Focuser options for an SCT

[allan gould]

Chris
I use a stepper motor controller with temperature compensation using the Sequence Generator Pro software with my moonlight focuser. Works a charm. Automatically re-focuses on a filter change and with every degree drop in temperature. Wouldn't be without it. The controller is in the DIY section of IIS by David Trewin in the UK.
Allan

[ChrisM]

Originally Posted by allan gould

Chris
I use a stepper motor controller with temperature compensation using the Sequence Generator Pro software with my moonlight focuser. Works a charm. Automatically re-focuses on a filter change and with every degree drop in temperature. Wouldn't be without it. The controller is in the DIY section of IIS by David Trewin in the UK.
Allan

Thanks Allan,
I'll check out controller too.

One other general question: Is there any compelling reason to go for a 2.5" focuser? I note that the diameter of the visual back on the Meade (and Celestron too I think) is 3" or 3 1/4 inches. I was thinking that a 2.5" focuser would be stronger, but with a downside of being heavier.
Chris

[Astro_Bot]

Originally Posted by ChrisM

3" or 3 1/4 inches.

3 1/4" on: C11, C14, EdgeHD 925 > 1400, Meade 10" > 14"
4" on: 16" Meade

IIRC, the Meade 3 1/4" is actually 3.27" x 16tpi while Celestron is 3.29" x 16tpi (but someone might have to double check that).

As for the focuser width, for visual, there's no advantage I can see of 2.5" over 2" since 2" eyepieces are the widest you can go*. For imaging, I don't know ... thinking out aloud, with the distinct possibility of being completely wrong ... I suppose it depends on the sensor size, but that'd have to be a big sensor, and probably a short focal length tube, to produce a light cone that needs 2.5" at the base of the focuser. ... and come to think of it, the ID of the SCT baffle is ~51mm in Meade SCTs, at least in ones I've seen.


* Just make sure you're using the 3 1/4" flange, as using the 2" SCT threaded adapter will vignette the image (for widefield 2" eyepieces) due to the ID being only about 40mm in Meade and 38mm in Celestron.

[ChrisM]

[QUOTE=As for the focuser width, for visual, there's no advantage I can see of 2.5" over 2" since 2" eyepieces are the widest you can go*. For imaging, I don't know ... thinking out aloud, with the distinct possibility of being completely wrong ... I suppose it depends on the sensor size, but that'd have to be a big sensor, and probably a short focal length tube, to produce a light cone that needs 2.5" at the base of the focuser. ... and come to think of it, the ID of the SCT baffle is ~51mm in Meade SCTs, at least in ones I've seen.


[/QUOTE]

Thanks Astro Bot. I found a page on on the Starlight Instruments web page giving recommended FT focusers for various types of SCT. For the M14, the FT2015 was listed (for a GEM); otherwise a much shorter draw tube was prescribed. I have also asked them if a 2.5" dia focuser would be of any advantage.

Chris

[Starblazer99 (Brian)]

2.5" Moonlight on a EDGE 14 with reducer only one flange available on A 2.5 large format focuser