LX90 adapter advice

[Trevor-Austin]

I've got the Meade 5000 2" diagonal with my LX90 and that is fitted to a 2" adapter screwed into the back of the OTA. That adapter has an extension piece with it I haven't used yet that will extend the adapter approx another 2" and therefore "push" the diagonal back 2".

What effect will this have on the Diagonal and my views?

Thanks

[Merlin66]

Adding the extra length won't have any effect on the image. You should be easily able to refocus. There's no benefit in doing it you may get more slop. I think that type of "extension" is to replace the length "lost" when you work without the diagonal.

[Trevor-Austin]

OK, that makes sense, so if I connect my dslr directly or similar.

Thanks

[r3i]

Normally extensions are used with scopes to compensate for insufficient capacity in the focus travel. SCTs usually have shed loads of back focus and extension tubes are typically not required to achieve focus. However it might be handy with your DSLR to distance it from the rear cell so that the DSLR doesn't clash with the focus knob should that be a problem.

[Trevor-Austin]

Great, thank you. Just need to find the adapter to downsize the thread on my 6900 to suit the SCT thread then. Is that 55mm on the back of the SCT and the extension tubes?

[Magnum]

It will affect your focal length slightly as the mirror will be in a different place to achieve focus.

Lee

[brianb]

It will affect your focal length slightly

Indeed, if you measure the focal length you'll probably find you're working at f/11 - f/12 with the 2" diagonal and an extension. If you have the ACF scope (which has corrector elements in the baffle tube) the correction will be dependent on having the distance between the corrector and the focal plane reasonably near to the design value. I'd definitely not use the extension tube with a 2" diagonal.

The extension tube fitted to the eyepiece end of a barlow might be useful to get a higher magnification e.g. 3x from a 2x unit. Or fitting it instead of the diagonal when imaging might be required to get the corrector backfocus right if you have ACF optics.

[Merlin66]

Hmmmm

The finepix 6900 doesn't appear to have the ability to remove the lens.

You'd have to image using the "afocal" method ie behind an eyepiece. If this is the case, your looking for an eyepiece to camera adaptor...

[Trevor-Austin]

no you're right it doesn't, I sort of hoped I could have acheived focus screwed directly to the back of the OTA in place of the diagonal as it does have filter adpater thread in front of the camera lens. Am I oversimplyfying?

[Merlin66]

Yes.

The camera lens focused at infinity, needs to "see" a collimated beam - this is not the case in the scope - the light is obviously converging to the focus.

The easiest way is to use an eyepiece positioned at focus - this gives a collimated beam towards the eye - in your case towards the camera lens. Finding the right focal length eyepiece and the adaptor which is long enough to clear the moving zoom element is a challenge ( I had a ol' Olympus 3030 zoom and managed to get the long spacer adaptor to work with a 32mm plossl....)

[Trevor-Austin]

OK, but will my camera focus not be able to focus as well using its focussing capabilities.

[Merlin66]

Ideally it should be focused at infinity....

You can still use the zoom capabilities to get a good match with the exit pupil from the eyepiece.