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MMTF Observing Guide
Version 0.2, 01 Dec 2006
Version 1.0, XX Yyy 2007
Summary

This guide provides instructions to the observer for operating the MMTF. It outlines basic principles, observing strategies, and detailed procedures for calibrating the etalon.

Outline

Basic Principles
The Advantages of Tunable Filter Imaging
Applications
System Components
Fabry-Perot Physics

On the optical axis (R = 0), for effective plate spacing d:

  d = n * lambda / 2
  FSR = lambda / n
  FWHM = FSR / Finesse = lambda / n * Finesse
  lambda = A + B*z

(list FSR, FWHM dependence in Z-space)

n = order number, lambda = wavelength transmitted, finesse depends on the reflectivity, flatness and parallelism of the plates. A and B describe the wavelength-z calibration.

Off the optical axis at radius R:

  lambda(R) = lambda(0) / sqrt( 1 + R^2/F^2 )

where F is the focal length of the camera.

At a given plate spacing, the FSR varies as lambda^2, thus if the FSR is 300 A at 6600 A, it should be about 460 A at 8200 A. This is not exact because the coating depth varies with wavelength, changing the effective plate gap.

Note that z is really controlling the plate gap, i.e.

  d = E + G*z, E and G are independent of wavelength, 
  where A = 2E/n, B = 2G/n; 

A and B depend on the order you're working in.

Since B is what we also call dw/dz, this implies that dw/dz ~ 1/n, so at a given plate spacing dw/dz is proportional to wavelength, i.e. if dw/dz is 0.30 A/z-unit at 6600 A, it should be 0.37 A/z-unit at 8200 A.


Information for Prospective Observers
Wavelengths

Observers have full freedom to tune the central wavelength of the MMTF bandpass within the range allowed by a particular blocking filter. The wavelength can be tuned from one exposure to the next, or within a given exposure (see Charge Shuffling). A blocking filter is required to select only one transmission order (a given plate spacing transmits multiple wavelengths at a single location, each corresponding to a different order).

The currently available narrowband blocking filters are listed below. For each filter, we list the central wavelength and full-width at half-maximum in the IMACS beam.

Wavelength (Å) FWHM (Å) Transmission (plot) Transmission (data) Remark
5102 150 plot data
5290 156 plot data
6399 206 plot data
~6600 ~260 plot data FWHM > free spectral range
6808 218 plot data Unavailable; broken in shipment
7045 228 plot data
8149 133 plot data
9163 318 plot data

If you are considering the purchase of a narrowband filter not listed here and are interested in making it available for use with MMTF, or have other questions about filters, please contact Sylvain Veilleux.

Changing filters between exposures is straightforward. However, each filter requires its own set of calibrations to determine the wavelength solution and etalon parallelism. It is thus important to plan ahead and find wavelength solutions for each filter during the day before a run. [What about parallelism?]. Though it is technically feasible, we caution against pushing the envelope by trying to observe in more than 2 or 3 filters during a single night.

Bandpass

The nominal range of bandpass full-widths at half maximum is 6-30 A. The bandpass depends on the coarse plate spacing and the wavelength. It is likely that the bandpass will not be allowed to change during an observer's run.

Sensitivity

The MMTF sensitivity at 6600 A is known from flux standard measurements. Measurements at other wavelengths are ongoing, and will be posted here as they are available.

The MMTF sensitivity at 6600 A was calibrated with observations of emission-line flux standards taken during the June 2006 commissioning run. We assume a (moderately dark) sky brightness of R=20.5 AB mag/arcsec^2, which may be a few tenths of a magnitude brighter than actual dark time at LCO. (The actual calibrations were done at full moon, when the sky brightness was R=18.5 mag/arcsec^2.) For an etalon bandpass of FWHM ~ 25 A, an unbinned, 900 second exposure yields 102 counts from the sky and 49 counts of read noise in a 2 arcsec diameter aperture. To achieve a S/N of 5, one then requires 575 counts from a source. We find that this corresponds to an incident emission line flux of 6.9e-17 erg/sec/cm^2.

This sensitivity is for a single exposure of an emission-line object, where the etalon is tuned to match the wavelength of the emission line. The calibration relies on the total throughput, including CCD, instrument, telescope, and atmosphere (for an airmass of 1.2-1.3).

Charge Shuffling

The charge shuffling mode enables the observer to switch among 2-3 central wavelengths during a single exposure. The procedure is as follows. Each IMACS CCD is first divided into three equal segments by an aperture mask. The aperture allows light only onto the center third of each chip.

