RC8" associated with CCDT67 front end configuration

[alpheratz06]

Hello

I would like to know what the configuration shall be in front of the focuser, that is to say between the back end of the tube and the focuser.

Is it necessary to add M90 extension  tubes or not? If it is necessary , what is the conect amount of extension needed?

Thanks for your support

Chan skies !

[vlaiv]

I don't think that you'll need M90 extension tubes, but it might also mean that you'll need to counter sink the reducer further down the focuser tube - depending on what reduction factor you decide to go with

Use this calculator to get rough idea:

https://www.wilmslowastro.com/software/formulae.htm#FR_a

Just a few pointers - CCDT67 has around 303mm of focal length. Working distance is about 10mm shorter than actual lens to sensor separation, so if you put sensor at ~91mm from M48 thread - you'll be at 101mm optical distance.

That will give you factor of x0.67. Depending on sensor size you are using, this might be too much. People often choose to go with x0.72 - x0.75 reduction instead.

For that you will need to use 1 - X / 303 = 0.75 so it will be X = ~76mm. This is optical distance, actual distance will be 66mm. In that case, focus won't move so much inwards - it will be only:

-25mm (so one 25mm extension removed.

But do keep in mind that if you have threaded connection with your focuser (you replaced stock focuser) - it will be difficult to reach focus even in this configuration as distance from front reducer thread to camera will be around 100mm.

It is best to either sink whole lot into focuser tube (loosing threaded connection part) or if you have 2.5" focuser - do some fiddling with different adapters / threads.