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#1 Durden

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Posted 29 September 2013 - 06:48 PM

I've been fumbling around with the collimation of my 12" Skywatcher collapsible dobsonian for a few weeks now, I understand how the process works but I keep getting conflicting reports from the tools I use. The tools in my collimation repertoire include a collimating cap, a cheshire sight tube and a Hotech laser.

The problems I've been having mainly relate to getting the secondary in the correct position over the primary. The skywatcher manual says to get the reflection of the primary under the secondary in such a way that you can see all three primary mirror clips with equal size and spacing. With the collimating cap this is no problem, with the cheshire this is not possible as the view is too narrow to see all the mirror clips. If I try and pull the cheshire out a little bit and then tighten the focuser screws to give me a wider view, there is waaaay too much focuser slop to get any kind of accurate reading. So the collimating cap is definitely the best manual way to go for this step for me.

Now the problem I've been having - If I get the secondary centered over the primary according to the collimation cap and then throw in the Hotech laser, the beam from the laser is way off, I'm gonna say close to a full centimetre. If I make the adjustments according to the laser and then throw the cap back in it looks noticeably out of alignment. See the attachment for the difference, don't worry about clip sizes or spacing this is just a rough sketch to show the difference in space around the reflection of the primary.

Now here's the kicker, my laser is not out of alignment as the beam doesn't move when rotated in the focuser. Also, this is my second laser that has given me this exact type of result, my first was an Orion lasermate deluxe which I sent back because I figured it was faulty. I also went to the store where I purchased my scope and tried collimation with one of their floor models of my scope with a different laser and I got the exact same result - two different scopes of the same model, two different lasers, same result!!

So either Skywatchers instructions are off and the primary mirror reflection isn't supposed to be centered, maybe due to a slight offset of the secondary?? Or, something is going wrong in the laser process that I can't figure out.

It would make sense to me that what I can see with my eye to be centered would be the correct way, but the laser says I'm way off. Very frustrating.

I know that the star test is the best way to tell if something is out of alignment but to be honest I guess I haven't quite perfected either process yet because when I do a star test after each method of collimation the rings aren't exactly symmetrical. The only response I've gotten from Orion, and the vendor where I purchased the orion laser, and a different vendor where I purchased the Hotech is that "the laser is probably correct", this does not instill a lot of confidence.

Any input would be greatly appreciated, sorry for the long post. Maybe someone has had a similar experience to mine with collimating skywatcher dobs.

Thanks in advance!
Brian

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#2 christheman200

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Posted 29 September 2013 - 07:07 PM

The only thing I can think this could be is that your centre spot is off on your primary mirror. Check that.

#3 Durden

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Posted 29 September 2013 - 07:29 PM

I was hoping to avoid this =( I'm a little sceptical about removing the primary and putting it back in. But I suppose nonetheless it would be a good learning experience for a first time dob owner.

#4 Vic Menard

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Posted 29 September 2013 - 07:44 PM

...If I try and pull the cheshire out a little bit and then tighten the focuser screws to give me a wider view, there is waaaay too much focuser slop to get any kind of accurate reading. So the collimating cap is definitely the best manual way to go for this step for me.

Actually, considering your results, neither is getting the job done. Ideally, you should be aligning three circles (the bottom edge of the Cheshire/sight tube, the actual edge of the secondary mirror, and the reflected edge of the primary mirror). If you only align two circles (the actual edge of the secondary mirror and the reflected edge of the primary mirror), the focuser axial alignment is strictly hit or miss--as your laser demonstrates.

You can proceed with your alignment procedure if you limit certain adjustments to certain tools. For laser alignment, limit any secondary mirror adjustment to the three tilt alignment screws. For collimation cap alignment, limit the secondary adjustment to fore and aft (middle mounting screw between the three tilt adjustment screws) and rotation. Repeat as many times as necessary until both tools demonstrate good alignment (both errors should progressively be reduced after each alignment round if you limit the adjustments to specific tools).

FWIW, the views you posted are usable. The HoTech is delivering axial alignment (focuser) and since you can see the entire primary mirror reflection in the collimation cap, you're getting good illumination in the center of the field of view (the illumination will be unbalanced as you move to the edge of the fov.

