2003 meade ds2114 - cannot collimate

[derherr65]

I'm about ready to throw the towel in. I started with a scope that had an egg shaped view through the eyepiece but fairly clear. After multiple laser and visual collimations I have gotten nice circular and concentric views through the eyepiece, but stars are blurry and planets have multiple lobes. I'm at a loss.

[sixela]

Through the focuser, there is supposed to be a "egg-shape": the sihouette of the reflection of the secondary *should* actually appear offset with respect to the rest (towards the primary), so my guess is that you "unfixed" something by trying to make too many things concentric. And as this is a fast scope, it should appear offset a lot.

What tools do you have exactly, and what protocol are you following to collimate?

[Peter Drew]

Sounds like you may have a "pinched" primary mirror. Check to see if it is reasonably loose in its cell.

[derherr65]

I am beginning to agree. My results are more consistent, just not effective.

The laser collimator is by "ANTARES." I check with a micrometer to make sure the secondary is centered in the 4.5 aperture, and inline with the focuser by making sure the mirror clip is directly opposite the focuser. I sit the laser in the focuser, usually bottomed out, screws just tight enough to prevent wobble. The corrector lens in the focuser turns the laser football(American!) shaped. I've found that the laser also scribes a circle on the primary as it turns. I collimated the laser resulting in a smaller circle. So I declare center the center of the circular path. Adjust the three screws on the secondary to center the laser on the primary mirrors center dot. (also installed by micrometer) Then I adjust the primary to get the laser "dot" to circle the focuser. It is usually half in the focuser hole and half out as it scribes it's circle.

I have not found any rule as to how far in out out the secondary should be so it is entirely possible it has wandered while I attempted to visually center the secondary under the focuser.

I have measure the focal length of the primary mirror, and found it to be somewhere inside the focuser. I suppose the corrector lens only works correctly if the focus is at some exact point?

Is there a way to determine why the laser scribes a circle? I'm sure wobble contributes, but it's too consistent for random wobble. Is it likely the corrector lens is not centered in the focuser? The football shaped laser tumbles as the laser is spun in the focuser so that one flatter side continually points toward the middle.

[derherr65]

Lol, as I reread that, thinking spherical mirror, corrector lens, wobble in focuser, possible corrector lens misalignment(glued in naturally!), If I were giving the advice I'd be struggling no to suggest ditch the OTA and put a decent one one the mount.

[sixela]

Then I adjust the primary to get the laser "dot" to circle the focuser. It is usually half in the focuser hole and half out as it scribes it's circle.

A bit ineffective, because it will miscollimate your scope by half the forward error. Also, the fact there's a corrector also makes the forward beam error important.

And as long as that beam scribes circles, it's not a good tool to collimate. You really have to collimate it so that it no longer scribes a circle at all, and you may then still have to shim it to make registration in the focuser good enough.

To collimate it, use a v-block and a distant wall that magnifies the error (and not your scope because the corrector makes everything harder).

Instead of a V-block, you can also do the dirty'n'cheap trick with screws:

Is there a way to determine why the laser scribes a circle?

Probably because it's miscollimated - the beam doesn't exit parallel to the axis of the tool.

Have you checked it with a v-block (or just four screws on a block) on a distant wall?

The football shaped laser

That's because the corrector turns the nice beam into a cone coming from a virtual point source, with the outline football shaped because the original beam is rectangular (which means the laser collimator doesn't have a good aperture stop). The fact very small changes on where the beam hits the corrector are magnified makes it impossible to use the forward beam at all, really.

You'd be better off with a sight tube and Cheshire - the fact you cannot remove the corrector means that really, you cannot use a laser collimator's forward beam. And the Cheshire is in any case more precise than a laser collimator's return beam (which is only a correct reference if the forward beam is circular and perfectly hits the centre of the primary).

For setting the tilt of the primary, you're even better off with a simple collimation cap. Even if it's hard to read because the corrector makes the centre spot reflection as seen through the focuser a lot smaller.

Do you know anyone who has passive collimation tools you could borrow?

Otherwise, I'd just star test. Do your best with the tools you have, and then use a star test to try to null coma by adjusting primary tilt (i.e. making the diffraction ring pattern extremely close to focus at very high powers symmetric), and live with any possible residual astigmatism that you can still see.

By the way, that all has nothing to do with the appearance of the secondary outline when you look through the focuser - that's something that has to do with diagonal placement and not axial collimation (which is what you're trying to do with the laser collimator and is independent of where the secondary is, but just depends on what plane its face is sitting in). You evaluate that with a sight tube, but you don't have one, I gather.

Yes, they're not the friendliest scopes to collimate (which is why I tend not to suggest those catadioptric scopes to beginners), and in fact if the corrector isn't glued correctly you can't both correct coma and astigmatism at the same time. It would be better if the corrector was actually screwed in somewhere and removable, but alas...

But persevere and you'll make it as good as possible.

[derherr65]

I have done a rough v block collimation of the laser, but intend to do better. Halving the laser error actually made very little difference in the size of circle seen in the scope, hence my suspicion about the corrector lens. Will report back as soon as I have confidence in lasers collimation.

[sixela]

The thing is that the barlow effect of the (diverging!) corrector means that a very tiny offset in where the beam hits the corrector gets amplified by it before if hits the primary (and it makes the spot very blurry, too).

[derherr65]

Ok, laser collimator now accurate to 1/2cm over 12m... or less that 1/4" over 40'. The nearest tape measure happened to be metric. Any closer is going to require bearings and a driver motor.

[derherr65]

For those of you keeping score, still definitely scribing a circle, though slightly smaller than last time. Now, on to scope collimation.

[derherr65]

Made some progress tonight. In and Out focus have similar views, and have reasonable clarity when focused. Ran into a mechanical issue so did not fully test focus. Due to tracking a smaller circle I realised the shadow of the center spot on the primary does not circle and should be centered in the laser receiver regardless.

I think sixela is right. The corrector lens makes this model highly sensitive to even small laser errors, and the sloppy focuser aggravates that even more. What sort of laser accuracy is required in my 40' span?

[johnnychaos]

Did you ever get this sorted? I'm really struggling to collimate one of these my self. It all seems lined up fine, but views are still bad and out of focus.

Worse thing is it's not even mine so doesn't matter if I can't sort it. But I don't like to be beaten! lol