Cross/square shaped stars Esprit 120 0.77x reducer

[Mickconn77 (Mick)]

Spent all night playing with backfocus and tilt last light, trying to dial it in as best as I could. Although I made considerable improvement, things are far from perfect.
Im left with a strange abberation at the extremities of my image, and given the sensor is not even aps-c size, I feel something definitely needs to be addressed.

Details:
Skywatcher Esprit120
Skywatcher 0.77x reducer
Optolong L filter/ Astrodon 5nm Ha
Asi1600mm

Melbourne based

[By.Jove (Jove)]

Looks like astigmatism

[Mickconn77 (Mick)]

Thanks Jove.

I did think the same. I’ll post a better image showing all corners and centre shortly.