Why does a OAG need an EP???

[kirkster501]

^^^

I mean, don't need an EP when using guiding on my ST80 - just the QHY5 at prime focus...

Was looking at the Celestron OAG for use on a potential C9.25

http://www.widescreen-centre.co.uk/Products/Celestron_Off-Axis_Guider.html

And it mentions an EP required.....

Just wondered why???

Rgds, Steve

[ronin]

At a guess and that is all it is you will set the scope up for a sharp image at the camera sensor, this could well be different to the guider sensor. In effect you put a small set of optics (guider eyepiece) in to enable what goes through the guider section to focus on the guider sensor. That way you have a sharp image on both so image is good and the guiding is good.

Could have the guider camera on an adjustable piece but if the "eyepiece" approach is cheaper then that is what a manufacturer will go for.

If not that then no idea why or what it does.

[r3i]

I would guess the product description is somewhat dated. In the past before electronic imaging became readily affordable, these OAG's were used with film cameras and the guiding achieved manually by the user with an eyepiece. That must have taken incredible patience and dedication.

I guess nobody has bothered to bring the desciption up to date with the equipment that today's astronomer would use with an OAG.

[Gina]

Never heard of using an EP on an OAG Put the guide camera on it and agjust prism to get stars then focus. I don't really see that an EP helps but then I don't have an OAG but I never used an EP on my guide scope. Just put camera on it and focused until I saw a collection of stars, then defocused very slightly (works better than sharp focus).

[kirkster501]

Put the guide camera on it and agjust prism to get stars then focus.

Indeed. The key point there Gina being "then focus". How to focus the guidecam? Can't move the main imaging train so what component "gets turned" to adjust the focus falling on the guidecam?

[IanL]

As others have noted, the eyepiece reference is from the age of the dinosaurs (i.e. prior to the 90's). Long exposure imaging was done by means of film and a reticule eyepiece (on a guidescope or through an OAG). You sat there looking through the eyepiece and pressing buttons to keep a target star centred. Not much fun I'd guess! Even when CCDs became available to amateurs, they were pretty expensive so people would still manually guide them until affordable guidecams (and webcams) came on the scene.

I think a lot of the obsession with periodic error of mounts, PEC and perfect polar alignment comes from that era since the better your mount tracked, the less work you (the guider) had to do and the less chance of ruining an exposure due to inattention (and only discovering it once you had processed the film). These days it is not as much of an issue as people/adverts make out it is, especially at shorter focal lengths. If you are imaging at longer focal lengths or your mount is dodgy, good PA and minimising PE are going to help (and PA is important in terms of minimising field rotation on longer exposures or at long focal lengths).

How to focus the guidecam? Depends on the design of the OAG, but generally you would focus the main imaging camera using the scope's focuser. You'd then use some kind of adjustment on the OAG to move the the guide camera (or eyepiece) in and out to achieve focus. Simplest is just sliding a nosepiece in and out of a holder and locking it with a screw (like an eyepiece), in which case fitting a parfocalising ring once you have focus means you just have to drop the guider in and focus the main imager thereafter. Alternatively you might have some kind of screw in/out mechanism to give a small adjustment combined with extension tubes to get it close enough to focus to start with.

[ollypenrice]

As above, it goes back to guiding by eye. I like Ian's point obout PE and PEC as well. The NEQ6 has abominable PE, about five times worse than the premium mounts, but how much does this matter with an autoguider?

Olly

[kirkster501]

Thanks guys. I think I follow it now ! Going to get an OAG for a C9.25 when I get one.

I will start a new thread regarding which OAG to get.

Thanks again, steve