Author Topic: Zhumell Z12 Collimation woes  (Read 533 times)

quiterhardpho

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Zhumell Z12 Collimation woes
« on: December 24, 2017, 07:41:57 PM »
I am having no end of trouble attempting to collimate my new (used) Z12.

The best I can tell, the secondary is centered on the focuser tube.  My laser collimator appears to be very we'll collimated.  I place it in 2" to 1.25" adapter, held it firm against a solid object with all the laser shining on a wall a few feet off.  Spinning the collimator kept the laser stable in 1 spot.

Place it at the focuser and moved through the steps.  Adjusted secondary to find laser in circle of main.  Adjusted primary till laser disappeared in center of goal of collimator.

Then when searching thru focuser without an eye piece, I will see the circle on the main and the spider vanes.  If I transfer my eye into line the cross of the spider with the circle on the main, its noticeably not centered in the focuser tube.  It centers a little toward the open end of the tube.

2nd issue, not about collimation, it appears the focal length is somewhat too long for the focuser.  With the 2" eyepiece (30mm) set all the way into focuser,I can't focus out enough.  The focuser reaches its limit prior to the stars or moon is in focus.  If I pull on the eyepiece out about 3/4" and lock down it (not in its own groove), then I am in a position to focus out enough to focus on the moon.  Same issue with all the 9mm 1.25" eyepiece from the 2" adapter.  But maybe not quite as bad, since the adapter adds some length.  I guess I want to use a focuser tube extension to solve this?

Any ideas or thoughts on these 2 issues?

Thank you



breakagalkit

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Re: Zhumell Z12 Collimation woes
« Reply #1 on: December 30, 2017, 09:50:25 PM »
About the out-focus problem, I don't know your particular telescope, but I had a similar problem with a smaller Newtonian. In my case, over the course of many collimation adjustments, the mirror cell gradually was adjusted further and further up toward the secondary leading to the same problem you are having. I fixed this by turning all three collimation screws so the mirror moved back down the tube away from the secondary. That fixed it. After that I always used only two adjusting screws, and always the same ones, to collimate. That way the mirror doesn't creep back up the tube.
Hope this works for you.

coatiorachin

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Re: Zhumell Z12 Collimation woes
« Reply #2 on: December 31, 2017, 07:14:52 PM »
I'm not sure I understand your description of the collimation problem, but you could try using a sight tube or Cheshire to check if the secondary is lined up with the focuser tube. The secondary is supposed to be slightly off-set in a well collimated Newtonian. Could that be what you are seeing?

Did you try the scope under the stars? Maybe it isn't all that bad. If you put Polaris in the center of your field of view, and you defocus slightly, do you see concentric rings (check both sides of focus). If the rings aren't perfectly concentric, you are not properly collimated.

There are many excellent collimation guides on this and other websites. I like the Barlowed laser method myself.

Another approach would be to seek out a local astronomy club where someone would be able to help you.

subhymerlo

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Re: Zhumell Z12 Collimation woes
« Reply #3 on: January 04, 2018, 05:29:26 AM »
Quote
Then when looking thru focuser without an eye piece, I can see the circle on the primary and the spider vanes. If I move my eye to line the cross of the spider with the circle on the primary, its noticeably not centered in the focuser tube. It centers a little toward the open end of the tube.

It is unclear to me what you meant by "the circle". Did you mean the secondary mirror "shadow" or silhouette -- that is, were you referencing the dark disc indicated by the two black arrows?

<p class="citation">Quote

2nd problem, not about collimation, it seems the focal length is slightly too long for the focuser. With the 2" eyepiece (30mm) set all the way into focuser,I cannot focus out enough. The focuser reaches its limit before the stars or moon is in focus. If I pull the eyepiece out about 3/4” and lock it down there (not in its groove), then I'm able to focus out enough to focus on the moon. Same problem with the 9mm 1.25" eyepiece in the 2" adapter. But not quite as bad, since the adapter adds some length. I guess I need to use a focuser tube extension to resolve this?
[/quote]

One way to improve the above is my tightening all 3 primary mirror knobs then collimate without touching the knob under the focuser. Just use the other two knobs. This method will ensure the primary mirror is located as far as possible from the secondary mirror.
In addition, it is possible to have the effect you described when the secondary mirror is too low. That will push out the focal plane away from the OTA.
One more, ensure the secondary mirror stalk is centered in the OTA -- that is, make sure the stalk is not closer to the focuser.

For your scope, you should not have a problem with 1.25" EPs. However, it is possible to continue to have an issue with some 2" EPs. For these problematic 2" EPs, you might need to use a 2" O-Ring which you would slide up the EP barrel to keep it slightly above a fully racked draw-tube.

