JamesB303

4 hours ago, ollypenrice said:

Focal length alone is not relevant to the guiding precision you require. It's the combination of focal length and pixel size which determines your image scale, the area of sky landing on each pixel. The unit is arcseconds per pixel, usually written "PP or similarly.  For a rule of thumb, high resolution would be around 1"PP, medium res. would be around 2"PP and low res. around 3"PP. There is no official definition but that's about right.

At high or very high resolution an OAG is a safer bet, though I use a guidescope to image at 0.9"PP without difficulty. However, I'm using a refractor. If using a reflector the huge advantage of an OAG is that it is guiding on the same light cone as the imaging camera and, if the mirror moves (as they do) this movement will be guided out. Anyone shooting at high res. and with a reflector should surely choose an OAG.

With refractors and at medium or low resolution it will be easier to use a guidescope. (I'm not sure how you'd use an OAG with camera lenses at all.) When setting up a guidescope there is no need to use adjustable rings. They are a source of flexure as Michael has pointed out. The idea has arisen that their purpose is to allow the guidescope to be aligned with the main. In truth the reverse is true. They were designed to allow the imager to search off axis for a guide star for early and insensitive guide cameras. Since modern guide cams always find stars (at least in guidescopes and usually in OAGs) you should just bolt them down hard pointing more or less on axis.

I use cheap and cheerful Skywatcher ST80s on both of our imaging rigs and have done so for years. I made sure that everything was mechanically tight on the OTA and focuser and, to extend the tube to reach focus, I fitted and epoxy-bonded a de-lensed Barlow into the focuser. There is no need to search for critical focus in a guide scope. The author of PHD guiding says that a slightly soft focus is best.

Have fun,

Olly

I'll very likely be using refractors with around and under 800mm focal length, with a DSLR attached, for the foreseeable future - so it sounds like a guidescope would be the way to go. The info on pixel size and guiding was very helpful--thanks for that!

40 minutes ago, michael8554 said:

The bane of guiding on long focus setups, over 1000mm perhaps, is Differential Flexture. 

No matter how well you attach the Guide Scope to the rig, it moves microns relative to the imaging scope as you're imaging. 

Delrin tipped screws on adjustable rings are typical sources of flex, and guidecams mounted in the finder slot.

Given camera pixels are measured in microns too, you end up with oval stars.

OAG will minimise Diff Flex because the guidecam and imaging cam are on the same scope.

But for your shorter FL setup a separate guidescope and mounting rings will suffice, but may be more expensive to purchase than a budget OAG and adapters.

Michael

Thanks for the info. I'm not worried about cost too much for this particular setup as far as the guidescope and mounting system goes - it should all be less than $200 either way - I just wanted to have an idea of which path to go down. Now it's about finding the right tandem bracket for the side-by-side (I think I prefer this over mounting directly on the scope, mostly for aesthetic reasons).

Hey all, 

I'm new to the community and the hobby and I'm excited to be here. Thanks for having me!

I wanted to get started right away with a question I haven't seen the exact answer to that I'm looking for, and it concerns auto guiding and off-axis guiding systems. 

Considering that I will be doing wide-field up through intermediate range (500-700mm focal length) AP using DSLR with lenses up through a small-medium scope (to start), here are my question(s):

1. Does either system have an objective advantage over the other, or is it simply personal choice?

2. Does either one make more sense for wide-field up to a particular focal length (or maybe weight?), at which point the other makes more sense, or are the pros and cons pretty much uniform regardless of the setup?

3. Do you have any OAG's you recommend or don't recommend?

4. Any recommendations for guide scopes which would be useful for my range of uses?

***Note: The setups I am comparing would both include a mini monochrome guide camera similar to a ZWO ASI120mm

Thanks for any advice or info you can provide.

- JamesB303