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Collimation question -- apologies in advance!

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#1 castorpolu11

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Posted 16 October 2020 - 11:54 PM

I've reviewed Vic's pinned instructions and I'm a bit lost. Here's what I did today, my first time trying to collimate my new to me 8" firstlight dob:

 

1. Placed the new sbvony laser collimator into the 1.25" eyepiece holder

2. Found that the laser hit my hand when I placed it in front of the tube

3. Adjusted the secondary mirror until the laser dot was in the middle of the donut on the primary mirror

4. Went around to the back end of the scope to begin working on the primary mirror alignment and expected to see a red dot off target on the collimation tool in the eyepiece. Instead I saw this

 

IMG_1815.jpeg

 

I tried adjusting the laser but it just shows all red on the target. What am I doing wrong? Thanks!



#2 Jim Waters

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Posted 17 October 2020 - 12:26 AM

FYI

http://www.astro-bab...nian-reflector/

https://www.youtube....h?v=8G98RTP6jbY

 

Put laser in focuser and turn on.

Adjust laser dot to hit center of primary mirror but adjusting secondary.

Adjust primary mirror so red dot hits bullseye of laser.

 

There's no red dot?



#3 castorpolu11

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Posted 17 October 2020 - 12:34 AM

FYI

http://www.astro-bab...nian-reflector/

https://www.youtube....h?v=8G98RTP6jbY

 

Put laser in focuser and turn on.

Adjust laser dot to hit center of primary mirror but adjusting secondary.

Adjust primary mirror so red dot hits bullseye of laser.

 

There's no red dot?

I mean, you can see the picture. It doesn't appear to be a red dot! It's the first time I'm doing it so I wasn't sure what to expect but it looks nothing like what I was expecting. The whole target is just diffuse red. Wasn't sure it it's something simple I'm doing wrong. 



#4 Jim Waters

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Posted 17 October 2020 - 12:38 AM

You may be partially hitting the hole in the laser and getting scattered light.  What happens when you adjust the primary a lot?

 

Watch the YouTube video.


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#5 bradbaker7

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Posted 17 October 2020 - 12:42 AM

Try turning down the brightness of your laser. I have the same model and I find anything over 3 or 4 is too bright for me to judge the accuracy very well.
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#6 TOMDEY

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Posted 17 October 2020 - 04:00 AM

What's happening is that the returning dot is hitting somewhere on the folding flat, but too far from the middle to make it through the ~1-inch hole in the bottom of the laser gizmo.

 

Just aim the scope at a wall. Tip-tilt the PM to find the edges of the folding flat and chase the return to the centerish, whence the return will appear on the target.

 

Nother technique is to take a piece of paper with a quarter-inch hole in it and hold it over the bottom of the focuser inside the tube so the exiting beam goes through it. You will see the badly off-center return dot on that paper. Tip-tilt the Primary Mirror to drive it toward center. Now take the paper off and you will see that dot on the target and can then finesse it in to perfection. That's all there is to it!

 

That's all there is to it!    Tom

 

[I did optical alignments at work on giant aerospace telescopes. That is complex, with Prime, Nasmyth, Cass, Coudé foci all needing alignment for all rotations of the axes. Even on those things... the most common alignment tool is --- a shard of paper! ... special Clean Room Paper...]    Tom

 

~click on~ >>>

Attached Thumbnails

  • 67.1 Tom's Jumbo Lambertian Calibrtion Screen.jpg

Edited by TOMDEY, 17 October 2020 - 08:55 PM.

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#7 Men2Boyz

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Posted 17 October 2020 - 12:02 PM

I recognize your problem through my own misadventures in collimation. After reconfirming that your narrow laser beam is still centered on the primary mirror, turn off the laser and remove the device from the focusing tube. Look into the empty focusing tube. Can you see the tiny center spot of your primary mirror? It’s probably way off from where it needs to be. If you can’t see the center spot of your primary mirror, you may have to move your scope around to catch right amount of light so that it is visible. What you need to do is make adjustments with your primary mirror collimation knobs to place the primary mirror center spot in the middle of the smallest circle as seen in the reflection of your secondary mirror. That small circle is the reflection of your focusing tube. You will see your eyeball reflection there. That is where your return laser beam is going to be when we’re done with these rough adjustments.

