RY, DN, DR Tau with Star Analyser

Re: RY, DN, DR Tau with Star Analyser

Postby Andrew Smith » Mon Feb 11, 2013 7:09 pm

Thanks Robin - I do appreciate you taking the time to do these comparisons. I will look at my calibration methodology and see if I am doing something wrong. I suspect it is in part due to not getting good focus on the spectra. I focused on the zero order and then did an offset that I had previously calculated. I had tried to step in the focuser and measure the change in line sharpness but got inconclusive results. I noticed your comment elsewhere that on focus your zero order is a doughnut so I may not have got in far enough. I have just got a copy of Rspec which should help considerably with its focus option. I just tried it with my Lisa and it worked very well in focusing the Ne lines.

Having said all that, I am pleased at the level of correspondence in our spectra once the calibration offset is corrected.

Thanks for your help. If you have any additional advice I would be happy to have it. I will return to these stars next season and practice refining my technique in the mean time. If only we had more clear nights!

Thanks again Andrew
Andrew Smith
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Re: RY, DN, DR Tau with Star Analyser

Postby Robin Leadbeater » Mon Feb 11, 2013 7:33 pm

Hi Andrew,

I have recently been discussing various aspects of this with David Boyd who is also interested in this campaign and you may have come across through the BAA. Here is what I suggested to him about focusing. The attached conversation relates to measurements of Nova Cep but applies generally. I think the doughnuts were move obvious this time as I deliberately focused on H alpha for the nova, using Gamma Cas (This was very easy to do and I think I might use this technique more often. Focusing on H alpha did not seem to degrade the focus significantly at the blue end and brought out some nice emission lines in the far red in the nova)

I had a closer look at how much background signal there is compared with the spectrum. Attached is the result for DN Tau without background subtraction and with the background normalised to 1. It is worse than I had imagined and I like to think I live under reasonably dark skies :(


"I seem to lucky enough to have a pretty good eye for focussing in general
and rarely use aids. (It is probably not by chance that the sharpest star
analyser spectra tend to be produced by users experienced in planetary
imaging). I usually use the Balmer absorption lines (usually H beta
initially) in a bright A or B star. In this case though I used the H alpha
emission line in Gamma Cas which made things very straightforward. Other
active bright Be stars should work too eg Delta Sco etc. You can find
potential Be star targets and see how strong H alpha is currently here
Also P Cygni of course. Note that focussing on H alpha will throw the
shorter wavelengths out of focus to some degree so there has to be a
compromise setting for a wide range of wavelengths. There is some
astigmatism in the spectrum so concentrate on the sharpness of the line
rather than the thickness of the spectrum. Other techniques used are:

Deliberately trailing the spectrum to make it wider eg by switching off the
drive can make the lines more obvious. This is Christian Buil's favourite
technique eg

If you are fortunate enough to have a focusser with a digital setting you
could note the offset between the zero order focus and the spectrum for
different wavelengths and dial in the correction after focussing on the
zero order.

The software RSpec can generate spectra in "real time" even from astro CCD
cameras which can be used to tune focussing based on the actual features in
the spectrum.
DNTau_20130206_incl_background.png (10.55 KiB) Viewed 4722 times
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
Robin Leadbeater
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Re: RY, DN, DR Tau with Star Analyser

Postby Andrew Smith » Mon Feb 11, 2013 8:47 pm

Thanks Robin - Good advice. I try to automate as much as possible as I don'r seem to have good eye for anything astronomical. I use the Paramount to slew to the target, pate solve with the Sky X (yes it even works with slitless spectra) to check the field (normally redundant) and work out which star is which. I use @focus2 & Robo focus to focus the zero order and now plan to use Rspec to derive an offset from the zero focus to various lines. Then all I will need to focus on the zero order and offset from there.

That is the theory anyway I will let you know how it goes.

On the sky background I am probably much worse off than you with glow from Manchester and Liverpool in addition to local light polution.

Regards Andrew
Andrew Smith
Posts: 317
Joined: Fri Sep 30, 2011 12:23 pm

Re: RY, DN, DR Tau with Star Analyser

Postby Andrew Smith » Tue Feb 12, 2013 9:14 am

Robin - I have discovered the reason for the 50A offset. I have been using the zero order line in ISIS on the "Calibration" page and I discovered that if I forget to check the "Emission Line" box I get the 50A offset. I have checked the spectra I submited to Francis Teyssier and I did process them correctly.

Regards Andrew
Andrew Smith
Posts: 317
Joined: Fri Sep 30, 2011 12:23 pm


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