Mini guide scope & scope FL question

[Kirkus (Kirk)]

I'm just looking around for a guide scope and I see an Orion Mini 50mm set up. I'd be using it on an 8" SCT. The text for the guide scope says the following:

Compact mini guide scope designed for use with astrophotography telescopes up to 1500mm focal length

Can anyone tell me what the FL of the main scope has to do with the ability of the guide scope to work correctly?

The 8" SCT has a 2032mm FL. How would that determine the usefulness of the guidescope?

[riklaunim]

OAG for SCTs are best. I did try a 60mm finder/guide scope and that partialy worked - there was some flexture between the guide scope and the SCT OTA at some positions.

OAG setup:
http://oi52.tinypic.com/zufbs2.jpg

[Kirkus (Kirk)]

Originally Posted by riklaunim

OAG for SCTs are best. I did try a 60mm finder/guide scope and that partialy worked - there was some flexture between the guide scope and the SCT OTA at some positions.

OAG setup:
http://oi52.tinypic.com/zufbs2.jpg

Of course! I was a beginner when I had to quit for a few years and there's so much more I need to learn. Thank goodness for IIS!

An OAG is a great solution. Certainly less expensive than a new guide scope, rings, etc. Plus it will keep the weight down.

I don't have any experience with an OAG. How exactly do you search out a guide star while keeping the scope trained on what you're photographing? Is the mirror inside the OAG adjustable?

[Visionoz (Bill)]

Originally Posted by Kirkus

...

I don't have any experience with an OAG. How exactly do you search out a guide star while keeping the scope trained on what you're photographing? Is the mirror inside the OAG adjustable?

With great difficulty I'm afraid except for those who have successfully done it, but once mastered is quite "easy"!
- in answer to your question, there is a pick-off prism which (as its name imply) that you use to pick off the guide star that you can target on thru the guide camera

Some other posts recently you can check where the tips were given as to how to achieve success - http://www.iceinspace.com.au/forum/s...ad.php?t=81343

With regards to the main OTA FL - there is a relationship because if the ratio of difference is too great then you're going to have a hard time getting the guiding results that is good - it used to be when I first started that the figure of 1:3 is about optimum (http://www.wilmslowastro.com/tips/autoguiding.htm) - eg 2000mm main, 600~800mm would work out fine but like all things these figures are only a guide (not a pun); I've been having success using a 9x50 SW finder scope with my 1900mm OTA - it's good to try-and-see and what works will go towards the knowledge database!

HTH
Cheers
Bill

[Kirkus (Kirk)]

Thanks for that, Bill! Using an OAG seems like a good alternative to a separate guide scope. Both flexure and weight can be reduced.

My previous setup had my inexpensive mount overloaded. The mount simply wasn't able to handle everything. So, while researching and planning on purchasing my new setup, I'm keenly aware of keeping the load as light as possible. Perhaps to a fault.

Catching up on the thread you linked to gave me a wealth of information! Thank you for that (I didn't see it when searching the forum). I've not even decided if I'm going to use an OAG, but I did have visions of me already drop kicking the thing across the room.

[Kirkus (Kirk)]

I'm still having trouble understanding why the FL of the guide scope is related to the FL of the main scope.

If you have a guiding set up that accurately controls your mount, why does it matter what the main scope is? The guiding setup either accurately controls the mount or it doesn't? Right?

[adman (Adam)]

With long focal length scopes, a very small movement of the mount results in a significant movement of whatever you are trying to image because the field of view is so small. If you couple that with a short focal length guide scope, the same amount of movement when seen through the guide scope may not even be perceptible.

Adam

[Kirkus (Kirk)]

Originally Posted by adman

With long focal length scopes, a very small movement of the mount results in a significant movement of whatever you are trying to image because the field of view is so small. If you couple that with a short focal length guide scope, the same amount of movement when seen through the guide scope may not even be perceptible.

Adam

A-ha! Thank you so much, adman. I totally get it now.

A smaller guide scope is only going to provide the amount of accurate guiding that is dependent on it's ability to see. Obviously a more powerful scope will provide the guide camera with a better visual image in which to guide the mount.

So there is no direct relationship between the guide scope and the imaging scope. It's more a judgment by the user as to what accuracy he/she finds acceptable for his/her imaging scope.

I was reading it as "this guide scope will not work if your imaging scope is too big". Which isn't true. It will still guide the mount regardless of the size of the imaging scope. But if you want acceptable imaging from a 16" SCT, don't expect a 50mm guide scope to make you happy.