 

Loosen the primary mirror locking screws. Turn the collimation knobs of the primary mirror to get the center spot to move towards your eyeball reflection. This is easier if someone turns the knobs while you are looking into the focusing tube but you can do it without assistance by adjusting a knob and swinging the scope around and checking inside the focusing tube after each adjustment.  When you are as close as you can get placing the center mark into the smallest circle, place laser device back into focusing tube and turn it on. Now you should be able to see a bright narrow laser beam shining somewhere on the side port of the laser device fairly close to the target center. Continue to adjust collimating knobs on primary mirror using the laser device until the narrow beam disappears into the center target.

 

When you get the thin laser beam to disappear into target, lightly tighten all knobs. As you tighten the locking knobs, your laser beam may shift a bit. Keep adjusting again to put beam back into target until all knobs are not loose and each knob is at least tightened enough to keep primary mirror from shifting while scope is being used. Now rotate your laser device while it is in the focusing tube. If the beam is not staying in the target, your laser device itself may need calibrating or there may be too much play in the focus tube to keep the laser device from shifting. Still, my limited experience has shown that even with this bit of inaccuracy, you can have decent viewing at this point. To get even better collimation, try this modified Barlows technique.  Place a 2X Barlows in the focusing tube and then place your laser device through it. This combination will project a diffuse red ring with dark center onto the side target port of your laser device. Make more primary mirror adjustments to center that ring around the central target and afterwards rotate the laser device in the Barlows. Now the red laser ring should not shift. You can confirm your efforts with a collimation cap and other tools but essentially you’re done unless you have more advanced equipment to assist you. I am always able to see Cassini divisions of Saturn rings and south polar caps and albedo markings of Mars at this point.



#8 Tuktu

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Posted 17 October 2020 - 02:03 PM

First you should try a white 1 1/4" film container with a small 1/4" hole in center it is all I use. Or find a similar 1 1/4" plastic cap in your house use white tape or paint it on bottom if it is not white. Or you can buy a collimating cap if film container not available like this one:
https://agenaastro.c...t-eyepiece.html

Now you just line up dot on mirror with your pupil and your done 1-5 minutes. Do it inside house looking at a white wall or I prefer ceiling. Have someone else adjust knobs for easier adjustment. The no larger than 1/4" Hole can be made with a knife doesn't need to be perfectly round just so you can see through it and line your pupil with dot on primary. Your done go enjoy your scope do a startest to confirm. It is very easy and I get perfect results everytime confirmed by a startest, I get pinpoint stars, and can see cassini division through my 8" f6 dobsonian. I check my collimation with film container before each use usually it is fine.

I assume you did your secondary mirror first. First before you adjust primary you should make sure secondary mirror is lined up first. Look through focusser with and without film cap you should see the entire primary mirror centered in your view. Not any of the telescope tube. Secondary mirror should be centered in the primary. If not adjust secondary. Then collimate your primary with the film container.

Now that your scope is collimated,
I guess you could use the laser to fine tune after. Or now check laser to confirm it is working correctly. As noted above if laser is not collimated or not straight in eyepiece you'll be off. So I just use my film container. You can also sell your laser collimator and buy more eyepieces or accessories.

Good article in collmination:
https://garyseronik....-what-you-dont/
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#9 SteveG

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Posted 17 October 2020 - 03:07 PM

First you should try a white 1 1/4" film container with a small 1/4" hole in center it is all I use. Or find a similar 1 1/4" plastic cap in your house use white tape or paint it on bottom if it is not white. Or you can buy a collimating cap if film container not available like this one:
https://agenaastro.c...t-eyepiece.html



Good article in collmination:
https://garyseronik....-what-you-dont/

The collimating cap is so much better than the laser return beam. I highly recommend it, especially if you have the newer GSO or Synta black center ring on your primary.

