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Hi Clare & Peter, a low powered nose Barlow in front of the Quark( telescope side) will increase your focal ratio. The Quark's built in telecentric Barlow will still give you a telecentric beam into the etalon. Don't mask the aperture as you will lose resolution.
I stack Barlows when I use my ED72 for H alpha imaging. A focal reducer behind the Quark will bring your image scale to a more usable level.
There is a bit more glass in the image train and you may get some reflections, but this can be cured by using flats.
Martin
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