Jason


noneanoncrag

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Re: Zhumell Z12 Collimation woes
« Reply #4 on: January 08, 2018, 06:27:14 AM »
Aside from the good points cook made, my z8 came with a 2" long focuser extension tube, I haven't needed it yet. Originally I thought I did, but I learned that the screw in the bottom of the focuser closest to the telescope tube (there should be two) was stopping my focuser from coming out all the way. Remove that screw completely and it will let your focuser travel fully.

(the other screw adjusts the focuser tension)

bandretaco

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Re: Zhumell Z12 Collimation woes
« Reply #5 on: January 10, 2018, 02:30:19 AM »
Quote
Quote

Quote

Then when looking thru focuser without an eye piece, I can see the circle on the primary and the spider vanes. If I move my eye to line the cross of the spider with the circle on the primary, its noticeably not centered in the focuser tube. It centers a little toward the open end of the tube.

It is unclear to me what you meant by "the circle". Did you mean the secondary mirror "shadow" or silhouette -- that is, were you referencing the dark disc indicated by the two black arrows?
<p class="citation">Quote

2nd problem, not about collimation, it seems the focal length is slightly too long for the focuser. With the 2" eyepiece (30mm) set all the way into focuser,I cannot focus out enough. The focuser reaches its limit before the stars or moon is in focus. If I pull the eyepiece out about 3/4” and lock it down there (not in its groove), then I'm able to focus out enough to focus on the moon. Same problem with the 9mm 1.25" eyepiece in the 2" adapter. But not quite as bad, since the adapter adds some length. I guess I need to use a focuser tube extension to resolve this?

One way to improve the above is my tightening all 3 primary mirror knobs then collimate without touching the knob under the focuser. Just use the other two knobs. This method will ensure the primary mirror is located as far as possible from the secondary mirror.
In addition, it is possible to have the effect you described when the secondary mirror is too low. That will push out the focal plane away from the OTA.
One more, ensure the secondary mirror stalk is centered in the OTA -- that is, make sure the stalk is not closer to the focuser.

For your scope, you should not have a problem with 1.25" EPs. However, it is possible to continue to have an issue with some 2" EPs. For these problematic 2" EPs, you might need to use a 2" O-Ring which you would slide up the EP barrel to keep it slightly above a fully racked draw-tube.

Jason
I was referring to the little circle in the center of the primary mirror. If I look directly from the center of the focuser, that little circle doesn't line up with thee spider vanes. Maybe what I'm seeing is a non issue. It just doesn't seem to focus as sharply as my 60mm refractor.

Thanks everyone for the suggestion on the focal length issue. I'll give that a try. It does look like it has room to co e back a little.

I'm planning to take it to a local star party soon. May by tonight if I'm not still too sick. Maybe someone there can give it a once-over.

James Gruber

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Re: Zhumell Z12 Collimation woes
« Reply #6 on: January 10, 2018, 09:47:16 AM »
Concerning collimation... you should not be using your spider vanes as a collimation reference. Jason's diagram should give you a good approximation of what you should see when using a sight tube or combo tool.

You can also use a slightly defocused star as a collimation tool. Choose an eyepiece that will give you about a 1mm exit pupil. If the dark center is offset from the brighter outer ring/rings, then adjust your collimation until everything is concentric.

Clear DARK skies!

CB

ransgesislu

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Re: Zhumell Z12 Collimation woes
« Reply #7 on: January 10, 2018, 11:34:23 PM »
Quote

I was referring to the little circle in the center of the primary mirror. If I look directly from the center of the focuser, that little circle doesn't line up with thee spider vanes. Maybe what I'm seeing is a non issue. It just doesn't seem to focus as sharply as my 60mm refractor.


I see, you were referring to the primary mirror center spot (sometimes referred to as the primary donut).
What you see is normal. The center spot reflection that you see is located at around a focal length away from you whereas the spider vanes reflection is located close to infinity. When you move your eye left and right, the spider vanes reflection will hardly move unlike the center spot reflection that will move in the same direction as your eye movement. What you see is parallax.

Refer to the following diagram. The left diagram illustrates why objects located at the focal plane will appear as if they are located at infinity (in your case, the diagram explains the location of the spider vanes -- the same applies to the reflection of the collimation cap pupil). The right diagram illustrates how we view the center spot.
The following diagram emphasizes why objects located at the focal plane give the illusion they are located at infinity just like stars
Jason




stalafovkith

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Re: Zhumell Z12 Collimation woes
« Reply #8 on: January 11, 2018, 10:38:59 AM »
Looks like my laser collimator was a bit off. I built a jig and clamped it to a table, shining the collimator on a wall 25 feet away. Turning the Collimator produced a circle with about a 1" diameter. After adjusting, the circle grew to about 2 feet (oops), but I finally got it down to a single point once I figured out a method for the adjustments.

