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Another Collimation Question

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#1 diex

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Posted 02 July 2020 - 05:08 PM

I have been struggling to collimate my new 10 inch dob for over a week. I had purchased the Celstron Collimation tool but returned it and now have a Meade laser collimation tool. I have been reading many of the forum posts on this topic and followed one of the suggestions to finally get the red dot back to the center of the primary (looking down the tube). I did this by loosening the 3 screws and manually adjusting the secondary until the laser dot was near the center. I then adjusted the tilt until it was about perfect.  Looking through the focuser without an eyepiece it doesn't look great.

 

view2

 

So, having faith now that the red laser dot is in the center and that the secondary is facing the focuser tube properly, should I expect that adjusting the primary mirror is going to bring me home on this process? I have little faith that it will work. One of the problems I have been having when I go to adjust the primary is that the red dot on the Meade collimator target area is either not seen or does not look like a dot but instead is kind of blown up where the dot is not clearly defined. 

 

Also, based on the picture above, will a primary mirror adjustement really bring everything to the center when viewing through the focuser?

 

Thanks much. 



#2 Tangerman

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Posted 02 July 2020 - 05:18 PM

Have you checked the laser collimator? These often come misaligned. If you spin it around in the focuser and it traces a red circle on the primary, then the laser isn't coming quite straight out of the collimator and needs to be adjusted. I had to adjust mine with some very small hex screws.


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#3 SteveG

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Posted 02 July 2020 - 05:35 PM

 

Also, based on the picture above, will a primary mirror adjustement really bring everything to the center when viewing through the focuser?

 

Thanks much. 

No. If the picture is just looking into the focuser then we don't have enough info to determine how well it's aligned. There's no way to perfectly center your camera over the focuser. Do you have a simple collimating cap?


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#4 SteveG

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Posted 02 July 2020 - 05:36 PM

You need to start by making sure the secondary is centered under the focuser, but you no longer have a tool to help you determine that.


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#5 JohnBear

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Posted 02 July 2020 - 05:45 PM

Looks like you have been a member for a long time. I'm a bit surprised that you are having collimation issues. 

You did provide a good picture however.  Those of us that know how to do collimation could fix your scope in a matter of minutes - but telling you how to do online a VERY difficult task.

Collimation is easy - once you know how to do it. Learning how to do it is the hard part!  There is a easy way to learn, but that is not via forums with a dozen different people suggesting 20 ways on how to collimate a Newt.

 

Long story short: I'll just link to a similar newbie topic i replied to today.  Follow that advice and you will have your new dob perfectly aligned a ready to go in no time. This will save you hours of frustration and trial and error. Here is the link:

https://www.cloudyni...#entry10305551   

 

BTW, lasers really confuse people that try to teach collimation to themselves. Just use a collimation cap. Once you know how to do that, you can experiment with a laser - then use the collimation cap to fix what ever you messed up with the laser.  They do have a place, but not as a learning tool.


Edited by JohnBear, 02 July 2020 - 05:56 PM.


#6 MikeTahtib

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Posted 02 July 2020 - 05:53 PM

I would say your secondary is still not right.  I think it needs to be rotated.  You should be able to see the primary mirror in the secondary before adjusting the primary, or adjusting the tilt of the secondary with the laser.  If you have any sort of mirror retaining clips evenly spaced around the mirror, you can adjust until they all poke inot the collimator field fo view evenly)

So, without getting too crazy:

1.  Get the secondary aligned under the focuser

2.  Adjust the rotation of the secondary to get the primary centered in the secondary.  Fine tune with the adjustment screws on the secondary mount.

