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rgm40
super member

Reged: 04/15/08

Loc: Western Kentucky

Cloudynight collimation blues. S.O.S. new

new

#5055256 - 02/04/12 02:36 PM

I have a reasonable understanding of basic collimation (I think

). However, I am having a problem diagnosing an error. First, I currently only have an Orion combination tube with crosshairs. Second, I have a nice Moonlite crayford newtonian focuser on a home built 7.55 dob. Now, using the tube I have a nice concentric ring from the bottom of sight tube with regard to the outline of the secondary. But, here's what happens. No matter how I adjust the secondary screws, if I tilt the secondary to center the reflected primary center ring spot directly under the sight tube crosshairs, the reflection of the primary becomes non-concentric and looks left shifted toward the front of the tube. If I make the primary mirror reflection concentric with regards to the outline of the secondary, the center spot looks off (shifted towards the primary at the bottom of the tube)I made a crude drawing in Paint to help illustrate what I am seeing. Deleted the drawing. Not very good. See the two drawings in subsequent posts. Thanks.

Mirror not centered in the tube?
Bad collimation tool?
Mirror spot not at the optical center?

How would you deal with this one? Any help sincerely appreciated.

*

Edited by rgm40 (02/04/12 04:05 PM)

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bherv
scholastic sledgehammer

Reged: 03/10/06

Loc: WMass

Re: Cloudynight collimation blues. S.O.S. new

new

[Re: rgm40]
#5055294 - 02/04/12 03:01 PM

Most secondary mirrors are offset toward primary, that is why you don't see concentric circles when scope is properly collimated. I am sure that others will respond who have a more intimate knowledge and can give further help.
Barry

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rgm40
super member

Reged: 04/15/08

Loc: Western Kentucky

Re: Cloudynight collimation blues. S.O.S. new

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[Re: bherv]
#5055372 - 02/04/12 03:54 PM Attachment (36 downloads)

Thanks Barry. I drew two more crude drawings to help illustrate. I don't think I did a good job on the first one. Tried to simplify it a bit

. I guess what I am really asking is this: if all is aligned properly, if the reflection of the primary is centered within the secondary mirror, then shouldn't the reflection of the center spot of the primary be aligned with the intersection of the crosshairs in the sight tube? If not, can anyone point me in the right direction? Maybe I am missing the boat here.

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rgm40
super member

Reged: 04/15/08

Loc: Western Kentucky

Re: Cloudynight collimation blues. S.O.S. new

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[Re: rgm40]
#5055377 - 02/04/12 03:57 PM Attachment (23 downloads)

Another view when primary reflection is centered within secondary.

Edited by rgm40 (02/04/12 03:57 PM)

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E_Look
Post Laureate

Reged: 03/06/08

Loc: near New York

Re: Cloudynight collimation blues. S.O.S. new

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[Re: rgm40]
#5055418 - 02/04/12 04:24 PM

I'm not sure, but look up Jason D's threads and look at his avatar. It has the correct image in it. It looks like your first picture.

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Jason D
Postmaster

Reged: 10/21/06

Loc: California

Re: Cloudynight collimation blues. S.O.S. new

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[Re: E_Look]
#5055464 - 02/04/12 05:09 PM Attachment (11 downloads)

Quote:

I'm not sure, but look up Jason D's threads and look at his avatar. It has the correct image in it. It looks like your first picture.

Ed, I believe the OP is not referring to the secondary mirror silhouette which will indeed look shifted towards the primary mirror. I believe he is referring to the primary mirror reflection edge versus the actual secondary edge.

I will respond to the OP in the next post.

Jason

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Jason D
Postmaster

Reged: 10/21/06

Loc: California

Re: Cloudynight collimation blues. S.O.S. new

new

[Re: bherv]
#5055473 - 02/04/12 05:20 PM

rgm40,

Sometimes, adjusting the secondary mirror by rotation and adjusting the 3 set screws is not enough to meet the following two conditions simultaneously:

1- Getting the sight-tube cross hair to coincide with the center spot

2- Getting both the secondary mirror edge and the primary mirror reflection edge to coincide (or at least be concentric)

In this case, you will need to perform one or both of the following additional adjustments:

1- Adjust the secondary mirror center bolt to move the secondary mirror up and down the tube

2- Adjust the focuser or the spider vanes (or both)

Check the attachment animation I prepared few years ago. In each frame, the sight tube cross-hairs will coincide with the center spot but as you can see the primary mirror reflection moves around. I was changed the secondary mirror rotation and up/down/right/left movements between frames.

Examine the first few pages of the following link. There should be enough info to help.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3033065/page/0/view/collapsed/sb/7/o/all/fpart/1

Jason

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rgm40
super member

Reged: 04/15/08

Loc: Western Kentucky

Re: Cloudynight collimation blues. S.O.S. new

new

[Re: Jason D]
#5055525 - 02/04/12 05:57 PM

"I believe he is referring to the primary mirror reflection edge versus the actual secondary edge." That is correct. When the secondary is tilted so that the reflection of the primary edge is concentric with the actual secondary edge, the reflected center spot of the primary is not coinciding with the site tube crosshairs. At the site tube it appears to be off by the width of the paper reinforcement ring. What has me confused Jason is that the actual secondary alignment appears very concentric in relation to the sight tube. And the rotational adjustment of the secondary looks spot on. So, if I adjust the secondary center bolt forwards or backwards the secondary will be non-concentric in the sight tube. I was careful when I center spotted the mirror, but I am to the point where I may remove the mirror and cell to verify the center spot. I really hate to do that. If the center spot is off, it can't be by much. A millimeter perhaps. Thanks for the info by the way. Any additional comments appreciated. I am going to study your animations some more. Very nice work.

