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Collimation Questions...

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#26 GOLGO13

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Posted 05 December 2012 - 01:14 AM

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



Looking at picture 3 it does look a little crooked...but hard to tell with the flocking around it. That's an area worth looking at. Check screws and fitment.

#27 howard929

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Posted 05 December 2012 - 01:22 AM

I'm not a fan of using milk jug washers. They are made out of plastic milk containers and are inserted between the spider and the mirror stalk. They're supposed to allow for adjusting the secondary easier then without them but I found they just get chewed up by the screw ends. And it's a PITA to replace them when that happens.

Anyway. Plexi would work fine if you can shape it to be sized like the open end of the stalk and locate a hole for the center screw.

(slow typist and we noticed the same thing) That one photo of the focuser shown inside the OTA: is it the photo or is it titled to one side? Actually, I copied and pasted to a drawing program and rotated it. The focuser looks tilted but it may be the photo.

#28 TahoeNoob

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Posted 05 December 2012 - 01:29 AM

I'll check the focuser in the morning. I hadn't noticed that. (My guess is that it's just the flocking, that's making it look like that.)

Right now, I'm tired. I've been fighting with this almost all day.

Thanks for the help, everybody! I'll let you know what I find out, in the morning!

#29 howard929

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Posted 05 December 2012 - 01:38 AM

I'm heading off too. One last thought though. The bezel around the focuser hides the condition of the OTA right around the opening as does the flocking on the inside. I'm not saying you need to remove the focuser to see what the OTA looks like right around the hole. But if it is crooked, you may want to and hopefully it's not tilted.

#30 frito

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Posted 05 December 2012 - 02:09 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.


you are correct, it just kind of appears oval due to the offset but it is not in fact oval

here is one of Jason D's posts about it with a link to another thread discussing it.

http://www.cloudynig...ber/5503304/...

OP make sure that the secondary shadow is not whats making you think its not centered, many people get tripped up by this that much i know.

#31 tag1260

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Posted 05 December 2012 - 07:44 AM

If you can't find longer screws, you can get a couple of "fender washers" and use. They're just those big round washers with a small hole.I use one under a milk jug washer on mine. I use the fender washer to protect the secondary and the the milk jug washer makes the screws easier to turn (at least in my mind).

#32 JLovell

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Posted 05 December 2012 - 08:12 AM

Another thing to possibly try to make it easier to see the mirror and the edge of the peep sight against the flocking background is to put a piece of paper in there across from the focuser. You'll only see the paper around 2/3 or so of the secondary. The edge of it and the stalk will hide the rest. You are trying to center only the reflective surface, right? I'm guessing you are, but just want to make sure.

#33 TahoeNoob

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Posted 05 December 2012 - 09:52 AM

I looked at the focuser last night before I went to bed. It's fine. It just looks crooked because the bottom of the focuser is flat and it's sitting in a tube with a 10 inch diameter. (I think that think another possible answer to this problem is to put a shim under one edge of the focuser. I'm not sure how thick that shim would have to be and I don't like that idea though. I'd rather extend the secondary, further back.)

Yes, I'm only trying to center the reflective surface of the secondary in the eyepiece. It does get confusing though. Sometimes it's not easy to see where the mirror stops and the stalk begins. The paper trick helps a lot, but not in the area where the secondary's stalk is.

"Fender washers," I'll look for them when I go into town today. First I'm going to have to take the secondary out and get some good measurements though. :)

I was thinking that I could make a washer out of an old plastic cutting board. (The plexiglass idea bothers me a little bit. Plexiglass tends to crack. I don't want chipped plastic falling down onto my primary.)

#34 TahoeNoob

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Posted 05 December 2012 - 10:11 AM

BTW, I don't have a laser collimator yet. I haven't figured out which one I want to get, or how much I'm willing to spend. What I'm using is both a 2" peephole eyepiece, and a 1.25" Orion Cheshire/sight tube combo with cross hairs. (Both eyepieces seem to be quite high quality.)

I don't think a laser eyepiece would help at this stage of the collimating process, even if I had one.

When using my "manual" collimation tools, I rock my head side to side... and center everything, below, in the center of the two extremes of paralax view. In this way, I feel like I'm centering my view down the eyepiece as best that I can.

#35 tag1260

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Posted 05 December 2012 - 10:19 AM

Just my opinion. I have a laser but just bought the Catseye from Catseye Collimation. It's nice Chelshire eyepiece. While it may take a little time to get accustomed to it, I don't think I'll go back to my laser. After lining up with my laser, the Catseye showed being still out of collimation. It only took a couple of minutes to line everything up. The nice thing about the Catseye is that it NEVER needs aligned like the laser does. I hope to get the money for the rest of their collimating tools in the future.