   (These first 2 items are best done with a sight tube - I use something like this:  

https://www.astronom...-tool-972.html 

    I don't think you can really do these adjustments with a laser.  The tool above can do everything you need, quickly, cheaply, and easily, but it needs a little study first)

3.  Then use the laser to further fine-tune the aim of the secondary to the center spot of the primary

4.  Use the laser to adjust the primary

 

I just got a Meade laser collimator myself, and I have been impressed.  It seems perfectly collimated right out of the box, provides repeatable results, and was dirt-cheap.  Between this and the sight-tube, collimation is easier than ever. I had a much more expensive collimator before it, and it was terrible.  So dodn'dt be too quick to sell the laser you have or rip the gunk off the screws to try ot adjust it



#7 jupiter122

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Posted 02 July 2020 - 06:01 PM

Do you need a collimator to collimate your collimator? This is why I switched to a refractor. 😀

#8 MellonLake

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Posted 02 July 2020 - 06:01 PM

You actually can't fully collimate a secondary mirror with a laser, it will often lead to rotational errors.  Basically, if you don't have the rotational position perfect before adjusting the tilt, you will be essentially correcting the rotational error with the laser.  

 

To check if you laser is correctly collimated put the laser in the focuser and only lightly clamp it in.  Then rotate the laser while watching the beam on the primary.  If the beam goes in a circle, the laser is not collimated sufficiently.  An incorrectly collimated laser will also lead to both secondary and primary mirror issues.


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#9 SteveG

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Posted 02 July 2020 - 06:26 PM

I would say your secondary is still not right.  I think it needs to be rotated.  You should be able to see the primary mirror in the secondary before adjusting the primary, or adjusting the tilt of the secondary with the laser.  If you have any sort of mirror retaining clips evenly spaced around the mirror, you can adjust until they all poke inot the collimator field fo view evenly)

So, without getting too crazy:

1.  Get the secondary aligned under the focuser

2.  Adjust the rotation of the secondary to get the primary centered in the secondary.  Fine tune with the adjustment screws on the secondary mount.

   (These first 2 items are best done with a sight tube - I use something like this:  

https://www.astronom...-tool-972.html 

    I don't think you can really do these adjustments with a laser.  The tool above can do everything you need, quickly, cheaply, and easily, but it needs a little study first)

3.  Then use the laser to further fine-tune the aim of the secondary to the center spot of the primary

4.  Use the laser to adjust the primary

 

I just got a Meade laser collimator myself, and I have been impressed.  It seems perfectly collimated right out of the box, provides repeatable results, and was dirt-cheap.  Between this and the sight-tube, collimation is easier than ever. I had a much more expensive collimator before it, and it was terrible.  So dodn'dt be too quick to sell the laser you have or rip the gunk off the screws to try ot adjust it

That link is a guy that has a focus problem, not collimation. I agree he probably has his collimation off, but there’s nothing useful to the OP in that link.



#10 SteveG

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Posted 02 July 2020 - 06:27 PM

Do you need a collimator to collimate your collimator? This is why I switched to a refractor.

No. You simply spin it in your focuser. If it draws a circle, then it’s off. If the dot stays put, then it’s collimated.


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#11 Stardust Dave

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Posted 02 July 2020 - 06:33 PM

Been using a machinist's V- block and a wall across the room to check my laser. 



#12 MellonLake

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Posted 02 July 2020 - 07:13 PM

Learning collimation can be frustrating but once you got it,  you got it.  

 

Read up on the milk jug washer modification which helps make secondary rotation easy (be careful if you do remove the secondary not to touch the mirror or drop it or worst of all drop it on the primary). Also note that if the secondary collimation screws have been over-tightened they can put dents in the secondary stalk that cause the secondary to want to take up only one position (this can be fixed with a steel washer).  

 

Make sure the tube is always horizontal so you don't drop tools in.  

Put a piece of paper behind the secondary on the tube wall (I prefer orange, blue, green (not white)).

 

To collimate with a laser!!!:

1) Put a collimation cap in the focuser tube and look at the secondary.  Slightly loosen the secondary collimation screws (only very very slightly)  Rotate the secondary such that it is visually a perfect circle and has no visible oval shape when looking through the collimation cap (Make a collimation cap if you don't have one).   Don't worry about what the primary looks like in the reflection yet.  Gently tighten each of the secondary collimaiton screws so they are just barely tight. 

2) Check and set the rotational position again.  The rotational position is very important.  

