Collimation Revisited

[mbyrr]

Hi folks

Just some questions about collimation:

Is it possible to still see something if the mirrors are not aligned properly?

I'm asking because the other night I took the dob out to look at the moon (not the Super Moon unfortunately) and I looked down the tube and noticed that the primary mirror was out of whack. So I started collimating outside and while I was aligning the primary mirror using the knobs at the back I saw the collimator's laser light out the front of the tube, so presumably when that happens then the mirrors are aligned (or near so), is that correct?

Because when I collimate centering the laser in the middle of the primary mirror using secondary mirror adjustments and centering the laser in the target of the collimator by adjusting the primary mirror, I do not see the laser light out the front! This doesn't make sense to me at all.

If the mirrors are not aligned properly then could this affect magnification of things? Like the moon seemed to look clear, but other objects seem really distant. Again I'm asking because the first night I took the dob out, after buying it, I saw a very large red glowing object and since then I have not seen anything that big, besides the moon of course. Just wondering if this is all related...

cheers
A

[Dodie]

I am only new to this, but I'll give you my thoughts.

The laser should not be coming out the front of your tube. The course the beam should take is this - laser goes into EP slot, hits secondary mirror - bounces onto primary - bounces back to secondary and then should hit the target on the laser collimator in the centre.

Damien

[mbyrr]

Actually that does make sense, thanks Damien.

So it seems it is collimating correctly then, well at least I'm doing something right!

I wonder what that big orange thing was then... UFO!

[Robh]

Hi Antony,

The object of collimation on your Dob is to make sure all the light coming from the primary mirror reaches the centre of the eyepiece. To get there, all this light must be reflected by the secondary mirror. If the primary mirror is tilted off-axis, some (or even all) of the light could miss the secondary mirror i.e. pass back out of the tube. If the focal point of the primary mirror is off to the side in the eyepiece, you will get large distortions. If the focal point is way off the eyepiece altogether, you won't see anything.

The laser helps to align the secondary mirror with the primary mirror. Then by returning the beam to the centre of the collimator you put the focal point of the primary mirror in the centre of the eyepiece.

See attached diagrams.

Regards, Rob

[mbyrr]

Thanks Rob. It seems I have been collimating correctly then. I'd never seen the laser come out the end of the tube, so it made me think something was wrong with the way I was doing it, tho it seems it was the opposite. :lol:

[Jon]

If the mirror is not collimated you will still see stuff. Just with a variety of distortions/defects/lack of pin-point focus, and some loss of light-gathering.

Large glowing red object ... sounds like an aircraft warning beacon on top of a tower to me :-)