 

You just place it in the focuser, and adjust the primary so the black dot (pupil) is centered in the black center-spot ring on your primary.

 

crop.jpg



#10 philinbris

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Posted 17 October 2020 - 04:02 PM

Is it worth trying the Barlow technique?

I have been trying that over the last few days. I use a 2x on my cheap $30 eBay laser and I can make out the donught in the 45 Deg thingo in the laser.

I quite like that method, trying to work out if its worth throwing $140 at the Howie Glatter TuBlug now.

Cheers



#11 sixela

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Posted 17 October 2020 - 07:27 PM

The TuBlug is more precise -- the fact that the screen is at the correct side of the barlow makes the reading simple and more accurate (and makes it easy to guess when the virtual point source is close to the focal plane) and eliminates some sources of small additional errors.

Whether it's worth it, that's left to everyone's own appreciation -- it is indeed not that cheap, and it's not more precise than a well calibrated Cheshire (i.e. one mated to a centre spot well matched to it).
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#12 castorpolu11

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Posted 18 October 2020 - 12:18 AM

Thank you for all your help and advice. I was able to loosen the laser collimator a bit and found that a red dot appeared if I pressed the device against one side of the eyepiece hole. Based on seeing that, I was able to figure out how to turn the screws sufficiently to eventually retighten the collimator and get a dot on the outer portion of the target. From there it was simple to follow through to basic collimation. Thanks again!


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#13 philinbris

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Posted 18 October 2020 - 05:15 AM

The TuBlug is more precise -- the fact that the screen is at the correct side of the barlow makes the reading simple and more accurate (and makes it easy to guess when the virtual point source is close to the focal plane) and eliminates some sources of small additional errors.

Whether it's worth it, that's left to everyone's own appreciation -- it is indeed not that cheap, and it's not more precise than a well calibrated Cheshire (i.e. one mated to a centre spot well matched to it).

That means I will probably get one then.

The problem I have with a Cheshire is my brain tricks me a bit with all the images of sec and pri mirror etc.

I do like the Barlow or TuBlug idea.

 

@ the OP - glad it worked out for you.

Cheers


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#14 philinbris

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Posted 23 October 2020 - 05:12 PM

And I did.

One Howie Glatter 1.25" coming my way. Really keen to see how good they really are.


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#15 e7FvPDZR

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Posted 23 October 2020 - 05:43 PM

The collimating cap is so much better than the laser return beam. I highly recommend it, especially if you have the newer GSO or Synta black center ring on your primary.

 

You just place it in the focuser, and adjust the primary so the black dot (pupil) is centered in the black center-spot ring on your primary.

 

attachicon.gifcrop.jpg

I've got to agree with Steve. By using a collimating cap (for aligning your primary mirror) you remove problems with the alignment of the focuser from the equation.

 

I use a laser to align the secondary, but to align the primary I wish to ignore any minor misalignment (which I can't really adjust) of the focuser tube. I am after all aligning the primary mirror, not the secondary mirror, or the focuser. All I want is a "pinhole" to look through at the center of the focuser.


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#16 philinbris

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Posted 23 October 2020 - 11:12 PM

I've got to agree with Steve. By using a collimating cap (for aligning your primary mirror) you remove problems with the alignment of the focuser from the equation.

 

I use a laser to align the secondary, but to align the primary I wish to ignore any minor misalignment (which I can't really adjust) of the focuser tube. I am after all aligning the primary mirror, not the secondary mirror, or the focuser. All I want is a "pinhole" to look through at the center of the focuser.

Isn't that a bit tough with a Newt like the Dob?



#17 SteveG

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Posted 24 October 2020 - 02:46 PM

Isn't that a bit tough with a Newt like the Dob?

What do you mean?


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#18 e7FvPDZR

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Posted 26 October 2020 - 12:01 PM

Isn't that a bit tough with a Newt like the Dob?