3) Take the collimation cap out and look down the focuser draw tube.  Make sure that the secondary is not angled up or down in the focuser (the secondary screws being threaded in unequal distances can lead to the secondary being askew up or down).  If it is angled, loosen all three secondary alignment screws and make sure that when you tighten them against the stalk (again only slightly tight) the secondary does not go askew and all screws are threaded in about the same amount (thread them in slowly one turn each on each screw at a time).  If you found the secondary askew and have corrected it go back to #1.

4) Using the collimation cap make sure the secondary is correctly rotated such that it is round (again the secondary collimaiton screws should still be just barely tightened).  Once the secondary is round.  Then adjust the secondary collimaiton screws by gently tightening them until the reflected image of the primary mirror clips are equally visible (you can move the focuser in and out so the clips appear at the edge of the secondary mirror).

5) Check to see if the secondary is centred in the focuser tube. See if there is equal amounts of coloured paper visible all around the secondary between the edge of the secondary and focuser tube (if you move the focuser in and out you can make the ring of colour smaller (note: a portion of the ring of colour will be missing at the secondary stalk).   Usually if the secondary is out of position, it will need to move toward or away from the primary mirror.  If you do need to move the secondary loosen off the secondary collimation screws and tighten or loosen the centre screw to move it up or down the tube (clockwise is away from the primary and counter clockwise is toward the primary).  You will need to hold the secondary stalk (without touching the mirror) to prevent it from rotating.  I would do only a half turn on the centre screw at a time.  If you have to move the secondary position, repeat 4 and 5 (you might have to do this a couple times).  Note: this process only has to be done once!!! Once the secondary is centred you should not need to adjust its position again. 

 

Note: if the secondary is too far up or down with respect to the focuser tube, the spider may not be sufficiently centred (ask again here if you need to do this).  

 

6) Put the laser in the focuser and by only adjusting two secondary collimaiton screws get the beam to the centre of the primary mirror (this should not take very much turning of the secondary screws as it should be very close at this point.). During this process the secondary collimation screws should be slightly tightened to "lock" the secondary in place.  Do not over-tighten or you will put dents in the secondary stalk.  

7) Put the collimation cap back in and check the primary mirror clips are still visible at the edge of the secondary mirror.  If the clips are not equally visible, you still have a slight tilt and/or rotational error and may want to go back and do 4 and 5 again.  This does not have to be perfect.  If 75% of one of the clips is visible and the other two are at 100% your are fine (or 2 are 75% and one is 100%) You may have 4 clips (I have 3).  Work on perfection another day.  

 

Just as a side note, a sight tube (long collimation cap) is the best tool for centering and rotating the secondary mirror.  It allows you to more easily centre and round the secondary.  The laser can then be used to tweak the secondary tilt only very slightly. 

      

8) Once you are confident your secondary is in position, only then try to collimate the primary by using the collimation knobs on the back of the mirror cell to get the laser reflection to the centre mark on the laser collimator target. 

 

If you have a barlow consider doing a laser barlow primary collimation (put the laser in the barlow and centre the shadow of the centre marker of the mirror on the laser collimator target).  

 

9) Do a star test to see if you have it correct.  Using the above method my star tests are usually perfect.  If the start test is not perfect collimate the primary using the star.      

 

 

Everyday collimation is just doing steps 6 to 9 above.  

 

 

Just a note on the laser and secondary collimation.  The laser will never show if the secondary is in the correct rotational position. If you try to collimate the secondary with a laser when the mirror is not in the correct rotational position, the laser will not address the rotation error.  The first step in collimating the secondary is always to get the rotation correct.


Edited by MellonLake, 02 July 2020 - 08:45 PM.

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#13 MikeTahtib

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Posted 02 July 2020 - 08:49 PM

That link is a guy that has a focus problem, not collimation. I agree he probably has his collimation off, but there’s nothing useful to the OP in that link.

You quoted the wrong post.  The link in my post (that you quoted) is to a sight tube combination collimator on the Astronomics website, which I think you will agree would be very useful for the original poster to have. 

The link to the guy who can't get to focus was in the post above mine.


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#14 diex

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Posted 03 July 2020 - 12:38 AM

Op here.

 

Thanks for all of the suggestions. I will keep reading them over to take it all in. Something tells me I should have kept the Celestron Cheshire. The collimation was so bad that I thought it would be easier to use the laser version.

