UFTI + FP Manual

UFTI + FP Manual

The 2 micron (K-band only) Fabry-Perot was commissioned with UFTI in late January 2000. The information presented below is an update of the characterisation data originally obtained with the FP on IRCAM3 (by Tom Geballe).


The low resolution, 50mm diameter Fabry-Perot etalon for the K band was commissioned at UKIRT and had its first successful observing run with IRCAM3 in March, 1995. The etalon has a spacing of 40 microns, a Finesse of Q~25 across the K band, and a nominal resolution with UFTI of 400 km/sec when properly aligned (R = mQ ~ 750 for order m~30). The stability of the FP is very high compared to older etalons; during nights on which it has been used with UFTI the thermal drift has been less than one resolution element. Approximately the central 70 arcsec circular diameter field of the UFTI array, as shown below, is unvignetted.  

FIG.1. A raw image of the planetary nebula IC418 taken with UFTI through the FP tuned to Br Gamma. These data have not been dark or sky-subtracted.

The phase shift between the centre and edge of the usable 70 arcsec field of view is ~20-30 FPZ steps – equivalent to about 70-100 km/s; between the centre pixel 512 and pixel 700 (distance 17 arcsec) the phase shift is only ~10 FPZ steps. Thus, one setting of the FP is sufficient to accurately image an unresolved line across the entire unvignetted field. For the same reason sky OH lines will either be transmitted or rejected across the entire field (i.e. no sharp rings) and will not cause serious flatfielding problems. The separation of adjacent orders corresponds to dlambda/lambda ~0.027; hence it is possible to use narrow band filters with percentage bandpasses of up to about that value without transmitting an unwanted order.

Note that because of the FP coating, it can only be used in the K-band. However, it can be used in conjunction with IRPOL to perform spectropolarimetry on extended line emission.

The Fabry-Perot may be controlled from an ORAC sequence using an FP iterator. A number of ORAC reduction recipes specific to the FPnow exist, and Template Sequences are also available in the ORAC-OT Template Library. Further details are given below.

Performance Summary 

Plate Spacing                   ~40 microns
Finesse                         ~25 across the K window
Phase shift                     ~10 FPZ steps from centre to
                                      radius of 17 arcsec
Resolution - Velocity           400(+/-20)km/sec (FWHM) at 2.1um
Resolution - Wavelength         0.00288(+/-0.0001)um (FWHM) at 2.1um
Resolution - FPZ steps          100 (+/-5) FWHM at 2.1 um when 
                                     best aligned
Wavelength change/FPZ step      2.62 x 10^{-5} microns at 2.15um  
Plate Spacing change/FPZ step   0.000500 microns
Order spacing                   ~0.06 um (~120 cm-1) or 2115 FPZ steps
Allowed range of FPX,FPY,FPZ    -2047 --> +2047 steps
Drift with temperature          approx. -8 FPZ steps per degree C 

Note that because the FP is not situated in the focal plane of UFTI, the converging beam will result in a larger velocity resolution than has been measured previously with IRCAM3 (with IRCAM3 this was ~ 350 km/s). The phase shift across the array is much shallower, however.

The following sensitivity figures have been measured for UFTI (all data were obtained through the 2.122 micron narrow-band filter).

Zero Point (measured in 6" aperture):  17.27  

Sky + Instrument background:     18.2 mag/pixel
                                 13.0 mag/arcsec

Sensitivity (extended source)   
              1-sigma 1 second:  18.2 mag/pixel
              1-sigma 1 hour:    22.7 mag/pixel
                                 (1.5 x 10^{-21} W/m2/pixel)
                                 20.1 mag/sq. arcsec
                                 (1.7 x 10^{-20} W/m2/sq.arcsec)

Sensitivity (point source - 2" aperture, 0.8" seeing)
              1-sigma 1 hour:    18.2 mag
              5-sigma 1 hour:    16.5 mag

Notes: The UFTI pixel scale is 0.091 arcsec.  IMPORTANT: the above
       sentivity figures do not take into account time spent on sky 
       OR on continuum (off-line) wavelengths, i.e. these are the
       on-source, on-line sensitivities.  For the observing modes
       described below, integration times must therefore be doubled 
       or quadrupled.  The extended source sensitivity is only thought 
       to be accurate to within a factor of 2.

With UFTI’s very small pixels, getting background-limited performance from the array can be very difficult with the narrow bandpass of the FP. Use of the “High-Gain” readout mode is therefore recommended.

