Frequently Asked Questions

At what wavelength does the performance of the RTTF match that of the BTTF?
The rigorous answer is 680nm. However, note that the TTFs have been designed so that both BTTF and RTTF work well around H-alpha. This is important as H-alpha is a fundamental diagnostic against which all other lines must measure up. It provides a point of reference for comparing blue and red diagnostics. I have used the RTTF at 630nm but the highest resolving power I could reach was R ~ 210 due to the declining coating finesse.

Can we obtain an image anywhere over the optical range?
Only at low spectral resolution. At high resolution, you need to use one of the TTF blockers.

Is the broad-narrow shuffle ready yet?
Not quite. We found a bug in the electronics.

Can we shuffle between a broadband filter and a TTF band?
With the new broad-narrow system once fully commissioned.

What is the phase shift over the field and does it depend on wavelength?
The phase shift at 5' radius is 18 A, independent of wavelength. Consult Bland & Tully 1989, AJ.

Does the charge shuffling do anything strange to the data that we should know about, like pixel colour terms?
I have not detected anything like that. My biggest concern was always the increase in charge transfer inefficiency at low light levels. However, there is no detectable effect that I have seen, say at 400nm in a narrow band, which is quite remarkable really.

How often should one shuffle?
The U Minnesota aeronomy group have looked at the OH power spectrum. Its form is 1/f with no preferred scale. TTF observers have been getting good results by shuffling charge every minute or so.

Is the TTF stable?
At the beginning of TAURUS runs, we see a steady drift of up to 5z before the instrument settles down. This is due to the warm-up of the electronics. It is really important to get the telescope venting going in the late afternoon as the sudden drop in temperature produces a z shift. In Feb 00, the BTTF was stable for 18 nights right from the first night. This was a warm observing season and we suspect the small thermal differential contributed to the stability.

Nominally, TTF should be stable to high accuracy, say +/- 0.5z per night or better.  In practice, when TAURUS is on for long periods, we generally find that it settles down to a slow drift of about 0.1z per hour. So it is advisable to obtain an arc every hour or so. If observing at high airmass, it is certainly advisable to get a new calibration before you start observing. Some observers (SLL, Apr 00) have reported shifts of up to 2-3z which have returned to the previous calibration once back at zenith.

More recently, there have been reports of  drifts like 5z during the night. In some cases, this was caused by powering off TAURUS electronics or fiddling with hardware boards. In other cases, small files which were part of sausage cubes were being lost over the network, such that the arc had missing spectral bins! The new reduction scripts watch out for this now.

Concerning runs in Sep 99, there is an important point to realize.  The calibration lamps must diffuse the optics from a good distance, i.e. up in the chimney, rather than being stuck on a stick close to the camera. You need only budge the stick by 0.1mm to get a phase shift in the arc. We have worked hard with UCLES to find bright lines in the UV and still do not have an optimal set for work below 400nm. We are working hard to have this resolved before the EEV is commissioned since the BTTF will be an awesome machine below 400nm once we get a decent blue detector.  If anyone knows of UV line lists for Ca, Kr, Rb and U, please let me know.

Can we really observe something useful through cloud?
Yes indeed, providing some photons get through. If you have cast your measurement as a differential problem, say, Mg vs. Fe absorption in spirals, you will find that the absolute photometry is hopeless but the differential variation is real.

Can we do multi-slit or long-slit spectroscopy with TAURUS?
Yes indeed. Send me email for details.

Is there service observing with TTF?
See the AAO home page under Observing. TTF has been getting about 2-3 nights per semester for this.

Can we do TTF observing from the Northern Hemisphere?
You could up until 1998 but TAURUS was taken off the telescope at that time. Contact R. G. McMahon at Cambridge for details on progress in getting TAURUS back onto the telescope.

Are there key areas where TTF is not being exploited?
Yes. Some that come to mind are absorption line imaging, and exotic line diagnostics like [FeX], HeII/HeI, and so on.