Eyepiece focus with an RC8.

[Auspecial]

Hello all, I recenly purchased a GSO RC8 and have been having good fun learning to do astrophotography but I also bought a 26mm 2" eyepiece and cant seem to find focus. I have zero experience with visual astronomy. Do eyepieces focus at the same point as a camera? Is there any way to work out where the focus point will be on a GSO RC8 for an eyepiece?


Cheers!

[Stonius]

No, they don't. Typically you'll need to use some extension tubes to get the eyepiece far enough away. Mine uses large rings that screw in between the telescope and the focuser, so the focuser assembly sits about another 5 inches away from its normal position.


Markus

[Auspecial]

okay great thanks mate

[Stonius]

Wait, sorry, I think I have that the wrong way round. The EP needs to be *closer to the telescope than the camera. It's the camera that uses extension tubes. Are there any bits that unscrew?

[Auspecial]

I used it with all of my big extension tubes and the focuser and couldnt get focus, then used it with zero extension tubes just the focuser and still couldn't find focus. Not really sure where to go from here :lol:

[Stonius]

If you point it at the moon, then hold a piece of paper in front of the (empty) focuser you can measure the scope's focal plane, where the camera would sit and go from there.



Also if there is not enough backfocus, you could try insering a Powermate to move the native focus out a bit.


Markus

[Auspecial]

great thanks for the tips mate appreciate it

[N1]

Eyepieces generally sit further from the objective than cameras*. Start with the focuser screwed directly to the tube. Point the telescope at a bright star, wind the focuser all the way in, put the eyepiece in then start racking out the focuser and observe the donut formed by the defocused star. If it's getting smaller, you're on the right track. If you hit the end before things are sharp, add extensions (or a diagonal). If you see no donut to begin with even after making sure it's pointed at the star, add one of the two M90 25mm extension rings, or a 2" extension before the eyepiece and restart the process as above.

If, however you are still too far out even with no extensions or diagonal in the path, and the focuser all the way in (i.e. donut gets bigger as you rack out from there), then I suspect your mirrors are not at the correct spacing from each other. That's very unlikely though, especially if the RC is new and your cameras focus fine*. But if that's the problem you'll want to get that fixed because the distance between primary and secondary is absolutely critical in RCTs. Beforeyou do anything like that, you must read up on the subject or it will become a major headache.

*More simply, you could establish roughly where the camera's senor comes to focus, and move out from there. The EP should focus a few to a few dozen mm behind that spot, depending on its design.