Like I stated in the beginning, this is only MY OPINION!!!!

#36 TahoeNoob

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Posted 05 December 2012 - 10:50 AM

Your opinion is, of course, welcome... and at this point, I tend to agree with you! :)

I don't think a laser would help me with what I'm fighting with, anyway. For me, right now, I think this is a process that I just have to go though. The good news that I think I understand collimation much better now than I did yesterday... and probably better than I would've if I had been dealing with a fancy laser "toy."

I really do think there's something, physically, wrong with my scope. No laser is going to fix this problem.

BTW, that video/link posted above is awesome! It ought to be required viewing for every new newtonian owner!

As a beginner, I tend think that every problem is "user introduced." Nothing could possibly, ever, be wrong with my scope! Oh no! Not that! I'VE gotta be WRONG! In this case, I think I was wrong. The scope IS fouled up. Can't win! LOL

#37 tag1260

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Posted 05 December 2012 - 11:02 AM

I just bought a large dob myself after only using a 4.5". There is something to be said in the long run on having problems. You won't be afraid to tear into your scope for other things now as you will have already done so.

I would check out the fender washers as I couldn't find any screws long enough THAT HAD THREADS far enough down to use. Most longer screws only have about an inch of threads and these being metric usually limits your choices in the hardware store even more. You'll need one or two almost the same size as your secondary's base. That's providing your main screw is long enough. Just put these against the secondary, under the spring, when you assemble and let the adjusting screws push against them. I also used a Milk Jug Washer as I think your screws will turn easier that when against the metal.

#38 JLovell

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Posted 05 December 2012 - 11:34 AM

One other thing you could try that may help if/when you remove the secondary would be to carefully blacken the edges with a sharpie. That would make it a little easier to discern where the reflective edge ends and the side begins. It may also help to put a piece of paper of another color down the tube a little way so that it is what you see in the secondary instead of all the reflections from the primary.

#39 lamplight

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Posted 05 December 2012 - 02:26 PM

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


Makes senseofsomemissinconcepts! Thanks

#40 TahoeNoob

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Posted 05 December 2012 - 07:42 PM

I'm still not collimated, but I think I solved one problem:

Today I made a plastic bushing to use as a spacer. It allowed me to center my secondary in the eyepiece. I also bought a longer center screw, so that I'd have more threads holding the secondary in place. (Better to be safe than sorry!)
Posted Image

The shim worked as planned, so now the three collimating screws are long enough to give me some adjustment play:
Posted Image

Now, on to today's problems... I think I have the scope perfectly collimated, except for the minor fact that it's blowing out the side of my OTA! I'm not quite sure where I should start, trying to fix this. I have two drawings that show what I'm seeing in my eyepieces.

One of the things that I don't understand is why it is that with the 2" peephole, I can see that the whole primary image is hitting the secondary mirror. If that's the case, why can't I see the whole image with my Cheshire Sight Tube, even if it is aimed out the side of the scope? (Both of these drawing were made without making any changes to my collimation settings.)

If it's not clear that I'm totally confused, it probably should be. :help:

This is what I see though my 1.25" Cheshire Sight Tube, with the built in cross
hairs:
Posted Image

This is what I see though my 2" eyepiece:
Posted Image

Why am I seeing the whole primary image in one eyepiece, but not the other?

#41 Vic Menard

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Posted 05 December 2012 - 08:28 PM

Welcome to the fine art of secondary mirror alignment!

Your drawings illustrate why a combination sight tube/Cheshire collimating tool is valuable when sorting out the secondary mirror alignment. In your first drawing, the sight tube crosshairs (yellow) should be aligned to the primary mirror center spot (focuser axial alignment) before you try to align the bright Cheshire ring to the center spot (primary mirror alignment). But before you correct either of these, you need to determine just how "centered" the secondary mirror is under the focuser.

The signature alignment for an optimally placed secondary mirror is defined by three circles:
The bottom edge of the sight tube, surrounding
The real edge of the secondary mirror, surrounding
The reflected edge of the primary mirror seen in the secondary mirror

Normally, you use the combination sight tube/Cheshire inserted flush in the 2- to 1.25-inch adapter. But if your secondary mirror is slightly over sized, you may not be able to rack the focuser out enough to see the real edge of the secondary mirror. If you find this to be the case, it's OK to pull the combo tool an inch or so out of the adapter to see the edge of the secondary mirror surrounded by the bottom edge of the sight tube. If you can't separate the edge of the secondary mirror (dark) from the bright reflection of the primary mirror, you can lay a piece of white paper behind the secondary mirror against the inner tube wall and the edge of the secondary mirror should be clearly visible.

