lines anyway so you do not need high resolution to bring these out.)
Just as in comet imaging, Aligning and stacking as many long exposure images as possible is the key, picking exposures where the comet is clear of other bright stars. If the comet image is trailed in your individual long exposure images then rotating the camera plus grating so the movement is 90 deg to the spectrum direction will stop the movement blurring the spectrum. Note that the best focus setting for the spectrum is not the same as for the zero order image so you need to focus on the spectrum. This is very difficult for a comet spectrum where the features are naturally broad so to get best focus, first set the focus up on the Hydrogen lines in a bright A type star before moving to the comet.
A slit spectrograph has the advantage of being able to isolate a slice of the coma, useful to get a good spectrum when it is not so condensed and a slit also keeps the sky background out, increasing contrast.
(You can even build a slit spectrograph around the Star Analyser eg http://www.threehillsobservatory.co....roscopy_18.htm )
but they are tricky to use as you have to find the target, place it on the slit and keep it there, which is not trivial
Here are a few (mostly rather old) examples of mine of comet spectra taken with different arrangements of the Star Analyser
Positioning the brighter (1st order blazed) spectrum horizontally across the CCD chip reduces the likelihood of adding unwanted artefact's during subsequent processing.
Thanks for coming in Robin & Ken, I'm a bit out of my depth!
Quote:
Originally Posted by robin_astro
A slit spectrograph has the advantage of being able to isolate a slice of the coma, useful to get a good spectrum when it is not so condensed and a slit also keeps the sky background out, increasing contrast.
Because a comet is an extended object, without a slit the spectrum becomes self-contaminating. For the broader cometary spectral features of a low-res spectrum it probably doesn't matter that much (ie the signal of the bright central condensation at relatively large scales will largely swamp the fainter coma signals) but for decent quantitative information & fine detail a slit is essential.
