Focal reducer for SCT,advice please.

[paul mc c]

Hello,can a focal reducer be used with a dual speed focuser on an SCT,and if so where would it be placed.

Thanks Paul

[Peter Drew]

The SCT focal reducers are designed to screw on to the rear cell of the SCT, the dual speed focuser should follow this which can cause problems with some focusers. There is also an optimum distance between the focal reducer and the  camera sensor. Sometimes there is no other option than to return to the basic focusing facility aided by one of the manual reduction or electronic systems. Either of course, leaves you faced with potential mirror shift issues.  

[ChrisLX200]

It's true - the focal reducer need to be a specified distance from the sensor, and with a secondary focuser on the back of the SCT that distance lies within the focus tube. That's fine if the focuser has the capability to accept the reducer internally but mine didn't. The only solution for me was to machine down the outside diameter of the reducer and thread the end to screw onto a 2" extension nosepiece - then it was usable! This is not a recommendation - just my own solution, it does reduce the s/h sale value of the reducer slightly

Turned and threaded:

Screwed onto the end of the nosepiece:

Now it sits internally within the focuser body:

ChrisH

[RayD]

My Moonlite on my 8" SCT has the FR inside the tube.  As noted not all of them can, but Moonlite make the adaptor and the tube specifically for this purpose.

Other than this I believe there are a few dual speed focusers (Feathertouch) which replace the stock one, so doesn't affect the FR placement.

[Dave In Vermont]

Here's something you may find quite informative. It's written around the MallinCam-line of video-cameras, but the principals are the same in any case. And it's a good read - even for fun:

http://www.mallincam.net/uploads/2/6/9/1/26913006/focal_reduction_for_dummies.pdf

I find strange things amusing,

Dave

[paul mc c]

Thanks all,a bit more complicated than i thought with the set up i have,probably not worth the hassle.

Paul

[RayD]

Just now, paul mc c said:

Thanks all,a bit more complicated than i thought with the set up i have,probably not worth the hassle.

It's really depends what you are trying to achieve Paul.  ChrisH above has produced a superb solution that most wouldn't even attempt (I'm impressed), but the easy route if that's what you're after is a dual speed focuser in place of your standard one, but this of course does not solve the mirror shift problem with some SCT's.  Perhaps look at a motorised focuser which will help with reducing vibration and settling time after focusing?  With either of these options you would fit your FR directly to the OTA with the T adaptor connected to that (or other imaging equipment).