## Focusing

- Details
- Written by Husser, Tim-Oliver

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...

## Current weather & roof status

- Details
- Written by Husser, Tim-Oliver

On the 'Status' page you now always see the current weather status at both sites together with an explanation, why the roof is closed at the moment:

We hope that this clears up a little on why the telescope is not observing under certain weather conditions. Keep in mind that we have a lot more closure rules than those that are plotted in the weather plots!

## Wind sensor not working

- Details
- Written by Husser, Tim-Oliver

During a heavy hail storm two days ago, the weather station at McDonald Observatory was partly damaged. Due to this, we don't have a working wind sensor on the mountain at the moment. Therefore we will take the robotic system offline, until this problem is solved. We apologize for any inconvenience.

## New system for observation constraints

- Details
- Written by Husser, Tim-Oliver

We've just updated the constraints system, which now allows much more flexibility, e.g. you can now set min and max values for each constraint. Here is how it looks now:

For a new task you will be presented with a default set of three constraints. You can add new ones by selecting them in the dropdown list at the bottom and clicking "Add". Constraints can be removed by clicking on the red X button next to each text field.

Please be aware that we are now enforcing some constraints, i.e.:

- Min elevation of target: 10
- Max elevation of sun: -10
- Min on-sky distance to moon: 5

So when you edit an existing task, what can happen is that your task is not valid, although you haven't changed anything. This is because the formerly default values are outside the new limits. You can fix this easily by clicking on the button that pops up, whenever one of the enforced constraints are not met:

## ImageJ timestamp problems

- Details
- Written by Hessman, Frederic

The public domain image processing programme ImageJ is recommended for schools in our "Astronomie & Internet" project. When the exact time of an observation is important, ImageJ reads the FITS header to produce a Julian Date at the time of mid-exposure. The old STELLA FITS headers contained a "JD" card which was useless and resulted in incorrect times. This card has been removed from the FITS headers in all data (including images obtained in the past). If you need to correct old data files, we offer a special ImageJ plugin which renames "JD" to "JD-END" and thereby forces ImageJ to calculate correct times.

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