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Issue with secondary alignment

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#1 espnerhus

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Posted 18 September 2021 - 08:53 AM

Hi!

It has been a lot of posts from me recently, because of some trouble with my new reflector, a GSO 12" dob.

It is so nice to be able to post on this forum and ask for help, and I really appreciate the answers I get!

"Luckily" there are a lot of cloudy days (and nights) at the moment, so I have many days to sort out the issues.

 

My question is about aligning the secondary.

When collimating I first align the secondary under the focuser. I have tried this both visually and with laser collimator.  When measuring by eye I have noticed a tendency where the secondary is always skewed towards one side of the focuser, as if its axis (the axis that is paralell to the tube) is tilted. I use various concentric lines in the focuser as guiding lines.

 

To correct this I have to constantly fight against the secondary holder, turning one of the 3 collimation bolts as far in as it will go, while only gently adjusting the other 2.

It does not feel right to assymetrically overtighten bolts in an optical system.

 

I have checked the spider itself, and the secondary holder is in the center of the tube.

 

However, when using the laser, the laser beam will only hit the center mark on the primary, if I allow the secondary to be skewed! After having used the laser, I remove it from the focuser, and check the secondary visually. It is always skewed! Multiple degrees it seems...

 

Is this due to cheap chinese mechanics? Or is it fixable?

It seems so weird to me that the only way collimation works is if the secondary is obviously not centered onder the focuser drawtube.

 

Below is an image where I have drawn circles showing focuser and secondary outlines. The yellow line is the axis that the secondary seems to follow.

 

Best Regards,

Espen

 

242096505_828945331128741_5636875925266610001_n.jpg


Edited by espnerhus, 18 September 2021 - 08:55 AM.


#2 spaceoddity

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Posted 18 September 2021 - 10:22 AM

You don't use a laser to center the secondary under the focuser. The best tool for this is a sight tube. If you don't have access to a sight tube, then you just have to eyeball it the best you can.  Assuming your camera is perfectly centered, it looks like yours is a little bit off but not too bad. Without a sight tube to get a more accurate assessment, I probably wouldn't touch it. If you do want to adjust it, this is done with the center bolt on the secondary holder. Depending on which direction you go, you may need to loosen the tilt adjustment allen screws. Be careful because they work in tandem with the center bolt in a push/pull type fashion and you will need to have a good hold on the secondary as it will come loose and rotate. The center bolt is also used to rotate the secondary. You will want to adjust it so that it is centered along the horizontal axis in the focuser and rotated so that it is round. They usually come from factory close enough for visual use and like I said, I probably wouldn't touch it without a sight tube unless it was obviously way off.

 

The 3 allen screws adjust the tilt of the mirror. This is the 2nd step of collimation. If you have a laser you just adjust them one at a time until the dot is centered in the primary mirror donut. There are many collimation experts that will chime in when they see this post and they will give you better detail and help get you squared away. Good luck.


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#3 spaceoddity

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Posted 18 September 2021 - 10:33 AM

If it's the black shadow throwing you off, that is just the reflection of the spider/secondary holder from the primary mirror. Ignore that. For centering the secondary the only 2 things you want to look at are the outer circle of the secondary and the edge of the focuser draw tube, or the edge of the sight tube if you have one. Some people block off the reflection with a piece of cardboard to make it easier to see. 


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#4 Vic Menard

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Posted 18 September 2021 - 10:34 AM

My question is about aligning the secondary.

When collimating I first align the secondary under the focuser. I have tried this both visually and with laser collimator. 

As you have found, "visually and with a laser collimator" doesn't work. You need to add another tool, either a simple collimation cap or a combination Cheshire/sight tube collimating tool. Either tool will accurately center your eye above the focuser, which is critical for assessing the secondary mirror placement. If you don't have either, you might consider the Rigel Aline  https://agenaastro.c...t-eyepiece.html

 

...when using the laser, the laser beam will only hit the center mark on the primary, if I allow the secondary to be skewed! After having used the laser, I remove it from the focuser, and check the secondary visually. It is always skewed! Multiple degrees it seems...

