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Secondary Mirror Offset (Collimation Problems... :))

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#1 Usman

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Posted 28 July 2021 - 12:58 PM

Hi,

 

I'm trying to align the secondary mirror of my brand new Skywatcher 8 inch Dobsonian (I haven't reached the stage yet where I'm supposed to adjust the primary mirror smile.gif). The thing is, it looks like a circle when I look down my Celestron Chesire collimation tool and I've managed to use the tiny hex screws to get all the primary mirror clips on the secondary mirror. But as you can see in this photo, the secondary mirror, even though it looks like a nice circle, isn't centered in the focuser tube. It's displaced towards the left which means that there's a large gap on the right befween the Cheshire tube boundary and the edge of the secondary. 

 

Secondary Mirror...jpg

 

I tried using the central screw to move the secondary mirror towards the primary (in the right-hand direction in this image) but then it becomes just a little loose and susceptible to rotation. When I tighten it, it moves back away from the primary and towards the open end of the OTA (as is the case in this photo).

 

Can you please let me know how I can adjust the position of the secondary towards the primary mirror (a little towards the right as seen from the focuser) without compromising its position (and tightness)? Sorry, I know this is a basic question but this is my first attempt at collimation smile.gif

 

Thanks! smile.gif


Edited by Usman, 28 July 2021 - 12:59 PM.


#2 Starman1

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Posted 28 July 2021 - 03:07 PM

To move the secondary downward, toward the primary:

Loosen the center screw and tighten the 3 collimation screws on the secondary.

Repeat to move it more.

 

The picture you post shows a rotation problem as well.


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#3 Asbytec

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Posted 29 July 2021 - 10:26 AM

"The thing is, it looks like a circle when I look down my Celestron Chesire collimation tool and I've managed to use the tiny hex screws to get all the primary mirror clips on the secondary mirror. "

First, do as Starman1 says to fix the position "left and right". That is along the tube longitudinal axis toward and away from the primary mirror. You can also tilt the secondary downward a little to even the gap at top and bottom. The rotation Don mentioned is seen in the dark reflection of the secondary silhouette, it appears to bulge toward the upper right. We can fix that, too.

So once closely centered, then rotate your secondary so the primary reflection is centered "up and down" as seen in your image (rotation only moves the primary reflection in that direction). The secondary may look round, but proper rotation is best done by putting the primary reflection centered on the secondary major axis (approximately centered on the horizontal spider vane - notice how it nearly bisects the focuser, too). The primary center marker will also be on or nearly on the secondary major axis (and close to the bisected focuser axis, as well). Secondary rotation is the final part of step 1, secondary placement. (Hat tip to Starman1).

Don't worry about the clips just yet. They are not a collimation signature. Anyway, the clips will fall into place when your secondary is centered under the focuser and the focuser axis is aligned to the primary center. This is because when the focuser axis is aligned to the primary center, the primary reflection will necessarily be concentric with the focuser, too. If your secondary is also concetric under the focuser, those three circles are your collimation signatures. Then the clips will be easily seen in the secondary when the focuser axis is collimated.

Edited by Asbytec, 29 July 2021 - 10:50 AM.

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#4 Usman

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Posted 29 July 2021 - 11:58 AM

Hi Starman and Asbytec,

 

Thank you for your advice. :) I'll try it and then report back.



#5 Vic Menard

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Posted 29 July 2021 - 01:23 PM

Here's a quick annotated look at your current alignment. It looks like your sight tube cross hairs (green) are not quite inline with my annotation (light blue), but you're pretty far off, so there might be some parallax. The red circle is my best approximation of the reflected edge of the primary mirror (using the primary mirror clips for reference). The red cross hair shows where the primary mirror center marker should be (I couldn't get it to show with my software). The actual edge of the secondary mirror appears to almost coincide with the reflection of the primary mirror, which tells us that you're very close to the apex. So, as noted by Don and Norme, the secondary needs to move toward the right, and maybe a little down--but the sight tube cross hairs should at least be aligned to the primary mirror center marker (secondary mirror tilt adjustment) and then the bright Cheshire ring should be brought into alignment with the primary mirror center marker (primary mirror tilt adjustment) and then you should post another image showing the overall alignment.

