Help with Alpy Calibration
Posted: Tue Sep 08, 2015 8:08 am
I'm experiencing some strange things working through Christian's Alpy calibration methods guide a part of commissioning a new Alpy.
Using the Balmer lines method on a nice A0V standard I get a very good result - an RMS of 0.42, and a set of Balmer lines within an angstrom or two (eye-balling) on my resulting profile. So far so good.
Then I try the pre-defined Alpy calibration module mode. Now I'm getting an RMS of 12, and resulting Balmer lines that are upwards of 15Å from where they should be. My wavelength fit deviations are all over the place, and I'm wondering what's going on here. Of note, my spectral image has quite severe fishtailing at the short end, and the corresponding neon image shows significant 'softening' at the blue end. I'm wondering if this is the cause - as during testing with the Calibration Wizard, I get an error message informing me that my "calibration error is too high" (UV on or off).
I have managed to manually compute a quite acceptable dispersion from my neon using 5 lines from 4200 to 7635 giving me an RMS of 0.052 using a 3rd order polynom, but my resulting spectra are not as 'aligned' with an A0V reference as I'd like - to within a couple of pixels, but not quite right. In fact, my aforementioned 'Balmer lines' version appears more accurate when comparing with a database A0V, hence the alarm bells (that and a reported R of 850 @ 5852Å which seems a bit too good to be true).
I'm using an 80mm fluorite doublet - so I'm not sure whether the fishtail is due to optics or my focus (though I did try pretty hard to narrow down the focus to within the Alpy's seemingly 'quarter turn' tolerance while observing the solar spectrum).
Advice or thoughts on any of the above would be most appreciated. All in all, I'm loving the Aply experience (coming from a LhiresIII). And, as always, much appreciation for the excellent information on Christian's web site.
Cheers,
Paul
Using the Balmer lines method on a nice A0V standard I get a very good result - an RMS of 0.42, and a set of Balmer lines within an angstrom or two (eye-balling) on my resulting profile. So far so good.
Then I try the pre-defined Alpy calibration module mode. Now I'm getting an RMS of 12, and resulting Balmer lines that are upwards of 15Å from where they should be. My wavelength fit deviations are all over the place, and I'm wondering what's going on here. Of note, my spectral image has quite severe fishtailing at the short end, and the corresponding neon image shows significant 'softening' at the blue end. I'm wondering if this is the cause - as during testing with the Calibration Wizard, I get an error message informing me that my "calibration error is too high" (UV on or off).
I have managed to manually compute a quite acceptable dispersion from my neon using 5 lines from 4200 to 7635 giving me an RMS of 0.052 using a 3rd order polynom, but my resulting spectra are not as 'aligned' with an A0V reference as I'd like - to within a couple of pixels, but not quite right. In fact, my aforementioned 'Balmer lines' version appears more accurate when comparing with a database A0V, hence the alarm bells (that and a reported R of 850 @ 5852Å which seems a bit too good to be true).
I'm using an 80mm fluorite doublet - so I'm not sure whether the fishtail is due to optics or my focus (though I did try pretty hard to narrow down the focus to within the Alpy's seemingly 'quarter turn' tolerance while observing the solar spectrum).
Advice or thoughts on any of the above would be most appreciated. All in all, I'm loving the Aply experience (coming from a LhiresIII). And, as always, much appreciation for the excellent information on Christian's web site.
Cheers,
Paul