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Hey all,

I'm considering the 250PDS. The main use is visual, and the second is EAA (I currently use a zwo183mm). 

What I'm hesitant about is the primary focus. since It's mostly for visual, I'm wondering on how much back focus it has compared to the standard Skywatcher 10" dobsonians? with the standard dobsonias with the same optics and focuser you already need an extension tube for most eyepieces...

Also, there's probably a "bigger" obstruction (bigger shadow on primary)? 

Edited by guydive
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  • guydive changed the title to Skywatcher 250-PDS anyone?

Pretty sure the obstruction will be larger - the oversized secondary to avoid vignetting was a definite feature of the DS models.

Back focus on the 200PDS at least is plentiful but with a decent extension tube (the Tele Vue ones are great, but others are available - just do upgrade from the Sky-Watcher one!) that's no issue really, and if you swap the focuser for something a bit nicer or put e.g. Baader Clicklock on the focuser that takes up a good amount of backfocus anyway, and you'll probably want a coma corrector eventually which will take up a bit more...

Edited by discardedastro
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On 22/02/2021 at 23:56, discardedastro said:

Pretty sure the obstruction will be larger - the oversized secondary to avoid vignetting was a definite feature of the DS models.

Back focus on the 200PDS at least is plentiful but with a decent extension tube (the Tele Vue ones are great, but others are available - just do upgrade from the Sky-Watcher one!) that's no issue really, and if you swap the focuser for something a bit nicer or put e.g. Baader Clicklock on the focuser that takes up a good amount of backfocus anyway, and you'll probably want a coma corrector eventually which will take up a bit more...

do you happen to know how big is the secondary? Vendor doesn't have this info at the moment.. 

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On 24/02/2021 at 09:18, guydive said:

do you happen to know how big is the secondary?

Hi I made a template when making a secondary dew heater, its roughly 80mm by 57mm see the attached image.  The Quattro is considerably larger for the reduction in F ratio to F4

secondary.jpg

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9 minutes ago, mikeyj1 said:

Hi I made a template when making a secondary dew heater, its roughly 80mm by 57mm see the attached image.  The Quattro is considerably larger for the reduction in F ratio to F4

secondary.jpg

Thanks! 

if that's an oversized secondary what the size of the classic dobsonian's secondary??

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