I suspect you're trying to use the secondary mirror tilt adjustment screws to make the laser hit the center mark on the primary mirror. Another way to make the laser hit the center mark is to rotate the laser. For the record, combined tilt/rotation errors are often referred to as "skew" errors.

 

...Is this due to cheap chinese mechanics? Or is it fixable?

It seems so weird to me that the only way collimation works is if the secondary is obviously not centered onder the focuser drawtube.

It's fixable. To get an idea what the different secondary mirror placement errors look like, you should read the first 5 posts here (the images in post #3, 4 and 5 are animated--you might have to click on them).

 

https://www.cloudyni...rror-alignment/

 

With a laser and a collimation cap, you can sort out your secondary mirror placement using this procedure:

https://www.cloudyni...ment/?p=5260727  (you might also find this post useful  

https://www.cloudyni...ment/?p=5255758


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#5 Elroy

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Posted 18 September 2021 - 10:41 AM

Don't know where I found this, but it always helps me. Hope it helps you.

Collim_steps_M.jpg Collim_steps_M.jpg


Edited by Elroy, 18 September 2021 - 10:43 AM.

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#6 Vic Menard

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Posted 18 September 2021 - 10:57 AM

...For centering the secondary the only 2 things you want to look at are the outer circle of the secondary and the edge of the focuser draw tube, or the edge of the sight tube if you have one. Some people block off the reflection with a piece of cardboard to make it easier to see. 

While centering the secondary mirror relative to the bottom edge of the focuser (or sight tube) will get you close, the alignment isn't finished until the reflection of the primary mirror is also centered. This is why I prefer leaving the reflection of the primary mirror visible. (If the OP has a truss Dob, he could also place a cardboard disk (ideally, with a center mark) directly on top of the primary mirror. This effectively removes the reflections inside the reflection of the primary mirror but still keeps the third circle visible.) 

 

To achieve correct placement of the secondary mirror , these three circles (see attached diagram) should be concentric:

the bottom edge of the focuser drawtube (or the bottom edge of the sight tube), light blue circle,

the actual edge of the secondary mirror, green circle, and

the reflected end of the primary mirror, red circle.

(Note that when the reflected edge of the primary mirror is centered, the primary mirror center marker will also be centered. This means with a sight tube the primary mirror center marker will be aligned with the sight tube cross hairs, or with a simple thin beam laser, the outgoing laser beam/dot will be centered in the primary mirror center marker. For the record, I always start assessing the secondary mirror placement with the primary mirror reflection centered under the focuser--from this starting point (and from a centered pupil), the error(s) is easier to "see".)

Attached Thumbnails

  • offset2.jpg

Edited by Vic Menard, 18 September 2021 - 11:05 AM.

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#7 Starman1

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Posted 18 September 2021 - 01:49 PM

Hi!

It has been a lot of posts from me recently, because of some trouble with my new reflector, a GSO 12" dob.

It is so nice to be able to post on this forum and ask for help, and I really appreciate the answers I get!

"Luckily" there are a lot of cloudy days (and nights) at the moment, so I have many days to sort out the issues.

 

My question is about aligning the secondary.

When collimating I first align the secondary under the focuser. I have tried this both visually and with laser collimator.  When measuring by eye I have noticed a tendency where the secondary is always skewed towards one side of the focuser, as if its axis (the axis that is paralell to the tube) is tilted. I use various concentric lines in the focuser as guiding lines.

 

To correct this I have to constantly fight against the secondary holder, turning one of the 3 collimation bolts as far in as it will go, while only gently adjusting the other 2.

It does not feel right to assymetrically overtighten bolts in an optical system.

 

I have checked the spider itself, and the secondary holder is in the center of the tube.

