Takahashi FS60cb back focus confusion....

[Stuart1971]

Hi,

I was  looking at this data sheet for the Takahashi FS60cb, and am confused by the two measurements circled in red, why are they different....surely they should be the same, and am I correct in thinking that this is the measurement of where the sensor or eyepiece should be to get focus...

Can someone explain... 😀👍

 

[Kinch]

Not sure - but perhaps the Backfocus length (higher number) is the distance from the back lense. The one to work with is the Metal back focus otherwise known as metal back distance.

[Scooot]

There’s a slightly different configuration for imaging, I think that probably accounts for the difference.

 

Edited January 4, 2021 by Scooot

[Kinch]

If imaging  - you always use the metal back distance......simply because that is the one you can measure. It reality, it is only a guide because you can move the focuser in/out.....but it helps you decide what extension tubes (if any) you need.

[Stuart1971]

1 hour ago, Scooot said:

There’s a slightly different configuration for imaging, I think that probably accounts for the difference.

 

Yes but the distance is the same....

[Silent Running]

Hi Stuart,

I recently acquired an FS60CB but haven't had much chance to use it yet. I like the fact that Tak provides the system charts which not all makes do... but then I too found they are rather capricious! Have you been able to resolve the back focus issue? Could you post any pics of your imaging setup for guidance?

Thanks,

Jo

[Stuart1971]

3 hours ago, Silent Running said:

Hi Stuart,

I recently acquired an FS60CB but haven't had much chance to use it yet. I like the fact that Tak provides the system charts which not all makes do... but then I too found they are rather capricious! Have you been able to resolve the back focus issue? Could you post any pics of your imaging setup for guidance?

Thanks,

Jo

Hi,

No I am still a bit confused where all this is concerned....the focus point is apparently at 151mm behind the back edge of the focuser housing.

I saw your other thread about this backspacing issue, and see you have the reducer....the backspace for this is 56mm, but not sure where the reducer goes, directly on the back of the scope or after some spacers, the issue with Taks, is they use non standard thread sizes, which makes betting spacers really awkward...the threads  on the reducer are M56 on the scope side, and M52 on the camera side, but I can’t find spacers with either of these threads....☹️

 

[Silent Running]

Hi thanks for this. That is useful info on the threads on the reducers and explains why none of my other adapters fit!!! 

[Stuart1971]

56 minutes ago, Silent Running said:

Hi thanks for this. That is useful info on the threads on the reducers and explains why none of my other adapters fit!!! 

What I have found is a Baader M43 to M42 adapter, this is specifically for these scopes and allows M42 accessories to be fitted, if you look as the pieces that come on the scope, you will see that there is an M43 thread just after the two largest spacers, see red arrow on right of image below, from this point on you can then fit M42 adapters...

 

Edited January 11, 2021 by Stuart1971

[astro_al]

I am pretty sure 162mm is the correct figure. I use binoviewers so back focus is always a challenge with this focuser, same on the FC-100DC which is 1mm less. I use the Baader clicklock and the Baader prism diagonal to maximise available back focus. Don’t use it for imaging though.