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Collimation Questions...

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#1 TahoeNoob

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Posted 04 December 2012 - 05:48 PM

I received my new (used) Orion XT10i yesterday. When I received it, the collimation was way out, so this morning I sat down and started trying to figure out how to get it set up correctly. I fought with it for the longest time and made some dumb mistakes. I think I now have it collimated reasonably well... but something still isn't right and I don't know what's wrong...

First off, this scope was upgraded with "Dobs Knobs," so there's nothing that says it was close to being set up correctly when I got it. When I started this morning, I sat down with the directions and started from the beginning. (Everything at infinity was very fuzzy, so I knew something was wrong.)

It took me a long time to figure this out, but when I try to center the secondary mirror in the Cheshire Eyepiece, I can't! Either the three adjuster screws need to be longer, or there needs to be a shim between the back of the primary mirror and the ends of the three adjuster screws. I can't push the secondary mirror far enough back towards the primary mirror to center it in the eyepiece.

Something has to give here! This seems so obvious to me and, yet, I'm so new at this... I just assume I'm missing something obvious! (There's already about a 1/4" between the back of the spider and the back of the secondary mirror and I need slightly more! Can that possibly be correct? Am I doing something stupid?)

Another problem I had, which took me forever to figure out, is that I needed to use the 2" Cheshire Eyepiece instead of a 1.25" Cheshire/Sight Tube which I had bought. The 1.25" sight tube clips my view down enough that I can't see the whole secondary mirror... thus, I can't center it.

Any and all help is very welcome! :)

#2 Atl

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Posted 04 December 2012 - 06:27 PM

Adjust the focus tube in until you can see the whole mirror. If the secondary is reasonably straight you should be able to collimate the scope with the 1.25". If you can't see the whole mirror in any position then adjust your secondary in or away from the primary until it comes into view. I had this issue when I suddenly found myself in possession of a 12" dob...unexpectedly. I managed to cope easily until I got a 2" collimation eyepiece. Recently I have found I can very roughly collimate it by just staring down the focus tube. It is not exact at all, but you can get in the ballpark once you start understanding the process. Once you can collimate it I recommend you do so frequently rather it needs it or not...it is a skill that must be learned and it is not cut and dry at all.

http://uncle-rods.bl...ollimation.html

#3 howard929

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Posted 04 December 2012 - 06:46 PM

Let me see if I can help. Start off with a 8x11 piece of paper taped inside the OTA and behind the secondary mirror. This will give you something to reference off of when you look at the secondary mirror with your sight tube. I use a simple peep sight for this. Adjust the length of the center screw on the secondary stalk until the mirror is centered in the view. Adjust the secondary collimation screws so that it also appears round. Go back to the previous step and adjust if it's round but now not centered. For the most part, a small mis-adjustment of the secondary will only result in a small loss of light but do get it as close as you can to correct.

You may not need to and hopefully you won't but do you have an understanding of how to collimate the primary mirror?

Also, adjust the focuser out a bit until you can see the entire view of the secondary mirror.

#4 TahoeNoob

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Posted 04 December 2012 - 08:30 PM

>> Adjust the length of the center screw on the secondary stalk until the mirror is centered in the view. Adjust the secondary collimation screws so that it also appears round.

That's the problem, when I adjust the length of the center screw so that the secondary mirror is centered in the peephole... the three secondary collimation screws aren't long enough to hold the secondary in position.

The secondary mirror appears to be round.

The scope is collimated, except for the fact that the image of the primary isn't centered in the secondary. The secondary mirror needs to be moved further back into the tube. (Towards the primary.)

Is it possible that I need to put a shim under one edge of the focuser?

EDIT: I probably ought to mention that the previous owner upgraded the focuser. It's not original. (Perhaps it's not square?)

#5 TahoeNoob

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Posted 04 December 2012 - 08:50 PM

I suppose maybe moving the mirror down, away from the focuser, might cause the image to move up towards the center of the secondary.

EDIT: Well, that didn't work. Harumph! :bawling:

#6 uniondrone

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Posted 04 December 2012 - 10:28 PM

The scope is collimated, except for the fact that the image of the primary isn't centered in the secondary. The secondary mirror needs to be moved further back into the tube. (Towards the primary.)


Do you see all four of the mirror clips in the reflection of the primary in the secondary? If so, do they all appear the same distance from the edge? If not, you've got some tweaking to do. If you can't get the secondary to extend far enough to be under the focuser, you can pick up a longer screw from the hardware store. Also, I would recommend relocating the spring on the secondary holder to the other side of the hub, so that you actually get spring-action on the collimation screw adjustment. The original positioning doesn't seem to accomplish much.

#7 TahoeNoob

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Posted 04 December 2012 - 10:44 PM

UnionDrone, what you say makes sense to me...

I can see all four clips that hold the primary, but they're not centered in the secondary. The image is shifted towards the primary, which tells me that the secondary hub and mirror has to be moved further back towards the primary. (So the image is centered in the secondary.)

Ok, so I need longer screws. That's actually good news, because it means I was on the right track... I just couldn't believe it!

