SW ED72 backfocus

[Skipper Billy]

Just a head up for anyone thinking of using a SW ED72 as a guide scope.......

My plan is to use a SW ED72 with a ZWO ASI 290 MM USB 3.0 mono camera.

I wanted to have an all 'screw together' system and bought the adapter to convert the end of the focuser tube to T2 thread and the camera has a native T2 thread.

I knew I would have to play about with various T2 extension tubes to achieve focus with the focuser barely racked out to reduce focuser flop.

What took me by surprise was it needs 115mm of extension tubes !!!

This was to achieve focus on a tree 2 miles away so I cant see it being much different on a starry sky!

 

 

[RayD]

Good information as always, David.  Thanks.

[Tommohawk]

4 hours ago, Skipper Billy said:

Just a head up for anyone thinking of using a SW ED72 as a guide scope.......

My plan is to use a SW ED72 with a ZWO ASI 290 MM USB 3.0 mono camera.

I wanted to have an all 'screw together' system and bought the adapter to convert the end of the focuser tube to T2 thread and the camera has a native T2 thread.

I knew I would have to play about with various T2 extension tubes to achieve focus with the focuser barely racked out to reduce focuser flop.

What took me by surprise was it needs 115mm of extension tubes !!!

This was to achieve focus on a tree 2 miles away so I cant see it being much different on a starry sky!

 

 

So that's how they keep the OTA so short!!!

[Kelian98]

On 12/01/2019 at 12:30, Skipper Billy said:

Just a head up for anyone thinking of using a SW ED72 as a guide scope.......

My plan is to use a SW ED72 with a ZWO ASI 290 MM USB 3.0 mono camera.

I wanted to have an all 'screw together' system and bought the adapter to convert the end of the focuser tube to T2 thread and the camera has a native T2 thread.

I knew I would have to play about with various T2 extension tubes to achieve focus with the focuser barely racked out to reduce focuser flop.

What took me by surprise was it needs 115mm of extension tubes !!!

This was to achieve focus on a tree 2 miles away so I cant see it being much different on a starry sky!

 

 

Thanks ! I also put a ZWO ASI 120MM-Mini and I wasn't able to achieve focus. I was at 60-65mm... No I know it needs to be 2x that !