The NICMOSlook FAQ Page

How do I extract a spectrum from a NICMOS grism image without matching direct image?

If only a grism image is available, NICMOSlook can be used as follows:

• As usual, it is strongly recommended to estimate and subtract the background in the grism image as accurately as possible before running NICMOSlook. While NICMOSlook does subtract the local background, it is better to subtract the background before extracting the spectrum.
• First, load the the grism image. As usually, press LOAD button and select file.
• Next, you have to decide how to select spectra for extraction. If the zeroth order of the spectrum is visible, this is clearly the best choice. Otherwise, you might have to select a spectrum by its beginning, center or end point. Select you choice from the sub-menu under OBJECTS -> "Prepare Marking of Positions on Spectra".
• Next, you want to check whether the object has been identified properly. For that purpose, use menu Objects -> Mark Objects -> Spectra Location.
• Give the object a size e.g. with the Objects -> Size & Orientation -> Edit option.
• Probably, there will be the need to trace the object with Spectra -> Trace -> fit offset only option.
• Now Extract spectrum e.g. with the Spectra -> Extended Object Unweighted -> Specific Object not deblended option. Do NOT try to extract more than one object at a time.
• What follows are steps specifically needed if no direct image is available. The reason for these steps is that the location of the object can not be estimated precisely enough from the grism image only.
  • On the spectrum pop-up, press button "DN" to get spectrum without flux calibration.
  • Press "Response" to overplot with response function. You now will probably see a systematic offset of the spectrum compared to the response function. This much more obvious in high s/n spectra. Look at features in the response function and try to line it up with the spectrum by pressing the "Xobj+" and "Xobj-" buttons. You will see an additional shifted response function appear each time you press this button. If the plots gets too crowded, press "Re-plot" and "Response" again.
  • After the response and spectrum are lined up, press "Done". Now you have determined the position of the object much more accurately. But you have NOT yet extracted the spectrum with this more accurate position.
  • Extract the spectrum once more using the Spectra -> Extended Object Unweighted -> Specific Object not deblended option. This time, the spectrum will be much better flux calibrated.
  • You might need to iterate on the above procedure.