I haven't seen a spring on the secondary hub. While I was checking to see if the center screw was long enough, I had the secondary mirror completely removed from the scope. I still didn't see any spring. Where, exactly, should this spring be located?

Is it possible to run without the spring, because I think that might be what's being done now. How important is this spring?

#8 howard929

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Posted 04 December 2012 - 10:52 PM

That spring is supposed to be around the center screw to hold the mirror holder in place until the adjusting screws are tightened. On my GSO Newt, it's mostly useless. If you left it out and collimation is good, don't sweat it.

#9 TahoeNoob

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Posted 04 December 2012 - 11:12 PM

Thanks. I won't worry about the spring then. The three collimation screws hold the mirror just fine.

I believe the answer is that I need either longer screws, or to mount a custom shim/washer between the secondary hub and the spider.

#10 uniondrone

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Posted 04 December 2012 - 11:16 PM


I can see all four clips that hold the primary, but they're not centered in the secondary. The image is shifted towards the primary, which tells me that the secondary hub and mirror has to be moved further back towards the primary. (So the image is centered in the secondary.)


Hmmm... ok, not quite sure what you mean. Do you mean to say that the reflection of the primary is "off" in the direction of the primary? If so, that just means that the tilt of secondary needs to change, specifically the end closest to the primary needs to be tilted toward you.

Ok, so I need longer screws. That's actually good news, because it means I was on the right track... I just couldn't believe it!


It is possible that longer screws could help. Don't run out and buy them quite yet. If the "circle" of the secondary mirror's outer edge, as viewed through the focuser opening, doesn't appear to be directly under the focuser, then you might need to unthread some of the extra screw length out of the secondary holder to effectively lengthen it. When I say this, I am speaking specifically about the CENTRAL screw that holds the secondary holder in place. You should only need a longer central screw if you are getting dangerously near the end of its threads. As for the three collimation screws for the secondary, you should only need longer ones if you are near the end of the range of possible adjustment.

To see if the secondary mirror is centered, I compare the position of the secondary mirror's edge to the edge of the focuser draw tube. I find this easier (at least for the rough adjustment) when there is no collimation tool in the focuser.

I haven't seen a spring on the secondary hub. While I was checking to see if the center screw was long enough, I had the secondary mirror completely removed from the scope. I still didn't see any spring. Where, exactly, should this spring be located?

Is it possible to run without the spring, because I think that might be what's being done now. How important is this spring?


There normally is a spring there. Orion always puts it between the head of the central screw and the hub--which does nothing at all! If you were to place it instead on the central screw between the hub and the secondary holder, you would effectively spring-load the three collimation screws, which is very helpful.

If you don't have a spring, you can get along without one. In fact you'd be no worse off than having it where Orion originally placed it. However, if you can find a spring of the right size and tension, and then put it in the *right* place, it would make things easier for you. I find that the compression spring found in some staplers is about the right size.

#11 uniondrone

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Posted 04 December 2012 - 11:20 PM


Also,

if you are still having trouble with any of the steps of collimation, this video is pretty informative:

http://www.andysshot...ollimating.html

#12 TahoeNoob

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Posted 04 December 2012 - 11:39 PM

You're right, I got off track... and was no longer focused on the problem at hand. To be specific, I can tilt the secondary mirror and put the primary's image wherever I want it on the secondary. That's not the problem.

The problem is that the outside edge of the secondary isn't centered under the eyepiece, and the three collimating screws aren't long enough to push the secondary to where it needs to be. The center screw is long enough, it's the three collimating screws that aren't.

I checked the length of the center screw when I became concerned that I was getting myself into a position where I might strip the threads in the secondary.

#13 GOLGO13

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Posted 04 December 2012 - 11:52 PM

tahoe...please snap a few photos of your setup if u can. be careful not to extend your secondary holder so far it falls off and onto your mirror. i am afraid you maybe too far toward the primary if the screws are ok. some pics of the empty focuser may show us some clues.

#14 TahoeNoob

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Posted 04 December 2012 - 11:53 PM

Thanks for the video. It's a good one. My problem is Step #1. (Steps 2 and 3 work fine.)

#15 uniondrone

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Posted 04 December 2012 - 11:56 PM

You're right, I got off track... and was no longer focused on the problem at hand. To be specific, I can tilt the secondary mirror and put the primary's image wherever I want it on the secondary. That's not the problem.

The problem is that the outside edge of the secondary isn't centered under the eyepiece, and the three collimating screws aren't long enough to push the secondary to where it needs to be. The center screw is long enough, it's the three collimating screws that aren't.

I checked the length of the center screw when I became concerned that I was getting myself into a position where I might strip the threads in the secondary.


Ok, it seems that the collimation screws are not long enough. If I understand correctly, you can loosen the central bolt enough to allow the secondary to potentially reach the right position, but the three collimation screws won't reach even when they are threaded in all the way. If this is right, you'll very likely need to replace those collimation screws.

I would guess that the screws that you have are not stock, but are incorrect replacement screws put in by original owner?

You can try either Bob's Knobs (and order the ones with the proper length) or you can find screws to your own liking from a place like McMaster-Carr. With McMaster-Carr, however, be sure to know exactly what you want, as they have many options.