Attached Thumbnails

  • post-275768-0-37454600-1627494803_thumb.jpg

Edited by Vic Menard, 29 July 2021 - 01:25 PM.

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#6 Usman

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Posted 29 July 2021 - 07:50 PM

Wow!! Thanks, Vic! I really appreciate it smile.gif

 

I tried following the suggestions I had received and was able to more or less (though not perfectly) center the secondary under the Cheshire crosshairs:

 

IMG-20210729-WA0004.jpg

 

I then aligned the primary to the secondary:

 

IMG_20210729_214244807-B.jpg

 

Then I took it out tonight and tried to do a brief star test. The diffraction rings were almost perfectly circular but the problem is, the focuser tube is clearly acting as an obstruction - as shown in both the photos. When I was looking at any particular star in focus, I didn't notice anything but as soon I turned the focuser inside and outside of focus, I could see the focuser tube obstructing the diffraction rings. It's protruding quite deep into the OTA.

 

IMG_20210730_024940803-A.jpg

 

 

Can you please let me know how I can solve this problem?

 

 

Thanks.


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#7 Kunama

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Posted 30 July 2021 - 12:36 AM

Wow!! Thanks, Vic! I really appreciate it smile.gif

 

I tried following the suggestions I had received and was able to more or less (though not perfectly) center the secondary under the Cheshire crosshairs:

 

attachicon.gifIMG-20210729-WA0004.jpg

 

I then aligned the primary to the secondary:

 

attachicon.gifIMG_20210729_214244807-B.jpg

 

Then I took it out tonight and tried to do a brief star test. The diffraction rings were almost perfectly circular but the problem is, the focuser tube is clearly acting as an obstruction - as shown in both the photos. When I was looking at any particular star in focus, I didn't notice anything but as soon I turned the focuser inside and outside of focus, I could see the focuser tube obstructing the diffraction rings. It's protruding quite deep into the OTA.

 

attachicon.gifIMG_20210730_024940803-A.jpg

 

 

Can you please let me know how I can solve this problem?

 

 

Thanks.

Remove the extension tube you have on the eyepiece side and use a shorter one...... that way you will be winding the drawtube out of the field of view


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#8 Starman1

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Posted 30 July 2021 - 12:54 AM

Exactly.  Remove the tall adapter from the focuser and use a normal short 2" to 1.25" adapter instead.


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#9 Usman

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Posted 30 July 2021 - 02:51 AM

Thank you, Matt and Starman :)

 

What's disappointing is, the seller claims to have first collimated and then thoroughly checked the scope and its optics (with a star test among other things) before sending it to me. I can understand that the collimation got severely affected during transport but I am not sure how he could have done a star test with this focuser configuration. As soon as the diffractor rings start to appear you can see the tube obstructing them.

 

I also have a question about the diffractor rings. They appear to be very smooth and mushy on either side of focus (almost like a donut) and don't have a clear ring structure apart from the outer ring. Could this lack of contrast in the rings be due to this extension tube blocking a large amount of light from hitting the primary mirror?

 

Thanks once again.



#10 Asbytec

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Posted 30 July 2021 - 04:13 AM

I tried following the suggestions I had received and was able to more or less (though not perfectly) center the secondary under the Cheshire crosshairs:

 

You're getting much closer. Good job. Yea, it's not perfectly centered, but it (really) does not have to be. Close enough is good enough. So long as you can see the entire primary mirror reflected in the secondary at infinity focus. You can nudge it closer to center if you want. But, to see that, you need to be able to get the pupil of your Cheshire closer toward the secondary. To know for sure, focus down a little using the Cheshire. The secondary will appear larger and the primary reflection will appear smaller. So, the primary reflection will be more easily seen within the secondary. If so, your field is fully illuminated and secondary placement is good enough for government work and visual observing. Then it's a matter of axial alignment. 

 

A technical note. You mentioned centering the secondary under the cross hairs. Yea, okay, but it's the same thing to center it by making their edges concentric. Whichever works best. I prefer to work with the edges because those are collimation signatures. 

