Reaching focus with a 100mm refractor

[samir_ansari]

Hi everyone,

I have a Celestron 100ED and just wanted to know what sort of extension tube length to buy so I can achieve prime focus with my Canon DSLR? I have my eye on a 35mm but I am worried this won't be enough.

I currently have a 1.25" t mount for my camera already which I have been slotting into a Barlow and then putting that into the diagonal to reach focus, but if I want to go prime focus will I see any benefit from taking out the diagonal?

Thanks!

Samir

[Peter Drew]

The usual problem is not having enough IN focus. Your Barlow will improve that situation but it will increase the magnification. To get to the true prime focus you should just use the camera directly coupled to the focuser. If you remove the diagonal it will help with the IN focus issue.

[samir_ansari]

Thanks Peter! But if I just slip the camera straight into the focuser what sort of extension tube length do you think I will require? Will I even need an extension?

[Rossco72]

You will find benefit in using a 2" adapter as the 1.25 almost certainly will give you vignetting.

The diagonal should be removed and you will probably need an extension tube. Most 2" ones seem to be around 50mm in length and that should be plenty.

Adding a barlow increases the magnification but introduces vignetting and sometimes field curvature as well. Unless the target is small and you can crop the frame to just the central portion, this is often quite unsatisfactory.

Regards

[jgs001]

The barlow is a solution for lack of in focus on a reflector. Refractors suffer from the reverse, a lack of out focus, as they are designed to be used with a diagonal in the optical train for visual work. You have to replace the length of the diagonal with something else, the tube. The tubes I've seen are in two lengths, 50 and 80mm. 50mm would probably be sufficient, but, you can get a rough idea... using the 1.25" adapter, remove the lens cell from your barlow, and use it as a tube, fit the camera behind it... you should be then able to get focused, albeit vignetted, and you can measure the distance added by the adapters you've fitted.