When the exposure begins, the center of each chip is exposed at a single wavelength. With the shutter closed, the accumulated charge is then (almost instantaneously) moved down one-third of the chip length. The etalon's central wavelength is changed (again, almost instantaneously). The exposure proceeds at this second wavelength, again at the center of the chip, for a time equal to twice that at the first wavelength. The charge is then shuffled back up, and integration proceeds at a third wavelength atop the acculumated charge from the first part of the exposure for a time equal to that at the first wavelength. The procedure is then repeated. At the end of the exposure, each chip contains two images of the same field, each on one-third of the chip. The last third is empty.

The third wavelength may equal the first, to produce two monochromatic images. Alternatively, it may be unique from the other two. In this case one of the resulting images is the sum of images at two separate wavelengths. Each wavelength in this image experiences one-half the exposure time of the other image.

This procedure allows significant improvements in relative photometry. Time variations in transparency and seeing are matched between images at two different wavelengths, using a cadence chosen by the observer. A second advantage of charge-shuffling is that it allows accurate matching in wavelength space of the continuum of an emission-line source. If one image traces an emission-line, the other can trace the continuum at one or two immediately adjacent wavelengths.

Only a single filter can be in use, but within that filter any wavelength is allowed.


Observing Strategies
When to do calibrations
How to execute a scripts
Using charge-shuffling
Dithering
Making a data cube
How to use FPCALC


Calibrations
Wavelength Calibration

The MMTF produces an image whose bandpass varies from center to edge of the field (edge=bluer). The gradient is about 100 A at H-alpha. Adjusting the plate spacing with the Z parameter tunes the wavelength. Because you only see a short section of the spectrum in any given image, and because a line can be transmitted in different orders, the challenge in setting up the instrument is to identify which line is which.

For this reason we want relatively few lines transmitted by the filter and use one or two of the available gas-discharge lamps (neon, krypton, xenon, argon, helium) at a time. Use a lamp that has a few lines in your filter: neon for the 6600/260 filter, krypton and/or xenon for the 8150/150 filter.

(*** have a table of line wavelengths accessible here, and plots of the grism spectra of the lamps ***) We have rough linelists for the calibration lamps.

Insert the appropriate blocking filter into the IMACS beam. We will take a series of exposures, stepping the etalon through an entire free spectral range (order) and measuring flux at pixel X,Y as a function of the Z (spacing) axis of the data cube to build up a spectrum. To speed up this process, only read small sections of the CCD array.

The default subraster size is 32x32, so bring up the QL-tool and subraster menu item to change it to 100x100. Define a subraster file: select subraster from the Full/Subraster box in the IMACS CamGUI. You can type in the subrasters or use the QL-tool window to define subrasters: "a" adds a subraster at the cursor location, "r" or "d" removes it. Make a subraster on chip 2 from about x=1900-2000 and y=3900-4000, which is near the optical axis, and another on chip 1 from x=100-200 and y=3900-4000, far from the axis (and others to be at a range of radii, but only do one per chip). IMPORTANT: choose "minimal" not "full" for SaveMode, so that you only save the actual subraster, not the 2048x4096 full chip. Save the subrasters in a file, e.g. "sausage.sub", so you can load them quickly later.

Take a test exposure (*** length? probably 1-5 sec for the IMACS internal lamps?) to check flux levels. Then take a loop over somewhat more than a full FSR. A full FSR is about 700 in Z at H-alpha (and scales with wavelength). Do a loop stepping by dz=20 with ~50 images (*** instructions for taking a loop and changing Z with an IMACS script here ***).

To use an IMACS script to take a loop, you must have a file in your home directory called ".imacs_use_script", type "touch .imacs_use_script", then restart the CamGUI to see the script menu. Click "Create" to bring up the script creation window. Choose "Scan(seq)" and enter the starting z, dz (step size), and number of exposures, e.g. 2050, 20, 50. Give the script a name like "sausage1.script" and save it. Then make sure you are in subraster and minimal savemode, click "Run" to bring up the script execution window, and run the loop.

The spectrum through the Z axis is called a "sausage cube." To plot and fit the spectrum, use the program "fitsausage". First make a list of the input images with "makeimlist". If your cube started at image 30 and went to image 79 and you want to use the subraster from chip 2, type

makeimlist 30 79 2 list.cube1

to put the list of the fits files in "list.cube1". Now run

  fitsausage
  list.cube1
  /xs
  [hit return to accept the default box to sum pixels in]
  [hit return to skip fitting the peak]
  [hit q to quit]

This will plot the spectrum in flux as a function of Z (defaulting to averaging the flux in a box section of x=10-30 and y=10-30 out of the ~100 pixel square subraster that you set up). If you type /ps instead of /xs you can get a postscript file pgplot.ps that can be printed out.

