Afocal imaging eyepieces with the Stratus line

I have been doing some great video though cheaper eyepieces, so I am upgrading to a 21mm Stratus off Astomart. But I cannot find any info on the size of afocal adapter I will need and where it might be available. The Hyperion eyepieces are already threaded so all I need with them is a step ring. Could I be saving money by going with Hyperion's? :S