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Mak-Newt Collimation confusion

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#1 StargazerMax94

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Posted 20 June 2019 - 11:57 AM

I have a Comet Hunter 152mm Mak-Newt. Visually collimating with a Cheshire style eyepiece versus the star images I get are quite different and I’m not sure why. Can you guys point me in the direction on how to resolve and get this scope in tune for perfect collimation? 

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#2 spencerj

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Posted 20 June 2019 - 12:08 PM

Was the focuser in the exact same position for both pics?  Your focuser is probably not perfectly aligned with the optical axis so as you move through the focus range, it looks like collimation is changing.  Are there leveling screws on the focuser?  



#3 StargazerMax94

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Posted 20 June 2019 - 01:05 PM

Well the one photo is of the star is which is flipped for a Newtonian. But that also being said that the star image was inside focus as opposed to outside of focus. I’ve never tweaked the focuser. 



#4 spencerj

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Posted 20 June 2019 - 01:26 PM

You could insert the Cheshire and then move through the whole range of focus and see what you think.  Collimation should not appear to change.  If your focuser is not moving along the optical axis, you can still achieve good conditional alignment of the primary mirror with a Cheshire for that spot in the focuser travel (like you Cheshire pic), but when you move the focuser the primary mirror no longer looks right (like your star image).  To correct this, you can square the focuser to the OTA and then realign the secondary and primary mirrors.



#5 ImNewHere

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Posted 20 June 2019 - 01:34 PM

I use a laser to do mine then fine tune with a Cheshire 



#6 StargazerMax94

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Posted 20 June 2019 - 01:37 PM

You could insert the Cheshire and then move through the whole range of focus and see what you think.  Collimation should not appear to change.  If your focuser is not moving along the optical axis, you can still achieve good conditional alignment of the primary mirror with a Cheshire for that spot in the focuser travel (like you Cheshire pic), but when you move the focuser the primary mirror no longer looks right (like your star image).  To correct this, you can square the focuser to the OTA and then realign the secondary and primary mirrors.

When I use the Cheshire and move the focuser up and down on the travel, the Collimation doesn’t change from the top spot to bottom spot 


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#7 spencerj

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Posted 20 June 2019 - 02:02 PM

When I use the Cheshire and move the focuser up and down on the travel, the Collimation doesn’t change from the top spot to bottom spot 

That is good.  Is the OTA at a similar orientation when comparing both pics?  What do you use for a 2/1.25" eyepiece adapter?  Do you use it for both the Cheshire and the camera?  Could it be tilting either the Cheshire or camera?

 

And just to cover all the bases, did you check the primary center spot to be sure it is actually in the center of the mirror?  Daytime collimation with tools is very convenient, but you do need accurate tools with no "play in the focuser" and an accurate center spot.  Or you can do the best you can and then do a final touch-up of the primary on an actual star.



#8 photoracer18

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Posted 20 June 2019 - 02:49 PM

Well the first picture is definitely out of alignment. You should not be able to see the side of the secondary holder with the secondary centered. Secondary and primary are both out of alignment but to the opposite degree. That usually comes from trying to collimate in the wrong order (both are tilted in opposite directions so it looks sort of collimated but isn't). Its been awhile since I used to do Mak-Newts at Hands On Optics when we were Intes/Intes Micro dealers even though I have an MN56 in my collection I bought cheap last year because it has a loose secondary. I can fix that and align it I just have not done it because I don't have a mount so I can star test it (all mine are too tall or packed in storage).

 

As for yours do you have a well collimated laser? Or a cross-hair sight tube? You need to get the secondary in the proper relationship both rotationally and axially to the focuser and the primary. Since what you previously said I assume the focuser is not an issue. Was the secondary ever loose enough to rotate any? I assume not (unlike my scope). This is not really much different than a stock Newt, really. You get the secondary aligned so that a laser dot hits the center of the primary when looking down the main tube thru the corrector. I like to start out with the alignment screws on the secondary all about the same length (your design may vary) uncollimated unless this is just a tweek. You may need to move the secondary in or out in the tube if its not centering top to bottom. Then adjust the alignment screws to center the laser dot in the center of the primary. Then align the primary so the dot reflection goes back up into the center of the laser. I like to call this rough alignment because at least the relationships are correct. If when you look thru the Cheshire the relationships are all correct (all gaps are even and you don't see something you should not, like the side of the secondary mount) its ready for a star test. Your star test not only looks off it also looks like it has not cooled down enough as yet.

 

I am doing this from memory while at work right now.


Edited by photoracer18, 20 June 2019 - 02:52 PM.


#9 StargazerMax94

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Posted 20 June 2019 - 03:08 PM

So, this is the kit I use for collimation. I’ve never used the laser pointer part, just the eyepiece cap with the pinhole for collimating. The way I showed you is the way I used to collimate my 14” dobsonian and it seemed fine but maybe the Mak-Newt is a bit more different and challenging. 

