Focusing Limits

I just pulled out my old Canon EOS and bought a Meade variable projection camera adapter and the appropriate adapter ring to attach it to the front of my camera. I'm using a 8" Hardin Optical Dob (which I understand I can't go too high on the magnification anyway on dark sky objects because of the lack of tracking but I'd like to practice on lunar and solar photography until I can upgrade). The problem I'm having is if I move the focus knobs all the way in I can use a 25mm eyepiece but it's right at the edge of being focused - anything smaller in eyepieces and I can't achieve focus. Are there any adapters to help with this problem or do I have to change out the focusing assembly on the telescope side? Thanks in advance for any light you can shed on the subject.

This is a common problem and what you need to do is move the primary miror up in the tube a little (closer to the focuser). This pushes the focal point out of the focuser a little further so that the focuser doesn't need to be racked in all the way. You don't need to move it much, maybe an inch or less. If you move it too much then your eyepieces (in normal viewing mode) might have problems focusing.


Robert W.

Be aware that if you move your primary up, you may have problems bringing your eyepieces to focus from the other side--the problem is that your camera adds an inch or two to the length of the lightpath--if you move the primary up, the light path will be too short for the visual eyepieces.

One thing nobody has mentioned is that you might have just enough travel in your collimating screws to move the primary just enough.

Alex

If you move the mirror as others suggest you will increase vignetting, perhaps to the point that the secondary can no longer "see" the full aperature of the telescope. It is of course free. My solution to this problem was to buy a low profile focuser with a 1.5" extension tube to retain the overall focuser length to ensure eyepieces focus. The extension comes out for photography.