medfreaky I always wonder how the RA encoder is working if at all with the ASIair.
Mechanical gears are not precise enough for long exosure astrophotograpy.
To counter the gear errors, you can add a high resolution shaft encoder to the RA shaft, and use the encoder output to correct for any gear deviations, Yoy can also add an autoguiding system, by using stars as the reference position.
Now, the rub is this: the mount's firmware has to know how to add the autoguide correections to the encoder corrections! It is just a couple of lines of code adding an error accumulator, but many mount manufacturer misses this.
If the firmware is not correctly written, the two corrections will be "fighting" each other. Autoguide could try to send the mount one way, and the saft enocder could insists it is correct (instead of adding the autoguide corrections to itself).
Before buying a mount, one needs to investigate if an encoder version of a mount can be autoguided without the "fighting." If not, one of them has to be turned off (use encoder for shoft camera lenses, and autoguiding for long exposure DSO).
My RST-135E has no problem being autoguided. On clean windless nights, that strain wave mount (with a Renishaw encoder) gives me better than 0.3" RMS error on the RA axis (the error on the declination axis depends pretty much solely on the polar alignment error and the centroid estimation):
By itself, the Renishaw encoder in the RST-135E has a worst case error of about +/-2.5" peak (according to specs, but mine measured significantly better most of the time). It is prerfectly fine for long exposures with a 135mm camera lens, but definitely produces ovals on a 455mm focal length main scope when seeing is good.
So, we know that autoguiding is not fighting with the Renishaw encoder in the RST-135; there is no way the Renishaw encoder in the RST-135E by itself can produce 0.25" RMS error. The encoder is not useless, and allows you to use slower guide rates, although I stay with 0.5 second exposures to get guide rates of 2FPS with an ASI178MM guide camera.
Chen