...the laser says I'm way off. Very frustrating.

The laser says your secondary mirror placement is not optimal. Optimal secondary mirror placement delivers balanced illumination across the fov. Axial alignment delivers image performance (planetary detail, etc.). Axial alignments have tolerances--secondary mirror placement does not.

...The only response I've gotten from Orion, and the vendor where I purchased the orion laser, and a different vendor where I purchased the Hotech is that "the laser is probably correct"...

Implying that the collimation cap is not correct? That's actually kind of funny. The laser is showing where the focuser axis is pointing--but it's not very good at telling you whether or not the secondary mirror placement is correct. Similarly, the collimation cap shows where the primary mirror axis is pointing (when you align the collimation cap pupil to the primary mirror center spot)--but it also isn't the best tool for secondary mirror placement. The two tools used together with a systematic procedure can deliver "textbook" alignment (axial alignment and optimal secondary mirror placement), but a good sight tube will get you there quicker. I suspect the loose fit you've described is mostly due to the 2- to 1.25-inch adapter. If it really is too loose, an upgrade is the best solution. I've found that the Glatter Parallizer delivers consistent axial alignment results.

#5 Vic Menard

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Posted 29 September 2013 - 07:47 PM

I was hoping to avoid this =( I'm a little sceptical about removing the primary and putting it back in. But I suppose nonetheless it would be a good learning experience for a first time dob owner.

Don't worry about the center spot yet--even if it's 1/2-inch off center, it won't cause the error you've illustrated.

#6 Vic Menard

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Posted 29 September 2013 - 07:56 PM

We just had a similar discussion thread here.

#7 christheman200

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Posted 29 September 2013 - 08:38 PM

That thread should help you out a lot. I now realize that it is probably not the centre spot that is the problem. If you still need help, we can get together and fix this.

#8 Durden

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Posted 29 September 2013 - 09:36 PM

Thanks for the replies guys. Looks like I've got my reading material for work tonight ;) This site is freakin awesome and filled with helpful people. I've been blown away by the community support since I've gotten back into this hobby full throttle.

Anyways I will try the suggestions listed in the other thread and possibly upgrade my cheshire for one with a decent fit, or maybe a focuser that's been machined a little better.

Will report back soon!

#9 Nils Olof Carlin

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Posted 30 September 2013 - 01:57 AM

If I get the secondary centered over the primary according to the collimation cap and then throw in the Hotech laser, the beam from the laser is way off, I'm gonna say close to a full centimetre.



You don't say what it is off of :grin:
If you mean it does not hit the center of the elliptic face of the secondary, this is perfectly in order! This is due to the offset necessary to center the fully illuminated field - I'd guess 5-6 mm (not quite a cm) closer to the focuser side.
Thus, trust the collimation cap (if its peephole is indeed centered!). Tilt the secondary to center the laser beam on the primary, then check that the secondary appears concentric with the primary (a piece of white paper held behind the secondary can make this easier). A sight tube or Cheshire is helpful but not really necessary here, as long as you have a reliable laser (or lacking one, use the crosshairs of a Cheshire combination) to tilt the secondary.

Of course, watch out for a significantly rotated/tilted secondary holder...

Nils Olof

#10 Durden

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Posted 30 September 2013 - 02:04 AM

Sorry, I meant to say that the beam is way off the center mark on the primary. There's really no time when you actually look at the laser on the secondary if I'm not mistaken? To be honest I don't even know where the beam lands on the secondary mirror. :lol:

#11 Jason D

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Posted 30 September 2013 - 02:37 AM

Sorry, I meant to say that the beam is way off the center mark on the primary. There's really no time when you actually look at the laser on the secondary if I'm not mistaken? To be honest I don't even know where the beam lands on the secondary mirror. :lol:


Read the last post in the following page
http://www.cloudynig...eflectors/Nu...

Here is a quote from that post. Read the rest of the post.

"In addition to the collimation cap that comes with many mass produced reflectors, many beginners purchase a laser collimator and follow the proper steps to find out they can't see all the primary mirror clips via the secondary mirror. By proper steps I meant aligning the secondary mirror by redirecting the laser beam to the primary center then aligning the primary mirror by redirecting the laser beam back to its source. When beginners run into this issue they wonder if the problem is with the quality of their laser collimators. Then they realign the secondary mirror using the collimation cap to bring all of the primary clips to view. Now the laser beam no longer hits the primary mirror center. At this point frustration builds and no matter what they do, they just can't reconcile between the collimation cap and the laser collimator."

