Re the decreasing hydrogen amplitude ratios: I think it is real, and not a result of Ha saturating. I will be attending the RASNZ conference in Wellington next weekend and, fortuitously, Bill Allen will be presenting a paper on Eta Carinae. He has been observing this star for 30 years and knows a lot about its behavour ... in fact his paper puts forward the theory that it has a companion with a period of 5.5 years. I will discuss the H amplitude matter with him.
Don't rush in Bernard.
Here's a little comparison of 2 spectra captured last night. The blue profile is the 13.7s exposure, the pink is a failed attempt to capture the long exposure as an avi file. The exposure in the avi file was not as specified in IC Capture but was something less than 1/3.75 of a second (3.75 frames per second frame rate).
While the exposure on the avi isn't ideal, the ratio of Ha/Hb is clearly different. This suggests to me that the longer exposure was overexposed and the Ha line was clipped.
Al,
You could set the exposures of the DMK independent of the frame rate..
For some of my prom shots I need to go to 1/6 sec and capture AVI ( It says 30/sec on the screen.. but this is actually only the play back speed according to Stephen at IC)
You should, haven't tried it yet, be able to increase the individual frame exposure to 2 or 3 sec and get a better SN outcome. What do you think??
I thought it did that, Ken, but I couldn't get it to do it last night. I ended up writing it off as remembering how K3ccdtools works, and took a series of stills instead.
I was quite sure it wasn't working, because live view behaves right, and updates after each exposure, but when I chose capture, the image dimmed, and the frame count was ticking up at the right rate for 3.75 frames per second. Couldn't get it to do any different...
Al,
I think you are correct, which is a relief as I was scratching my head trying to come up with a viable physical explanation. The fact that your H alpha/beta ratio is around four for your pink/AVI curve while mine is just over three made me wonder if my Ha was clipped, or in the non-linear zone. Turns out the latter is probably the case. I've not yet got around to determining the linear range of my new DSI Pro III ... but I think I exeeded it with my Ha!
Al,
I think you are correct, which is a relief as I was scratching my head trying to come up with a viable physical explanation. The fact that your H alpha/beta ratio is around four for your pink/AVI curve while mine is just over three made me wonder if my Ha was clipped, or in the non-linear zone. Turns out the latter is probably the case. I've not yet got around to determining the linear range of my new DSI Pro III ... but I think I exeeded it with my Ha!
I think Eta Carinae poses a real problem for getting the exposure right because there is so little of the frame that is bright.
I gotta say, I'm learning heaps and I like the more analytical side of spectroscopy - results should be repeatable and testable, not just look good, and I really appreciate the help you guys have given me. Thanks guys!
It is good to be able to swap experiences and, as you say, the analytical side of spectroscopy is very rewarding ... as is also the case with photometry. I quickly found out that the science that amateurs can do iin astronomy is far more rewarding than taking 'pretty pictures'.
I tried to get an Eta Car BVRI and a spectra earlier this evening but as soon as I was ready to shoot the clouds arrived thick and fast
Al, and others-
We're all in this together, learning from each other..
" the pinnacle of amateur astronomy - spectroscopy" -Richard
" If optical spectroscopy had not been invented then fully 75% of ALL astronomical knowledge would be unknown today" -Kitchin
Let's continue to hold hands in the traffic and "boldly go where no amateurs have gone before" ( couldn't help myself!)
Al,
I only use the DMK21 for solar imaging, but I'll set it up and play with long exposure settings....keep you posted!!
Bernard,
I was lucky enough last year to get a full set of Schuler UBVRI filters from the Maria Mitchell Observatory in the US ( they were "upgrading")
I'll be honest, although they are now mounted in an AtiK -USB electronic filter wheel, as yet I haven't done any serious work with them.
Why do you ask???
I don't know of any photometrist that has been able to use the U filter successfully without having a large (at least 1M) scope and/or a back illuminated CCD. Many even have problems with the B filter.
