Off Axis Guiders

Michael,
If you were to get a OAG, dont get it new, I got mine at a reduced price on astromart, you'd be unwise to get one new.

Off Axis Guiders on SCs usually have quite a poor star image (due to off-axis aberrations of the optic design). It's definitely not "pinpoint". Also, they're limited to a relatively small "ring" of field in which you need to find a guide star. Not usually a problem in areas close to the milky way, but it will limit you when e.g. imaging clusters, galaxies. Some designs (the Taurus tracker, Van Slyke's slider and Lumicon's Easyguider) allow for more area to find a guide star by moving the pickup prism/mirror and/or the guideport, but it's still more difficult than with a guidescope. I have the Van Slyke slider.

The advantage that's usually named for off-axis guiders is they avoid flexure. I.e. optical axes of guidescope and imaging scope move during exposure because of "mechanical weakness", either in guidescope mount or in the moving mirror of the SC. Well, that's not really true anymore as their is, modern SCs allowing to lock the mirror. Second, for guidescopes there are "decentering" eyepiece ports available that allow for perfectly rigid mounting of the guiding scope and still plenty of freedom to look for a guidestar.

I would go for the guidescope, particularly a cheap chinese one at 80mm dia. short focal length. As long as your OTA is solid, no flexure.

BTW I use an old telephoto lens/homemade coupler to a video camera as a guidescope

I do use a Van Slyke OAG on another telescope, but not for the function as an off axis guider, but as a means of creating two different f ratio for two different applications (wide and narrow fields).

Eamonn A

That's interesting Eamonn. I appreciate your advice and will give it some thought.
Thanks again,

Michael - I got great images with the Skylux - as good as the ETX70. And was able to pick up good detail on the Blackeye...

In general, a good quality refractor will give you good views/images. But if its good wide-field images you want then you not only need a good quality refractor, but a good quality imager too

No good shoving a toucampro into a Megrez Apo...

I think your suggested course of action is best - forget about autoguiding for the mo and concentrate on polar alignment and PEC. You'll be surprised at how long it can take but when done, I'm sure you'll get 'minutes' worth of unguided exposure time. And all it'll cost you is time

Cheers and good luck!

Dave

Just incase you think that I'd consider nothing but a OAG, I also have a 60mm F15 refractor from years ago which I'll also experiment with as a guider, to see if its a viable alternative.

I agree with Dave Mac, now that you have a permanent mounting and observatory, you are in a much better position to apply more accurate polar alignment fix plus PEC. If you are in a position to use MaxPoint (which I think is a free download) at least mine was free. Its rather like using the poor man T Point from Software Bisque which you have to pay for anyway. Maxpoint is far better than T Point because typically for guys like me who are lazy I don't have to keep appying T Point, analyse the data and start doing manual corrections again for another round. MaxPoint will do that for you, it'll just keep improving the tracking as long as you keep using your mount. So you may end up never ever using a guide system. Of course its important within the equation to keep using PEC. Unfortunately I have no experience using a meade mount, so I don't know its idiosyncrocies.

My 3 cents worth

Eamonn A