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Primary mirror adjustment - collimation

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#1 coenie777

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Posted 25 October 2018 - 02:22 PM

You normally accomplish the attached by adjusting the primary mirror.

 

Without restricting yourself to "normal" collimation procedures, which other adjustments can you make besides touching the primary mirror in order to shift that secondary mirror reflection on the primary mirror? I am thinking extreme things like rather tilting the focuser or moving the focuser in any other way, or increase/decrease the secondary mirror's position on the stalk etc.

 

But you still want to keep the primary mirror dot under the crosshairs while doing so.

 

I would appreciate any views.

 

 

PMadjustment_zpshpw3oxdx.gif

 

 



#2 Starman1

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Posted 25 October 2018 - 02:33 PM

You are confusing the reflection of the collimation tool with the outline of the shadow of the secondary.

The tool will not appear centered in the secondary shadow on scopes <f/10.

In fact, this image shows a collimated f/4:

 

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#3 Starman1

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Posted 25 October 2018 - 02:34 PM

And this image a collimated f/5:

 

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#4 Starman1

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Posted 25 October 2018 - 02:39 PM

This points out what's going on:

https://www.cloudyni...ment/?p=3040728



#5 Starman1

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Posted 25 October 2018 - 02:42 PM

There will always be offset in the secondary shadow in the primary mirror reflection with short f/ratio scopes.



#6 TOMDEY

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Posted 25 October 2018 - 03:38 PM

Yes, the nominal signature looks offset as the guys are showing there.

 

But, to your generic question... it IS indeed possible to build a scope that has a non-adjustable PM... No decentering, tip-tilt or piston... none at all! In that case, the folding flat and focuser would "chase" the PM. That's actually been done... successfully!

 

Even more extreme... You can build a scope that has no adjustment of the PM or folding flat (?!) Then, just moving and tilting the focuser will get the PM focal point on axis and square, nominal draw. Those are all 5 degrees of freedom needed to form a good image/wavefront/Strehl. [doesn't address vignetting, which comprises two "soft" degrees of freedom.]

 

So, when designing a scope, Especially complex, multi-element one... the mechanical and optical designers have a Lot of freedom on how to execute it. Count up the degrees of freedom needed, rank them (critical, hard, medium, soft), and then decide where to put them as adjustments. That involves things like access, finesse, stability. Then come up with an alignment sequence that will converge most rapidly.

 

The way Newtonians are done, by us here... is really very good, intuitive, convergent, stable, accessible.  Tom


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#7 coenie777

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Posted 25 October 2018 - 04:16 PM

Hi Don

 

And this image a collimated f/5:

Don, I know there will be the offset. In the animation, and during collimation, you end up most of the times with the PM dot under the crosshair after adjusting the SM screws, but the PM dot is not aligned with the sight tube's annulus. You then tilt the PM to achieve this. The animation was not done with the same collimation session so there is some variables. The point I wanted to make is your final adjustment involves tilting the PM. The question is then how do I achieve the final collimated view without adjusting the PM? Forget about how good or bad the animation appears collimated as I just took two random images. I am trying to just illustrate the movement that I am looking for without tilting the PM.

 

 

Yes, the nominal signature looks offset as the guys are showing there.

 

But, to your generic question... it IS indeed possible to build a scope that has a non-adjustable PM... No decentering, tip-tilt or piston... none at all! In that case, the folding flat and focuser would "chase" the PM. That's actually been done... successfully!

 

Even more extreme... You can build a scope that has no adjustment of the PM or folding flat (?!) Then, just moving and tilting the focuser will get the PM focal point on axis and square, nominal draw. Those are all 5 degrees of freedom needed to form a good image/wavefront/Strehl. [doesn't address vignetting, which comprises two "soft" degrees of freedom.]

 

So, when designing a scope, Especially complex, multi-element one... the mechanical and optical designers have a Lot of freedom on how to execute it. Count up the degrees of freedom needed, rank them (critical, hard, medium, soft), and then decide where to put them as adjustments. That involves things like access, finesse, stability. Then come up with an alignment sequence that will converge most rapidly.