Getting the secondary correctly centered under the focuser and then properly tilted/rotated to center the primary mirror reflection is a little tedious, as each alignment affects the other alignment. I usually separate the secondary alignments like this:
Secondary mirror rotation and fore and aft adjustments for centering the secondary under the focuser
Secondary tilt for centering the primary mirror reflection in the secondary (when tilted correctly, the sight tube cross hairs will be aligned with the primary mirror center spot).
Repeat these two steps until all three circles are concentric.

(A slightly perverse side note. If the focuser body isn't "squared" to the optical axis traveling longitudinally through the OTA, you won't be able to make all three circles perfectly concentric! I put the word "squared" in quotations because the focuser doesn't actually have to be perfectly "square"--but the drawtube axis should at least be close to intersecting the optical axis and the angle the two axes make should be at least within a degree or so of 90-degrees. I only mention this because it's a pretty common alignment issue when someone upgrades a focuser without paying attention to this geometric requirement. That said, "close" will be good enough for your application.)

Getting the secondary mirror alignment sorted is the hardest part of Newtonian collimation and the reason for 80- to 90-percent of all collimation threads. Once you get it figured out, the last step is easy.

The last step--you adjust the primary mirror to move the reflection of the bright Cheshire ring into alignment with the reflection of the primary mirror center spot--and you're done. If you decide to go back and try to tweak anything that has to do with the secondary mirror, you must also go back and finish by adjusting the primary mirror to align the bright Cheshire ring and the primary mirror center spot.

#42 TahoeNoob

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Posted 05 December 2012 - 08:48 PM

I shall print this out and try again tomorrow. Thank you very much!

#43 TahoeNoob

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Posted 06 December 2012 - 01:24 PM

Chapter three... Hope, or no hope!

The following sketch displays the results of my latest effort. I think I have it all figured out, except for the part about getting the primary image centered on the secondary. (I can put the image there, but it causes everything else to be thrown out of position.)

Couple questions:

1) I tried offsetting my spider down a little bit, in an effort to get the primary image to strike the center of the secondary. Is offsetting the secondary downwards, otherwise known as "cheating," a bad idea?

2) For some reason I found myself trying to overlay the sight tube's crosshairs with the spider's image. Why I started doing this, I don't know... but it's not required, right? I see no reason why the spider needs to be centered. If I gave up on this, I might be able to tip the secondary down a little bit more... which would (maybe) allow the primary's image to hit further up towards the center of the secondary.

And now the picture:
Posted Image

When the fog lifts, I'll sight this thing across the lake to see if I've got a sharp image! :jump:

#44 uniondrone

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Posted 06 December 2012 - 02:27 PM

Definitely listen to what Vic says, he's the master at collimation!

As for your questions:

1) I assume that you mean that you adjusted one or more of the four screws on the outside of the telescope tube for shortening/lengthening each of the spider arms. Typically you want to have the mirror holder centered in the telescope tube, unless using some unorthodox method of collimation (Vic knows all about these).

2) Unless you are making the adjustment in #1, the reflection of the spider vanes should only move as the reflection of the primary moves. Ideally you want the cheshire crosshairs to align with the reflection of center spot. The crosshairs (if the cheshire is rotated to that particular position) may come very close to overlapping the spider, but probably won't overlap completely, for reasons that I'll leave for someone like Vic to explain.

As an excercise for learning more, I recommend that you try this: if there is any particular adjustment where you are in doubt about the effect on the view through the cheshire, adjust one screw out of alignment, observe the effect, then undo what you just did to return it to alignment. Repeat this with each adjustment in question. Very quickly you will get a very firm grasp of what does what.

#45 Vic Menard

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Posted 06 December 2012 - 03:38 PM

Let's see if I can straighten this out...

Your drawing shows a couple of problems, and I think I know why. First, while your secondary mirror cross hairs indicate alignment with the primary mirror reflection, we know the focuser axis is correctly aligned. Since the primary mirror reflection is not centered in the secondary mirror, we also know that the secondary mirror is still not correctly centered under the focuser. The second signature I see is the bright Cheshire ring surrounding the primary mirror center spot (indicating correct primary mirror axial alignment) and the dark silhouette of the secondary mirror is also concentric with the bright Cheshire ring, which it shouldn't be if the secondary mirror is correctly centered under the focuser. When you center the secondary mirror under the focuser, you automatically have offset the secondary mirror closer to the primary mirror. And if the secondary mirror is centered (= offset), then the silhouette of the secondary mirror surrounding the bright Cheshire ring should appear offset as well (wider on the side pointing to the primary mirror). If you try to keep the silhouette reflection concentric, you can't keep the secondary mirror centered under the focuser!