 

I then aligned the primary to the secondary:

 

This took me aback for a moment. Not sure what you mean. We do not align the primary to the secondary. We align the reflection of the primary center marker to the reflection of the Cheshire pupil/bright ring. The Cheshire pupil is on the optical axis, the primary center needs to be aligned with the Cheshire pupil. Not aligning the edges of the secondary and primary reflection (so you can see the clips). This concentric edge alignment of the primary reflection and secondary will happen automatically only if the secondary is very well centered under the focuser (yours is slightly off) and when the focuser axis is accurately pointing at the primary center mark. To align the primary mirror, tilt the primary so the center mark aligns within the Cheshire ring (or pupil). 


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#11 Usman

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Posted 30 July 2021 - 06:08 AM

Thank you for the detailed explanation. Yes, I guess I didn't describe the primary collimation properly :)

 

The thing that I find disappointing is, this scope was supposed to have been thoroughly checked and configured by the seller before dispatch. I realize that the mirrors can go out of collimation during shipping and transportation but the tall adapter tube in that position means that he didn't really check the optics as he claims to have done. I noticed the obstruction as soon as the view was inside and outside of focus which means that one can't do any kind of star test (with either a natural or an artificial star) with this current configuration.

I'd really like to know if I'm being unfair, though. :)



#12 Asbytec

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Posted 30 July 2021 - 07:16 AM

Here's a neat trick. Once the focuser axis (cross hair) is aligned to the primary center, the primary reflection must necessarily be concentric with the bottom of the focuser and the site tube edge. Since the primary reflection will be centered under the focuser, you can use the centered primary reflection to evaluate your secondary position. But, you should focus inward just enough to see the entire primary edge (just inside the "apex" where both appear to be the same size - where you appear to be in your images). The clips can give you a hint because they are on the primary edge, but the primary edge is the actual signature. Your clips look evenly spaced, so that's good. That means the primary edge is not far away. Still, we like to see the edge of the primary reflection. And when we do, it can tell us more about where our secondary "needs"(sic) "wants" to be. 

 

Notice in your second image how the reflection of the black secondary silhouette (offset with respect to the bright Cheshire ring, a good thing) now bulges toward the primary mirror (to the right) instead of around 2 o'clock as before. This indicates your secondary rotation is more refined. It looks very good. Now, it's a matter of aligning the optical axes by tilting the secondary so the reflection of the primary center mark appears to move onto the cross hair. Then tilting the primary mirror so the primary center mark (and cross hair) appear to move into dead center of the Cheshire ring. You look close already. 

 

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Disclaimer: The following is not a collimation signature and can be confusing if you're not closely collimated or the secondary is not well placed and rotated. But, it's just a visual cue /I use/ to understand what I am seeing. I can see "it" coming together in your second image. I mention it only in hopes of helping you understand what you are seeing. This is not a collimation procedure, just a neat visual cue. I cannot help but see it, I'm sure you will too. 

 

Notice how one spider vane (circled in your third image) is parallel to and appears to "bisect" the focuser. When your secondary placement is good and since the vane is parallel to and centered on the focuser, the reflection of that vane /can/ serve as a visual proxy for the focuser axis. Notice the reflection of that parallel spider vane also appears to emerge from /near/ the center of the reflected focuser draw tube and /appears/ to run approximately along the secondary major axis (if the secondary is well placed or even closely) and points directly at the center of the primary mirror (to the right). The reflection of the vane /can be/ a visual cue as to /about/ where your focuser axis should be.

 

Also notice your primary center marker (probably lost somewhere in the dark center of the Cheshire ring) is likely near the vane as is the dark Cheshire pupil itself. Your site tube cross hair will be on or very near it, too, as it nearly appears to be. Visually, this means all the moving parts are coming together in a plane. That plane includes the focuser axis (defined by the Cheshire pupil and cross hair), the secondary major axis (of a well placed secondary), and the primary center marker. That's a good thing when all these things begin to align in the same plane as the focuser axis. The view I am talking about shows me things are squaring up nicely. smile.gif


Edited by Asbytec, 30 July 2021 - 07:37 AM.

