Field Flattener - does chip distance vary with focus?

[ian_bird]

Hi All

As the subject says - with a Field Flattener (FF) - does the optimal FF to chip distance vary with focus?

That is to say - I have my H-18/Filterwheel/OAG/AO/FF as my imaging train. That then screw mounts (Tak 72mm) to the back of my Feathertouch focuser. So I have the distance from the FF to the CCD set as per specs and a bit of tweaking.

So my question is - as focus changes - do I need to adjust the FF to CCD distance in any way to allow for the change in focus? Please say "No"! Please say when it is set - you are done.

Cheers

Ian

[Earl]

Once spacing is correct and all is in focus and as flat an image that can be achieved. (that is the tweaking part)

Job Done that will be the correct spacing.

[ian_bird]

Hi Earl

"and all is in focus".

That's my question. If the focus changes - even slightly - will that affect the FF spacing - even slightly?

Cheers

Ian

[Earl]

No focus will not change once the spacing is correct, it will change a little while sorting this correct spacing

Sent from my Nexus 7 using Tapatalk 2

[ollypenrice]

Earl's right. No change.

Chip distance is tricky though, and affected by any filters in the line. I think the rule is to add a third of the glass thickness per filter. I'd look that up though.

Olly