What type of mirror is feasible for Schlieren photography setup?

I would like to build a Schlieren photography system with a single mirror setup^*. My question is – what type of mirror (parabolic/spherical) is good or at least feasible for this setup?

The theory says that spherical mirror is the right choice since a point source is needed to be focused back to a point at similar position. However, parabolic mirrors are easier to find and get and some people seem to use them. My question is – what conditions need to be met so that a parabolic mirror is sufficient? I’m loking for data – focal lengths, mirror diameters, etc. Personal experience counts.

Thanks for any answers.

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^*light from a point source goes through the tested area onto a mirror, is reflected, goes through the tested area again, is focused into to a single point where there is a knife edge and then ends up in camera