Set-up and Alignment 


Mounting the FP on ISU2:

Install the FP in front of UFTI. Plug in the cables from the CS100 (they should be tied up somewhere above UFTI), being careful that the X, Y, and Z cables are connected properly on the FP and that the loose cable does not block part of the beam. The micrometer settings on the FP mount should be set to:

        X (bottom right, when facing mounted fp) = 11.0
        Y (top left, when facing mounted fp)     = 9.0

Do not disturb these settings.

CS100 Operation:

The CS100 is located in the lower-half of the blue electronics rack mounted above UFTI.

  • Check the alignment settings on the CS100 front panel. The X, Y, and Z coarse (c), fine (f), and quadrature (q) values should be close to the following. Xc Xf Xq Yc Yf Yq Zc Zf Zq 0 1 4 -1 8 3 -3 9 3 (The fine scale is about 1/10 of the course scale. Hence the above Z coarse and fine readings can be thought of as -3 + 9/10 = -2.1. In fact, the Z setting on some occasions has been Zc=-2, Zf=1 (i.e.,-1.9), which is not very different from the Z setting above.)

Instructions for turning on and setting up the CS100 are given below; the following steps should be followed.

  • Turn on POWER. The yellow BALANCE indicator will illuminate.
  • Turn MODE switch to OPERATE. The yellow balance indicator should go out and the green OPERATE indicator should light up. The X, Y, and Z meters should all be on scale and close to zero. Adjust the COARSE or FINE X, Y, AND Z CONTROLS to zero the meters; probably only the fine controls will be required
  • Turn the METER DISPLAY switch to QUADRATURE ERROR and null out any offsets on the X, Y, and Z meters by adjusting the relevant QUADRATURE BALANCE CONTROLS (Xq, Yq, Zq).
  • Turn the METER DISPLAY back to OFFSET.

Now you are ready to align the FP plates with ORAC. Instructions on how to runup ORAC are available on KIKI and KAUWA; simply typerunup after you’ve logged in as observer.

Aligning the FP:

This should be done at the start of every observing run. The Z-axis should also be checked at the beginning of each night, and half-way through the night as the temperature changes. (Note that the waveplate tuning is sensitive to temperature).

Alignment is achieved by finding the “tip” and “tilt” settings (FPX, FPY) which give the maximum signal from an arc lamp line, usually the 2.11712um line of the krypton lamp. This is found by scanning through FPZ at different values of FPY and then FPX. NOTE THAT IN RECENT TESTS WE FOUND IT EASIER TO ALIGN THE Y-AXIS FIRST. The best values of FPX and FPY are determined (usually interpolated) from these measurements. This should be done before your first night of observing, using the ORAC FP sequences “FPY alignment” and “FPX alignment”; these are found in the UKIRT Template Library in the ORAC-OT.  Note: you may need to arrive in Hilo a day early so that you can align the FP with your Support Scientist the day BEFORE your first night of observing: discuss this with your SS! 

Images of the krypton lamp (through the FP perspex cover) when the FP is fully aligned, and when not aligned, are shown below. At present a 5 second exposure should give a few thousand counts when aligned.  

FIG.2. Images of the krypton lamp when aligned/tuned (left) to the 2.1171 um line and when not (right).

The complete alignment procedure is available as three ORAC Template sequences in the Template Library. The complete procedure involves running “FPY alignment”, then “FPX alignment” and finally “FPZ alignment”. Peaks in FPZ are found at different values of FPY, then FPX (details below). With the best FPX and FPY set, a final “FPZ alignment” sequence steps through FPZ to give the FPZ value appropriate to the Kr line. These sequences employ the QUICK_LOOK DR reduction recipe.

The Krypton lamp is mounted on a slider in ISU2. Position the lamp in the beam by loosening the locking screw and sliding it into position; its “in-the-beam” position is marked. Replace the FP perspex cover so that only a portion of the lamp can be seen (near the centre of the FP field of view on the array; see Fig.2 above). Also, without the cover the lamp will be too bright.

IMPORTANT: The lamp should illuminate the centre of the FP field-of-view, which is slightly offset from the centre of the UFTI array, so that you tune the centre of the FP to the correct wavelength. Moreover, the “alignfp” script used to analyse the alignment data (described below) only measures the signal in a box of size X=470-600 pixels, Y=580-650 pixels.