 

The way Newtonians are done, by us here... is really very good, intuitive, convergent, stable, accessible.  Tom

Tom, are you saying that by moving the focuser and SM you can replicate the effect of a tilting PM? If so, how would you do that? Has the SM placement got a lot to do with it?



#8 Starman1

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Posted 25 October 2018 - 06:03 PM

Coenie,

Why would you collimate a newtonian and not adjust the primary mirror, given that the collimation of that mirror is the most critical collimation?

 

As to your first point, if the crosshairs of a sight tube are centered on the primary center marker, the primary center marker IS perfectly concentric with the sight tube's outer edge.

Unless the crosshairs are off-center, that simply has to be.

 

Now, if the primary mirror is fixed and cannot be adjusted (hopefully the optical axis is coincident with the center of the tube), then the secondary must be mounted with a mechanical offset on whatever holds it so that

it is NOT centered on the spider vanes that hold it.  This illustration points out that the offset is at a 45° angle--away from the focuser and down the tube.  It will appear centered under the focuser but the secondary shadow will have the same offset as in my illustrations above:

The secondary is moved up and down in the tube to center it under the focuser.  If that's not adjustable, then the focuser has to be angled until the secondary is centered.

If that's not adjustable, the scope cannot be collimated.

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Edited by Starman1, 25 October 2018 - 06:04 PM.


#9 sixela

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Posted 25 October 2018 - 06:51 PM

Why would you collimate a newtonian and not adjust the primary mirror,

 

 

tl;dr: in a normal Newt your reference is the focuser axis and you collimate every element from the focuser inward to it, but you could also take the fixed optical axis of the primary as a reference and work _the other way around_, i.e. "catch" the optical axis be moving the secondary, then tilt it to point it to the focuser, then tilt the focuser to make its axis parallel to the reflected optical axis.

 

That's sort of what you have to do on a small Skyscanner, since it doesn't have a moveable primary. What's critical is not "to adjust the primary mirror", it is to make the focuser axis and optical axis cross at the focal plane. _Normally_ we do that by tilting the primary, since we also want a focal plane perpendicular to the focuser axis and we've set the tilt of the secondary for doing that (which prevents us from using _that_ in subsequent steps).

 

But if the primary mirror is fixed, then  you have to use a Cheshire and try to make things work using the tilt of the secondary, and live with the focal plane tilt that you end up with (which is doable for visual use, since the eye can focus at distances other than infinity and can accommodate for a slightly tilted focal plane.)

 

Both moving the secondary away/from the focuser and to/from the primary will also translate the optical axis seen at the focuser along one dimension, and rotating the secondary will move it in the other direction (but also change its angle with the focuser axis).

 

Since usually you want a centred fully illuminated field and you have too many choices, just as when you have no sight tube, on scopes like that you first do 'axial collimation' using tilt of the secondary --which in this case is a misnomer, since the axes will cross at the focal plane but will not be guaranteed to be parallel-- and then you evaluate whether the secondary is concentric with the primary's reflection and appears round. If not, rotate it and you move it towards where the primary's reflection appeared and start again.

 

In theory you could play with the length of the spider vanes and the centring of the secondary in the tube to get good axial collimation (the optical axis crosses at the focal plane _and_ the focal plane is perpendiculair to the focuser axis) but then you lose control over vignetting by the front of the tube.

 

If you can also move the tube opening...well, moving the tube opening (with the secondary attached to it) with respect to the primary is exactly the same thing as tilting the primary, so you could chase the primary with the tube rather than chase the tube (and the secondary centred in it) using the primary. On a truss scope, instead of tilting the primary, you could in theory work on the length of the trusses to collimate!


Edited by sixela, 25 October 2018 - 07:08 PM.


#10 TOMDEY

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Posted 25 October 2018 - 06:55 PM

Tom, are you saying that by moving the focuser and SM you can replicate the effect of a tilting PM? If so, how would you do that? Has the SM placement got a lot to do with it?

