Focal Extendor

[CW 392]

Can anyone please advise me.

I have a Celestron Inspire 100AZ. The scope is fine and I have upgraded the eyepieces to 25mm and a 9mm Celestron X Cel LX and they both perform very well

So instead of upgrading a to a Barlow I have a ES x2 Focal Extendor. Now the problem.

The eyepieces with a Focal  length of 660 give x26 and x73 respectively. The ES when inserted with either eye piece does not focus at all - it works on terrestrial viewing  - just when at the total limit of focus

I have tried both on the moon and get no focus at all but when its removed the eyepieces are fine

What has happened is the ES too big for the scope or not suitable   

Any advice would be appreciated

Thanks

[brantuk]

Does the extender work at all with the original supplied Kellners (20mm and 10mm)?

[ronin]

This going to be guesswork, just about total guesswork, and from a few things I have read.

Apparently unlike a barlow where the TE sits makes no difference, it will always produce 2x. Also TE's are talksed of in DSLR photography, and in that you cannot adjust the position of the sensor, it is sort of fixed in the DSLR body.

I am guessing therefore that a teleconverter applies a 2x magnification but does not move the focal plane of the scope, in effect at the primary focal plane you get the 2x image but it is still at the primary focal plane. A normal barlow usually moves the primary image outward a bit.

It may simply be that the ES TE does this but sort of adds some mechanical distance in the body of the TE and so pushed the eyepiece out and it cannot be moved in sufficent for the eyepiece object plane to coincide with the prime image plane. It would then be acting as if there is still at the focal plane an image, 2x in size, but because of the TE body a small pseudo extension tube is aded and this pushed the eyepies out too far for it to reach the prime image plane.

Terrestrial objects have there image further out and assuming the ES TE does not alter this then the eyepiece object and scope prime imge can coincide.

If you search you will find the Russ Will had one some time back, he may be able to offer a better idea.

But I am guessing that the TE makes a bigger image, 2x, but at an unchanged prime focal plane and that the TE body adds in a small amount of distance that moves the eyepiece out and so prevents the EP being able to get at the image. It may be the design of the eyepieces as some have their object plane inside the EP body. They therefore need a fair amount of inward travel.

I can see how it might do the above, what I am not sure of is how correct this thinking in.

[Cornelius Varley]

I think your problem is that the focal extender has moved the point of focus inwards, beyond the inwards travel of the focuser.  See this thread