Preparing for an observing run

To put your mind at ease, TTF operation without shuffle modes is very straightforward. With shuffle options, a little more thought is required. Here are some considerations to keep in mind before coming to the telescope. Feel free to email me (jbh@aao.gov.au) if you are concerned at all. I will contact you before the run in any event. One of the things we need to watch out for are TTF runs scheduled close together at the WHT and AAT. In certain instances, TTF runs can operate concurrently if, for example, the red TTF is required at the AAT while the blue TTF is required at the WHT.

Observing program. It helps to know before the run precisely which TTF filters you want to use, the wavelengths and bandpasses, and which if any broadband filters (UBVRI) might be useful as well. The available filters are shown here.  Don't forget that you can get a little more filter response with tilting, shown at the same web location. If your objects do not fall within the intermediate filter bands, consider going to lower resolution and using the UBVRIz broadband filters.
 
Detector choice and SNR calculations. Now that you have the telescope time, you might check your SNR calculations once again with the TTF signal-noise calculator. If your back-of-the-envelope calculations differ a little from mine, do not worry. There may be regions of parameter space that the calculator does not handle particularly well. Recall that the TTF calculator includes the effects of atmospheric emission and absorption. Consult the high resolution, atmospheric spectra used by the calculator. 

Both the AAT and WHT have a choice of detectors (see the respective web sites), so be sure that you are happy with the current selection.

Choice of input beam. Be sure to decide well before the run which top end you will need for the run at the AAT, and which TAURUS camera you will need at the WHT. In most cases, we recommend the f/8 at the AAT, and the f/2 camera at the WHT. (It is unlikely you will need the slower beams now.) Both require personnel and valuable time to change over.

Phase effects. This is a deep issue. It's best that you consult with me on this one. If we tilt the filter to remove ghost light, the phase effect over the filt increases quadratically with the tilt angle. If you are observing a field of point sources, or a source with an axis of symmetry, we will not need to tilt. If you are observing a highly irregular source which extends over much of the field, tilts may be necessary.

Shuffle frequencies. If your intention is to beat variations in the sky background, the power spectrum of OH fluctuations is complex both in a temporal and a spatial sense. See the article by Adams & Skrutskie (1996) . We have set up shuffle files for frequencies of 10, 20, 30, and 60 sec. If you are working near [OI], different rules apply. This issue is a can of worms. Let's try it on the night and decide then. (There will soon be a web tool for producing arbitrary shuffle files.)

At the AAT, a more important use for charge shuffling is to beat variations in seeing and atmospheric transparency. As a working principle, we find that 20 shuffles every 30 sec works well enough for a 10 min exposure in each of two bands. Curiously, you can even get useful comparative sets of images in fairly heavy cirrus and in bright twilight. The absolute calibration of the images may be useless, but we find the relative comparison to be quite the contrary.