News

Disabling portal

08 Jul 2019 - Tim-Oliver Husser

After years of successfully running the robotic software created from our colleagues in Potsdam for their STELLA telescopes (which are twins of MONET), we are finally going to switch to our own software. This new software is called pyobs and is developed under an open-source licence. The source code can be found here:

https://github.com/pyobs

This also means that the mode of operation will change quite significantly over the next couple of months: we will fall back to a more manual observation planning, especially we will disable major parts of this portal and you won’t be able to create new tasks here anymore. If you want to observe something, please contact us directly. Data download will still be available, but will also change over time.

Small changes in task handling

21 Feb 2019 - Tim-Oliver Husser

You are all used to first creating a new task, and then “activating” it. What this last thing always did, was actually sending the task to the telescope and waiting for a reply. Especially when activating/deactivating multiple tasks this took quite a long time.

From now on, we only store your “request” to change the activation status locally, and then do the actual change silently in the background. This of course means that we cannot show you errors directly, but they will pop up after a short period of time, either as status “error” in the task list on the project page, or on the task page itself like this:

Errors

And since we want to make (de)activating tasks as easy as possible for you, the buttons for doing so have also changed their position:

New task GUI

Note that you can now also edit a task while it is still active!

The “Status” page that showed the activate button before, has now becomes the “Observations” page that only shows previous observations.

MONET/South down for repairs/upgrades

15 Feb 2019 - Frederic Hessman

MONET/South has been down for about a week due to repairs to the roof (a failed proximity sensor and 2 small bearings in the roof mechanism).

From 15 Feb to about 10 March it will also be down for various upgrades:

Fabrication of the MORISOT spectrograph begins

08 Oct 2018 - Frederic Hessman

The new low-resolution, fibre-fed IFU “COLORS” (“COmmerical Lens Optical Reflection-grating Spectrograph”) spectrograph for MONET/South, is now being fabricated in Göttingen’s mechanical workshop. Here is an exploded CAD-view of the internal design, showing (in optical order from upper left to upper right to lower left) the fibre-feed, the shutter, a commercial collimator lens, the reflection grating, the commercial camera lens, and the CCD camera.

MORISOT

With a spectral resolution between R=600 (sky in two 200mu “bucket” fibres) and R=1500 (IFU with 31 80mu fibres), the MORISOT system of spectrograph, fibre-feed (behind the filter wheel of the science camera), and autoguider (science camera) will make it possible to perform robotic spectroscopy projects - e.g. monitoring the changing spectra of AGNs, young stars, accreting binary stars, or identifying the character of an object found in a photometric survey (where one usually just has a brightness in several different colours). Schools will be able to study the differences in spectra of stars and bright galaxies and even measure the expansion of the Universe! The IFU (“integral fibre unit”) is a compact hexagonal bundle of 31 fibres that produces a spectral image of a small region on the sky - handy for viewing the differences in the spectra of extended objects like nebulae and galaxies.

The spectrograph, a Masters project by Maksim Tkachenko (HAWK Göttingen), will be installed on MONET/South sometime in 2019.

Taking flats with MONET

30 Apr 2018 - Tim-Oliver Husser

Calculating optimal exposure times for flats

Taking flats with a robotic telescope like MONET is way more convenient than doing it manually. While exposing images, we can easily calculate the optimal exposure time for the next image. If C_opt is the optimal mean count rate in the images that we aim at (usually ~30,000), we can take the mean count rate C_last of the last image, and together with the mean count C_bias rate in a bias, we can derive the counts C_1=(C_last-C_bias)/T_last, which added to C_bias, is the number of counts we would currently get with a 1s exposure. Now it’s easy to find the optimal exposure time T_opt using the last exposure time T_last using the rule of three:

T_opt = T_last / (C_last - C_bias) * (C_opt - C_bias).

Of course, the optimal exposure time changes during twilight, but with short exposures, this is good enough.

Measuring time span in which taking flats is possible

But, there is one big problem: how do we find the best time to start taking flats? While in morning twilight we can just take images until we reach a given mean count rate in the images, this is impossible in evening twilight without creating tons of over-exposed images. So, what can we do about that?

We measure it!

Here is a plot with the optimal exposure time in 3x3 over the elevation of the sun over the horizon for several filters:

Scaled flat times

The lines are fits of exponential functions to the data, which we can use to predict exposure times. This tells us now that, for instance, in evening twilight we should not start taking B flats in 3x3 before the sun has dropped to ~4.5 degrees below the horizon.

But why does the plot only show times for 3x3 binning? Simple: we can scale the 1x1 and 2x2 times by multiplying them by 9 and 4/9 respectively!

Combining flat exposures

Unfortunately, our current pipeline has some problems processing the flat images (that’s one reason, why we’re working on a new one!). Have a look at the final master flat for observation 20180429S-0116:

Bad flat

One problem is the weird pattern, that we still need to work out. But there are also stars in the final flat! The reason for this is that we used the average to combine the images, which doesn’t completely remove the stars. It would be better to use the median. But we cannot do this, because we’re rotation the image by 180 degrees mid-way through the flat series.

To explain this, let’s do a combination using the median, but separately for the first and the last 15 images for the same observation as shown above (mirrored vertically thanks to AstroImageJ):

Median flat Median flat

Here you can actually see the reason for rotating the camera: there is a gradient in the sky during twilight! The first 15 images (left image) are taken with North up, the last 15 images (right image) with South up. And that’s why we cannot simply use the median for all images, since the pattern actually changes due to the rotation.

During the night we don’t have this gradient anymore, so we want to get rid of it. Which we can simply do by using the average of the two images above:

Average flat

And this is how we will process the flat fields in the future!