guiding

[MarkH]

Ok here's another of my advice threads. For those who don't know I have a meade lx90 with it's 8x50 finder which has rotating focuser, i am thinking of using this as a guide scope. After advice from Duncan about flexure between the two scopes I need advice before I go and spend more money un-neccesarily. How important are the following points.

1. Am I better off re-engineering the current finder mount and keeping it lightweight ( which I am more than capable of doing) as opposed to sacrificing photons by going off axis.
2. Does the roation of the immage matter in the feedback loop after focussing once guiding is under way
3. Would I be better off buying a dedicated guide scope, if so opinions please
4 could I use a 3rd mounted scope as a guider with a "hard" fix to the main tube and use whatever star I can find through that for guiding.

[The Thing]

1) Use the finder to start with as it's far easier. Since the Meade finder is in quite a solid mounting flexure won't be an issue you will notice to start with - I certainly didn't. Other factors were more in evidence e.g. getting collimated accurately, focusing spot on, polar aligned well enough etc etc!

2) No, guide camera rotation doesn't matter as long it stays firmly in place. The guiding program shouldn't care (PHD certainly doesn't). Your images will only rotate if your polar alignment is off and then only by a small amount.

3) No, you can always use your finder with a barlow to increase the effective focal length but you shouldn't need to esp. if using the f6.3 focal reducer (I seem to remember it has a 1.25" EP fitting???).

4) Yes but keep in mind you are using a fork mount and don't have a counterweight system. You will get clutch slip if things are too far out of whack and the load factors will change as the scope rotates. Keep it light. I used a Skywatcher AT80 on top of my OTA for a while but it was too much even with a 3D counterweight system. Then I went to a finder guider and started getting decent images...

[MarkH]

Duncan, as always the expert. Thanks mate, been there before me.

[MarkH]

Duncan, I wasn't sure what you had done to your mirror to prevent flexure. I stumbled upon this , might be of interest.

http://homepage.isomedia.com/~cvedeler/scope/mirrorlock.htm