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Unable to calibrate a spectrum in ISIS

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John Coffin's picture
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Unable to calibrate a spectrum in ISIS

I would be grateful for any help. I have previously successfully used ISIS with a DSLR and an Alpy 600. I've started using an Atik 460 binned x 2 with my Alpy. I calculated the pixel scale using the formula 3123/(difference in pixels between H alpha and H beta). In my case the pixel scale works out to 8.747. I checked the distance between H alpha and beta on both my calibration star, HR 6025 (A0Vn) and the target AG Draconis. 

I believe I have set everything up properly for ISIS but the calibration assistant fails to provide an adequate calibration. The RMS is 31.7.

I've uploaded lots of screen shots below. 

Here are the numbers generated by the wizard.

Lines search zone 

|  -8  |  375  |  0  |

|  -2  |  381  |  6  |

|  -4  |  400  |  4  |

|  0  |  423  |  8  |

|  0  |  473  |  8  |

|  -8  |  588  |  0  |

|  -2  |  786  |  10  |

|  -2  |  805  |  10  |

|  0  |  871  |  12  |

|  0  |  923  |  12  |

|  -4  |  1067  |  8  |

|  0  |  1114  |  14  |

|  -5  |  1196  |  9  |

Gaussian fit on : +/-5 pixels 

--------------------------------------------------------------------------

Reciprocical inverse dispersion equation 

Coefficient a4 : -8.933238E-10

Coefficient a3 : 1.693153E-06

Coefficient a2 : -8.668276E-04

Coefficient a1 : 4.84377

Coefficient a0 : 2091.448

--------------------------------------------------------------------------

Fitting deviation (wavelength)

point #1  x = 375.443  lambda = 3864.395  dlambda = -29.005

point #2  x = 380.701  lambda = 3889.225  dlambda = -0.175

point #3  x = 399.765  lambda = 3979.381  dlambda = -9.301

point #4  x = 418.000  lambda = 4065.822  dlambda = 35.928

point #5  x = 468.000  lambda = 4303.956  dlambda = 36.524

point #6  x = 594.604  lambda = 4914.239  dlambda = -52.899

point #7  x = 786.384  lambda = 5851.107  dlambda = 1.383

point #8  x = 804.791  lambda = 5940.925  dlambda = 3.905

point #9  x = 866.129  lambda = 6238.731  dlambda = 27.759

point #10  x = 921.493  lambda = 6504.380  dlambda = 2.150

point #11  x = 1067.002  lambda = 7176.223  dlambda = -29.183

point #12  x = 1113.173  lambda = 7377.372  dlambda = 6.578

point #13  x = 1196.010  lambda = 7717.422  dlambda = 6.338

--------------------------------------------------------------------------

RMS : 31.777352 (in angstroms)

Attachments: 
AttachmentSize
Image icon general.jpg146.11 KB
Image icon calibration.jpg179.08 KB
Image icon calibration wizard.jpg88.6 KB
Image icon hr6025.jpg163.92 KB
Image icon settings.jpg139.57 KB
Robin Leadbeater's picture
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try without calibration wizard ?

Hi John,

Does it work ok with just the calibration lamp?  ie without the wizard,ticking "predefined mode"  "ALPY (calibration module)" in "general" and entering the position of the 5852 line in "calibration"

Robin

John Coffin's picture
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Still not working

Thanks for your suggestion. The best calibration I can get with the method you suggested gives an RMS of 19.976 Angstroms. I hope I am using the correct neon line (marked below with an arrow). I have even updated the version of ISIS to the latest one. I have also tried adjusting the pixel scale by increments of 0.02, but no luck. 

John Coffin's picture
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OK in RSpec

i calibrated the same star in RSpec using a third order polynomial and with an RMS of 0.6539, just by using the Balmer lines. I’d really rather be able to use ISIS as it is so much more powerful and does all the stacking etc. 

Robin Leadbeater's picture
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using ISIS

Hi John

Yes that is the right reference line.  ISIS should definitely work with the camera you have and the method I suggested, I use it all the time. (The high errors means ISIS is failing to find and lock onto the correct lines)

Can you check that the smile correction has worked correctly please.  Run the calibration on the lamp spectrum itself instead of the target star, ticking sky background not removed. (you will need to put a 1 on the end of the file name so ISIS sees it as the first of a series)   If you then examine the image ("display image") in the page you get after pressing go, the lines should be vertical and straight.  