 

I made a collimation cap earlier in the day and after reading the replies here I gave it another try.

 

  • I used the collimation cap to get a nice round secondary in the cap view.
  • I then used the 3 rotation (?) screws to get a reasonable center view of the primary.
  • Then I used the laser to center to the primary target. I only needed to adjust the rotation screws just a bit and it centered right up. Previously it was hell to move the laser dot across the mirror.
  • At this point I have a near perfect round secondary, a primary in full view including the the 3 mirror clips (for the most part) and it all appears centered through the focuser (no eyepiece). However the focuser tube and my eyeball viewed on the secondary is just a bit off center.
  • I then adjusted the primary. Since I was having problems seeing the red dot on the target area in the laser collimator I decided to adjust the primary until the center target on the mirror was centered in the secondary which I was able to do.
  • Now the view through the focuser looks very close to the pictures that show what it should look like. The focuser tube/eyeball was still off by a bit. I adjusted the 3 screws just a bit to try and center it and came pretty close. I tweaked the primary to keep the center target on the secondary.

 

I put the laser collimator back in to look at the primary target area and low and behold I was able to see a clearly defined red dot which  was off target.  I hesitated to adjust the primary again because I knew it was going to ruin my nice looking center view through the focuser (no eyepiece) and it did. So I reverted back to my focuser view. I don't understand this contradiction.

 

I waited until tonight to test my setup. It may have been a mistake to test on the moon but it was a big target I couldn't ignore. I was using a Celestron zoom eyepiece at 24mm. I don't know if that was a mistake either. Should I have used my 2in 38mm?  In any case, it did'nt go well (the dob had been sitting outside for about an hour). The moon was big and bright and I could not focus to any level of detail, sometimes it was just a perfect pure white. At this point I did adjust the primary using the laser to the center target area and that seemed to make things worse. To eliminate poor seeing, I looked through my 80mm achro and got a fantastic view of the moon. I wondered if I should I have went for the 127mm refractor instead.

 

At the very lest I may have a telescope that is just slightly off collimation which is much better than were I was.  I'm going to keep reviewing the suggestions and definitely read through http://www.astro-bab...nian-reflector/ which looks pretty good.

 

This is disheartening though, I have this giant reflector, relatively speaking, that I can't get to work!  If all else fails I'll try and get some help from the KC Astronomical Society.

 

Again, many thanks.



#15 Asbytec

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Posted 03 July 2020 - 07:59 AM

Op here.

  • I used the collimation cap to get a nice round secondary in the cap view.
  • I then used the 3 rotation (?) screws to get a reasonable center view of the primary.
  • At this point I have a near perfect round secondary, a primary in full view including the the 3 mirror clips (for the most part) and it all appears centered through the focuser (no eyepiece). However the focuser tube and my eyeball viewed on the secondary is just a bit off center.

Lot going on here, let's take this a step at a time. I'll repeat a lot of advise above, but maybe say it a little differently. Let's start with your secondary position under the focuser. Use a white paper background behind the secondary and opposite the focuser so you can easily see the secondary against the bright background. Focus down to get the smallest white annulus you can achieve, but do not force the focuser all the way down. Doing so may cause the draw tube to tilt under the pressure.

 

So, you get a nice round view and it's reasonably centered. But, you are not (really) done with positioning the secondary. You are immediatly jumping into tilt of the secondary which is a collimation step, not (really) used to set the secondary position. You're not really ready to start (tilt) collimation until you complete the position step by refining the secondary rotation. I find it's not good enough just to eyeball the circular appearance of the secondary. Rather than leaving it at that, use the reflection of the primary to refine our secondary rotation.

 

Once it looks centered in the focuser draw tube (much easier with a proper site tube than a collimation cap), you need to refine the secondary rotation. Rotate the secondary so the primary is centered along the secondary major axis. This brings the primary center marker into the same plane as the focuser axis. Don't worry about the minor axis just yet, tilt collimation will take care of that. All we're doing is lining up the moving parts so they are easier to align. Do not hide the primary reflection, use it to refine secondary rotation such that the reflection of the primary, and the center marker, are on the secondary major axis. And everything is in the same plane as the focuser axis, that's a great starting point. 

