Tech – Telescope Alignment & Guiding

Soon after our First Hardware/Software Test we again went to finish what we could not finish before, namely the 16-point alignment to increase pointing accuracy, reading out PEC-Data and enabling auto guiding.

First we started with the alignment. Having read the corresponding section in the Sitech Controller manual we did the alignment by memory and not by the book, probably the first mistake we made that night. Getting 16 stars across the sky centered and saved took a while and the sky was already nicely dark when we finished, so we were already getting anxious to make some pictures. Calculating the pointing model however took no time, so we instantly applied it and thus made our second mistake that night.
We should really have tested the pointing accuracy because as it turned out later that night we seriously worsened the accuracy of the telescope with the 16-point model in comparison to a 1-star alignment.

Mounting the guide scope was too easy, so we knew there would be trouble ahead.

Not knowing this, we went on to installing the guide scope, a Skywatcher 80/400 Refractor. We planed to initially use an old ASI 120MM camera which we had lying around, and already good experience with. However, we could not get it to work with the PhD2 guiding we use on our telescope control server. After some At-Home investigation, we assume this has to do with a combination of the USB-Port handling of our virtualization and the known issue that the ASI 120MM sometimes does not like newer USB ports. Don’t get me wrong, this is the first time the camera does not want to work and our solution for this is using the ASI 120MM-S which is newer, has USB3.0 and simply works after plugging it in.

Beside the technical stuff we also wanted to make some pretty pictures. For this we chose NGC7635, the bubble nebula. We took 5 exposures for 30sec, 60sec and for 120sec exposure time for each LRGB filter. For the H-α (12nm) filter we took 20 exposures with 30 sec, 10 with 60sec and 5 with 120sec exposure time. And to be honest, due to the night being a bit cloudy and our technical problems the raw material did not look very promising.

During the testing we observed another problem: the RGB filters have a different focal plane then the H-α filter. Therefore we always have to refocus when changing between RGB and H-α. And this was quite challenging for the auto focus since we needed up to 3sec exposure times for each focus image. But the focus handled the situation quite well.

But after some DSS and FitsWork magic we arrived at this:

NGC7635, the bubble nebula taken with the KRO, composed by Günther Truhlar

One problem you can see in the picture is that the camera does not have a anti-bloom gate. Additionally we didn’t make bias or dark frames (jet), but will make some in teh future. Also the sky was a bit dusty, but the biggest problem was already mentioned above, the guiding did not work and the 16 point alignment worsened the accuracy, so we had to turn it off an re-shoot the 120sec exposure (because we did not check the alignment immediately).

So the next steps are to install an test the ASI 120MM-S as a guiding camera, but more importantly redo the 16-point alignment step by step according to the manual. After this is done, we can try out the automated alignment using plate solving (see here page 78).


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