#12 Durden

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Posted 30 September 2013 - 03:14 AM

Very nice detailed explanation of what's going on how to remedy the problem. Vic was bang on as well in his post above. I'm stuck at work so I can't try it out right now but I can say with 99% certainty that this is my exact problem and I feel very confident about finally getting the visual and laser cues to match up.

It makes sense when you think of how the laser works and what it's capabilities are. What got me stuck in the endless loop was that I was checking the secondary/focuser alignment as the first step by making sure the real secondary was a nice circle in the center of view.

Again thanks for the replies, will report back as soon as I have a chance to try this all out.

Brian

#13 bleep

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Posted 30 September 2013 - 09:01 AM

I have the Hotech laser with the 2" adapter and I've been using the adapter as my centering tool for the Cheshire with positive results. Just insert the Hotech. Tighten it down as it says in the instructions then loosen the ring holding the laser and pulling the laser itself out leaving the 2" adapter in the focuser for the cheshire. I did put some packing tape around the Cheshire first for a little tighter fit.

#14 Nils Olof Carlin

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Posted 30 September 2013 - 06:48 PM

Sorry, I meant to say that the beam is way off the center mark on the primary. There's really no time when you actually look at the laser on the secondary if I'm not mistaken? To be honest I don't even know where the beam lands on the secondary mirror.



I see now. Think of it like this: by tilting the secondary to land the laser on the primary's center mark, you center the primary on the "line of sight" from the focuser. If things look like your first sketch, fine, but I understand this is not so for you. In your second sketch, the primary is centered but the secondary isn't centered around it. Then it is the secondary that should be moved (you see what direction in the cap - but be sure you don't drop the secondary on to the primary!).

If the secondary is not centered on the line of sight (=laser beam), you can tilt it to center the primary in it (as in your first image) - but then both are off, and that is the explanation for your sketches, I believe now. *)

With a truss tube, you can see the laser beam on the secondary if you look from somewhere between the focuser and the primary (with a closed tube, not so easy!). You can see a spot from the down-going beam and (normally) from the reflected beam - you tilt the primary to bring the spots together, as a first step in collimating the primary. But always get the secondary right first!

Nils Olof

*) (E.g. Gary S in S&T has advocated first placing the secondary centered as best you can, then center the primary in it. This might work after a fashion, but I would not recommend it if you own better tools such as a laser or a crosshairs Cheshire combination tool).

#15 Starman1

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Posted 01 October 2013 - 06:13 PM

Look carefully at Jason's avatar. Notice how the shadow outline of the secondary is not centered where all the other circles are. That's an example of a well-collimated scope:

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#16 michaeldurban

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Posted 30 October 2013 - 03:23 PM


I also cannot see the six mirror clips in my SW 10" truss.
So I just use a cheshire, block off the opposite side of the focusser with white paper.

Then, I traced the bottom lid that comes with the truss. cut it out and found the centre.
I put the lid (with the circular cutout stuck onto it) back on the bottom tube.

Now I insert my cheshire, and make sure that the secondary is circular and centered in the focusser tube.
Then I tilt the sencondary to make sure the crosshairs of the cheshire and the centre spot on the cut-out coincide.

Only then I take the lid off and survey the situation.
I only needed to tweak the primary a little bit and the whole thing was in perfect collimation.

But, when I inserted my (collimated) Orion Deluxe lasr colli it did not hit the centre spot dead on.
So I tweaked the secondary centering the laser on the primary spot.
I rechecked with the cheshire and everything seemed exactly the same.

Maybe do it like that, make a cut-out (same diameter -or slightly less- ) of your truss lid, and stick it on.
It helped me a lot.

and otherwise..eh...ask Don or Nils... :bigblush:

#17 nypsirc

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Posted 30 October 2013 - 04:40 PM

Here's a link to a quick guide that is extremely helpful
Astro Baby's Guide to Collimation

This guy only uses a collimating cap and a cheshire.






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