5 December 2016 at 8:43 pm #573643
Not quite sure why but once I’ve put my spectra through Isis and tried to do comparison with miles or pickles star, hydrogen alpha lines up but the rest are all to the right of the cam parison spectra. I redid all my darks, bias, neon calibration and tried again, but got the same result.
I’m now confused where I’m going wrong. Could my star not be fully on the slit so I’m not getting enough light?
kate5 December 2016 at 10:42 pm #577702Steve CuthbertParticipant
Not quite sure whats going on there. I think the star not being fully on the slit would just reduce the intensiity I don`t think it would cause any line shifting,maybe someone else could chip in ?.
Steve6 December 2016 at 5:06 am #577703Paul LuckasParticipant
It sounds suspiciously like your wavelength solution is the culprit. Can you you elaborate on how you’ve performed the calibration (this is an Alpy, right?). If there was a way to share your spectral and calibration images I’d be happy to take a look in ISIS.
In the mean time, I created a guide some time ago on using ISIS’ calibration assistant that may, or may not, be useful to you:
Paul6 December 2016 at 9:14 am #577704
i am using an alpy without calibration module. Star is delta cas. I have now got things closer by changing my pixel value slightly.
Great guide Paul.
kate6 December 2016 at 8:49 pm #577707Andy WilsonKeymaster
It is one of the idiosyncrasies of ISIS calibration for the Alpy that you have to tweak the value of the pixel size. This goes against common sense as of course the pixel size is fixed.
I think this is because no 2 Alpys are quite identical, and is due to minor differences in focal lengths and chip to grism distances. François Cochard explained this at the BAA workshop far better than I can remember it now, but I remember that tweaking the pixel size fixes the problem.
Andy6 December 2016 at 10:06 pm #577709
Hi Kate, The pixels size is usually not too critical whe using the H alpha lines as there are no other lines nearby but it can be super critical with the calibration module where it needs to distinguish between some very closely spaced lines
I find Christian Buil’s tip here
works well for calculating the pixel value to use:
“A tip for calculate the scaling factor p (or virtual pixel size). Use the formula:
p = 3123 / dx
where dx is the distance between the H alpha and H beta lines measured in pixels along the horizontal axis in a raw image. Try to find the distance to the nearest pixel (the reading of the mouse pointer is sufficient). In the example, the H alpha line is at x = 882, while the H beta line is at x = 535. So dx = 882-534 = 347, and thus the pixel size to adopt is p = 3123/347 = 9.00 pixels.”
Another trick is to keep an eye on the RMS value that ISIS generates in the running commentary on the “go” page when it is running the calibration fit. If you keep on repeatedly running varying the pixel size slightly, it should be obvious when the program locks onto the right lines as the RMS will suddenly tumble to a very low value.
Finally double check you are picking the H alpha line and not the nearby Telluric band as the reference point. (It has been known !)
Robin7 December 2016 at 6:03 pm #577715
hi Andy and Robin,
i did the calculations and came up with p=4.66.
I tried this and got an RMS of 22.118.
I repeated it with 4.69 and got an RMS of 8.86, so will stick with that figure for now.
Thanks for the help
Kate7 December 2016 at 6:14 pm #577716
There is still something wrong I am afraid. The RMS fit should be better than 1 Angstrom. For example I have just run a Balmer line calibration on one of my recent reference stars and get the following results (ALPY 600 2xbin 4.54um pixels, pixel size set to 8.97A (3123/348) )
Wavelength fit deviation
point #1 x = 143.059 lambda = 3835.148 dlambda = 0.242
point #2 x = 154.045 lambda = 3888.984 dlambda = 0.066
point #3 x = 170.621 lambda = 3970.476 dlambda = -0.396
point #4 x = 197.253 lambda = 4101.992 dlambda = -0.242
point #5 x = 245.161 lambda = 4340.028 dlambda = 0.452
point #6 x = 349.413 lambda = 4861.473 dlambda = -0.133
point #7 x = 696.362 lambda = 6562.787 dlambda = 0.023
point #8 x = 762.564 lambda = 6872.013 dlambda = -0.013
Coefficient a4 : 3.601046E-10
Coefficient a3 : -1.409022E-06
Coefficient a2 : 1.183699E-03
Coefficient a1 : 4.63613
Coefficient a0 : 3146.763
RMS : 0.408720
If you email me your spectrum image fits file, I can see if i can get a better result if you like
EDIT: typo in pixel size corrected (4.54 not 5.45)
Robin7 December 2016 at 6:31 pm #577718
These are my numbers from the last run through
Wavelength fit deviation
point #1 x = 706.025 lambda = 3839.683 dlambda = -4.293
point #2 x = 724.012 lambda = 3879.507 dlambda = 9.543
point #3 x = 767.819 lambda = 3979.946 dlambda = -9.866
point #4 x = 816.480 lambda = 4096.430 dlambda = 5.320
point #5 x = 913.897 lambda = 4341.149 dlambda = -0.669
point #6 x = 1111.911 lambda = 4861.383 dlambda = -0.043
point #7 x = 1782.002 lambda = 6562.789 dlambda = 0.021
point #8 x = 1898.594 lambda = 6872.013 dlambda = -0.013
Coefficient a4 : 7.007126E-10
Coefficient a3 : -3.752157E-06
Coefficient a2 : 7.268176E-03
Coefficient a1 : -3.45201
Coefficient a0 : 3798.122
RMS : 8.861652
Kate7 December 2016 at 7:43 pm #577720David BoydParticipant
It looks like you are giving ISIS the wrong wavelengths for some of the Balmer lines so it is giving large errors on those lines.
David7 December 2016 at 7:48 pm #577721
What smile settings have you used? If you do not have a calibration module, you can measure the smile using sky lines. I tried your images assuming vertical lines with no smile (by setting the Y smile to the same as the spectrum position and the radius to a large number eg 999999) and got an RMS of 1.14 A using a pixel size of 4.68um
Wavelength fit deviation
point #1 x = 704.340 lambda = 3834.491 dlambda = 0.899
point #2 x = 725.984 lambda = 3889.299 dlambda = -0.249
point #3 x = 758.121 lambda = 3970.993 dlambda = -0.913
point #4 x = 809.533 lambda = 4102.375 dlambda = -0.625
point #5 x = 901.554 lambda = 4339.253 dlambda = 1.227
point #6 x = 1102.840 lambda = 4861.706 dlambda = -0.366
point #7 x = 1771.045 lambda = 6562.745 dlambda = 0.065
point #8 x = 1898.937 lambda = 6872.037 dlambda = -0.037
Coefficient a4 : 3.006774E-11
Coefficient a3 : -2.735230E-07
Coefficient a2 : 6.837095E-04
Coefficient a1 : 1.92972
Coefficient a0 : 2221.769
RMS : 1.116470
In his ALPY tutorial, Christian Buil suggests an RMS of 2-3 A maximum is acceptable for this calibration method
Robin7 December 2016 at 7:55 pm #577722
Hi David and Robin,
i set the smile using a neon light source.
kate7 December 2016 at 10:48 pm #577725
I am able to obtain an acceptable calibration (RMS 1.25 A) with your files using the neon file to measure the smile. I will email you a set of output files and screenshots which hopefully will allow you to reproduce it.
Robin8 December 2016 at 8:33 am #577726
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