Skywatcher 250-PDS anyone?

[guydive]

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 February 22, 2021 by guydive

[discardedastro]

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 February 22, 2021 by discardedastro

[guydive]

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.. 

[mikeyj1]

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

[guydive]

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

Thanks! 

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

[Darren Williams]

there may be more issue with inward travel on the focuser for my eos 1100d no coma corrector there is only about 10mm of inward travel at focus so no room for filter wheel or filter draw oag etc., a dedicated camera should be ok as sensor is near the front which gives extra 30mm approx,  if not enough you will need to space out the primary or shorten the tube  just checked secondary it's a bit hard to measure approx 53mm x 76mm 

this is a 120 sec at iso1600 with cls clip in filter and 128 processed just in case your interested 

i'm proberbly a bit late by now but hope this helps

 

[Darren Williams]

also master flat auto stretch