- Written by Husser, Tim-Oliver
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!
- Written by Husser, Tim-Oliver
tl;dr: for 3x3 images, please add an X offset of 2 pixels in order to get the images automatically processed by the reduction pipeline.
As you might have seen, we reactivated the pipeline and also started taking FLATs again, so that you can download fully reduced images.
For this to work, your images must have exactly the same dimensions as the calibration frames. This works fine in 1x1 and 2x2 binning. But in 3x3, unfortunately there is a little problem: The first 50 (unbinned!) pixels on the CCD are covered, so that they effectively serve as DARKs. Now we don't want a binned pixel to cross this border. This means that for 3x3 binning, we take the calibration frames with an X offset of 2 pixels, resulting in 48/3=16 binned pixels in the covered area.
Long story short: for 3x3 binning, your CCD settings should look like this:
Note: your reduced images will not undergo any dark/bias or flat correction, if the image dimensions do not match!
- Written by Hessman, Frederic
The open-loop (i.e. blind) guiding of the MONET telescopes is, well...., capable of being improved! As the pointing model gets better, the blind pointing will get better, but a large steel telescope just isn't designed to be so exact (small high-tech amateur telescopes can be so good that they essentially don't need any additional help in guiding). The best solution is to place an additional camera near the main science camera just for guiding, but we don't have enough focal-plane space for this solution. One of these days we'll get the piggy-back telescopes working, but a non-aligned guider whose camera has to be rotated differently from the science camera is quite difficult to develop.
For projects where lots of science-images are being taken (e.g. photometry of an eclipsing variable, motion of an asteroid,...), we can use the short exposures to correct the guiding while the series is being taken. Dr. Tim-Oliver Husser, our magical MONET-programmer, is testing such a system on MONET/South. Here is an example of how well the telescope can track over an hour:
As soon as the system is fully tested, we will add science-image autoguiding to all observations automatically. Observations using just a few long-exposed images obviously won't profit from this service, but most MONET projects will.
- Written by Husser, Tim-Oliver
tl;dr: Please avoid targets in the Azimuth range 0-90° (North to East). Due to shutter problems you will not get good images!
It seems that we're having some technical problems with the camera shutter at Monet/S: sometimes it opens only for a (too) short time and sometimes it doesn't open at all. Apparently that's connected to the position of the telescope, or more precisely, to that of the derotator.
For this test during the day with a closed roof we moved the telescope to different positions and took 5 pictures at each one. We expect some variations due to an inhomogeneous illumination of the roof, but for all mean count rates below 1000 we assuma that the shutter didn't open. In the following plot the mean count rates are indicated by the radii of the circles and those smaller than 1000 are marked red:
As one can see that is a vertical strip in the azimuth range 0-90° (North to East), where the problem occurs. The numbers in the circles are the absolute derotator positions. Plotting the mean counts over those positions, we get the following plot:
So it seems that there is some range in the derotator rotation that causes the problem, probably too much strain on some cable.
Therefore we recommend avoiding observations in that range for now. We will let you know as soon as the problem is fixed.
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