The FP alignment sequences are set to observe with the 2.122 micron H2 1-0 S(1) filter and NDSTARE (you may use the S1z filter, but it transmits more than one line, so you must be sure to find the stronger 2.11712 micron line). The sequences obtain series of images at different FPX, FPY and FPZ settings; by measuring and plotting the median average flux measured across the centre of the lamp image at each FPX/FPY/FPZ setting, one should be able to align the three axes so that the plates are parallel and the distance between the plates (FPZ) is “tuned” to the wavelength of the Krypton lamp line observed. A sequence of frames obtained with FPX and FPY fixed, though stepping through FPZ (see below), may be analysed using a simple script called alignfp . This shell script examines a sequence of frames and plots to the screen the FPZ and mean counts measured in the centre of the array (the area, specified in pixels, should only include the central quarter of the lamp emission/FP field of view) in each image. A plot of FPZ versus counts is also displayed (this may be printed out).

The script uses the Kappa commands:

>stats f2000_00001_raw'(400:600,470:640)' clip=3 | grep mean

Running up UFTI:

In the evening the TSS will run up UFTI and datum the motors. However, observers and support scientists will have to do this themselves during daytime X-Y alignment.

On kiki, type dm ufti.dl & and, from the main UFTI Epics display, open the “filter control” window (under “other displays”) and INIT the two filter wheels and the shutter. Remember, also, to open the shutter!

Complete alignment procedure from scratch:

On Kiki: fetch and run the FPY alignment sequence (you may need to adjust the FPZ range in the ORAC-OT).

On Kauwa: start up oracdr in the normal way; the DR will display the raw (QUICK_LOOK) images.

>oracdr -loop flag &

Note the ampersand. In this same xterm run the alignment shell script by simply typing


The script operates on “_raw” files in the reduced data directory (/ukirtdata/reduced/ufti/UTdate) set by the “oracdr_ufti” command (hence the need to run the script from the same xterm!). The script prompts for the UT date and the start and end frame numbers of your sequence. A plot of FPZ vs. flux is displayed by xmgr.

The ORAC-OT template sequences for FPX, FPY and FPZ alignment obtain frames as described below (use the sequence “FPY alignment” for steps 1-4, “FPX alignment” for steps 5-8, and “FPZ alignment” for step 9). Note: increasing FPY by 400 moves the FPZ peak by about -95; similarly, once FPY is fixed, increasing FPX by 700 moves the FPZ peak by +65.

  • 1. With FPX=0, FPY=-800 step through FPZ to find the maximum flux across the lamp image. (At least) 10 frames should be observed, with FPZ in the range -1200 to -450. Note the FPZ at the peak flux and the peak flux value.
  • 2. With FPX=0, FPY=-400 step through FPZ to find the maximum flux across the lamp image. (At least) 10 frames should be observed, with FPZ in the range -1200 to -450. Again note the FPZ at the peak flux and the peak flux value.
  • 3. With FPX=0, FPY=0 step through FPZ to find the maximum flux across the lamp image. (At least) 10 frames should be observed, with FPZ in the range -1200 to -450. Again note the FPZ at the peak flux and the peak flux value.
  • 4. With FPX=0, FPY=+400 step through FPZ to find the maximum flux across the lamp image. (At least) 10 frames should be observed, with FPZ in the range -1200 to -450. Again note the FPZ at the peak flux and the peak flux value.
  • From the counts measured at the peak in each of these four scans find the best FPY, i.e. plot the four values of FPY against peak counts. 
    This is your FPY setting.
  • 5. With FPY set to the “tuned” value found in steps 1-4, and FPX=+700, step through FPZ to find the maximum flux across the lamp image. Note the FPZ at the peak flux and the peak flux value.
  • 6. With FPY set to the “tuned” value found in steps 1-4, and FPX=+1400, step through FPZ to find the maximum flux across the lamp image. Note the FPZ at the peak flux and the peak flux value.
  • 7. With FPY set to the “tuned” value found in steps 1-4, and FPX=0, step through FPZ to find the maximum flux across the lamp image. Note the FPZ at the peak flux and the peak flux value.
  • 8. With FPY set to the “tuned” value found in steps 1-4, and FPX=-700, step through FPZ to find the maximum flux across the lamp image. Note the FPZ at the peak flux and the peak flux value.
  • From the counts measured at the peak in each of these four scans find the best FPX, i.e. plot the four values FPX against peak counts. 
    This is your FPX setting.
  • 9. Finally, with FPX AND FPY set to the “tuned” values found in steps 1-8, step through FPZ one more time. From a plot of flux against FPZ you should now be able to measure the FP resolution; the FWHM of this final plot should be about 100-120 in FPZ (note that the background signal – when not tuned to the lamp line – is NOT zero [see Fig.2]).

