Coma Corrector Question

[The-MathMog]

I am looking into getting a coma corrector at some point. It might be after I get a new OTA, as that one will likely have more coma, and that coma isn't the biggest of issues right now. BUT, I've been surfing around, trying to figure out how coma correctors actually work and what other effects they might have, and it has been a bit confusing to me, especially concerning focus.
I've read several places that some CC's will either require in-focus or even out-focus, even out-focus in such a case where prime-focus, which wasn't possible before, actually can become possible. I'll quote what I read, but I'd like people to give some comments on how that would work out:

    "I've found that the GSO requires about 10mm of focuser in-travel when using the supplied eyepiece holder and an additional 25mm of spacing between that holder and the optics unit.  For every additional 4mm of spacing, you save about 1mm of in-travel required.  So I guess if you put 110mm of spacing in there, you wouldn't change the focus point at all.  Of course, the correction would be awful.

If you screw the same 70-75mm of spacing onto the back of your 2"-1.25" adapter, you'll need about 1.5 inches of out-travel to reach focus.  Since my focuser won't go that far, I just pull the whole assembly up and out of the focuser about and inch or so.  Thus, you'll always be able to reach focus with 1.25" eyepieces and possibly even with cameras setup for prime focus photography via the T-mount adapter that wouldn't be able to reach focus otherwise." Louis D -

This was interesting to me, as I am actually not able to achieve prime-focus as of right now with my current OTA (Celestron 130SLT), and was wondering if acquiring a CC now instead of later, would be able to fix this for me? And if so, does the focal-length attributes of the CC determine this? Like some CC's acting as a 0.9x reducer or a 1.15x. And what CC's would you recommend that I eventually get? My current plan for the new OTA, is a Skywatcher 8'' Quattro CF, so I'll want a CC for it eventually, but if the CC can also "fix" focus issues with my current OTA, that would kill two birds with one stone as I currently have to use a barlow to focus my Nikon D5200.

So any insight into this or general advice would be very welcome!

-Mathias M. M.

[Waldemar]

Hi Mathias,

The focal distance from the CC or reducer/flattener to the sensor is critical. In most cases that distance is about 56 mm. your camera has a back focus of 44mm,  the T-ring and adapter about 11mm.  If that distance is changed to reach focus, the desired effect of the coma corrector or flattener will be nullified.
So if you cannot get into focus with that combination you will have to find out if you need more inward or outward movement and act accordingly by using a spacer or move the main mirror a bit closer to the secondary.