Calculating backfocus

[Hoges]

Hi guys,

I was looking at the SW Espirit 120 triplet apo and noted that it stated the visual backfocus of the scope listed at 225mm from the end of the focusser tube. Am I correct in assuming that, with the focusser fully racked in, 225mm is the extra distance the image would focus for a camera chip?

What about an eyepiece?

Would this also mean that my Denkmeier bino viewers (114.5mm needed) and a 90 degree star diagonal (95mm needed if I measured right) for a total of 209.5mm would actually come to focus (depending on eyepiece) without needing to modify anything?

[Astrofriend]

Hi,
It's always difficult to tell if there isn't any drawing how they measure the distance.

When I read here they say backfocus is 180 mm.

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p5123_Skywatcher-Esprit-120ED-Triplet-Super-Apo-Refraktor-Teleskop.html


Maybe there are different models or they measure it different.

But it must be when the focuser is full racked in as you say.

When they say visual backfocus I'm not sure. They could mean with a attached 90 degress prism. But then the backfocus from the diagonal should be shorter.

This telscope also have a big 3" focuser, normally there is a reducer to 2" when visual. From which are the visual backfocus?

Now I maybe just confusing you, but this is confusing and important to have the correct distance and how they measure that. Otherwise it can be expensive mistakes.

/Lars