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#13 Vic Menard

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Posted 30 July 2021 - 09:48 AM

Here's Usman's current axial alignment assessment (the red circles are concentric):

 

(For an 8-inch f/6 his alignments appear to be within the high magnification error tolerances.)

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  • post-275768-0-12174300-1627605256.jpg

Edited by Vic Menard, 30 July 2021 - 09:51 AM.

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#14 gene 4181

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Posted 30 July 2021 - 10:31 AM

Thank you for the detailed explanation. Yes, I guess I didn't describe the primary collimation properly smile.gif

 

The thing that I find disappointing is, this scope was supposed to have been thoroughly checked and configured by the seller before dispatch. I realize that the mirrors can go out of collimation during shipping and transportation but the tall adapter tube in that position means that he didn't really check the optics as he claims to have done. I noticed the obstruction as soon as the view was inside and outside of focus which means that one can't do any kind of star test (with either a natural or an artificial star) with this current configuration.

I'd really like to know if I'm being unfair, though. smile.gif

  Thats how they come, that other owner  just used what was provided. My 250  came with the longer 1.25 in adapter  so i moved the mirror up  in the tube .   Not the greatest  mechanicals, but very decent optics smile.gif ,   but most dobs need a little "loving" (work) to get the most out of them.   Skywatcher /Orion use the  longer  focuser tube into the light path as a baffle  .  Baader sells an 1..25 in eyepiece adapter to T ring  accessory that will screw onto your 1.25 in adapter  after unscrewing the  long piece .  


Edited by gene 4181, 30 July 2021 - 01:19 PM.

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#15 gene 4181

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Posted 30 July 2021 - 10:37 AM

Thank you, Matt and Starman smile.gif

 

What's disappointing is, the seller claims to have first collimated and then thoroughly checked the scope and its optics (with a star test among other things) before sending it to me. I can understand that the collimation got severely affected during transport but I am not sure how he could have done a star test with this focuser configuration. As soon as the diffractor rings start to appear you can see the tube obstructing them.

 

I also have a question about the diffractor rings. They appear to be very smooth and mushy on either side of focus (almost like a donut) and don't have a clear ring structure apart from the outer ring. Could this lack of contrast in the rings be due to this extension tube blocking a large amount of light from hitting the primary mirror?

 

Thanks once again.

It could be  that the mirror isn't completely cooled down  and / or bad seeing also.  



#16 SteveG

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Posted 30 July 2021 - 02:47 PM

Thank you for the detailed explanation. Yes, I guess I didn't describe the primary collimation properly smile.gif

 

The thing that I find disappointing is, this scope was supposed to have been thoroughly checked and configured by the seller before dispatch. I realize that the mirrors can go out of collimation during shipping and transportation but the tall adapter tube in that position means that he didn't really check the optics as he claims to have done. I noticed the obstruction as soon as the view was inside and outside of focus which means that one can't do any kind of star test (with either a natural or an artificial star) with this current configuration.

I'd really like to know if I'm being unfair, though. smile.gif

Many newt owners think they know collimation, but they don’t. It cannot be done by just using a star test. In addition, they do not come from the factory collimated, no matter what they say.


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#17 Asbytec

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Posted 30 July 2021 - 05:48 PM

The draw tube obstruction can get in the way of a good star test. IME, because it blocks part of the diffraction pattern. Any obstruction, including the spider vanes and the secondary mirror, will cause added diffraction. But, I don't believe the draw tube or any obstruction will cause mushy diffraction patterns on either or both sides of focus.

 

Normally with a little under correction you can get mushy patterns outside of focus, and with over correction you can get mushy patterns inside of focus. A bit of over correction is possible or likely if the primary mirror is not at thermal equilibrium with ambient temperatures. This is because the edges cool a little faster than the center causing the edges to pull back a little while the center bulges a little. A turned edge is a form of over correction causing the edge to focus long of best focus. So, either may cause mushy patters on either side of focus. But, it's unlikely you have both under and over correction on the mirror at the same time. Ones check for correction error is the relative size of the secondary obstruction shadow on either side of focus. If the shadow is noticeably different diameters on either side, there is likely some correction error. But, with correction error on the mirror surface, being mushy of low contrast should be confined to either side of focus. 