OK, question one. If you try to offset the secondary mirror "down" away from the focuser, you'll likely end up with spider vanes that are no longer perpendicular, which means additional diffraction spikes (not good). If, OTOH, you were trying to offset the secondary mirror "down" closer to the primary mirror, that's probably a good start. I say "probably" because I'm uncertain how much secondary mirror tilt you made to get the secondary mirror silhouette "centered" instead of leaving it offset and just finishing the primary mirror alignment (it's hard to tell in your illustration without the first circle showing--the bottom edge of the sight tube).

Second question--the intersection of the reflection of the spider vanes should be concentric with the silhouette of the secondary mirror reflection (assuming the secondary mirror mounting is centered relative to the spider). Since the secondary mirror silhouette is normally offset relative to the bright Cheshire ring, the spider reflection will also be offset to the bright Cheshire ring, and therefor the spider will not be aligned to the sight tube cross hairs.

Chuck was doing pretty well until question two. With the secondary mirror correctly centered and the focuser axis properly aligned, the three circles (the bottom edge of the sight tube, the real edge of the secondary mirror, and the reflection of the primary mirror), the actual close up sight tube cross hairs, and the primary mirror center spot will all be centered/concentric. But aligning the primary mirror moves the silhouette reflection of the spider, the silhouette reflection of the secondary mirror, and the reflection of the underside of the focuser (where you should also be able to see the distant reflection of the sight tube cross hairs). When the primary mirror is aligned, everything should appear centered/concentric except the silhouette reflection of the spider and secondary mirror, which appear offset toward the primary mirror.

FWIW, your scope is currently axially aligned (focuser and primary mirror axes are coincident) and will give excellent image performance. The incorrect secondary mirror placement will likely have no affect on visual performance as the unbalanced illumination at the edge of the field of view will almost certainly be beyond detection to your eye.

But, if the lack of offset bothers you, you should now (hopefully) have the information you need to fix the secondary mirror placement.

#46 TahoeNoob

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Posted 06 December 2012 - 05:19 PM

Ok. My head is spinning and I don't understand what you've said but, once again, I will print this out... then read it and reread it, over and over again, until I understand. Then, and only then, will I attempt to implement what you've said.

One thing that bothers me is this: You keep talking about the "inside edge of the sight tube." In theory, I understand what you're talking about. I see it in the illustrations, and it makes sense that it should be there. The thing is, I'm not seeing a hard sight tube edge. When I use the 1.25" sight tube, I see a sort of fuzzy white area that moves slightly when I shift my eye back and forth in the peephole. The fuzzy area goes right up to the edge of the primary image... AND the image is centered inside of that fuzzy area/sight tube. (It's only when I fall back to the 2" peephole that I can see the hard edge of the sight tube and that the primary image isn't centered in the eyepiece.)

I've tried racking the 1.25" sight tube all the way back, and pulling the sight tube back from its seat. No matter what I do, I still get the same fuzzy area. No hard inside edge of the sight tube. :(

Does that make sense? I'm not sure how to describe what I'm seeing any better.

You are correct. My test sightings look clear and sharp. I'd still like to get this right though. It has become a challenge! :)

#47 frito

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Posted 06 December 2012 - 05:23 PM

I only have experience collimating with a basic collimation cap but from what i can see its more or less the same.

my opinion based on my experience with a collmation cap and i have done my secondary from scratch completely (i even removed my spider once before to flock my OTA)is that your secondary is shifted too far to the right tword the primary and you need to loosen the 3 tilt screws and tighten the center screw a bit so that when you have the mirror correctly tilted you will be able to see the whole primary in it.

as others have said and i mentioned earlier on in this thread DO NOT concern yourself with the shadow of the secondary mirror, the black ring around the secondary's reflection. it will not be concentric with the rest of the optics. this is completely normal in Orion Dob's

#48 frito

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Posted 06 December 2012 - 05:36 PM

see attached. i pointed out the secondary's shadow in your photo and put a green arrow in the direction i think you need to move the secondary mirror.

also do not concern yourself with aligning the primary mirror at all until you get the secondary centered in the focuser and tilted so you can see all of the primary in the reflection. if you start playing with both at the same time you will never get it setup right.

Attached Files



#49 frito

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Posted 06 December 2012 - 05:43 PM

oh wait man brain fart. i see the outer circle is the edge off the secondary mirror. in that case it looks like you need to adjust the tilt of the secondary mirror so that the primary is centered in it. then and only then adjust the primary mirror so the center mark is centered.

#50 tag1260

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Posted 06 December 2012 - 05:54 PM

To me, it looks as though his bottom edge of the secondary needs to go backwards a little. Like frito said, ignore your primary until the secondary is aligned. I've read of putting a sheet of paper under the secondary (to block out the primary) to separate the processes. I went through this same sort of problem. I think sometimes we have a mental issue of seeing everything and sometimes not blocking out what we don't need to see and the paper helps eliminate this.






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