Focal reducer back focus

[diggy]

I have an Explore Scientific 3" .7 focal reducer/field flattener for my ES127ED refractor.  I also have a Moonlite focuser on the scope.  My problem is that the in focus amount is VERY small...so short, in fact, that auto focus routines fail because in trying to come inside focus, they run up against the 0 stop.  The only way I can think of to fix this is to lengthen the specified 55mm FR-Camera sensor distance, which in turn brings the focus point further out, giving the auto-focuser room to move in both directions.

My question is, what am I sacrificing by moving away from the 55mm back focus?  Does it affect the flatness of the field?  Reduce the image size?  Affect focus?  Does anyone know what sort of flexibility I have here?

 

 

[vlaiv]

Hi, welcome to SGL.

I'm afraid you don't have much room to work with here - field flatteners work at specified distance, and any alteration to it will result in poor star shapes at the edge of the field.

Is there any way you can move FF/FR forward? I'm guessing that you are using threaded connection to your focuser, and that FF/FR sits screwed at the end of focuser tube, but is there a chance that you can place it inside focuser tube? This will give you plenty of out focus (about the length of FF/FR body).

For example, my 2.5" focuser has M63 connection and I've got M63-M48 reduction with rotator. There are M48 both in front and at the back of this reduction, so I can choose to place my 2" reducer screwed after focuser, or inside the tube.

[x6gas]

You might find that you can increase the spacing a little without having too much impact on the field flatness across your imaging sensor.

If you add a spacer that gives enough headroom for your auto focus routine you can then take some test shots of a relatively even star field and run the results through CCD inspector to see how much curvature you have. Some flatteners are more forgiving than others and the longer the focal length the tighter the tolerance but it might be worth a go...

[ollypenrice]

I agree with Vlaiv: is there a way to get the focuser closer to the objective.

My other thought is that the Moonlite is not the ideal focuser for robofocus because, as a traditional Crayford, it may prove prone to slippage. I know several people who've found this problem. The robofocus doesn't know where the drawtube is but only how many steps in focus shaft rotation have been made. Will these always correspond precisely with drawtube movement?

Olly