Giles replied to the topic 'For those with focus issues' in the forum. 2 years ago

OK, so there are focus issues, we can put these down to:
* We have backlash
* We have slippage
When we say backlash we generally means that there is a delay when we change direction of the focuser that there are a certain number of steps that we take that have no effect, and it is until these steps have expired that focuser actually moves.
When we say slippage we mean that during he course of the movement of the focuser, some movement is recorded where no movement happens at all.

We can measure this, move your focuser from its normal in-focus position 10,000 steps in one direction, and move 10,0000 steps back, measure the difference of your starting position to your ending position, this is (more-or-less) a combination of the two added together.

Measuring backlash would be the same process, but with smaller steps (to eliminate the effect of slippage).

Now look at the settings under "Mechanics"

Initial Step Size - this needs to be a number that, not-withstanding the above values, creates a noticeable difference to the recorded HFR in an image.
Max Travel - this needs to be a number that the difference of gets you to within the focus value that you're looking for
Max Step size - this needs to be above the number of "Initial Step Size", if unsure, make it equal to.

20 seconds is pretty high for a focusing frame, the focusing algorithms will be able to detect differences far lower than the eye can detect, you don't need high gain settings, or high exposure settings for focusing, try the similar settings that you use for guiding, only raise them if needed.

You can't focus if seeing is bad, test these under optimal conditions, if they fail due to bad seeing later on, then maybe it was just a bad night for AP.


 

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