Can you also post the original image as a zip file please and I will see if I can get it to work this end.

Cheers

Robin

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pixel size

It could also be worth exploring pixels sizes more widely either side. With my setup (an ATK428, smaller but with same size pixels) I get a good lock and calibration (RMS 0.13A) for 2x binned pixel sizes between 8.90 and 9.06 for a nominal  4.54x2 = 9.08. Using the  3123/(difference in pixels between H alpha and H beta) formula, I get 9.02 which is towards the top end of the lock range

(I was going to attach an example lamp image for comparison but max allowed size is 2Mb and zipped files are not allowed. Can you send me your image via email to robin_astro (at) hotmail.com please)

Robin

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example lamp image

You can download my lamp image here 

https://drive.google.com/open?id=1I1UiZhmynSmkF19Fq3ce9foTsRF93ZgN

Robin

John Coffin's picture
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calibration problems

Thanks, am emailing you two zip files now. John

Robin Leadbeater's picture
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found the problem

Thanks for the files.  I now see the problem.  When ISIS does the geometric correction for smile, this shifts the X positions of the lines so the  entered X reference position for the line that ISIS works out the other lines from is not correct and it cannot lock on.  This does not give a problem if the lines are close to vertical, for example in my case the shift is only 2 pixels. If there is a significant slant though the shift can be significant (+15 pixels in your case)  If you add 15 pixels to the reference position, I think you should find the calibration runs correctly.  Can you try it and see what happens?  (It ran for me giving an RMS of 0.3A but I have not tried it combined with the Balmer lines using the wizard)

The slant can be corrected in the ALPY by rotating the grism to make the lines perpendicular to the dispersion direction. (It is simple to do. I did this with my ALPY when I received it and I will dig out the instructions)  but I am a bit surprised ISIS does not take this shift into account. I will check with Christian Buil via the ARAS forum

Cheers

Robin

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Calibration problem

Dear Robin, I tried checking that the smile correction is working but unfortunately I couldn’t get a result. I am obviously not doing it properly. I will look at your lamp image.

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geometric correction

Hi John,

See answer #9 above for the source of the problem.  Actually, I see now that the slant of the lamp  lines is partly because the dispersion is not exactly horizontal, as seen in the star image.  If next time you rotate the camera to make the star spectrum horizontal, the calibration might run ok without needing to change the X position entered or adjust the grism.

Cheers

Robin

John Coffin's picture
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Thanks Robin

Dear Robin, I hope you were up observing and not just working on my problem. I am most grateful for your help and will follow your advice.

Many thanks, John

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Shift of X position

i was surprised that ISIS doesn’t recalculate the X position if the spectrum is rotated. I knew the spectrum was not horizontal, it was my first one with the new camera, but assumed that when ISIS measured the tilt it would make the necessary corrections. Thanks again, I am babysitting today but will try your sugestions tomorrow.

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shift of X position

I agree.  I  found it by looking at the position of the lines in the geometric corrected spectrum image.  (This could be a work round by running the calibration a second time applying the offset seen in the image ) I am awaiting feedback from Christian

Robin

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running without geometric correction

As a further test, the calibration also runs ok with the original X position (though with slightly lower precision and resolution and poor sky background subtraction of course) if you run without the smile correction (by putting a very large value for the radius) 

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ALPY smile problem only

For any LHIRES /LISA users which might be following this, The error is specific to the smile correction used with the ALPY.  I have checked the slant correction that ISIS uses with LHIRES, LISA  and this works correctly. 

Robin

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Successful (ish)

Dear Robin, your method worked. Thanks a lot. The calibration is shifted to the blue by about 1.5 A, so I guess the lesson to learn here is for me to get my spectrum straight and try again. Here are my first spectra with this set up. 

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adjusting ALPY grism

That's great !.  If you still have problems with the spectrum horizontal, you can also adjust the ALPY grism to make the lines exactly vertical. If you look at the core module,you can see the grism held in place by allen head grub screws. If you loosen them off you can rotate the grism, viewing a spectrum by eye until the spectral lines are square to the dispersion direction.  (Take care not to loosen to much and hold the grism holder in place as there is a spring behind it)

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adjusting ALPY grism

It seems ALPY grisms out of adjustment is a common problem. I thought I remembered commenting on this here before

https://britastro.org/node/14085

Robin