O.K. I have probably worn out my welcome on this topic, but it is killing me..........

I have a Meade Lightbridge 8" Dob mounted Newtonian Reflector. I purchased a collimation tool to help out, but so far the instructions that came with the scope and with the collimation tool leave me bewildered! I have looked over the collimation info from several links posted on various sites (some helpful, some not so much).

1. In the center of the primary mirror there is what seems to be a sticker or marking that looks like a small white circle with a hole in the middle.
2. The instructions that came with the telescope say that due to the "fast" focal length (their words, not mine) the sweet spot for alignment is offset by 1/8" in two directions rather than centering perfectly. Say What??????
3. The collimation tool fits nicely into my focus drawtube with the eyepiece removed. When viewing through the collimation tool, the cross hairs DO NOT hit the center of the little white spot on the primary mirror. Should they? Or, does this offset that the telescope manual talks about come into play. I would think that they would offset the spot so you could align the optical system correctly, but who knows.

Anyway, if anyone is familiar with this scope and knows how this should be done, I would ABSOLUTELY LOVE some helpful input.

Thanks, as always, to eveyone here for their patience and understanding.

Frustrated.......................

MJ

You should have the secondary mirror exactly centered under the focuser, and you should adjust it so that you can see the entire primary mirror reflected in the secondary. It seems like you then should be able to adjust the primary until the collimating eyepiece's crosshairs line up.

However, I don't understand the concept of offsetting at all. I've heard of it before, but I do not understand it; and I have no idea how it affects collimation. My previous instructions could be in error because of this offset.

Yeah, the offset thing threw me as well. I would think that if they took the time to place a spot on the primary mirror, that it would have the offset compensated for. I am going to try the customer service at Meade and ask that very question. If it is compensated for and all I need to do is line up the cross hairs with the spot on the primary mirror, that would simplify things. I just want to make sure i don't change anything and do it wrong/make it worse........

MJ

Well, I just got off of the phone with Meade and they said the spot on the primary mirror does compensate for the offsets! Yeah! So, in theory, all I should have to do is just place the collimation tool in the drawtube and then line up the cross hairs with the center of the spot on the primary mirror. We shall see.

He did mention focusing on something in the day time and then getting it slightly out of focus. Then, once it is out of focus, refocus the object by adjusting the mirrors. That sounds a lot more cumbersome than just using the collimation tool though.

Man, I need a drink......

MJ

The offset is necessary to keep the secondary as small as possible. Without it you would need a larger secondary to catch the bottom of the light cone. This is because the lower part of the secondary intercepts the light cone when it is wider than when the top of the secondary sees it. Thus you'd need move the secondary away from the eyepiece and down the tube that 1/8th of an inch. This moves it along the same plane it is already in, see diagram at link below. This has been done for you. At slower f ratios the difference is small and hardly matters so is usually ignored except by the fanatics. Still the dot on the primary will center on the cross hairs when you are correctly aligned. In fact moving the secondary has no effect on this as long as its tilt isn't changed. Moving it just puts the reflection of the main mirror into the center of the secondary where it belongs.

See:
http://www.skyandtelescope.com/howto/diy/3306996.html
for a good explanation.

Rick

Thank you Rick! That explains it well for me. It sounds like as long as the primary mirror reflection is centered on the secondary when viewed through a sight tube in the focuser, the offset is dialed in. I base that on this paragraph from the link:

If you use a sight tube to center the secondary as described in Step 1, you have automatically offset the secondary toward the primary mirror, thus ensuring that the fully illuminated field is centered. (When using the sight tube, you make the near and far edges of the secondary appear to have the same angular size. This means that the distance from the far edge of the secondary to the optical axis is greater than from the near edge to the axis. This constitutes an offset.) Both partial and full offset conditions give good collimation.

Yep, it all happens automatically.

Rick