 

As Gene said, it may be seeing conditions. If the defocused pattern is not stable and is disturbed by seeing, it can be mushy or blurred on both sides of focus. We'd have to see some images to know more. Atmospheric seeing affects are usually very fast and appear almost as if water is washing across the image. It's almost like looking at a penny at the bottom of a fast moving stream. You can almost see the air rushing over the defocused diffraction pattern. You might get the impression it's kind of like a flag blowing in the wind. Thermal affects are much slower distortions with brighter and darker areas slowly moving across the defocused pattern. You'll see brighter caustic lines (like the bottom of a pool on a sunny day) slowly changing shape and moving randomly with darker areas randomly wondering over the image. The edge of the pattern may be spikey. 


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#18 Usman

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Posted 31 July 2021 - 08:45 PM

  Thats how they come, that other owner  just used what was provided. My 250  came with the longer 1.25 in adapter  so i moved the mirror up  in the tube .   Not the greatest  mechanicals, but very decent optics smile.gif ,   but most dobs need a little "loving" (work) to get the most out of them.   Skywatcher /Orion use the  longer  focuser tube into the light path as a baffle  .  Baader sells an 1..25 in eyepiece adapter to T ring  accessory that will screw onto your 1.25 in adapter  after unscrewing the  long piece .  

Thanks, Gene! That's good to know because I thought I might be stuck with this longer drawtube :).

 

 

Many newt owners think they know collimation, but they don’t. It cannot be done by just using a star test. In addition, they do not come from the factory collimated, no matter what they say.

Yes, I agree. Plus, he said he used the star test to check the quality of the optics but with this kind of a 'partial' star test, I don't think an accurate assessment of the mirror quality would be possible.

 

The draw tube obstruction can get in the way of a good star test. IME, because it blocks part of the diffraction pattern. Any obstruction, including the spider vanes and the secondary mirror, will cause added diffraction. But, I don't believe the draw tube or any obstruction will cause mushy diffraction patterns on either or both sides of focus.

 

Normally with a little under correction you can get mushy patterns outside of focus, and with over correction you can get mushy patterns inside of focus. A bit of over correction is possible or likely if the primary mirror is not at thermal equilibrium with ambient temperatures. This is because the edges cool a little faster than the center causing the edges to pull back a little while the center bulges a little. A turned edge is a form of over correction causing the edge to focus long of best focus. So, either may cause mushy patters on either side of focus. But, it's unlikely you have both under and over correction on the mirror at the same time. Ones check for correction error is the relative size of the secondary obstruction shadow on either side of focus. If the shadow is noticeably different diameters on either side, there is likely some correction error. But, with correction error on the mirror surface, being mushy of low contrast should be confined to either side of focus. 

 

As Gene said, it may be seeing conditions. If the defocused pattern is not stable and is disturbed by seeing, it can be mushy or blurred on both sides of focus. We'd have to see some images to know more. Atmospheric seeing affects are usually very fast and appear almost as if water is washing across the image. It's almost like looking at a penny at the bottom of a fast moving stream. You can almost see the air rushing over the defocused diffraction pattern. You might get the impression it's kind of like a flag blowing in the wind. Thermal affects are much slower distortions with brighter and darker areas slowly moving across the defocused pattern. You'll see brighter caustic lines (like the bottom of a pool on a sunny day) slowly changing shape and moving randomly with darker areas randomly wondering over the image. The edge of the pattern may be spikey. 