 

When I put the laser pointer in the focuser, the beam hits just outside the donut on the secondary. So you’re suggesting to adjust it to the center of the donut, and then how do you determine how much the primary needs adjusted from there??

 

 

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#10 David_Ritter

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Posted 20 June 2019 - 03:27 PM

Yes, I think you are on the right track.

 

With the laser in the focuser, you first need to adjust the tilt screws on the secondary so the laser beam hits the very center of the spot on the primary.

 

Then once that's done, you can adjust the tilt screws on the primary so the reflected beam bounces back up the tube and hits the center of the target on the angled face of the laser collimator.

 

This is assuming your laser collimator has an angled face with a target printed on it. I use a Hotech and it (and other similar laser collimators) have an angled face with a target printed on it. This is to provide a target that the returning beam is supposed to hit. But the photo of yours does not show an angled target face so I am not sure if this is applicable in your case.

 

If it does have the target, when you insert the laser in the focuser, you need to ensure the target is facing towards the back (the primary) and it is sitting pretty much exactly parallel  the optical axis of the tube. That way the returning beam can properly bounce off the secondary and onto the target where you can see exactly where the dot lands.



#11 Vic Menard

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Posted 20 June 2019 - 05:29 PM

At first glance*, the focuser axis (secondary mirror tilt) is obviously out of alignment (blue cross hairs should be aligned to the primary mirror center marker). Secondary mirror placement (green circle) is also incorrect (which is important for a Mak-Newt**). The primary mirror alignment looks good (primary mirror center marker is centered inside the bright Cheshire ring), but the primary mirror center marker may not be precisely centered (compare red cross hairs, which are aligned to the primary mirror center marker, to the red circle, which should be the outer edge of the primary mirror). To be fair, the bright background which looks like the face of the primary mirror is probably the boundary of the meniscus, so the primary center marker may be centered better than it appears.

 

*The blue circle and cross hairs are the reference for the other circles.

 

**If the offset is incorrect, the axial alignment will not be centered relative to the meniscus corrector, which is critical for Mak-Newt performance.

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Edited by Vic Menard, 20 June 2019 - 05:49 PM.


#12 Vic Menard

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Posted 20 June 2019 - 05:44 PM

...With the laser in the focuser, you first need to adjust the tilt screws on the secondary so the laser beam hits the very center of the spot on the primary. Then once that's done, you can adjust the tilt screws on the primary so the reflected beam bounces back up the tube and hits the center of the target on the angled face of the laser collimator.

 

This is assuming your laser collimator has an angled face with a target printed on it. I use a Hotech and it (and other similar laser collimators) have an angled face with a target printed on it. This is to provide a target that the returning beam is supposed to hit. But the photo of yours does not show an angled target face so I am not sure if this is applicable in your case.

 

If it does have the target, when you insert the laser in the focuser, you need to ensure the target is facing towards the back (the primary) and it is sitting pretty much exactly parallel  the optical axis of the tube. That way the returning beam can properly bounce off the secondary and onto the target where you can see exactly where the dot lands.

Your alignment procedure works with any simple thin beam laser (the Hotech holographic cross improves the alignment read). If you don't have the windowed laser, you can see the reflection of the alignment in the primary mirror.

 

The target doesn't have to sit parallel to the focuser axis, but it should be centered relative to the focuser axis. It's also important to know that when you use the return beam for the primary mirror alignment, the outgoing beam alignment (where the beam meets the primary mirror center marker) should be as perfect as possible. one half of any residual error (outgoing beam) is added to the final primary mirror alignment read. This is why a collimation cap (or other Cheshire derivative) is an important tool for primary mirror alignment. Cheshire derivatives (including the Barlowed laser) are relatively insensitive to residual focuser axial errors, and the final Cheshire read magnifies any residual primary mirror alignment error 2X.

 

For a common Newtonian, small errors in secondary mirror placement (residual rotation and offset errors) have little or no significant impact on image performance. But the OP has a Mak-Newt--and the meniscus lens needs to be centered (and perpendicular) to the primary mirror axis--so the secondary mirror placement is an important part of the optical alignment.



#13 StargazerMax94

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Posted 21 June 2019 - 03:47 AM

I tweaked the Collimation tonight. How does this look? I know it’s not PERFECT but compared to before, how is this and am I getting closer? 

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#14 RaulTheRat

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Posted 21 June 2019 - 05:31 AM

Looking good. There's a really big thread about mak-newt collimation at https://stargazerslo...onlite-focuser/ which contains some useful information.

Your latest Cheshire photo looks good, so imo don't touch it for the time being until you can star test, but when you do check by moving the defocused donut around all the corners of the field - if you have different donut images in different parts of the field then it'll be because you've got the meniscus and primary out of parallel - it's hard to tell using conventional daytime collimation techniques if that's the case, but that thread has quite a useful technique (among others) using 3 strings to square the primary to the tube axis.
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