If you, or anyone, is interested in variable star work you should join AVSON (Austral Variable Star Observer Network) http://tech.groups.yahoo.com/group/AVSON/. I set this forum up a few years back as a meeting point and target information dissemination centre for all those interested in southern hemisphere variable star work. While there are less than 100 members they include just about anyone doing amateur astro science in this hemisphere.
I'm no physicist, but in a "normal" Balmer Hydrogen series, I think the intensity of the Hb is 50% of the Ha...so any diffference in the ratio Hb/Ha shows something else is happening... correct Bernard???
I don't know, Ken. I vaguely remember reading about this (in Kaler?). If I remember correctly the relative heights of the different lines did depend on temperature. When star temps get high enough to reveal the Balmer Series, Ha is dominant, but as temperature increases more of the H is energised high enough to bring Hb and Hc up. It made sense when I read it, but left me a little confused about the significance of the relative strengths of the lines.
I remember reading about a secondary method of temperature determination based on the relative strength of the lines, but it wasn't explained and gave the impression it wasn't a simple method.
I'll have a look in Kaler tonight... see if I can find it.
I don't know, Ken. I vaguely remember reading about this (in Kaler?). If I remember correctly the relative heights of the different lines did depend on temperature. When star temps get high enough to reveal the Balmer Series, Ha is dominant, but as temperature increases more of the H is energised high enough to bring Hb and Hc up. It made sense when I read it, but left me a little confused about the significance of the relative strengths of the lines.
I remember reading about a secondary method of temperature determination based on the relative strength of the lines, but it wasn't explained and gave the impression it wasn't a simple method.
I'll have a look in Kaler tonight... see if I can find it.
Al.
I've had a scratch through Kaler...
Refer Section 4.3 - 3rd paragraph. It's on page 91 of my paperback copy. He describes a method of comparing the relative strengths of absorption lines to determine temperature.
I'm not sure whether I've read the same w.r.t. emission lines, but I see no reason why there wouldn't be an equivalent temperature effect.
In Robinson's Book "Spectroscopy -the key to the stars", p97 he talks about the "Theoretical Balmer decrement" which is based on the ratio of Hb to the bluer emissions of Hgam, Hdel etc etc
To quote " The Ha is often so strong that lines further up the series would have very small values indeed if the Ha was used as the standard"
The other thing I came across the the ratio of the FWHM of the Ha/ Hb ( see Robinson, p144) to determine the total amount of Hydrogen gas in the object, when compared with similar ratios for other elements we get ..."Cosmic Abundances"....
Kitchin ("Optical Astronomical Spectroscopy") ,p233 talking about Cataclysmic variables and Novae
"...During the outburst..the spectrum changes from that characteristic of a star to that characteristic of an interstellar nebula. The change arises from the enormous expansion in the outer layers of the star that are thrown off during its explosion... As the outburst progresses, the spectral type changes towards lower temperatures and emission lines start to appears. The emission lines become stronger and stronger,and the continuous component weekens, until emission lines dominate the spectrum."
Last edited by Merlin66; 18-05-2009 at 06:49 PM.
Reason: kitchin quote added
Robin Leadbeater has shown that you can mate a slit to the SA100; reduces background light etc etc. http://tech.groups.yahoo.com/group/s..._90mm_350D.jpg
I've been playing around with "standard" adaptors/ spacers and found what I think is a pretty good solution.
I'll upload in two parts
I've just trialed the set up's visually and they both work 100%
I'll get the webcam/ Canon/ DSI connected to get some images.
Oh, and finish filing the Surplus Shed slit assembly!!!
I've never really understood the various attempts to convert the simple grating into a 'real' spectrograph (slit with collimated beams), other than for its technical challenge or budget reasons. For me the main, and a very big advantage, of the simple uncollimated grating is that it is quick and easy to use. No need for difficult slit guiding (a real plus) and it can easily be used in a filter wheel ... which also allows for automated 'quick look' spectra. I see the ideal amateur spectroscopist's equipment being the 'quicklook' grating (RO, SA or Baader) plus a slit spectrograph for detailed work (SBIG, Lhires, etc).
Couldn't get it to do any different...
, and I really appreciate the help you guys have given me.
Thanks guys!
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