 

When the secondary is well centered and rotated, the secondary major axis will be (nearly) in the same plane as the focuser axis along with the primary center marker. Now, when you install your (hopefully collimated) laser, the laser dot will hit the primary somewhere on the primary near the center mark and in the same plane as the focuser axis (because it defines the focuser axis). Everything is well lined in the same plane and ready for tilt collimation. We rely on tilt for axial collimation rather than positioning the secondary. For the latter, use only fore and aft movement toward or away from the primary and secondary rotation to the extent possible. Small amounts of orthogonal tilt are okay, but you may have to correct some induced rotation error to keep the primary reflection centered. 

 

Now, with the primary reflection on the secondary major axis, you are ready to begin tilt collimation. You will quickly notice (most of) your tilt collimation will be in the same plane and one of the three secondary adjustment screws will also be in the plane of the focuser axis. Use that one screw to tilt the secondary, and use the other two to allow the tilt by turning both of them in the same direction by the same amount. Tighten one, loosen the other two, or visa versa. Use the secondary adjustment screws so they work together to allow tilt, rather than independently working against each other. Try to avoid the urge to use them independently like we do the primary mirror, it's best to approach axial collimation from one direction rather than three directions. So much easier. 

 

It's helpful to actually finish the secondary position step by refining your secondary rotation using the primary reflection as a guide prior to begining tilt collimation. Centering the primary reflection on the secondary major axis puts you in a better position to start tilt collimation. Because you do not have to use a lot of orthogonal tilt trying to collimate the focuser axis. Excessive orthogonal tilt pushes the secondary away from the focuser center and induces some rotation "error". Small amoounts of tilt with the other two screws are okay to nail collimation, as needed. But if your primary reflection starts at some random place as viewed in the secondary, it requires a lot of orthogonal tilt and causes confusion. It may be collimated, but it may not look or feel right. 

 

Get the primary centered along the secondary major axis first (finish the position step), then use tilt to collimate the focuser axis using the laser. The laser dot should move pretty much in a straight line onto the primay center with the seondary screws working together instead of coming at the center mark from three random directions with the secondary adjustment screws working against each other. This was the defining epiphany for me to finally understand how folks can collimate their scope in 10 minutes rather than two frustrating weekends. It'll look and feel right.


Edited by Asbytec, 03 July 2020 - 08:25 AM.

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#16 JohnBear

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Posted 03 July 2020 - 09:51 AM

 

This is disheartening though, I have this giant reflector, relatively speaking, that I can't get to work!  If all else fails I'll try and get some help from the KC Astronomical Society.

Actually the KC Astronomical Society should have been your first place to go for help.  They probably give classes on collimation for newbies. A lot of newbies go thru this frustration when attempting collimation without direct experienced guidance. The word "epiphany" is quite appropriate once you "get it", but for a lot of people it becomes a nightmare.

 

When someone "Shows" you how it is done, collimation can become intuitive and quite easy.  And that can be done literally in just a matter of minutes. 

 

Collimation  is a step-by-step process that needs to be done carefully and in the right order (if not then you muck things up) - and it really helps to mentally understand what you are doing to the optical path with each step. The secondary mirror has to be properly positioned and "aligned" first. Only then is the primary is adjusted.  Normally the secondary almost Never need to be adjusted, and that is usually the newbie's fatal mistake

 

Adjusting the secondary mirror is particularly insidious because after getting it properly centered directly below the focuser (no easy feat to even visually confirm for many), once the screws are loosened you have to control 3 directions of tilt plus rotation all at once based on what you see down the focuser tube using a collimation cap/Cheshire - and describing what you need to see is very difficult to do in words (or even static images), but once you see it and understand what to look for it becomes "obvious".  

 

The collimation process has been described above quite well at least twice, but it is still no substitute for "showing" a newbie how it is done.  If you take appropriate Covid-19 precautions, I am sure there is a local astronomer near you that would be willing to drop by and show you how to do collimation properly. 