The peak in each FPZ scan usually appears near FPZ = -700, although the peak may be considerably offset from this value (by a few hundred FPZ steps), especially when FPX and FPY are far from alignment. FPZ at peak signal is dependent on the setting of the CS100 and the temperature in the dome.

Note that in all images (near alignment) the lamp should appear as a diffuse horizontal bar across the centre of the array (see Fig.2 above). Peak counts should not exceed 6000; if the counts are greater than this saturation will affect your ability to reach a sharp peak in flux across the scan. 

A plot of FPZ against signal should look something like the plot in Fig.3 below. You should be able to find the peak to within 20 FPZ steps in each case.

FIG.3. A plot of FPZ against flux measured with the krypton lamp.

The optimum values of FPX and FPY should now be entered into your FP sequences via the ORAC-OT. The values of FPZ (the spacing between the plates) appropriate to the 2.11712 micron krypton line observed is temperature-sensitive. This value will need to be re-measured at the beginning of the night and again later in the night after the temperature has dropped. You may want to take a deep sky flat at the beginning of the night with FPZ set to an estimated value BEFORE you properly align FPZ. This will give time for the dome to cool down, and the flat is probably not sensitive to FPZ.

Recent daytime tests yielded values of:

  31 Jan '00   FPX = +300,  FPY =  -80,  FPZ = -630
  20 Apr '00   FPX = +730,  FPY =  -50,  FPZ =  --
  24 Apr '00   FPX = +700,  FPY =  -50,  FPZ = -720
   9 May '00   FPX =  +20,  FPY = -375,  FPZ = -700
   5 Jan '01   FPX = +380,  FPY = -240,  FPZ = -990
   1 Jun '01   FPX = +100,  FPY = -200,  FPZ = -830
  23 Jun '01   FPX =    0,  FPY=  -100,  FPZ = -755

Remember that all values are affected by the settings of X, Y and Z on the CS100.

Quick check of alignment: 

The “FPZ alignment” ORAC sequence is avaliable for quick alignment of the FPZ axis during your observing run. The sequence takes a series of about 20 frames at different FPZ settings (in the FPZ range of about -1000 to -250) appropriate to the 2.1171 micron krypton line. The sequence can be found in the ORAC-OT Template Library and used whenever the temperature changes by more than a few degrees to recalibrate FPZ specific to the lamp line.  Remember to set FPX and FPY to your measured values in this and all other FP sequences! The lamp (and cover) must first be installed in front of the FP of course. Once you’ve obtained the sequence of frames, run the shell script alignfp (as described above) to get a plot of counts against FPZ setting. The sequence could also be used to tune FPZ directly to the H2 or Br Gamma emission line wavelengths via observations of a celestial object (e.g. bright PN, OMC-1), although the FPZ range used by the sequence would have to be changed.

The FWHM measured should be close to that recorded after the full alignment procedure (see above). If the FWHM is ~110-120 steps the FP is properly aligned. If it is much greater than 120, you will probably need to realign the FP.

An example of the FP’s lorenzian profile (i.e. counts measured while stepping through FPZ) is shown in Fig.3 above.

Set-up for imaging 

At 2.1 – 2.2 micron:

Put the Krypton lamp in the UFTI beam as described above and turn it on. With the S1 filter installed and the FPX and FPY set to their aligned values (see above), the strong Kr 2.1171um line (vacuum wavelength) should be present at FPZ~-700 and FPZ~+1400. The two orders are separated by 2116 FPZ steps. Determine the FPZ value, FPZ(Kr2.117), for peak signal on the lamp by running the FPZ alignment sequence described above. The following formulae then give the correct FPZ setting for VACUUM wavelength, lambda, for your target. (Note that you must choose a formula which gives a result in the range -2047 < FPZ < +2047.)

  • If the Kr order near FPZ=-700 (usually the best choice for the H2 S(1) line or Br gamma) is used, (i) FPZ(lambda) = FPZ(Kr2.117) + (37380.*(lambda-2.11712))
 [or the above result plus or minus 2116*(lambda/2.11712)]

This is equivalent to 0.000026752um per FPZ step.

This is equivalent to 0.000026752um per FPZ step.If the Kr order near FPZ = +1400 is used, the formula is

(ii) FPZ(lambda) = FPZ(Kr2.117) + (38356.*(lambda-2.11712))

This is equivalent to 0.000026072um per FPZ step.

