Autofocus and SCT mirror shift

dark energy

Hi,

as we all know, SCTs have the mirror shift "feature" when reversing the focussing direction. Also, the focus point should be approached by "pushing" the main mirror. Is the new focus routine in V1.5 able to cope with this behavior?

Thanks in advance

Markus

w7ay

Hi Markus,

From what I have been seeing in V1.5 Beta, the ASIAIR always start the AF process from a "high" EAF step location and works its way down to get an approximate curve fit. It then resets back to a "high" value and move down towards the estimated point using smaller step sizes on a second pass.

This has so far compensated for the "backlash" on my EAF/focuser connection -- both focuser and the flexible coupler on the EAF create a hysteresis.

However, I have noticed a problem with their second pass where it has consistently underestimated the EAF location by about 30 EAF steps on my WhiteCat51 belt-driven focuser (60 µm error) and about 20 EAF steps on my FSQ85 rack-and-pinion focuser (80 µm error). I.e., if I blindly reduce the EAF location by 30 and 20 steps respectively after the ASIAIR completes autofocus, I get a smaller HFD. In both cases, the autofocus errors place the focuser outside of the critical focus region.

Chen

dark energy

Hi Chen,

thanks for your reply. The way you describe it, this should also work for my RASA 11". No need to go for V2. :-)

Markus

pruntwhat

Chen,

Which belt-driven focuser are you using with your WhiteCat and ASIAIR?

Thanks,

Wade Prunty

Texas, USA

w7ay

Hi Wade,

I bought a ring gear for the RedCat51 from DeepSkyDad that he had sold with his electronic focuser. I also got the pulley from him (he says 3D plastic works a bit better than the aluminum pulleys for this purpose). His focuser uses the same shaft diameter as the ZWO EAF.

I then mounted the WhiteCat and the ZWO EAF on a 6mm thick aluminum plate (fabricated by a web machine shop in Seattle, WA). I also had to get a different EAF mounting plate fabricated (the one that makes up the short leg of the "L" shaped mounting plate).

However, since that time, Pavle has decided to support mounting of not just his own focus motor, but also the ZWO EAF. So it should be much easier to get an EAF mounted on a Red/White/Gray Cat now.

https://deepskydad.com/autofocuser/redcat/eaf

With the gear ratio, I am getting about 2.0 µm per EAF step (the WhiteCat51 + ASI2600 pixel size's critical focus region is between 19 µm and 52 µm, depending on whose formula you use :-).

DHL from Slovenia takes just 2+ days to the west coast of the USA via Frankfurt. Might be even faster for you if you live in east coast USA or Europe.

Good luck,

Chen