Hello all RASA users!
I recently bought a RASA 8 that had been in use for several years. After the first tests I didn’t notice any problems, but when I moved to 15-minute exposures, the stars turned into streaks. It feels like the guide scope is not following the main scope’s movement. In an hour the drift is about 60 microns, i.e. 1 micron per minute. I checked the guide scope’s mountings, but the problem wasn’t there. In the end the only remaining option was mirror shift in the primary mirror. However, I wasn’t able to eliminate this movement in any way. When I exposed the focuser knob shaft, I could move it up and down quite freely. I use EAF, but I have removed without any change.
Here are a few points related to the situation:
- The problem is not in tracking, as tracking stays mostly at 0.3–0.5" RMS on both axes without large jumps.
- The guide scope’s attachments have been checked and also tested with another guide scope.
- The telescope has been kept for a long time in the same position it will be used for imaging, so that the mirror would settle into place. This has had no effect. The movement is still here after 3 hours!
- The movement is, in the long run, steady or at most slowly decelerating. Still, in the short term, occasional jumps are visible.
- When the scope rotation is adjusted so that, when looking at the image, the RA axis is horizontal and the Dec axis vertical, the movement is largely sideways (but not totally) when imaging near the southern meridian.
- The movement observed near the southern meridian is the same as the up/down movement of the focuser knob shaft.
In the end I disassembled the back of the scope and noticed that not only does the focuser knob move, but also the part attaching behind the mirror rotates relative to the mirror holder. Is the focuser knob’s movement or the above-mentioned rotation normal? Any ideas what might be wrong?
It is worth noting that with 5-minute exposures the stars showed only slight elongation, but at 15 minutes it became very pronounced. The telescope is excellently collimated and tilt has been carefully adjusted to be nearly eliminated. I’m using a small-pixel monochrome camera (ASI183MM), which also makes errors more apparent. I know that many people use this scope in city environments and typically shoot with exposure times of no more than 5 minutes, so they may not even notice if there are issues. I, however, image under very dark skies and want to get the most out of the scope’s optics. With short exposures everything looks fine, but with longer ones the problems appear.
Does anyone have an idea what could be done?
I have integrated 4 x 15 minutes exposures in this image without dithering so you can see the movement.

And image from focuser knob shaft which moves up and down without resistance.

I recently bought a RASA 8 that had been in use for several years. After the first tests I didn’t notice any problems, but when I moved to 15-minute exposures, the stars turned into streaks. It feels like the guide scope is not following the main scope’s movement. In an hour the drift is about 60 microns, i.e. 1 micron per minute. I checked the guide scope’s mountings, but the problem wasn’t there. In the end the only remaining option was mirror shift in the primary mirror. However, I wasn’t able to eliminate this movement in any way. When I exposed the focuser knob shaft, I could move it up and down quite freely. I use EAF, but I have removed without any change.
Here are a few points related to the situation:
- The problem is not in tracking, as tracking stays mostly at 0.3–0.5" RMS on both axes without large jumps.
- The guide scope’s attachments have been checked and also tested with another guide scope.
- The telescope has been kept for a long time in the same position it will be used for imaging, so that the mirror would settle into place. This has had no effect. The movement is still here after 3 hours!
- The movement is, in the long run, steady or at most slowly decelerating. Still, in the short term, occasional jumps are visible.
- When the scope rotation is adjusted so that, when looking at the image, the RA axis is horizontal and the Dec axis vertical, the movement is largely sideways (but not totally) when imaging near the southern meridian.
- The movement observed near the southern meridian is the same as the up/down movement of the focuser knob shaft.
In the end I disassembled the back of the scope and noticed that not only does the focuser knob move, but also the part attaching behind the mirror rotates relative to the mirror holder. Is the focuser knob’s movement or the above-mentioned rotation normal? Any ideas what might be wrong?
It is worth noting that with 5-minute exposures the stars showed only slight elongation, but at 15 minutes it became very pronounced. The telescope is excellently collimated and tilt has been carefully adjusted to be nearly eliminated. I’m using a small-pixel monochrome camera (ASI183MM), which also makes errors more apparent. I know that many people use this scope in city environments and typically shoot with exposure times of no more than 5 minutes, so they may not even notice if there are issues. I, however, image under very dark skies and want to get the most out of the scope’s optics. With short exposures everything looks fine, but with longer ones the problems appear.
Does anyone have an idea what could be done?
I have integrated 4 x 15 minutes exposures in this image without dithering so you can see the movement.

And image from focuser knob shaft which moves up and down without resistance.
