Main / autoguider scope alignment accuracy question.

[Bucky1379]

Hi

I'm working out what I need to do for astrophotography and have a question regarding alignment of the main and guider 'scopes.

I would like to be able to use both 'scopes seperately as well as in their combined autoguiding / photographic roles and without having to align them both perfectly again when I want to use them together so I was wondering if anyone had any ideas how close to perfect their alignment needs to be.

Ideally, I would like both to be fitted with standard tube rings for their independant roles (rather than the guide 'scope having 3 point adjustable rings). I then plan to mount both side by side on a plate that is bolted to my mounts' dovetail.

I feel that I should be able to align them very closely using shims and slightly oversize holes in the plate (if that is required) and by marking the positions of the guidescopes rings against the plate and also how many shims are under each ring, I figure that I should be able to seperate them and then re-assemble the combination and get a quite accurate (and adequate?) re-alignment without going through a full alignment process.

Does this sound like a reasonable hypothesis? From my investigations it seems that rigidity between the two scopes (provided by a non flexible plate) is the most important factor in successful autoguiding so I feel that this approach should work but I don't have any experience to call on.

I also feel that an added bonus of mounting the two scopes like this rather than putting the guidescope on top of the main 'scopes rings would be to provide a more centralised total mass (ie closer to the mount's axes) which I also figure would help the whole process.

Does anyone have any thoughts that might help me decide if this approach could work?

Thanks

Steve M

[citivolus (Ric)]

http://www.robincasady.com/Astro/TGAD.html

Not cheap, but does what you are after when used on a side by side setup.

[Geoff45 (Geoff)]

Don't quite follow the question. Are you suggesting that the two scopes have to be perfectly aligned in order to get good guiding? In that case the answer is that the scopes don't have to be aligned at all. That's why guide rings work--you can move the guidescope around until you find a good guide star. Both scopes track whatever they are pointing at, so if you keep one accurately tracking an object, the other one will also track its object accurately, even if it is a different object.

[g__day (Matthew)]

As Geoff said - your scopes don't have to be aligned, but you do have to have no differential flexure (think very, very slight bending) as you move two heavy scopes around.

I must admit that I go for Off Axis Guiding with a sensitive guide camera even though I have 3 scopes on my mount (1 piggy back, 1 side by side). Differential flexure and/or mirror shift made long duration, long focal length guiding impossible with either of the other two scopes, whereas with an OAG - but only if you have a sensitive enough guide camera (i.e. a colour Meade DSI I won't cut it in my opinion - a Mono DSI II Pro does)!

[Bucky1379]

You've answered my question.

I was under the impression that the two scopes needed to be pretty much aligned (I thought this was the reason for the 3 point rings) and you just had to hope that there was a reasonable star in the FOV to guide on (remember, I'm a newbie to this) but what you've said (and the example in the link) tells me exactly what I thought - the relative positions have to be flex free but perfect alignment isn't required. In fact I now know that you normally want to "misalign" them to be able to make use of a good guide star. Cool! One less thing to worry about It all makes sense when it is explained to you.

I estimate that this lowers the number of things to worry about from around 1,675 to 1,674. It's now "easy peasy".

Thanks again