 

If that cannot be done, then here is an old Orion video that demonstrates the Newt collimation process quite well. Use it in conjunctions with the instruction above given by Asbytec, and together they may shorten your learning curve by a few days. 

 

But please disregard the laser collimation part at the end of the Orion video until you really know how to do it the with a collimation cap. Laser collimation (IMHO) is best used for "checking" and fine tuning your collimation after you understand how to use  the collimation cap method - otherwise a laser just introduces another tool and factors that can make it difficult for a newbie to understand what is going on.


Edited by JohnBear, 03 July 2020 - 11:42 AM.

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#17 sixela

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Posted 03 July 2020 - 10:43 AM

The secondary in the original post is quite obviously misrotated, and you compensated that with tilt (which gives you good axial collimation, but the secondary will quite brutally clip the light bundles).

 

One word of advice that I haven't seen yet: look at the scope from the front; severe rotational errors are going to be quite obvious. The major axis of the secondary (the line from the point of the secondary closest to the tube opening to the one closest to the primary) should, from the front, be pointing at the focuser.

 

If you have severely skewed things then rotating the secondary to make it appear round from the focuser opening and *then* setting tilt to centre the primary's centre spot may actually yield another rotational error, but if you iterate a few times it will converge very rapidly (i.e. if the secondary itself looks like a skewed ellipse it'll be less bad the second time around, and usually close to OK the next time around).

 

In short: look at the secondary from the front of the scope and through the focuser (it helps putting a coloured piece of paper on the opposite tube/UTA wall to avoid confusing the edge of the secondary and the edge of the primary's image in it). It's only when you use a laser collimator and don't look at these different vantage points that you're likely to introduce large rotational errors.


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#18 SteveG

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Posted 03 July 2020 - 02:15 PM

Op here.

 

Thanks for all of the suggestions. I will keep reading them over to take it all in. Something tells me I should have kept the Celestron Cheshire. The collimation was so bad that I thought it would be easier to use the laser version.

 

I made a collimation cap earlier in the day and after reading the replies here I gave it another try.

 

Again, many thanks.

Can you take a picture through the collimating cap and post it here?



#19 diex

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Posted 04 July 2020 - 05:57 PM

Many thanks again for all of the replies. This is hands down the best forum on the internet.

 

I have developed a blister on my thumb from the rotation screws. I am quite certain I am closer than ever to  good collimation. To achieve this I have only used the collimation cap and viewing through the focuser.

 

  • I placed a white sheet of paper in the tube to cover the primary reflection.
  • I placed a colored piece of construction paper behind the focuser.
  • This was a breakthrough. I finally saw the focuser stalk and the secondary mirror clearly. With the rotation and center screws loose I adjusted the secondary to get a round view of the mirror. Unfortunately there is some subjectivity as to how round it actually is but it's the best I can do for this round.
  • I removed the papers and adjusted the tilt screws to get the secondary centered on to the primary. Another breakthrough, from the Astro Baby website, "IGNORE ALL OTHER VISUAL INFORMATION !  All you are trying to do at this stage is get the secondary mirror centered on the primary mirror." This was the most difficult part, trying to get the primary mirror clips at equal distance to the edge of the secondary. I got the blister and called it a night. It centered up today. I fear all of this movement may have changed the secondary alignment by a bit.
  • At this point I am instructed to use a Cheshire which I don't currently have. I have the laser collimator but am reluctant to use it.
  • Using the collimation cap and viewing down the open focuser, I adjusted the primary mirror to bring the primary center spot to the center of the secondary/spider vanes (target).

I know these images are approximations but they may have some value.

 

View through collimation cap.

colcap
 
View through open focuser.
focview

 

I must be close now. I certainly have more experience with the process. I am going to reorder a Cheshire which should be helpful now that I'm a bit more knowledgeable.

 

As far as the laser collimator goes, I have put all of this effort in and to my eyes everything  looks about right. I am hesitant to have the laser ruin it all. I am most certainly going to use it but I want to test what I have now.

Currently,  the laser dot when viewed through the top of the ota is maybe an inch of center. I don't  see anything in the collimator target area for the primary, it may be perfect in the center. I could really benefit from a Cheshire eyepiece.

