ISIS error “Spectrum Y coordinate is not valid”

[David Trowbridge]

I have been trying to use ISIS- V6.1.1 to process some spectra, but I am coming across an error that is blocking further progress: “Spectrum Y coordinate is not valid.” I am using MaxIm DL v6.28 to capture images and I have sets of bias, dark and flat frames, as well as calibration frames using a mercury/argon gas discharge tube and a cosmetic correction file. Whether or not I perform wavelength calibration, I am getting the error in “Go for process” right after this appears in the console:

Correct the slant angle : -0.34 degree(s)
Transverse registration at intermediate Y coordinate = 149.00
Spectrum Y coordinate Y 1 = 115.00
Spectrum Y coordinate Y 2 = 79.50
Spectrum Y coordinate Y 3 = 72.50
Spectrum Y coordinate Y 4 = 72.50
Spectrum Y coordinate Y 5 = 76.50
Spectrum Y coordinate Y 6 = 72.50
Spectrum Y coordinate Y 7 = 74.50
Spectrum Y coordinate Y 8 = 74.50
Spectrum Y coordinate Y 9 = 44.50
Spectrum Y coordinate Y 10 = 7.49

I don’t understand what the “Y coordinate” is referring to.

Any suggestions?

David

[David Boyd]

Hello David,

This could be because you are trying to measure a very faint spectrum and ISIS is failing to detect the spectral trace in each image. If that is the case you should tick the box labelled Fixed Y value for sequence in the General tab and untick the box labelled Vertical coordinate Auto in the Calibration tab. Then manually adjust the contrast sliders at the bottom to make the spectrum visible in the Calibration tab and manually position the dotted red line in the Graticule over the spectrum. Then press Go and it should correctly locate the spectrum in each image and measure it correctly.

This assumes your guiding is good enough to keep the star at the same position in the slit (which corresponds to the Y coordinate in the calibration window). If the star is too faint to guide on successfully then the spectrum may be at a different Y coordinate in each image and the above recipe will not work. In that case you may have to put the target in the slit manually then guide on a brighter star in the guider window.

Hope that helps.
David

[Robin Leadbeater]

If you try ticking the fixed Y coordinate and sky not removed boxes and rerun the calibration, then click “display image” in the go tab, you will get the sum of your spectrum images with the binning zones overlaid so you can see what is happening. The objective is to have the spectra overlaying each other with the region
with the combined spectrum as narrow as possible. The spectrum binning zone should include all the spectrum region and the sky zones should include none of the spectrum region. If the spectra are not in the same Y position, with the fixed Y coordinate box unticked ISIS will attempt to track the Y movement and overlay them but as David says it may not lock onto faint spectra or with large movements between spectra.

Cheers
Robin

• This reply was modified 2 weeks, 5 days ago by Robin Leadbeater.

[Robin Leadbeater]

If the spectra are too weak for ISIS to lock on to but overlay each other well in the combined image, you can just leave the fixed Y coordinate position box ticked. (My guiding takes care of the movement in Y so I normally operate this way)

Cheers
Robin

• This reply was modified 2 weeks, 5 days ago by Robin Leadbeater.

[David Trowbridge]

David and Robin,

Your suggestions are spot on and I no longer have that error. Processing now runs to completion. Very helpful.

For some reason, some of my spectral images appear with two parallel traces close together, but at different Y-values. I suspect it may be a tracking problem — as if the mount is being over-corrected, causing the telescope to oscillate between two slightly different directions.

Thank for your assistance.

David

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