#16 JLovell

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Posted 04 December 2012 - 11:58 PM

As far as a shim or washer is concerned, some people use a washer cut from a milk jug in there. Some sdecondaries develop little divots where the adjusting screws make contact, and can keep the secondary from rotating properly to make the reflection go straight up the focuser tube. You rotate the secondary until it looks round, rather than oval shaped. The milk jug washer keeps the screws from settling down into the divots (well it helps anyway).

If you can't see the entire secondary, rotate the focuser knob to extend the focuser. If the focuser is all the way out, pull the sight tube tool out some more, and tighten the set screw when you can see the whole thing. I'd actually use the 1.25" tool if most of your eyepieces are that size. I think collimation is more critical at high powers than at low, and high powered eyepieces are 1.25"

Don't look at any of the reflections in the secondary until you get it centered under the focuser and looking round instead of oval shaped. You'll mainly use the center screw and rotate the secondary around the center screw to do this. Then, use the three outer screws to adjust the tilt of the secondary up, down, left, right, to get the primary mirror centered in the secondary. Don't worry about what is reflected in the primary yet, just get it centered. When you get all of that done, you are through with the secondary. All the remaining adjustments are done at the primary.

#17 uniondrone

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Posted 05 December 2012 - 12:05 AM


An additional thought: double check to make sure that your eye position over the focuser isn't giving a false impression of the secondary's position. It's worth a double check, just to make sure.

#18 TahoeNoob

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Posted 05 December 2012 - 12:19 AM

I'm not sure if I should post the pictures or the links. (I can edit if this breaks some kind of rule that I don't know about.)

Pictures...

That's a 3/8" gap between the Spider Hub & the Secondary Hub:
Posted Image

No more room for adjustment on the 3 secondary collimation screws:
Posted Image

The tube is flocked. Could the upgraded focuser be mounted crooked:
Posted Image

This is deceptive, but the secondary isn't centered under the focusing tube:
Posted Image

The focuser:
Posted Image

#19 panhard

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Posted 05 December 2012 - 12:32 AM

Chuck the milk jug washers should help you out. If it Doesn't I am going to replace the 3 collimation screws with Bobs Knobs. Just send me a private message with your name address etc. I will send them to you no charge.

#20 howard929

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Posted 05 December 2012 - 12:36 AM

Lets for a moment assume the possibility that the adjusting screws are as long as the original screws that came with the telescope. You mentioned that the focuser had been replaced. Is it centered and pointing directly across the OTA without any cantering/tilted towards the primary mirror?

#21 GOLGO13

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Posted 05 December 2012 - 12:39 AM

First off it looks like a nice scope! Flocked, nice GSO two speed focuser. Good stuff.

Hmm...I would agree with you that it appears to be short screws. Let's maybe try the ATM/DIY forums with these pictures and the situation. Maybe they have some good solutions for this issue.

#22 GOLGO13

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Posted 05 December 2012 - 12:44 AM

That's a 3/8" gap between the Spider Hub & the Secondary Hub:


mine looks to be closer to an inch...maybe a little less than an inch.

Hopefully there an easy fix to fill that gap without needing new screws.

#23 frito

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Posted 05 December 2012 - 12:48 AM

i'm pretty sure if you make a few milk jug washers you'll get the extra distance you need and it will make fine adjustments much easier as well. i ditched my spring and made milk jug washers last weekend for my XT8. rotating and adjusting the angles are now super easy to do and i don't even have bob's knobs i'm using an allen.

on another note your secondary is A LOT farther off the holder base than my XT8's this may be a result of the GSO focuser though i have no idea.

also another thing to note do not concern your self with the shadow of the secondary mirror, it should look somewhat oval shaped and not at all centered and offset tword the primary mirror. this is completely normal. i'm surprised Jason hasn't crashed this thread yet to talk about that one :)

#24 howard929

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Posted 05 December 2012 - 12:53 AM

also another thing to note do not concern your self with the shadow of the secondary mirror, it should look somewhat oval shaped and not at all centered and offset tword the primary mirror. this is completely normal.


I beg to differ. Offset towards the primary, yes but still centered and round.

#25 TahoeNoob

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Posted 05 December 2012 - 01:09 AM

I can't say why this is going on. It's entirely possible that this is the result of the upgraded focuser. It also crossed my mind that maybe the scope was dropped, or something like that, but I don't believe that to be the case. The scope is in very good shape. No signs of abuse...

Tomorrow, if it doesn't snow (hard), I'll drive into town to see if I can find some longer screws. I think I'll try to find a longer center screw too, just to be safe. I doubt the hardware store will have what I want but there's only one way to find out.

What exactly are "milk jug" washers? I don't think a washer made from a plastic milk jug is going to do me much good... unless I start stacking washers. Would plexiglass hold up to the collimation screws? I've got some plexiglass. :)

In the end, I'll probably have to contact Bob's Knobs. The thing is, I think these might actually be Bob's Knobs. I was told the screws were upgraded. I'll have to go back and read the CraigsList ad again... I think it was mentioned there.

:tonofbricks:






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