Weird Focus Curve

[Saturn488]

Recently I have been a 'W' shaped focus curve instead of the classic 'U' or 'V'.

Currently happening with my narrowbands which are 6.5nm and the focus time per filter is 20 seconds.

Any idea what could be causing this effect? Some sort of backlash?

Step size is around 30 steps with 9 focus points.

[Drac0 (Mark)]

Try reducing the exposure time, it can help with this issue. Note that with my 3nm filters my AF exposures are only 5 seconds, but I use different software so your mileage may vary.

Cheers,
Mark

[Saturn488]

Originally Posted by Drac0

Try reducing the exposure time, it can help with this issue. Note that with my 3nm filters my AF exposures are only 5 seconds, but I use different software so your mileage may vary.

Cheers,
Mark

Thanks Mark. 5 seconds would be handy!

I reduced it to 15 seconds and increased the step size and results are a little better.

My curves were getting better during the night so unsure if seeing conditions had anything to do with it.

Other possibilities would be an obscure star it’s focusing on?

[The_bluester (Paul)]

Is that focusing on a single focus star or the full field? As I recall it, SGP used full field focus but I have not used it for a couple of years so that could have changed to allow for a single focus star.

I was thinking with a single star, no change it has picked a double is there with a dimmer partner? I know with Voyager it might occasionally pick a double star and it can cause some odd results and when I see those I add that star to the list of "Bad" focus stars so it does not use it again.