A typical TTF observing night (if only that was true)
 

The night of 29/30 August, 1998 was an excellent time for TTF observing. The conditions remained photometric from sun up to sun down. We obtained data for exactly 12 hours. The seeing went from 2" at dusk, to 1" at midnight,  and up again to 2" before dawn. However, this did not concern Jaffe & Bremer who wanted to detect H-alpha and [NII] emission in a sample of `cooling flow' clusters with the TTF 2-shuffle mode.

Below, I give the exact command sequence for the entire night, with explanatory notes. Bring up the TTF control system in the normal way.


CuAr arc spectrum

The first image 29aug0003  is a CuAr arc spectrum (80-shuffle image). See  here  for how this image was used to determine the instrumental parameters. This image will also be used to determine TTF stability with later arcs. In the SMS control window, click on run / run_ccd / shuf_multi which brings up the shuffle window. Ensure the following parameters are set.

CCD charge shuffling multi line
Csr_Default disk$user:[observer.cs_files]*.csx
Csr_Filename eighty
Preexpose_info 3_1000_100
Cycle_count 1
Bias True     False
Setup_Array True     False
Etalon_Start 0
Etalon_Increment 7
We set this running with PF1.


CuAr sausage cube


Another way to obtain the same information is with a "sausage" cube described in detail  here .    This method was used to obtain the images 29aug0005 - 29aug0084. In one minute, the data were stacked and the lines fitted to obtain the lambda-z relation z_1 = 1.41792 lambda - 9539.50. This was used to determine the "z" values which control the spacing of TTF.

Sky flats


The programme entails observations through the R1, R3, R and I filters. Since sky flats were already obtained for most of these on previous nights, we obtained 2-shuffle flats for just the R1 workhorse filter, 29aug0086 - 29aug0087.  (The R and I sky flats do not need to be shuffled as these are just straight images.)
2-shuffle data


The basic observing programme is to shuffle between an off-band and a redshifted H-alpha setting repeated for each cluster, and again for an off-band and a redshifted [NII]6583 setting, also repeated for each cluster. In the SMS control window, click on run / run_ccd / shuf_multi which brings up the
shuffle window. Ensure the following parameters are set.

CCD charge shuffling multi line
Csr_Default disk$user:[observer.cs_files]*.csx
Csr_Filename twostep60m
Preexpose_info 3_1000_100
Cycle_count 15
Bias True     False
Setup_Array True     False
Etalon_Start 428
Etalon_Increment 54
We set this running with PF1 to get 29aug0088. We perform the operation twice more with Z=514,542; 29aug0089 - 29aug0090 complete a set started on an earlier night.


CuAr sausage cube


The next object requires the R3 filter, so we need to dash off a sausage cube to calibrate the lambda-z relation at 760nm (29aug0091 - 29aug0170). Again, the data were stacked and the lines fitted to obtain z_3 = 1.31272 lambda - 9526.2. In principle, the z_3 relation can be
scaled from z_1.


Broadband data


For the next cluster, the observers require broadband images (29aug0171 - 29aug0172).
2-shuffle data


We repeated the same as before, remembering to type in the new "z" values, to get two more shuffled images (29aug0173 - 29aug0174).
CuAr sausage cube


The tiny images 29aug0175 - 29aug0254 comprise another sausage from which we obtain z_1 = 1.40949 lambda - 9473.13. If you compute the "z" values for the earlier wavelengths, you discover that the TTF has drifted by half a spectral bin (dz=7, FSR_z=373) during the last 4 hours. It is commonplace for Queensgate hardware to drift by this much in the first half of the night, and that's with the Stonehenge capacitor! The second half is not expected to drift at all.


Data cube


The observers requested a demonstration (from the safety of Europe) that the TTF calibration and the published cluster redshifts put the emission lines in the right place. So we took a series of small images (29aug0255 - 29aug0261) in short exposures which stepped through both the predicted H-alpha and [NII] lines. After stacking them in iraf, we bin up the spatial dimension 10x10 to produce a decent spectrum on the galaxy and a neighbouring star. and this is the outcome:

Note that the H-alpha and [NII] lines occur precisely where the lambda-z relation predict them to be.


2-shuffle data, CuAr arcs & broadband data


The remainder of the observing programme comprised twinned, shuffled pairs of images for three more clusters, interspersed with CuAr arc images and a few broadband images. The arcs demonstrated that the TTF was rock steady for at least the last six hours of observing. Summary: Satellite trails remind us that the sun lurks below the horizon. In fact, the last image was saturated.
Flux standards were taken on earlier nights and on the following night.


Flux standards


In principle, a flux standard observed at a single "z" setting for each filter is sufficient to flux calibrate. But in practice, it is only slightly more effort to observe the standard at all settings and filter combinations and tilts. Unlike the dome flats, it is convenient and unnecessary to shuffle on flux standards, although this assumes you have shuffled fewer than 100 times.

The images 30aug0164 - 30aug0183 were obtained through the R1 and R3 filters, one set of which was for a tilted filter. The "z" values used for the various filter/tilt combinations are:

R1/tilt=16:     317     345     374     407
R1/tilt=0:       513     540     571     598     597     611     641     669     510     562     593     620
R3/tilt=0:       407     475     506     532

(Anyone who checks the logs will note some discrepancies with the actual "z" values used. This is because the TTF was recalibrated on the following night.
What matters is that the "z" values correspond correctly to the desired wavelengths.)

Since these values are not evenly spaced, much the best way to operate TTF is to use the run / run_ccd / run_file option under the TAURUS II control window. You should see

Etalon stepping via z values in a file
Filename                                                                       disk$raw:[observer]joss1.lis
We initiate this with PF1.

Place each set of values above in a different file under VMS and run each sequence in turn. Here are the parameters we used for one of them:


Dome flats


For the broadband images, sky flats are perfectly adequate (i.e. with no TTF in the beam). For the TTF blockers, we take two kinds of flats: (i) sky flats through the filters without TTF in the beam, (ii) dome flats for all "z" settings used above, i.e. with TTF in the beam. This needs to be done for all "z" settings, filter combinations and tilts. The 2-shuffle dome flats were obtained with: using all "z" pairs used to obtain data. With the dome flat lamps turned up to full, 3 cycles of 1 sec
appears to be sufficient.

CCD charge shuffling multi line
Csr_Default disk$user:[observer.cs_files]*.csx
Csr_Filename twostep1m
Preexpose_info 3_1000_100
Cycle_count 3
Bias True     False
Setup_Array True     False
Etalon_Start 438
Etalon_Increment 29
We set this running with PF1. This produced images 29aug0282 - 29aug0292; we close with a final flurry of arcs, 29aug0293 and 29aug0294. A magnificent night of observing.