Imaging using extension tube.

[Cloengaa]

Hey all.

I am a little confused about the concept extension tube.

Can someone tell me what this is good for ?

And how I would actually use it. I have a Canon Eos 1100D which has a screw on extension tube I can make longer or shorter.

I have tried it but can not find focus.

So how do you use one of those things ?

Scope is skymax 127.

[JamesF]

Normally you'd use an extension tube when the focuser travel was insufficient to achieve focus by itself.  With a 127 Mak that's unlikely to happen.  You should be able to reach focus with the camera pretty much anywhere within reason.

You might also choose to use one between a camera and barlow to achieve a larger multiplier than quoted for the barlow.

There is another peculiarity of Maks and SCTs however that the further the camera is from the back of the OTA, the longer the effective focal length when the image is in focus and therefore the larger the image on the sensor.  For example if I put my 450D straight onto the visual back of my 127 Mak I find that lunar and solar disks pretty much fill the height of the frame.  If I put a 50mm extension between the visual back and the T-ring then the image is too large to fit on the sensor when in focus.  You might therefore use an extension tube to increase the size of the image on the sensor.  I'd personally probably only do that if I wanted to image sunspots or specific areas of the Moon though.

James

[Cloengaa]

I was thinking of using it to make images of Jupiter, Saturn and maybe Uranus and Neptune bigger ?

[JamesF]

Then for use with the 127 Mak a 2x barlow would probably be your best bet with the DSLR, or perhaps a 3x-ish barlow with the SPC900.  With the SPC900 you could use a 2x or 2.5x barlow with an extension tube, but I'd start at lower multipliers and work your way up.

I'm not sure that Jupiter, Saturn and Uranus are particularly viable targets for some time though.

James