As you might have noticed, the focus of MONET/North got worth and worth over the last couple of weeks. The problem is that we're not actually focusing at the moment, but only use a temperature model for the focus, i.e.:
Focus = slope * Temperature + offset
For the temperature we currently use that of the secondary mirror M2. All this works under the assumption that a change in the focus is only caused by the telescope structure expanding and contracting with the ambient temperature. This means that the slope is completely independent of the camera and the filter! So thanks to Paul Breitenstein and his students we have a pretty good value for this.
Now of course there's also the offset, which needs to be measured, because it does change with camera and filter. So today we re-measured this offset and therefore the image quality should increase significantly.
But a few words about how we focus.
There is a parameter called A4 that can be calculated from second-order moments of an image (see e.g. equation (5) of Tokovinin & Heathcote 2006). If we measure this A4 parameter for different focus values, we get something like this:
The resulting curve is a hyperbola with its extremum at the optimal focus. We can now measure the slope of its arms, which should be linear far away from the extremum. This has been done in the plot above, showing that of course both slopes are identical - apart from the sign.
For a quick focusing all we need to do now is to measure the A4 at an intra-focal point (i.e. somewhere on the left arm) and at an extra-focal point (i.e. somewhere on the right arm). With the known slope we can now immediately calculate the position of the optimal focus. With just two images. :-)
We will make this available to everyone soonish, so that you can get a perfect focus for your observations. The only problem with this is that the pointing is still pretty bad, so we cannot exactly predict star positions on the images. But we're working on that...