 

One last question, what's the best way to clean the secondary?? It's a mess right with spots of bluish glows. I have tried an eyeglasses cloth and small amount of eyeglass cleaner.

 

Thanks

 

 

 


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#20 JohnBear

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Posted 04 July 2020 - 08:57 PM

For what you are doing right now the Cheshire and collimation cap won't make much difference. The picture isn't very good, but from what I can see your "placement" of the secondary is 'close' (probably "good enough "at this point), BUT you still have some (i.e., a bit too much) rotation error and tilt error.

 

See my approximation of a "Vic diagram" below.  Do the circles look concentric and evenly spaced to you? 

 

Collim-3.gif

 

I would consider the secondary "placement" OK for now.  It is reasonably centered under the focuser

What I see is:

1. the secondary should first be "rotated" a bit in the direction of the red arrow to center the mirror surface "laterally". Do this by just "very slightly" loosening the center bolt on the secondary so that you can rotate the mirror to move it in the direction of the red arrow. Then re-tighten the center bolt and recheck it via the collimation cap. If you do this carefully and right nothing else should have changed and it should now appear more concentric.

 

2.  Then (AFTER correcting the rotation error error), slightly adjust the "bottom-most" of the 3 secondary adjustment screws to see how tightening or loosening it shifts the mirror image to move it in the (tilt) direction of the green arrow and get the viewed mirror image more centered. 

 

Important Note : Adjusting the the 3 secondary screws involves a "dance of the screws" where loosening one screw more that "very little" will require tightening one or both of the other two screws to keep the proper tension that holds the secondary in place while the .   

 

Once the above is done and the 3 circles are concentric and evenly spaced, you can start working on "centering the dots" with the primary adjustments.

 

I hope this will be helpful to you.   

 

BTW a colored paper placed below the secondary and opposite the focuser will make the view thru the collimation cap much easier to "read" and understand. Black on black is usually hard to distinguish.


Edited by JohnBear, 04 July 2020 - 09:06 PM.

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#21 diex

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Posted 04 July 2020 - 10:02 PM

For what you are doing right now the Cheshire and collimation cap won't make much difference. The picture isn't very good, but from what I can see your "placement" of the secondary is 'close' (probably "good enough "at this point), BUT you still have some (i.e., a bit too much) rotation error and tilt error.

 

See my approximation of a "Vic diagram" below.  Do the circles look concentric and evenly spaced to you? 

 

attachicon.gifCollim-3.gif

 

I would consider the secondary "placement" OK for now.  It is reasonably centered under the focuser

What I see is:

1. the secondary should first be "rotated" a bit in the direction of the red arrow to center the mirror surface "laterally". Do this by just "very slightly" loosening the center bolt on the secondary so that you can rotate the mirror to move it in the direction of the red arrow. Then re-tighten the center bolt and recheck it via the collimation cap. If you do this carefully and right nothing else should have changed and it should now appear more concentric.

 

2.  Then (AFTER correcting the rotation error error), slightly adjust the "bottom-most" of the 3 secondary adjustment screws to see how tightening or loosening it shifts the mirror image to move it in the (tilt) direction of the green arrow and get the viewed mirror image more centered. 

 

Important Note : Adjusting the the 3 secondary screws involves a "dance of the screws" where loosening one screw more that "very little" will require tightening one or both of the other two screws to keep the proper tension that holds the secondary in place while the .   

 

Once the above is done and the 3 circles are concentric and evenly spaced, you can start working on "centering the dots" with the primary adjustments.

 

I hope this will be helpful to you.   

 

BTW a colored paper placed below the secondary and opposite the focuser will make the view thru the collimation cap much easier to "read" and understand. Black on black is usually hard to distinguish.

JohnBear,

 

Your post is incredibly helpful. The picture really illustrates the problem. Now looking at my photo I can see the alignment issues. One question, when I carefully loosen the center screw, I am to rotate the mirror by slightly moving the stalk, correct? Looking through the focuser it would be a movement towards the 'left' thus a lateral movement.

 

I can't overstate how helpful that picture is.

