Getting Started



Figure 1. The input frames and result plots from the testoptextr procedure.

Preparing Data

The most straightforward use of the package requires six inputs: a data image, an initial variance estimate, the gain and read noise of the array, and pixel locations bounding the spectrum in the spatial direction. The data image is the object frame from which you wish to extract the spectrum. The spectrum must run in the vertical direction, and lines of constant wavelength are assumed to be parallel to the horizontal direction. The initial variance estimate is an array of the same size as the data image, but containing each pixel's variance estimate. Horne suggests

variance = abs(Data) / Gain + ReadNoise ^ 2

as a good estimate, where gain is in units of electrons per data number (EPADU) and ReadNoise is in units of electrons. The spatial bounds are pixel locations in the horizontal direction. Only data between the bounds contribute to the final spectrum. All pixels in the vertical direction contribute to the spectrum. The bounds should be set widely because the algorithm minimizes the influence of noisy pixels far from the peak of the profile.

If part of your preprocessing is sky subtraction (e.g., in the infrared), set skyvarim and rnsky to the sky variance image and sky read noise, respectively. These values will be added to the variance image as it is refined to reflect the increase in noise attributed to sky subtraction.

The importance of an accurate variance image can not be overstressed. The variance image is used to calculate which pixels are off because of noise, and which are cosmic rays. It is also used to weight the pixels for the final optimal extraction. To accuratly predict the noise of the data, the skyvarim must include noise estimates for all pre-processing steps that might introduce noise. Any noise source must be included except read noise for the data image and photon noise from the calculated background and object spectrum. If the data have additional noise sources that are not represented, the routine will mask good pixels. If it overestimates noise sources, cosmic rays can slip through, and the final spectrum will be inaccurate.

Trim out any bad regions where spectrum extraction is impossible, such high-variance or unilluminated areas. If that is not possible, then mask off that area, or make sure the variance image is correspondingly high. Otherwise the routine will incorrectly try to fit these data.

Example/Test Data

In the optspecextr/images directory there are four example FITS files. These files provide input for the following three example runs. Run each of these tutorials from the optspecextr/images directory.

1. Frame with flat background and perpendicular trace

This is the most basic case. First, read the FITS file: 

	IDL> frame1 = readfits('ex1.fits', header1)
Next, locate the read noise and gain levels for the CCD array. This FITS file contains that information in the header. 
	IDL> Q =  sxpar(header1, 'EPADU') 
	IDL> rn = sxpar(header1, 'RDNOISE') / Q
Now, we need to select the object spectrum limits. This can be done interactively using a plot of the frame. The trace is perpendicular, so any row will do. 
	IDL> plot, frame1[*,0]
	IDL> plot, frame1[200:300,0]
It seems the spectrum spans columns 240 to 270. 
	IDL> x1 = 240
	IDL> x2 = 270
Now we need to prepare the data and variance images. Since there is no sky frame, the data image is the object frame. The inital estimate for the variance image is easily calculated. 
	IDL> dataim = frame1
	IDL> varim = abs(frame1) / Q + rn^2
With all of the data now prepared, run optspecextr. 
	IDL> opspec1 = optspecextr(dataim, varim, rn, Q, x1, x2)
	IDL> plot, opspec1

Figure 2. Object frame and extracted spectrum from a simple frame.

2. Object and Sky Frames

When working with infrared data where a sky frame is to be used for sky subtraction, that information needs to be passed on to the optimal extraction procedure.

The preparation is identical to Example 1. 

	IDL> objframe2 = readfits('ex2obj.fits', objheader2)
	IDL> skyframe2 = readfits('ex2sky.fits', skyheader2)
	IDL> Q =     sxpar(objheader2, 'EPADU') 
	IDL> rn =    sxpar(objheader2, 'RDNOISE') / Q 
	IDL> rnsky = sxpar(skyheader2, 'RDNOISE') / Q 
	IDL> x1 =    240
	IDL> x2 =    270
The data image is object frame - sky frame. An additional sky variance image must also be created. The initial variance estimate must reflect the additional image processing step. 
	IDL> dataim = objframe2 - skyframe2
	IDL> skyvar = abs(skyframe) / Q + rnsky^2
	IDL> varim =  abs(skyframe + objframe) / Q + rnsky^2 + rn^2
The call for optspecextr uses the SKYVARIM keyword: 
 opspec2 = optspecextr(dataim, varim, rn, Q, x1, x2, $
	                           skyvar=skyvar)
	IDL> plot, opspec2

Figure 3. Object and sky frames, with resulting spectrum.

3. Frame with Curved Trace

A frame with a curved trace cannot fit the profile using a polynomial fit along the wavelength dimension because the trace intercepts some columns more than once and is poorly fit by a polunomial. Instead try either the boxcar filter or Gaussian fit.

The preparation begins as above: 

	IDL> frame3 = readfits('ex3.fits', header3)
	IDL> Q =  sxpar(header3, 'EPADU') 
	IDL> rn = sxpar(header3, 'RDNOISE') / Q
However, the curved trace makes setting spectrum limits more difficult. Take a look at the frame to get the general idea. 
	IDL> tv, frame3
The minimum occurs and the top and bottom of the frame, while the maximum occurs about halfway down the frame. 
	IDL> plot, frame3[200:300,0]
	IDL> plot, frame3[200:300,255]
	IDL> plot, frame3[200:300,511]
It spans from about 240 to 280. 
	IDL> x1 = 240
	IDL> x2 = 280
Now run optspecextr with either the FITGAUSS or FITBOXCAR keyword. 
	IDL> dataim = frame3
	IDL> varim = abs(frame3) / Q + rn^2
	IDL> opspec3gauss = optspecextr(dataim, varim, rn, Q, x1, x2, $
	                                /fitgauss)
	IDL> dataim = frame3
	IDL> varim = abs(frame3) / Q + rn^2
	IDL> opspec3rsa = optspecextr(dataim, varim, rn, Q, x1, x2, $
	                              /fitboxcar)


Figure 4. Exctraction using FITGAUSS and FITBOXCAR

4. Straightening the Trace

A curved frame may also be corrected using the straightening. This technique upsamples the data frame by 7 pixels, shifts each row by an estimated center correction. Then the usual fitting methods can be used to fit the expanded data. Before use the profile is downsampled.

The preparation begins just like before. 

	IDL> frame4 = readfits('ex4.fits', header4)
	IDL> Q =  sxpar(header4, 'EPADU') 
	IDL> rn = sxpar(header4, 'RDNOISE') / Q
In order to show the correction more clearly this frame is only curved in the x direction. The shift4.fits file contains the shift amount 
	IDL> traceact = readfits('shift4.fits')
        IDL> plot, traceact
It seems to span from 235 to 270 
	IDL> x1 = 235
	IDL> x2 = 270
Now run optspecextr with adjfunct keyword equal to adjgauss. This will tell the program to send the data frames to adjgauss before smoothing the profile. Adjparms.centerest returns the estimated shift amount. With the expantion, straightening may take much longer than other techniques. 
	IDL> dataim = frame4
	IDL> varim = abs(frame4) / Q + rn^2
        IDL> adjfunc = adjgauss
	IDL> opspec4 = optspecextr(dataim, varim, rn, Q, x1, x2, $
	                           adjfunc=adjfunc, adjparms=adjparms)
        IDL> plot, adjparms.centerest
        IDL> plot, (adjparms.centerest - 4) - traceact

Figure 5. Comparison between actual center shift and estmated.