Note that if you forgot "minimal" save mode and your images are in "full," you use much more disk space, but fitsausage will still work. Find where your subraster is and choose an xmin-xmax ymin-ymax range within it, e.g. if your subraster is x=1800-2000 and y=3800-4000, typing "1830 1850 3930 3950" when fitsausage asks you for the box to extract will work.

The spectrum over a full FSR should show one or more emission line peaks. Compare to example MMTF spectra to identify lines (see below). Currently examples are only available for the 6600/260 filter.

fitsausage also will fit data in an x-axis range that you specify and print the best fit parameters. The fit is a Voigt profile and the parameters are:

1. continuum
2. intensity (area under curve)
3. location of peak (in units of the x-axis)
4. sg = gaussian sigma (in x-units)
5. sl = lorentzian sigma (in x-units)

It also prints the total sigma = sqrt(sg^2 + sl^2) as an estimate of the width, since there is covariance between sg and sl. And it computes the 0, 1, 2 moments of the data in the specified x-axis range. You may find that the 2nd moment (dispersion) is more robust than the total sigma from the fit if the profile is very asymmetric (which usually indicates non-parallelism).

Once you have identified several line peaks, you have several wavelengths each with a Z and order. If you measure lines in more than one subraster, you also have a range of different radii from the optical axis. These can be used to find the parameters in the Fabry-Perot equations:

 W(z,R=0) = A + B*z
 W(z,R)   = W(z,R=0) / sqrt(1 + R^2/F^2)

A = zeropoint of wavelength-z relation
B = dw/dz
R = distance from optical axis (in pixels)
F = focal length of camera (in pixels)

As a quick solution to get on the sky the first evening, you can just find two lines (in the chip 2 subraster near the optical axis) of known wavelength and Z to get a crude estimate of A and B. For example, if Ne 6598.95 is at z=2674 and Ne 6678.3 is at z=2887, then B=dw/dz ~ 0.3725 and A=5602.9.

Generally dw/dz is predictable from the wavelength you are using and the Z-coarse setting (*** need table, see appendix), and F ~ 23670 pixels.

For a better long-term solution, you can use the program "fpsolve" which takes a number of wavelength and z pairs and solves for the parameters. fpsolve can also handle lines that are in different orders and at different radii, if these are specified in the input file. (*** need more information on fpsolve input and output)

(*** need to specify how to find the radius given chip#, x and y)

Example spectra

We present here arc lamp spectra taken with the MMTF by scanning the etalon spacing through somewhat more than 1 free spectral range (FSR). The spectra are provided at a number of coarse etalon setting in order to aid in line identification when performing wavelength calibration.

These spectra also serve to demonstrate order overlap in an MMTF spectrum. Order overlap occurs when the FSR is smaller than the FWHM of the blocking filter. (There can also be order overlap when there are bright arc lamp lines at the edges of the filter transmission profile, even if FSR > FWHM). Because the lamp emission lines repeat themselves from order to order, lines can sometimes appear in a sequence that is not monotonically increasing. Most of the filters have been designed so that FSR > FWHM for most coarse plate spacings. However, because FSR (in wavelength space) decreases with increasing plate spacing, some spectra with the highest values of Zcoarse may still show this effect.

As an example, the 6600 Å filter has a FWHM of 260 Å. For Zcoarse = +2, the FSR is 200 Å ; thus FSR < FWHM. The neon line at 6507 Å appears at a wavelength that you expect to be 6707 Å (6507 Å + FSR), but in a different order than the 6707 Å light also transmitted by the filter. Light at 6507 Å and 6707 Å can thus appear at the same position for a given etalon spacing (hence, "order overlap"). Some of the linked spectra below illustrate this phenomenon.

In each of the following plots, the x-axis is lamp intensity and the y-axis is the fine etalon spacing, Zfine, in computer units. The spectra are collected using a 400 pixel2 subraster near the optical axis.

Filter Arc Lamp Zc=-2 Zc=+1 Zc=+3 Parameter Table
5100 Ne plot plot plot table
5300 Ne plot plot plot table
6400 Ne plot plot plot table
6600 Ne plot plot plot table
7050 Ar plot plot plot table
8200 Kr+Xe plot no data plot table
8200 Kr no data plot no data table
8200 Xe no data plot no data table
9150 Xe plot plot plot table
Checking for wavelength drift
Parallelism
Flat fields