 

Thanks.



#22 JohnBear

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Posted 04 July 2020 - 10:44 PM

Vic Menard deserves the thanks. He is a major guru on collimation and this type of diagram is his contribution to all of us. He literally wrote the book on collimation for most of us - you should consider getting it: Perspectives On Collimation by Menard & D'Auria .

 

 

One question, when I carefully loosen the center screw, I am to rotate the mirror by slightly moving the stalk, correct?

That is correct if you mean rotating it around the axis of the central bolt. If the the other 3 adjustment screws stay "snug enough" (and do not bind) you should be able to rotate it without disturbing the left-right/and up-down mirror tilt much, if at all. 

 

Unfortunately some people over-tighten the 3 adjustment screws and they dig into the secondary bearing surface a bit - which makes small delicate rotations very difficult. There is a "milk jug washer" solution for this, but then you have to take the secondary apart and start all over again. You can look that up in the forums or via google. But you are so close now , I'd recommend just trying to get the remaining tweaks done right - then go out and have some fun with the telescope.

 

Have you tried to contact.your local astronomy club as I mentioned earlier?  


Edited by JohnBear, 04 July 2020 - 10:45 PM.

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#23 pjmulka

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Posted 04 July 2020 - 11:04 PM

When I’m way out of alignment I’ll de-focus a star and then work to get the secondary shadow in the middle. Fine collimation is easier for me when I’m already close.

#24 JohnBear

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Posted 04 July 2020 - 11:26 PM

One additional note to the OP:  Your collimation cap picture down the focuser tube does not show the edges of the primary mirror (usually defined by the seeing mirror clips), so adjust you focus tube in/out to just frame the entire primary mirror reflection in the secondary. In the above discussion I have assumed that the whole primary would be visible if you adjusted the focuser. 

 

This ensures that the secondary mirror is pointed right at the approximate center of the primary, AND that all light reflected from the Primary mirror is fully reflected by the secondary up the focuser tube to your EP.   Being able to see the whole primary mirror outline reflected in the secondary is essential for good collimation alignment.  

 

Best!


Edited by JohnBear, 04 July 2020 - 11:28 PM.

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#25 Asbytec

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Posted 04 July 2020 - 11:47 PM

JohnBear,

 

I can't overstate how helpful that picture is.

 

Thanks.

 

Yes, once you rotate the secondary so the red circle moves in the direction of the red arrow. Then the primary reflection (and the center donut) will be centered on the long axis of the secondary. A little tilt using one of the three screws will tilt the secondary so the primary reflection moves toward the green arrow. As John describes the "dance of the screws", tighten one and loosen the other two slightly, or visa versa, to "walk" the secondary tilt into place. At this point, your laser should be inside the center donut with the red circle (primary reflection) concentric with the blue (focuser draw tube). The focuser axis will be collimated. The green circle (secondary) is already concentric, so no need to move the secondary position. Just a little tilt and rotation. 

 

With some adjustment of the red circle using secondary tilt and rotation, as John described, all three collimation signatures will become concentric with each other. Your laser should be inside the primary center donut. It does seem each time you post a picture of your collimation, we (or just me) presume you did so with the laser dot in the primary center donut. Yet, each time your red circle is off. This likely means your laser is not true. If so, you may need to check the accuracy of your laser as stated in the first post in this thread. Either rotate it in the focuser to see if the dot remains pretty much stationary or use a V block shining the laser on a wall and rotate it. I prefer the V block to avoid any focuser slop (registration error or sloppy fit in the focuser). The laser dot should remain pretty much stationary as it is rotated.

 

If the laser is true, the red circle (primary reflection) should be "automatically" centered in the blue circle (focuser draw tube). The reason is, the laser defines the focuser mechanical axis and, of course, the donut defines the primary center. When the focuser axis points at the primary center, visually the centers are coincident and so are their respective edges (red and blue colored circles).

 

<Also notice the reflection of your focuser is not as skewed as it was in your earlier pictures. Looking much better. I am confident your offset will also look good as seen in the reflection of the secondary, too.> 


Edited by Asbytec, 04 July 2020 - 11:54 PM.

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