|
Curt,
The SIPS focuser from Starlight Instruments mounts a TeleVue Paracorr onto the bottom of the focuser for convenience. It has to be removed when collimating, of course. And the focuser moves in and out while the lens stays stationary.
The ideal use of a coma corrector requires having the focal plane of the eyepiece a set distance from the lens. That means that, because every eyepiece's focal plane is a different distance inside the eyepiece, you have to be able to vary the distance from between the eyepiece and the coma corrector to obtain the best correction. Attaching the lens to the focuser would prevent the lens being the correct distance from the primary, however, which would defeat the purpose of the coma corrector.
So here is the best way to use and set it up, and it will require parfocalizing rings for your eyepieces:
--start with an eyepiece in the coma corrector. Focus. Examine the edge of field stars for coma.
--pull the eyepiece out 2mm and tighten it down. Refocus. Examine the stars at the edge.
--Repeat until you fine one position where the coma is well corrected.
--Pull the eyepiece out, add a parfocalizing ring, reinsert and, without using the focus knobs, focus the eyepiece by pulling it out of the coma corrector until it is in focus. Slide the parfocalizing ring down on the top of the coma corrector and lock it in place.
--next time you use this eyepiece, simply insert it until the eyepiece's ring hits the coma corrector, focus, and the coma corrector will be in the right place for best correction for that and all your eyepieces.
--add parfocalizing rings to all your other eyepieces. Without refocusing the telescope, insert each eyepiece into the coma corrector and pull it out until it is in focus. Slide the parfocalizing ring down and lock it in place.
Now, all your eyepieces will be the correct distance away from the coma correcting lens, they will all be parfocal, and the coma corrector will automatically be the right distance from the primary.
Cool, huh?
If any of your eyepieces need to be pulled out a lot, add barrel extenders to those for safety reasons (to leave enough barrel in the coma corrector).
If any of your eyepieces don't come to focus and need to be moved out of the coma corrector, you can solve that with a long barrel extender.
More likely, if any of your eyepieces don't come to focus because they need to insert into the coma corrector more than the top of the coma corrector will allow, then just use that eyepiece inserted all the way and refocus the scope. It won't be ideal for coma correction, but if coma is 90% corrected, that's a lot better than zero%. This was the situation with the 31 Nagler in the first TeleVue Paracorr, and yet the correction was pretty good, so just insert the eyepiece all the way and refocus.
That eyepiece won't be exactly parfocal with all the others.
In practice, even using parfocalizing rings, you may need a touch of focus travel to get the eyepiece sharply focused. But, after you're done, it'll be really minor, like +/- 1mm.
|
GSO Coma Corrector
