Identifying focal point

[alistairsam]

Hi
I have a 10inch f4 mirror in a scope I'm building and wanted to mark the position of the uta with around 30mm in focus travel
If I focused during daytime at a terrestrial target say a km or so Away, how far would this be when focusing on a star

Also, would anyone know the distance to the sensor of a canon 1000d

Thanks

[Merlin66]

The standard backfocus for a DSLR with a T2 adaptor is 55mm
If you focus >1000mtr there will be little movement between that and infinity.The greater the distance obviously the better. (If it does move a little then it will move inwards towards the mirror.)
EDIT:
Just worked it out for you....the difference between focussing at 1Km and infinity is less than 1.0mm on your scope.

[alistairsam]

Thanks for that ken

Good to know that the change is minimal, gives me a bit of confidence before cutting my trusses!

I just realized that the canons have a white line on the body denoting the position of the sensor, so I've racked out about 10mm, and then focused on a building about 2ks away and marked the position of the uta.


Cheers