 

However, when using the laser, the laser beam will only hit the center mark on the primary, if I allow the secondary to be skewed! After having used the laser, I remove it from the focuser, and check the secondary visually. It is always skewed! Multiple degrees it seems...

 

Is this due to cheap chinese mechanics? Or is it fixable?

It seems so weird to me that the only way collimation works is if the secondary is obviously not centered onder the focuser drawtube.

 

Below is an image where I have drawn circles showing focuser and secondary outlines. The yellow line is the axis that the secondary seems to follow.

 

Best Regards,

Espen

 

attachicon.gif242096505_828945331128741_5636875925266610001_n.jpg

1) measure to be certain center bolt of secondary is dead center in tube.  If not, adjust opposite spider vanes until centered.

2) center secondary under focuser using collimation screws.  Ignore laser for this.

3) rotate secondary until reflected image of primary mirror in the secondary is centered in the up and down (short axis) dimension of the secondary.

4) install laser and adjust tilt to center laser on primary (assuming laser is collimated, itself)

5) Then Collimate primary.

6) Return to step 3 and repeat through step 5.

7) If necessary, repeat steps 3-5 again.  Each time, a smaller amount of adjustment should be necessary.

 

If the collimated scope still shows the off center secondary, it is perfectly acceptable to shim the focuser to aim it at the secondary.  In your case, a couple very thin washers between the focuser and the tube on the bottom two focuser attachment screws would do it.  You will have to rotate the secondary slightly and recollimate after doing so.


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#8 Asbytec

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Posted 18 September 2021 - 08:35 PM

Espen, you have some good recommendations already. We all say pretty much the same thing except for some minor differences in the process to get there.

 

You have measured the spider to be centered in the tube looing down the upper end toward the primary. Great. That should mean your spider vanes are straight and the central bolt is also centered in the tube and close to the tube's longitudinal center line. It's good to know the primary mirror center marker is also approximately on the tube longitudinal axis. Being close to the tube center line is close enough, we're not shooting for mechanical perfection. We do not align to the longitudinal axis, we align the "moving parts" with each other in order to achieve optical alignment.

 

I am struck by the amount of tilt you need to bring the secondary under the focuser by, "fighting with the secondary holder", and "turning one of the collimation bolts as far as it will go". It does not feel right to you, you really should not need to fight with anything. As Don mentioned above, one way to fix this is to normalize the three tilt adjustment screws. When you do, the yellow line (secondary major axis) will also be parallel to the tube longitinal center line. Then instead of "forcing" the secondary under the focuser, you can shim the focuser to capture the secondary's natural position roughly centered on the tube center line.

 

When you do this, the spider and center bolt are already close to the tube center line and, interestingly, so is the primary center marker. Normalizing the secondary tilt adjustment screws brings the secondary major axis (yellow line) toward and parallel to the same tube center line. Then, importantly, shimming the focuser to capture the secondary mirror brings the focuser axis into the same plane as the tube longitudinal axis. Now that all the "moving parts" we want to see during collimation are approximately in the same plane. Close enough. And this was done rather simply by squaring the "moving parts" with each other so they all sit naturally in the same plane. It's a great starting point to begin optical alignment, it will be a little easier and more intuitive.

 

I agree with Vic, you will need some visual tool with a peep hole, like the collimation cap he linked above, to keep your eye centered over the focuser - your eye will be on the focuser axis. Or as Spaceoddity recommended, a site tube or combo tool. I also agree to not hide the primary reflection with a piece of cardboard. As Don explains above, you will need to see the primary reflection to rotate your secondary. Secondary rotation is the final part of step 1, Secondary Placement (which includes both centering and rotation). Now are you ready to begin Step 2, aligning the focuser axis to the primary center using your laser. The Laser and Collimation Cap method Vic linked above will be fine.


Edited by Asbytec, 18 September 2021 - 08:39 PM.

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