Actually, it is easy, using a collimation cap. After the secondary mirror is aligned, I find that the primary mirror is the one that must be checked after moving the telescope. I use a Farpoint collimation cap, or "Cheshire", to align the mirror using the center mark on the primary mirror and the reflective "donut" on the bottom of the collimation cap.

 

Check out the video at about 4 minutes 19 seconds:

Farpoint Quick and Easy Field Collimation Method


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#19 Vic Menard

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Posted 26 October 2020 - 01:45 PM

What do you mean?

I'm thinking he means having to make fine adjustments at the rear of the scope and then having to move to the focuser where you can then check the fine adjustment(s) you just made. I do it all the time with an 88-inch focal length Dob, but I'm used to traveling back and forth to zero in the alignment, it only takes a few minutes. If there's somebody nearby that is willing to help, then the adjustments at the primary mirror can become "voice activated".


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#20 Jon Isaacs

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Posted 26 October 2020 - 01:53 PM

And I did.

One Howie Glatter 1.25" coming my way. Really keen to see how good they really are.

 

This is the image of the center marker of my 2 inch TuBlug.

 

Screenshot_20200202-190129.jpg
 
It's worth it.  Saves a lot of trips, I can generally get an image centered like this on the 22 inch F/4.4 with 2-3 trips to double check.
 
Jon


#21 philinbris

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Posted 27 October 2020 - 03:15 AM

That's very cool.

Mine turned up today, only to discover that the tiny Barlow lens was missing.

Spoke to the supplier - end result is its going back for replacement.

Cheers



#22 philinbris

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Posted 27 October 2020 - 03:16 AM

I'm thinking he means having to make fine adjustments at the rear of the scope and then having to move to the focuser where you can then check the fine adjustment(s) you just made. I do it all the time with an 88-inch focal length Dob, but I'm used to traveling back and forth to zero in the alignment, it only takes a few minutes. If there's somebody nearby that is willing to help, then the adjustments at the primary mirror can become "voice activated".

That's exactly what I meant - sorry I should have been a bit more descriptive.I do prefer to stay at one end myself.

Cheers



#23 apollo100

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Posted 27 October 2020 - 05:00 AM

WOW, a most timely topic for me.  I just purchased a 10"GSO newtonian and have been having the same type of issues.  The discussion here has helped.  I do have additional questions however.  I noticed that the primary mirror will move (shift to side) if I move my telescope.  I am guessing this has to do with the tensioning knobs? Also those who have been working with this type of telescope, what up grades would you recommend, like stiffer adjustment springs behind the primary?  Thanks in advance



#24 e7FvPDZR

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Posted 27 October 2020 - 12:21 PM

I'm thinking he means having to make fine adjustments at the rear of the scope and then having to move to the focuser where you can then check the fine adjustment(s) you just made. I do it all the time with an 88-inch focal length Dob, but I'm used to traveling back and forth to zero in the alignment, it only takes a few minutes. If there's somebody nearby that is willing to help, then the adjustments at the primary mirror can become "voice activated".

It didn't occur to me that is what he meant.

 

I have to agree that is it easier if you don't have to go back and forth from the focuser to the primary mirror adjustment screws. After a few collimation sessions, I memorized which way the mirror moved with each screw with respect to the focuser view. But, yes, back and forth...



#25 SteveG

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Posted 27 October 2020 - 03:43 PM

WOW, a most timely topic for me.  I just purchased a 10"GSO newtonian and have been having the same type of issues.  The discussion here has helped.  I do have additional questions however.  I noticed that the primary mirror will move (shift to side) if I move my telescope.  I am guessing this has to do with the tensioning knobs? Also those who have been working with this type of telescope, what up grades would you recommend, like stiffer adjustment springs behind the primary?  Thanks in advance

Yes. In fact, at 8” or larger, it is the first upgrade I do to an Chinese dob. Get heavier springs, and tighten them down fairly tight. With these, you don’t even need locking screws.


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