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Ive used the 120MM-S on both 6” and 8” metal tube newts ( fl 900mm and 1000mm respectively ) at two different sites ( Bortle 7/8 and Bortle 3 ) for years with excellent PHD2 guiding results. I use an Orion 60mm guide scope fl 240mm with helical focuser on both rigs. Guide cameras use ZWO native driver too ( Main imaging camera uses latest Ascom driver )
When it comes to guide scopes for guiding the only reason why you might have to upgrade is to ensure your image scale ratio between main scope and guide scope is less than x5
When I bought my 10” Carbon fibre newt ( fl 1280mm ) I chose the 290MM as due to the smaller pixels gave me an image scale ratio of 4x which is fine for the big newt and it’s focal length. Also use an Orion 60mm guide scope with helical focuser. Obviously if my focal length was +1500mm then I’d opt for an OAG but the Carbon fibre tube has very little flexure and thermal expansion compare to metal tube so the guide scope works fine.
I did notice a big difference in sensitivity with the 290MM compared to the 120MM especially when the moon is up , you do have to adjust your Gain to dampen that noise from moon glare.
At your relatively short focal lengths ( <500mm ) I wouldn’t bother upgrading to the 290MM , it’s not going to make you guide any better. IMO
The 120MM in my case does a stellar job at focal lengths up to a 1000mm with plenty of guide stars for Multistar guiding to take full advantage
Atmospheric conditions at the end of the day is the primary driver of how well our rig can guide , image scale ratio / Optics would be second , mechanical limitations and inefficiency third and pilot error fourth.
Unless you want to dabble in planetary with the 290MM I’d save your money , the 120MM-S is a fine camera and has served me well for many years.
Cheers
Martin
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