Mark Aston replied to the topic 'Tips for Auto-Focus' in the forum. 1 year ago

Thanks John, this is most helpful information.

I have been playing with EKOS focus module for quite a while now and your, Peter and Nigel’s comments have been very helpful. However, it appears there are two specific attributes of my RC setup that have exposed difficulties:

(1) The iOptron stepper motor belt-driving the focuser is quite course in real terms
(2) The central obscuration of an RC is bigger than those of newts, cass, SCTs, and Maks. This means that the diffraction donut is a much greater % of the out-of-focus image area and becomes more apparent, more quickly, than other designs as you move thro’ focus.

The courser step resolution of the iOptron set-up exacerbates the issue, and I may well just move to a ZWO EAF as installed on my large refractor and Mak-Cass.

However, given the success Peter has with a large SCT even pushing the boundaries, as you put it, it does beg the question if EKOS focus module could include an option to measure only the outer diameter of the diffraction annulus, once the HFR starts to flatten out, in a future update?

I have no doubt I can further refine the settings to better focus my RC set-up, and will actively play with this (once I get a clear night!); it is important to note that I also use NINA and a couple of other autofocus programmes on my observatory PC that do noticeably work better than EKOS. This appears to be mainly due to EKOS detecting sub-sections of the annulus (as you noted) sooner in the thro’ focus process than the other programmes.

I really wanted to move to the more stable implementation of EKOS on a RasPi and retire the PC that struggles in a remote coastal observatory, hence am very keen to see if EKOS can work with my RC - it does focus nicely with the other scopes by the way…

All the best