Yep, we build/align a lot of satellite optical trains like that: Generically, we are allowed one, Exactly ONE group that has no alignment adjustments, whatsoever. Every other group "chases" that reference coordinate system. And, because the PM is most often the Biggest, Meanest, Massivest, Delicatest one in the lot... we may choose to reference all else to it... moving and tilting them around... to "chase" the PM!

 

Most high-performance satellite imagers comprise the 3-element, all-reflective, off-axis, flat-field Anastigmat. Huge aperture, giant field, diffraction-limited all fields, compact... really delivering it ALL! ... as the customers demand.

 

To your question: Yeah, you could e.g. translate the spider/SM laterally, to mimic tilting the PM. Other equivalents available; many ways to do it. I'm not Recommending it; just pointing out what the generalized challenge is and that there are myriad ways to skin a cat!

 

Anecdote: I fondly recall excluding one entire Test Set, that had been provided for, at extreme expense (time, cost, complexity, risk) as being entirely redundant, based on the above "one free lunch" philosophy. That launched one associate into a Tasmanian Devil Whirling Dervish TYRADE... much to my, and others' amusement. Ummm... We did it my way... and it Worked!   Tom


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#11 coenie777

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Posted 26 October 2018 - 02:04 PM

Thank you Don, Sixela and Tom.

 

 

Coenie,

Why would you collimate a newtonian and not adjust the primary mirror, given that the collimation of that mirror is the most critical collimation?

 

I know that I will be lambasted for admitting this but it is unfortunately true that I am still battling with the same scope for more than a year now. I want to keep the primary mirror stable because the telescope in question is a Maksutov Newtonian. As Sixela would be able to vouch for (from reading many of his past posts), the key here is keeping the primary axis aligned with the corrector axis.

 

Problem is I moved my secondary up and down the central bolt as well as the focuser so I lost the factory setting. Sky Watcher advised that they would need the scope on a bench in order to get it back into position.

 

After many iterations I now know how to center the primary with the corrector axis using only a sight tube.

 

I also figured out that you adjust the focuser position up and down the tube (it can slide) when you do final adjustments. All of this is done to do final adjustments that normally would involve just tilting the primary. I can get the primary dot centered in the annulus by just sliding the focuser but can then see that it results in the corrector cell cutting off part of the primary (a matter I went into detail before on this forum, prior to figuring out the alignment of the two axis matter).

 

 


 

If you can also move the tube opening...well, moving the tube opening (with the secondary attached to it) with respect to the primary is exactly the same thing as tilting the primary, so you could chase the primary with the tube rather than chase the tube (and the secondary centred in it) using the primary. On a truss scope, instead of tilting the primary, you could in theory work on the length of the trusses to collimate!

Would something similar, albeit on a much smaller scale from an impact perspective, be moving an offset secondary laterally on its central bolt? If an offset secondary is moved up and down the tube, does it change the relative intercept of the primary axis in such a way that it could move the primary mirror dot as required? I cannot move the front of the scope without misalignment the primary and the corrector but maybe you are on to something involving what happens to the secondary when you would move the front cage of a truss design?

 

 


 

To your question: Yeah, you could e.g. translate the spider/SM laterally, to mimic tilting the PM. Other equivalents available; many ways to do it. I'm not Recommending it; just pointing out what the generalized challenge is and that there are myriad ways to skin a cat!

 

 

To Tom's point above then as well (if I understand what you are saying correctly), would moving an offset secondary mirror up and down the tube be the same as "translate the SM laterally"?

 

Since this scope has an adjustable focuser there is a dedicated factory set position for the secondary away from the corrector and I suspect that when one hit that sweet spot, the planets align (SM dot, PM dot, Sight tube crosshair while maintaining PM and corrector optical axis).

 

Something that I will try tomorrow is set up the scope until that PM adjustment is required and then measure how far off the PM dot is still from the annulus. I will then try to drop or retract the SM on the corrector by that much and see what the effect is when I then set up the scope again and get to the point where the PM adjustment was required.