Note that you can use Br gamma or H2 in a bright planetary nebula (eg NGC6572, BD+303639, NGC6572, IC2149, IC418), or H2 emission from a Herbig-Haro object (a fainter, though pure line-emission source), to tune or even align the FP. Some useful references for potential targets include Kastner et al. 1996, ApJ, 462, 777 (PNe) or Davis et al. 1997, A&A 324, 263 (HHs). Orion-OMC1 (RA=6hrs) would be another good target for alignment purposes.

  • Using Br gamma or H2 as the reference the above formula (i) becomes:(iii) FPZ(lambda) = FPZ(BrG) + (37380.*(lambda-BrG)) (iv) FPZ(lambda) = FPZ(H2) + (37380.*(lambda-H2)) where BrG and H2 is the wavelength of Br gamma (2.166167um in vacuum) and H2 1-0S(1) (2.121833um in vacuum) respectively. Note, however, that the line emission will be slightly shifted due to the PN or HH’s radial velocity and the earth’s motion about the sun (20 km/s = 5 FPZ steps).If your wavelengths are in air, the above formulae will work if you substitute the air wavelength of the Kr line (2.11655um), the Br gamma line (2.1655um), or the H2 line (2.1213um).

At 2.2 – 2.3 micron: 

Turn on the Kr lamp and install it in front of UFTI and the FP. With the 2-1 S(1) 2.248 micron filter installed and the FPX and FPY set to their aligned values the bright Kr line at 2.2492um (vacuum) should appear at about FPZ= -300 and at FPZ = +1950. The peaks are 2248 FPZ numbers apart.

  • Find the peak of line near FPZ = -300 and note the value, FPZ(Kr). The following formula then gives the correct FPZ setting for vacuum wavelength lambda: (iv) FPZ(lambda) = FPZ(Kr2.249) + (35436.*(lambda-2.24919))
  [or the above result minus 2248*(lambda/2.24919)]

This is equivalent to 0.000028220um per FPZ step. 
If you use air wavelengths, substitute 2.24858um.

FPZ drift with temperature 

The 400 km/sec FP drifts by about 8 steps (~ 30 km/s) per degree Celsius. Consequently, as the night progresses and the temperature drops, the FPZ setting for your source will need to be increased (i.e. add 8 from FPZ for each degree C decrease in temperature) if you are to remain tuned to the desired wavelength.

By monitoring the temperature in the dome (you may open a copy of the TSS’s weather display on KIKI by typing dm weather.dl &), you can compensate for the drift and keep the FP set to the correct wavelength. If the temperature changes by more than a few degrees, however, it is advisable to retune the FPZ axis by once again peaking up on a Krypton line or on Br gamma or H2 in a bright planetary nebula. Note that the dome temperature is not necessarily the same as the FP temperature!

Controlling the FP with ORAC 

There are a number of Template Sequences in the ORAC-OT UKIRT Template Library; observers should copy one of these sequences to his/her own programme and edit this to suit his/her personal requirements. A description of how to use the OT to build up an observing programme is given in the main UFTI pages

With ORAC, FP settings are treated much like spatial telescope offsets in that an iterator must be used; the FP settings are not set in the UFTI instrument configuration. Available template sequences include a basic observing mode that obtains object/sky pairs at on-line, off-line(blue), on-line and off-line(red) FPZ wavelength settings. This sequence, “Basic(FP)”, uses the ORAC-DR recipe FP

A similar, though considerably more involved sequence called “Jitter 5 (FP)”, obtains the same block of eight images, though with the sequence repeated 5 times at slightly jittered positions on-source (40 frames – plus a dark – are obtained!). This sequence uses the FP_JITTER recipe and can be rather time-consuming! Note, however, that fewer than 5 jitter positions are allowed by the recipe.

Lastly, the “Jitter 5 no sky (FP)” sequence assumes that the sky background is negligeable and so does not need subtracting; no sky frames are obtained (and so only 20 frames – plus a dark – are observed). This sequence uses the FP_JITTER_NO_SKY recipe. 

The choice of whether to use FP_JITTER or FP_JITTER_NO_SKY is left to the observer, although note that for very faint sources taking sky frames may be prudent. In raw exposures, users will notice “reflections” in the FP field of view. These diffuse bands are low-level and fairly stable, so in most cases they will flat-field out. Subtracting these reflection bands out may be a better option, however. A raw 120sec HiGain exposure of a faint target is shown below; the reflection bands are labelled.  