Yes, I guess the seeing also affected the image. I was also looking at Saturn through my Celestron C90 Mak (at about the same magnification) but the C90 was showing a much clearer and contrasty image. But I reckon that's only because smaller scopes get way less affected by seeing than larger scopes. :)


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#19 Asbytec

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Posted 01 August 2021 - 02:44 AM

 

 

Yes, I guess the seeing also affected the image. I was also looking at Saturn through my Celestron C90 Mak (at about the same magnification) but the C90 was showing a much clearer and contrasty image. But I reckon that's only because smaller scopes get way less affected by seeing than larger scopes. smile.gif

 

 

Well, a C90 can give a nice high contrast image, and it is less susceptible to the same seeing conditions for both scopes on a given night. But it should not out perform an 8" Newt/Dob in terms of both resolution and contrast - planetary detail. The larger 8" mirror may need to cool a little more before it's images come into their own. Seeing also varies greatly from night to night. Keep at it, give it another chance. All scopes need to be prepped for observing, that is they need good collimation and thermal stability. And the observing conditions need to be favorable. 


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#20 Usman

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Posted 10 August 2021 - 11:12 AM

Hi,

 

I tried really hard to improve the collimation by doing some fine-tuning; I tried to center the secondary in the focuser tube and adjusted the tilt, etc. So far, this is the best that I have been able to achieve. I know that it's not perfect but considering the telescope specs (f/6), would this be OK for now or do I need to make any further minor adjustments? If so, please let me know which ones smile.gif

 

I have attached three photos of the same mirror configuration. Minor differences might be due to the parallax error when taking these photos.

 

Thanks again.

 

 

 

 

Attached Thumbnails

  • 1..jpg
  • 2..jpg
  • 3..jpg


#21 Starman1

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Posted 10 August 2021 - 11:35 AM

I can't gauge primary collimation because I cannot see the center marker on your primary.

Your secondary collimation tolerance, though, is about 0.03D, where D is the diameter of the primary mirror, or 6.1mm for an 8" and 7.6mm for a 10"

I'd be happier if you could at least get the crosshairs centered somewhere in the dark center of the Cheshire reflection, which is about that number of mm in diameter.

It's outside the dark center in all 3 photos.


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#22 SteveG

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Posted 10 August 2021 - 05:05 PM

Agreed. Use the top and bottom tilt screws to slightly shift that secondary downward until the crosshairs are in the center.

 

Try illuminating the little cut-out in the side of the Cheshire.


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#23 Usman

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Posted 12 August 2021 - 01:01 PM

I can't gauge primary collimation because I cannot see the center marker on your primary.

Your secondary collimation tolerance, though, is about 0.03D, where D is the diameter of the primary mirror, or 6.1mm for an 8" and 7.6mm for a 10"

I'd be happier if you could at least get the crosshairs centered somewhere in the dark center of the Cheshire reflection, which is about that number of mm in diameter.

It's outside the dark center in all 3 photos

Thank you, Don. The primary center marker is almost directly above the dark center of the Cheshire reflection. That's why it's almost impossible to see it.

 

 

Agreed. Use the top and bottom tilt screws to slightly shift that secondary downward until the crosshairs are in the center.

 

Try illuminating the little cut-out in the side of the Cheshire.

OK. Thanks for the tip, Steve.



#24 Usman

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Posted 14 August 2021 - 10:36 AM

Sorry, I keep getting back to you guys about the same problem. smile.gif I tried to do some fine-tuning once again but there's still a small problem. Clip C in my last two photos is barely visible. I can clearly see its edge in the sight-tube when the focuser is fully inserted but it's nowhere as prominent as Clip A or even Clip B. The primary center marker (the tiny circle) is almost perfectly under the center of the crosshairs and so is the dark center of the Cheshire reflection. But there must be a small misalignment somewhere. Can you please let me know once again how I can fix this?

 

Focuser fully withdrawn (no flashlight):

 

CCC..jpg

 

Focuser fully inserted (no flashlight):

 

AAA..jpg

 

Focuser fully inserted (flashlight pointing at the collimation tool cut-out):

 

BBB..jpg

 

 



#25 Starman1

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Posted 14 August 2021 - 11:16 AM

It might not be your collimation at this point, but perhaps a slightly under-sized secondary mirror.

You should be able to see the entire primary mirror plus some space around it with the focuser racked in.

 

Might I ask which brand/model of combination sight tube/cheshire you are using?


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