ST-402ME Filter Wheel Positioning Errors

Greetings,

I'm trying to get a custom filter wheel to operate properly inside the ST-402ME. The issue I'm having entails the rapid shutter/filter/shutter cycle during an exposure. Inspecting the process close up with the ST's face plate removed, I can see that the stepper motor controlling the filter wheel is not able to properly stop, and or start the wheel rotation with its current, hard-coded acceleration/deceleration values. As a consequence, the wheel’s motor is losing steps and overshooting, causing severe misalignment after one or two short exposures. I gathered that the newly added mass of the filters near the periphery of my new wheel has caused this issue.

The first wheel I made was designed around 27mm diameter, 2mm thick glass filters in a machined aluminum disc. I knew that these filters were larger in diameter and thickness than those of the SBIG CFW-402 RGBC, which obviously works in this camera, so when the design failed to operate properly I designed a new filter wheel. The second attempt was with a machined carbon fiber disc, 1mm thick, with weights applied at each filter window to replicate the mass of the filters used on the CFW-402 RGBC. My weight estimate of the SBIG filter was based on a 20mm diameter, 1mm thick glass blank. Surprisingly, this setup showed the same positioning issues, although not as acute.

Reading more into the specific issue, I came across the motor phase selection in the advanced menu of CCDOps5, which I have been running throughout, and tried each motor phase setting with no positive outcome. I made sure I cycled power to the camera between each EEPROM write session. Furthermore, on the advanced menu, I manually energized the shutter and felt the holding torque of the wheel's motor. It seemed quite adequate. I know that the running torque will be significantly lower than the holding torque but it showed me that the stepper was relatively healthy.

To be clear, the original, unpopulated filter wheel is working fine during exposures. Something else I found while debugging the custom wheels, was that selecting a filter from the filter menu prior to an exposure works perfectly. even the heavier wheel would quickly rotate and stop on a dime in the correct position every time. I find it hard to believe that the wheel positioning system is that sensitive to small weight changes.

As this problem only occurs during an exposure, I suspect that the post position de-energizing delay may be the issue. That's admitting that I assume the acceleration/deceleration values would not change between use of the wheel in a manual positioning event, or an automated exposure event. at any rate, I doubt this is something that can be changed in firmware.

Any thought or insight would be greatly appreciated.


Thank You,

Andrew

 

Investigating further, I found that my calculations for the filter disc weight was based on a 2mm thickness, not the 1mm of the SBIG filters. Taking off half the weight at each filter window seems to allow the system to work properly. Perhaps it is just very sensitive to disc mass. This of course does not make me happy because I cannot seem to locate reasonably priced 1mm thick LRGB filters. I guess I could try to boost up the stepper voltage and see how well it would handle the thicker filters. So long as the holding torque is released during an exposure, which it seems to be, I should not have to worry too much about inducing more thermal noise. A larger step motor would most likely do the trick as well but there is precious little room for that.

Andrew

 

Thanks for the specifications on the filters Doug. I may indeed end up purchasing a wheel in the end. One might be hard pressed to find a set of high quality filters for less than the entire assembly offered by SBIG.