 

I do apologize for frequenting this forum so often with the same issue. A lot of the progress so far was figuring out the uniqueness of collimitating a MN scope. The other owners who did not move their SM from its factory set position is able to easily collimate and re-collimate. I also have very infrequent time to attend to the matter and you loose a lot of the insights gains made between adjustments. I hope that I am starting to pin point the solution down to just one or two last issues to resolve so I can again start imaging! I also do not want to spend money on a holographic laser as I believe you can reach the required settings with a sight tube if you know what you need to move when.

 

I appreciate your patients and assistance all!


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#12 Oberon

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Posted 26 October 2018 - 03:30 PM

Tom, are you saying that by moving the focuser and SM you can replicate the effect of a tilting PM? If so, how would you do that? Has the SM placement got a lot to do with it?

It is done very easily with adjustable trusses, creating a Stewart Platform or hexapod. Here is an example, this thread details construction of the scope, this ATM thread further develops practical ideas and tips for amatuer construction. From every point of view - structural engineering, rigid mirror support, simplicity of use, simplicity of build, ease of collimation - thise who have tried it consider the method superior.



#13 Oberon

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Posted 26 October 2018 - 03:35 PM

So far as a Mak is concerned I wouldn’t hesitate to rebuild a Mak to exploit the Stewart Platform, caveat being that the solid tube would be redundant and would need to be discarded.



#14 TOMDEY

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Posted 26 October 2018 - 08:51 PM

It is done very easily with adjustable trusses, creating a Stewart Platform or hexapod. Here is an example, this thread details construction of the scope, this ATM thread further develops practical ideas and tips for amatuer construction. From every point of view - structural engineering, rigid mirror support, simplicity of use, simplicity of build, ease of collimation - thise who have tried it consider the method superior.

Just so happens... here from my archives... >>> Tom

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#15 coenie777

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Posted 27 October 2018 - 01:15 PM

So far as a Mak is concerned I wouldn’t hesitate to rebuild a Mak to exploit the Stewart Platform, caveat being that the solid tube would be redundant and would need to be discarded.

I would prefer to not even start to think of something like this considering my technical skills. That "Merope" scope you build is an absolute work of art. Congratulations on that!

 

Looking at some of your technical drawings you posted in the build thread, any ideas on a CAD type program one can use to set up a two mirror reflection model through which you can simulate various adjustments? I have used AC3D so far but this cannot simulate reflections. I would much rather try these possible iterations via software than to keep putting my scope through all the adjustments.



#16 Jason D

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Posted 27 October 2018 - 02:49 PM

Can a reflector be collimated without primary mirror adjustments?

 

Yes. Basically, the burden of collimation will shift solely on the secondary mirror -- as if adjusting the secondary mirror is not confusing enough without the extra burden wink.png

The secondary mirror will have to chase the optical axis of the primary mirror. See attachment:

 

Figure A: Fixed primary mirror with axis skewed towards the focuser.

Figure B: Fixed primary mirror with axis running along the OTA axis.

Figure C: Fixed primary mirror with axis skewed away from the focuser.

In all the above, axial alignment can be achieved PERFECTLY!!!!

 

HOWEVER, to be able to accommodate axial alignment in all of the above three scenarios, the secondary mirror adjustments  will need to have a generous freedom of movement in all directions as shown in figure D. We know that typical scopes can't accommodate such a generous freedom of movement; therefore, typically we will end up with figure E when the fixed primary mirror optical axis is significantly off-center and we have a secondary mirror with limited movement. We will end up with a tilted focal plane at the eyepiece.

 

Jason

 

Fixed_PM.png


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#17 sixela

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Posted 27 October 2018 - 04:11 PM

For a Mak-Newt the visual presentation for most "design" collimations is the same as on a Newt.

The problem is that any rotational error or offset away from the focuser that isn't the one expected will result in  a reflected focuser axis that isn't square to the corrector.

