Guide scope or Off Off axis?

[Paduan]

I am getting into AP and have an 8 inch SCT, i also own a ST80. with the focal reducer on the SCT the "F" ratios are similar between the scopes 6.3 sct and 5 st80. Is it more beneficial to have the guide scope at the same "f" ratio or greater. I am thinking about buying an off axis guider to keep the"f" ratio the same and reduce the weight/setup time. As the O.A.G. has a mirror in the focal stream does it impact on the quality of the images? it is all sitting on a HEQ5 so weight is not really an issue.


P.S. there was only supposed to be one Off in the title

[cometcatcher]

I've used both OAG and guide scope in my old film days. Each has it's advantages / disadvantages. Now things have changed in the 20 years I've used them so I'm not sure how relevant my comments will be in the digital age. But since no one else has commented....

OAG advantage is that it eliminates differential flexure between guide scope and main imaging scope and internal flexure of the imaging scope. It's one of the reasons I got one. I could never completely eliminate flex at long focal lengths otherwise. But then I was using "flexible" scopes.

Disadvantage used to be the limited number of guide stars to choose from. I don't know if this is still a problem these days with CCD guiding.

With a full 24x36mm frame there was also some vignetting of the frame from the OAG mirror. Would not be a problem with smaller area imaging chips.

Guide scope. Biggest hurdle is differential flexure causing star trails. Making everything as solid as a rock helps but when there is a reflector telescope in the equation, especially at long focal length, it can be hard to completely get rid of.

I'm not sure why you are trying to match f/ratio? I'm not sure what the correct formula is for guide scope focal length. It may depend on the tracking chip, or in my case my eye. :)

[jenchris]

I haven't tried a piggy back scope so I'm biased, but once set up the OAG is a breeze and I seldom have a problem finding a guide star and if I do, the seeing is probably rats anyway.

If you get your alignment close, you'll have a decent chance of great images - I reckon mine are ok for an 8"SCT on a wedge

[Paduan]

i will be using a 5D so the entire CCD chip will be used hence my question if the OAG mirror interferes. Maybe i was thinking too deep about it with regards to the "f" ratio. i have just graduated from a pentax "ist" which does not use the whole CCD. after seeing the image above it seems to not affect the image but does the 450D use all of the available CCD. The main reason is that i am really close to the maximum weight for the mount, being piggybacked ( it has been like that for a while) i am starting to notice some unusual vibrations in the subs. maybe a slight tune up is required, but if it has done it once it will do it again.

[[1ponders]]

Well as luck should have it, I'm trying to image atm using a guide scope with an 8"sct imaging at 1800mm fl. I normally use an OAG with np (occassionally have trouble with guidestars but 8-10 sec exp soon takes care of that ;) ) and now I remember why. Last few nights with the guide scope .... np good images... tonight, can't guide worth a fig because of flexure and movement in focuser. Going back to OAG if I can''t sort it out quickly.

[[1ponders]]

fyi gone back to the OAG ;)

[cometcatcher]

If they still make OAG's like they used to 20 years ago, then I'd say there's a good chance part of your full 24x36mm frame will be obscured by the mirror.

[cometcatcher]

Oops! I just checked my old OAG. It's only 1.25 inch version. If you can get a 2" you may not get any mirror shadows with full frame imagers. Sorry for any misinformation!