Need advice on Coma Corrector

I've been using a 13.1 inch, f4.5 newtonian recently and found coma abberation, which I understand is not uncommon for fast newtonians. I've collimated the scope but still see those light cones when slightly out of focus. I've heard of a 'coma corrector' and assume it is some sort of lense, but have no idea of the size, cost, where it attaches or where to get one. My focuser is a 1 1/4 inch. Any advise, suggestions??????

I have a Televue 1.25" coma corrector and it is probably one of the few remaining 1.25" units as they are now made only in the two inch size. The unit functions somewhat similiar to a barlow lens and seems to work well with meade widefield eyepieces. Premium eyepieces, such as the pentax units and the speers-waler 10mm as well as those from televue can handle a short-focus scope to f-4 so if you upgrade to one of these eyepieces a coma-corrector may not be necessary. I have a Discovery 12.5" f-5 truss dob and an Astrosystems 16" f-4.5 dob and neither require a coma corrector with premium eyepieces.

There are two possible sources of aberration in fast
scopes (F5 or less). It is probably useful to mention
both.

One is eyepiece aberrations -- notably
astigmatism near the field edges -- which you will
see especially in non-premium widefield eyepieces
like Erfles, TeleVue Wide-Fields, and Meade Super
Wide Angles, etc. Stars will be blurry or elongated
near the field edges. Premium eyepieces like TeleVue
Panoptic, Nagler, Meade Ultra-Wide will show only
slight amounts of this aberration.

Then there is actual coma from the main optics.
This will be seen even in the premium eyepieces,
and causes stars near the edges to look like
little cones or seagulls. A coma corrector
will fix this. As Rod says, I think TeleVue is
the only one who made a 1.25" Coma Corrector.
Its like a Barlow, and usually has a 2-element lens
in the bottom. I think it has been discontinued,
but they sometimes appear on astromart. Two-inch
coma correctors are still readily available.

My suggestion is to borrow a couple high-end
eyepieces (i.e. Naglers) and verify that indeed
you are seeing coma in the main optics. And
then get a 1.25" corrector, or possibly consider
switching to a 2" focusser and a 2" coma
corrector, if it really is coma.

If you do decide to get a coma corrector -- one
thing to check is the amount of "in travel"
you have available on the focusser. Most of these
coma correctors need at least 1/2" of inward travel
in the focusser to reach focus. Put your
favorite eyepiece in the scope, focus, and
then see if you have at least 1/2" of travel
left running the focusser into the scope.
If you don't have the 1/2" of travel, you may
have problems reaching focus with the coma
corrector in place.

Good luck!