Quote:
Originally Posted by Camelopardalis
I guess it depends on your definition of OK 
With the reducer, the spot size increases from 3.8um in the centre to 8.7um at 21mm off-axis, and they’re not round. You prolly wouldn’t see that with something like a 6D with large(ish) pixels and a CFA. With a mono ASI6200/QHY600 you might. Assuming good conditions when the atmosphere isn’t smearing the shape of the stars anyhow 
The 1x flattener keeps the stars tighter, and with almost no vignetting... |
In this case "OK" would mean mostly usable in real world conditions (the ones we're likely to get around here).
I agree, the flattener is the most optimal way to go with it, but with the reducer and a full frame camera, you might have to crop a bit, or perhaps binning might work?