Edited by rgm40 (02/04/12 06:00 PM)

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rgm40
super member

Reged: 04/15/08

Loc: Western Kentucky

Re: Cloudynight collimation blues. S.O.S. new

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[Re: rgm40]
#5055572 - 02/04/12 06:21 PM

After looking at your animations some more, I am wondering if my secondary is as concentric as I think it is. I blackened the edge of the secondary so the part toward the tube is really black. I may try lining up the crosshairs with the primary center spot then making a slight adjustment to the center bolt and bring the secondary slightly toward the front of the tube. Don't think it would take much. Think I will give this another look.

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rgm40
super member

Reged: 04/15/08

Loc: Western Kentucky

Re: Cloudynight collimation blues. S.O.S. new

new

[Re: rgm40]
#5055667 - 02/04/12 07:33 PM

I'll tell you what guys. When it clicks, it clicks. Jason, your animation helped me hit a home run. I just spent about the last thirty minutes in the garage tweaking the center bolt and spider screws. For the first time EVER, I think I have a very close to perfectly collimated scope. Now, granted this is all depending on my center spot (which I feel good about), and my collimation tool, but I have finally achieved that Jason/Vic textbook view through the sight tube. I can't thank you guys enough. Now if it will just hold, and if we can just get some good weather

Edited by rgm40 (02/04/12 07:36 PM)

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cliff mygatt
Pooh-Bah

Reged: 01/27/09

Loc: Kitsap County, WA

Re: Cloudynight collimation blues. S.O.S. new

new

[Re: rgm40]
#5055671 - 02/04/12 07:34 PM Attachment (35 downloads)

Is your focuser squared to the tube? I have attached a great procedure from Howie Glatter that I used and my collimaton is spot on. Good Luck!

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Jason D
Postmaster

Reged: 10/21/06

Loc: California

Re: Cloudynight collimation blues. S.O.S. new

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[Re: rgm40]
#5055770 - 02/04/12 08:39 PM

Glad to hear things worked out for you

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rgm40
super member

Reged: 04/15/08

Loc: Western Kentucky

Re: Cloudynight collimation blues. S.O.S. new

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[Re: Jason D]
#5055865 - 02/04/12 09:42 PM

Only bad thing about this is: What am I going to read about now on cloudynights

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star drop
Snowed In

Reged: 02/02/08

Re: Cloudynight collimation blues. S.O.S. [Re: rgm40]
#5056122 - 02/05/12 12:43 AM

Quote:

Only bad thing about this is: What am I going to read about now on cloudynights

Read about new eyepiece$?

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FirstSight
Postmaster

Reged: 12/26/05

Loc: Raleigh, NC

Re: Cloudynight collimation blues. S.O.S. new

new

[Re: star drop]
#5056144 - 02/05/12 01:05 AM

Quote:

Quote:

Only bad thing about this is: What am I going to read about now on cloudynights

Read about new eyepiece$?

Now that your scope is well enough collimated to make you soon become dissatisfied with the eyepieces you currently have, we can safely forecast that we'll soon be advising you on some new eyepieces to solve this problem. Helping other people spend their money is something we are really good at here at CN.

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Fireball
professor emeritus

Reged: 03/24/06

Re: Cloudynight collimation blues. S.O.S. new

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[Re: FirstSight]
#5056220 - 02/05/12 04:58 AM

This behaviour typically is caused by not having the center bolt adjusted in a way that the secondary is exactly centered under the focuser. This is the very first step and should be done very carefully to avoid any headaches as described above ...

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Jason D
Postmaster

Reged: 10/21/06

Loc: California

Re: Cloudynight collimation blues. S.O.S. new

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[Re: Fireball]
#5056653 - 02/05/12 10:58 AM

Quote:

This behaviour typically is caused by not having the center bolt adjusted in a way that the secondary is exactly centered under the focuser. This is the very first step and should be done very carefully to avoid any headaches as described above ...

This is one possibility. Other possibilities are misaligned focuser and/or misaligned spider vanes and/or secondary mirror rotate/tilt error.

Jason

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rgm40
super member

Reged: 04/15/08

Loc: Western Kentucky

Re: Cloudynight collimation blues. S.O.S. new

new

[Re: Jason D]
#5056969 - 02/05/12 02:08 PM

I have one of the 3-vane name brand spiders/holders. I find it a little more difficult to make fine adjustments with a 3-vane vs. the 4 vane. I chose the 3-vane for improved contrast and lower overall diffraction. Don't want to start another discussion here on that topic

. Making a slight adjustment on the center bolt required me to make a slight side to side adjustment on the spider vanes. Once I carefully made those adjustments, whilst concentrating on keeping the center spot within the crosshairs, one final slight tilt adjustment brought it home. Lastly, I had to make only a very slight primary adjustment. Don't think I was very far off to begin with. I have one of the Hotech SCAs ordered and will use the barlowed laser trick to verify the primary. I know I will have to verify the collimation of the laser tool. Thanks again. BTW, I have plenty of reading to do yet

Edited by rgm40 (02/05/12 02:12 PM)

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bherv
scholastic sledgehammer

Reged: 03/10/06

Loc: WMass

Re: Cloudynight collimation blues. S.O.S. new

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[Re: rgm40]
#5057056 - 02/05/12 03:11 PM

I just finished installing my 3 vane spider in my 8" f/6 dob. It took me awhile to get everything centered. Still not perfect but it is collimated. Have to get it out under the stars.
Barry

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rgm40
super member

Reged: 04/15/08

Loc: Western Kentucky

Re: Cloudynight collimation blues. S.O.S. new

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[Re: bherv]
#5057329 - 02/05/12 06:35 PM

Been watching the cloud line closely tonight to see if I will be able to try it out after properly collimating. As usual, it looks like I will miss it by just a couple of hours

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