Guide Scope vs Off-Axis Guider

[Uncle Peter]

Hello,

I was wondering, what exactly is an off-axis guider? Is it better than a guide scope, and would it work with a CCD camera?

Clear Skies,

[toolmaker]

An off-axis guider is a device that sits on the imaging train usually just in front of your main imaging camera. Using a small prism or mirror it directs a small percentage of the light that would otherwise make it to your imaging camera off on a 90 degree angle so it can be captured by a guide camera/eye piece.

There are advantages and disadvantages to using an off-axis guider. The main advantage is that your guide image is taken directly from your main image completely eliminating the need for a second guide scope and the resulting flexure it may create. The main disadvantage is that the off-axis guider only redirects a small percentage of light to you guide camera resulting in a much fainter guided image and a much smaller FOV making it difficult to locate a suitable guide star.

You should be able to use a CCD camera with an off-axis guider no problem. Just keep in mind that you will be adding more weight to the imaging train making the back end of the scope relatively heavy and potentially more difficult to balance in dec.

Theoretically you can take much longer exposures with an off-axis guider it just requires more planning and setup.

[Uncle Peter]

Thanks Dave, is the loss of light significant?

I was planning on using it with my Celestron C6 SGT, with a 6.3 focal reducer. Would the amount of light lost cancel out the focal reducer?

[toolmaker]

I'm not sure exactly how much light is lost and how it will interact with a focal reducer. It WILL be more difficult locating guide stars than compared to a dedicated guidescope, however you're resulting images should have tighter stars.

This is one of those situations where you're going to have to look at the specifics of your setup, the goals you have for exposure times and your wallet, and then decide which approach will work best for you.

[jimt]

You just need enough light reaching the guide camera to lock onto a star. As long as the image is focused and bright enough to do that, your guide system is doing its job.

Don't worry how much light is reaching the guide camera if it can do the above. That's my take on it anyway.

Jim

[Uncle Peter]

This is one of those situations where you're going to have to look at the specifics of your setup, the goals you have for exposure times and your wallet, and then decide which approach will work best for you.

It is worth it for me to buy the off-axis guider, since I'd have to buy tube rings and a doveplate for a guide scope, since I have a SGT.

Jim, since I'll be using PHD guiding and a Orion SSAG, I could also set the camera exposure of the SSAG to a higher number, maybe 5 or 10 seconds. This should bring out dimmer stars that I can't see through the SSAG.