FIG.1. A raw 120sec exposure of a faint galaxy; note the faint diffuse reflections. These data have not been dark-subtracted, sky-subtracted or flat-fielded.

All FP recipes require appropriate dark exposures. A separate, blank-sky flat-field must also be obtained. The Template sequence “Make_Skyflat for FP” (which uses the DR recipe SKY_FLAT_FP) may be used for the latter.

Available ORAC sequences and a description of the associated recipes is given here. A more comprehensive description of ORAC, the OT and QT, is available here, though note that this is a general description and so is not specific to use with UFTI.

Standards and Sky Flats

Since the filter transmission may vary at different FPZ settings, it may be a good idea to obtain a separate flux standard mosaic at each of the FPZ values used on the target. For example, one could observe a 5-point jitter pattern at each of the FPZ settings; with these separate mosaics (one per FPZ value) target frames can then be individually calibrated before “continuum” images are subtracted from “line” images. To obtain separate mosaics, nest the 5-point offset iterator within the FPZ iterator in your ORAC-OT sequence. Then reduce the data with BRIGHT_POINT_SOURCE. This will use a pre-obtained flat (the same flat used by the FP recipes; it must be obtained through the same NB filter) and will produce separate mosaics named “_mos_0.sdf”, “_mos_1.sdf” and “_mos_2.sdf”. The template sequence “Flux Standard for FP” in the ORAC-OT UFTI+FP template library may be copied and edited for this purpose. 

Choosing a standard can be tricky. As a rough guide, a good signal (one to two thousand counts) should be obtained on a 7-8th magnitude A-type standard star with UFTI and the FP in 5 seconds integration. Note, however, that the UKIRT Faint standards are generally too faint (10th-13th mag.) and the stars in the Bright-star catalogue are too bright (4-6th magnitude)! Observers should probably use a star from the Elias “Faint” standards list (1982) or the catalogue of Maiolino, Rieke and Rieke (1994); both lists are taken from the IRTF web pages. Most of these stars are 7-8th magnitude at K – just right for the FP!

What about Sky Flats? Getting a few hundred counts on the array with the FP is very difficult. Even with HiGain mode, a two-minute exposure during the night will give only a hundred counts or so; this is barely background limited, and certainly not enough for a “noise-free” flat-field. Observers have two options. The first is to use much longer exposures: the “make skyflat for FP” sequence in the Template library is set up to obtain four dithered sky frames (with FPZ set to the same on-line, off-line(blue), on-line and off-line(red) values as used on the target) with a 500sec exposure time. Alternatively, it is possible to get a good flat at Sunrise/Sunset. Recent observations indicate that a 60sec HiGain exposure within 5-10mins of sunset (with the telescope near zenith) will give 500-1500 counts on the array. Be prepared; the sun sets/rises very quickly, so you’ll only have 20-30mins before the opportunity has passed! Also, please be careful not to “fry” the array – latency/persistence effects, resulting in rectangular bias structures across the array, could hamper subsequent observations.  These test observations should give you some idea of exactly when to take your blank sky exposures. Remember to “flush” the array with lots of short dark exposures after any observations that involve high flux levels (e.g. sky flats, lamp observations [during alignment] or standard star observations).

And lastly, remember that you must obtain sky flats before you observe your target and/or standard star if the FP DR recipes are to work.

And finally, a few (more) things to remember… 

1. The alignment procedure described above measures the lamp signal near the centre of the FP field-of-view; this region will be tuned to your chosen wavelength; Note, however, that there is a phase shift towards the edge of the array, so you should keep your jitter-pattern small!

2. For extra-galactic sources where line emission profiles may be broad, take care to set the FPZ values for off-line (continuum) images at sufficiently large offsets so that the FP lorentian profile (FWHM ~ 100 FPZ steps, or ~ 400 km/s) doesn’t overlap the broad source line-profile. Off-line FPZ values that are 300 or 400 steps more/less than the on-line setting should be used.

3. Consider the blocking-filter profile when choosing off-line FPZ settings. The filter transmission may drop off at blue or red-shifted FPZ settings, particularly if you are looking at extra-galactic sources where lines that are shifted into a filter bandpass don’t coincide with the centre of the filter profile. The safest thing to do is observe a flux standard at the same FPZ settings (on and off-line) and flux-calibrate the target continuum and line images before you subtract the former from the latter.

4. Beware of latency (and rectangular bias structure) after alignment with the arc lamp. Be prepared with a sequence of 10 short (4sec) darks; this may then be used to flush the array before you embark on lengthy target exposures.