Takahashi 645 0.72x reducer

[Martin Pugh]

Hi folks
for those of you who might own this reducer, perhaps you can clarify something for me.

My google reading yields 2 backfocus distances for this reducer. One is 73.5mm and the other 56.2mm

I have found a diagram that clearly shows how both are calculated….the 73.5mm is from the actual lens element to the focal plane and the 56.2mm from the physical end of the reducer….the so called metal back distance.

The fact that you cannot attach anything to the reducer on the camera side, would suggest that when working out your optical line up, you should use the 73.5mm as the ‘working distance’.

Agree?
thanks
Martin

[Tilt (Michael)]

Hi Martin

I'm familiar with the reducer and I will throw a third backfocus distance into the mix.

Probably the easiest way is using the 90.4mm back focus distance. Look at the system diagram and locate this.

For instance TKA36581 (645RD Adapter) ---> CAA ((CCA-250) Part 6A) will put you at 90.4mm to go until you reach the focus point. I am assuming Takahashi are using the middle of their focuser travel for this calculation.

With all that being said, the reducer is in a fixed position, so you only need to put yourself into the +-10mm range of the focal point once your EFW, OAG, Tilt Device (oh don't forget that part ) is configured. Getting nice and close to that 90.4mm distance is great and all however its not critical, your focuser should have enough travel to pick up focus and keep it there all night.

Michael

[Martin Pugh]

thanks

[Martin Pugh]

Michael….it turns out that 90.4mm is the distance to consider.

Having worked on the 73.5mm distance, I put the focuser to the estimated focal position. It was way off…..so I extended the focuser until the camera came into focus and I remember roughly what the focus position count was. Having applied the same calculations, the theoretical position taking into account 90.4mm backfocus, was very close to what the focuser count was.

cheers
Martin

[Tilt (Michael)]

Originally I was confusing myself with the 73.5mm and 56.2mm distances, therefore I decided to work from 90.4mm.

When all said and done I think the camera sensor ended up around the 91.5 - 92mm mark, which turned out to be quite close to focus. Final adjustments were made with focus travel.

Michael