Business overview (Details) $ / shares in Units, $ in Thousands		12 Months Ended
	Aug. 06, 2018 USD ($) $ / shares shares	Sep. 28, 2019 USD ($) country	Sep. 29, 2018 USD ($)	Sep. 30, 2017 USD ($)
Class of Stock [Line Items]
Number of countries where products distributed | country		50
Underwriting discounts and commissions		$ 585	$ 3,950	$ 0
Preferred stock automatically converted into common stock (in shares) | shares	32,482,590
Convertible preferred stock, conversion ratio	1
IPO
Class of Stock [Line Items]
Shares sold in IPO (in shares) | shares	15,972,221
Public offering price (in dollars per share) | $ / shares	$ 15.00
Net proceeds from IPO	$ 90,600
Underwriting discounts and commissions	5,300
Offering costs	$ 4,600