Most of the time the hub tilt screws are indexed in a way that identifies the major axis of the secondary (which allows you to avoid rotational errors) and the offset away from the focuser is correct if the hub is centred in the corrector.

But of course if Skywatcher glued the secondary slightly wrongly or decentred the corrector and/or one/both mirrors with respect to the tube and _then_ fixed it by collimating things differently, then you're between a rock and a hard place.

 

If there _are_ primary collimation adjustment screws, I'd collimate it like a Newton (taking care about centring and rotation of the elements, and but with a lot more care about ensuring that the secondary is optimally placed and viewed as circular) and then null coma using the primary mirror's adjustments in a star test (or for photo applications use an app that evaluates focal plane tilt, because that's more important than nulling coma).

 

The problem is that there is no visual clue other than a star test that tells you how the corrector is oriented, as far as I know. On a Schmidt corrector the back is flat, which leads you to ClownFish's excellent collimation trick, but on a Mak-Newt the meniscus doesn't allow the same trick.


Edited by sixela, 27 October 2018 - 04:13 PM.


#18 TOMDEY

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Posted 27 October 2018 - 06:59 PM

Can a reflector be collimated without primary mirror adjustments?

 

Yes. Basically, the burden of collimation will shift solely on the secondary mirror -- as if adjusting the secondary mirror is not confusing enough without the extra burden wink.png

The secondary mirror will have to chase the optical axis of the primary mirror. See attachment:

 

Figure A: Fixed primary mirror with axis skewed towards the focuser.

Figure B: Fixed primary mirror with axis running along the OTA axis.

Figure C: Fixed primary mirror with axis skewed away from the focuser.

In all the above, axial alignment can be achieved PERFECTLY!!!!

 

HOWEVER, to be able to accommodate axial alignment in all of the above three scenarios, the secondary mirror adjustments  will need to have a generous freedom of movement in all directions as shown in figure D. We know that typical scopes can't accommodate such a generous freedom of movement; therefore, typically we will end up with figure E when the fixed primary mirror optical axis is significantly off-center and we have a secondary mirror with limited movement. We will end up with a tilted focal plane at the eyepiece.

 

Jason

 

attachicon.gif Fixed_PM.png

Yeah, that's what I was getting at. One free static group always permitted. Must think of the optics as independent of the rest or the structure.

 

Here's even Further Embellishment, though! >>>
If we are willing to provide Some added quality surface, beyond the designed Clear Aperture of some/most/all or the groups... THEN we get this luxury: >>>

 

>spherical mirror 3 critical degrees of freedom (e.g. Dahl SM)
>flat mirror 3 critical degrees of freedom (e.g. Newt Fold)
>axi-symmetric aspheres 5 critical degrees of freedom (e.g. Newt PM)
>entirely asymmetric groups 6 critical degrees of freedom

 

The luxury is that there are myriad morphed alignments that all perform perfectly. Flat folds are especially fungible for packaging/compacting optical systems. This can allow for More than one free static groups! Tom



#19 Oberon

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Posted 28 October 2018 - 05:17 AM

I would prefer to not even start to think of something like this considering my technical skills. That "Merope" scope you build is an absolute work of art. Congratulations on that!

 

Looking at some of your technical drawings you posted in the build thread, any ideas on a CAD type program one can use to set up a two mirror reflection model through which you can simulate various adjustments? I have used AC3D so far but this cannot simulate reflections. I would much rather try these possible iterations via software than to keep putting my scope through all the adjustments.

Thanks...but sorry no, I’m not the CAD expert here, I just use Adobe Illustrator which is aimed at technical graphics not engineering.



#20 coenie777

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Posted 28 October 2018 - 12:41 PM

But of course if Skywatcher glued the secondary slightly wrongly or decentred the corrector and/or one/both mirrors with respect to the tube and _then_ fixed it by collimating things differently, then you're between a rock and a hard place.

 

What equipment would Sky Watcher have used to correct a possible incorrectly glued secondary? So far I could not find an answer as to whether they would use anything other than a laser and a sight tube. Any thoughts?

