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:
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.
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
And since we want to make (de)activating tasks as easy as possible for you, the buttons for doing so have also changed
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
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:
- re-machining the camera flange in preparation for the MORISOT spectrograph pick-off unit
- installation of the piggyback telescope at the 2nd Nasmyth focus
- replacement of the roof controller
- restoring the connection to our weather station
- installment of a new interior webcam
- alignment of the telescope optics
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.
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
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:
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:
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):
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:
And this is how we will process the flat fields in the future!