Then colimated the Z12 again. The secondary needed adjustment after fixing the collimator. I took the advice above and adjusted the primary all the way back (gained about 1/2") and then aligned it using only 2 knobs.

Now, when I look through the focuser w/o and eyepiece, it looks "exactly" like Jason's avatar. Much closer than what I was seeing before.

Also, as wrnchhead suggested, that second screw in the focusor was stopping it from fully extending. Loosening it more allowed it to extend another 1/4" or so. I think that and the primary mirror adjustment may be enough to fix that focal length issue.

I don't know if I'll make it to the star party tonight, but I'll at least haul this outside tonight and look at the moon and the few stars available in my white light dome.

Thanks for all the advice!

James




subhymerlo

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Re: Zhumell Z12 Collimation woes
« Reply #9 on: January 11, 2018, 12:06:23 PM »
My Zhumell laser would not stay collimated. I spent a lot of time doing
and redoing it over and over before I upgraded. You can still get good
results with it, but if it was anything like mine, you can treat it as gently
as possible but it would not stay collimated. My Z12 though stays collimated
very well requiring only an occasional tweek.

calfkommomu

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Re: Zhumell Z12 Collimation woes
« Reply #10 on: January 13, 2018, 09:16:15 AM »
Quote
Looks like my laser collimator was a bit off. I built a jig and clamped it to a table, shining the collimator on a wall 25 feet away. Turning the Collimator produced a circle with about a 1" diameter. After adjusting, the circle grew to about 2 feet (oops), but I finally got it down to a single point once I figured out a method for the adjustments.

Then colimated the Z12 again. The secondary needed adjustment after fixing the collimator. I took the advice above and adjusted the primary all the way back (gained about 1/2") and then aligned it using only 2 knobs.

Now, when I look through the focuser w/o and eyepiece, it looks "exactly" like Jason's avatar. Much closer than what I was seeing before.

Also, as wrnchhead suggested, that second screw in the focusor was stopping it from fully extending. Loosening it more allowed it to extend another 1/4" or so. I think that and the primary mirror adjustment may be enough to fix that focal length issue.

I don't know if I'll make it to the star party tonight, but I'll at least haul this outside tonight and look at the moon and the few stars available in my white light dome.

Thanks for all the advice!

James
jig.jpgdots.jpg

I have a collimating laser that looks exactly like that one, and I'm sorry to say it is worthless. The fit within a 1.25" adapter or focuser is very loose. I can put it in the focuser 5 different times, and the dot will hit the mirror center spot in 5 different locations without any adjustment. You should consider getting a better laser collimator that includes a barlow attachment, which is far superior to the return beam method.

thesaroha

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Re: Zhumell Z12 Collimation woes
« Reply #11 on: January 13, 2018, 01:01:21 PM »
Your best bet might be to trythe barlowed laser method thatcookjaiii mentioned in post 3 and right above, that takes care of a lot of lousy laser problems.

You also might be reasonably well collimated and not know it, have you done any kind of star test? When you look at the moon tonight and a few stars, see if the stars are very round and concentric, slightly in and out of focus. Google star test or search for star test here.

You're probably already reasonably well collimated, have fun.

wordpuzzlesubc

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Re: Zhumell Z12 Collimation woes
« Reply #12 on: January 15, 2018, 01:39:18 PM »
Too many clouds tonight. Completely overcast.

Ray Gibas

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Re: Zhumell Z12 Collimation woes
« Reply #13 on: January 18, 2018, 12:00:45 AM »
Our Zhumell collimator has been reliable, even after being dropped few times slippingout the rear of refractor that had full range ofdiagonal/focuser/objective alignment and collimation issues. I get the same results with it as using the 2" Glatter/Tublug (Barlow.)

One of the reasons it has worked well is that it has always gotten a good centered/flush fit in the adapter and focuser. It is such a good pairing with the compression ring adapter that I began using it in the 20" that way before I picked up the Glatter. The old adapter in the 20" is the old set screw style as is the focuser (2 set screws for it.) Where I was having difficulty with the 20" was the old Kendrick 2"slant face insert collimator. It was a challenge to get a consistent non-tilted position with it due to loose tolerances/heavy cantilever, although I had enough practice to know when it was on and when it was off. As Steve says, if the fit within the focuser or adapter isn't good and consistent, then it makes the tool hard or impossible to use. Incidentally this isthe same problem I have with the optical site tube...loose fit, so I don't trust it at all.

twenuvtentu

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Re: Zhumell Z12 Collimation woes
« Reply #14 on: January 22, 2018, 11:34:33 PM »
Quote
Too many clouds tonight. Completely overcast.

Isn't that always the way?!!

Well the clouds can't stay forever. Let us know how it performs when it clears.

I've read great things about Z12s, and there is a thread here on CN devoted to Z12 modifications. I was truly bummed when I learned they were discontinued.