The fundamental idea behind the ONAG was to provide a much wide FOV without any flexure, and also eventually a real time auto-focus capability (SharpLock).
To reach those goals the guider focuser as been designed first to avoid any flexure using a full body compression ring, not just a thin brass ring. The result is even full extended the guider focuser assembly is as rigid as one solid piece of aluminum. But it come at a price.
I have worked and still working to improve the product and find a better solution for the guider focuser. Beside flexure, which is very tricky to avoid (trust me), few microns could ruin your life, there are other requirements, such as back focus and the capability to remove the guider focuser drawtube for using directly 1.25" nosepiece or any camera body of same size (this may be useful on some configurations). this is important if you have a ST-4 for instance (I have customers still using a ST-4).
There are applications where the ONAG is used for NIR imaging, with guiding in visible, this is why the XT version have an integrated corrector inside the guider drawtube. However to be able to use SharpLock the drawtube needs to be rotated to create some astigmatism.
Experimental designs with Crayford focuser style did not go well, there is simply not enough room to have a dependable rigid solution, but I do not give up yet.
I which this one would have worked though. But I do not want to compromise on rigidity and back focus.
However I may have another solution which could improve the guider fine focus, in design phase.
From personal and customer experiences I would suggest the following approaches for fine focusing:
When close to best focus with the guider camera you could tight a bit the focuser more, enough to keep it from sliding, but not too much. Then twist back and forth the guide camera while pushing or pulling it, this helps to move it by just a little.
Another option is to use the T2 thread (M42) and locking ring, screwing or unscrewing the guider camera. We have added extra length to the thread for this. This would act like an helicoidal focuser. The guider camera body will roated some, but auto-guiding algorithms, as well as SharpLock, work with any guider camera orientation. T2 is an M42 x 0.75mm thread system which means a full rotation (360 degress) is 750 microns, likely you would need less than 360 degrees.
I am totally aware that I could make an even better product, I am certainly committed to this goal and I am opened to any suggestion.
It is just that some time good ideas could be more difficult than expected in the reality. He toke me more than 5 years to come to the current design on the market and I had to overcome many problems. At glance an ONAG seems an easy device to make, but in practice this is a totally different story (trust me on this too).
Finally I would like to thank you all for the feedback and suggestions made in this forum, I do value them. I am an astronomer first and my goal it to come with new innovative solutions to make our hobby even more fun to do and enjoy!