 

 

The problem is that there is no visual clue other than a star test that tells you how the corrector is oriented, as far as I know. On a Schmidt corrector the back is flat, which leads you to ClownFish's excellent collimation trick, but on a Mak-Newt the meniscus doesn't allow the same trick.

I read somewhere that a star test for fine tuning collimation is not an accurate way to go about things on this scope as it contains a spherical mirror for which the corrector compensates already. Apparently due to this the doughnut seldom appear de-centered. I have seen this myself where I cannot recall once seeing a defocused star with doughnut not centered. Yet in none of these instances did I achieve the image I wanted. I followed and tried Clownfish's tutorial and it is true that De Sitter's "blue ghost" fine tuning is not easy to see on the curved corrector.



#21 coenie777

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Posted 28 October 2018 - 12:43 PM

Thank you Jason for the as always very clear visual explanations. I must have close to a lever arch file full of your useful advice build up.



#22 coenie777

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Posted 28 October 2018 - 01:01 PM

Just want to check something that was touched on in some of my previous posts:

 

If the secondary is oversized, would I still HAVE to aim for a rounded secondary under the focuser? The reason for this question is that on this scope, with the secondary completely rounded under the sight tube, I require a large primary mirror tilt to get the secondary reflection centered.

 

The 3mm offset dot on the secondary mirror can also not be centered under the crosshairs with the secondary mirror rounded.

 

I actually need to drop the focuser (move it towards the primary on its slides) so as to have the secondary mirror appear too high up (large gap towards primary, cut off towards the corrector), in order to get the secondary mirror dot under the crosshairs. Once this is done the primary reflection is towards the lower half of the secondary mirror face (towards the primary).

 

Here is a view of the SM perfectly rounded under the focuser (I hope). Note where the dot appears:

 

Centered%20and%20rounded_zpselt1ybcu.jpg

 

And here is a rough example of what I mean what happens to the primary reflection when you center the SM dot under the crosshairs:

 

Centered%20SM%20dot_zpscjbxqge2.jpg



#23 Starman1

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Posted 28 October 2018 - 04:12 PM

That's because the secondary dot is centered on the mirror and the optical axis does not and should not hit that spot, but a spot a few millimeters closer to the upper end of the secondary.

The first image in post 22 is correct with a centered perspective.

The 2nd image shows what happens when you tilt the secondary so the laser hits the small circle on the secondary.

And here's the thing--it doesn't matter whether the secondary is offset down AND away from the focuser or simply down and NOT away from the focuser, the result will be the same--

the optical axis will not hit the center of the secondary mirror, but a spot farther up the surface toward the top end of the secondary.

What gave it away is your comment that if you dropped the focuser down so the beam would hit the center mark on the secondary, it resulted in the reflection of the primary being offset downward

toward the lower end of the secondary mirror.

 

What that all means is that the mark on the secondary must and should be ignored in order to attain good collimation on this scope.

--center the secondary under the focuser

--adjust the tilt until the laser is in the center of the primary's center marker

--adjust the primary until the Hotspot is in the center of the Cheshire's dark center

--If you want to follow up and eliminate some tiny residual collimation errors, use an autocollimator on the secondary, alternating with the Cheshire on the primary until both agree at the same time.

 

Sorry the marker on the secondary is wrongly positioned.  I can imagine how frustrating that must have been.



#24 coenie777

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Posted 28 October 2018 - 04:50 PM

That's because the secondary dot is centered on the mirror and the optical axis does not and should not hit that spot, but a spot a few millimeters closer to the upper end of the secondary.

The first image in post 22 is correct with a centered perspective.

The 2nd image shows what happens when you tilt the secondary so the laser hits the small circle on the secondary.

And here's the thing--it doesn't matter whether the secondary is offset down AND away from the focuser or simply down and NOT away from the focuser, the result will be the same--

the optical axis will not hit the center of the secondary mirror, but a spot farther up the surface toward the top end of the secondary.

What gave it away is your comment that if you dropped the focuser down so the beam would hit the center mark on the secondary, it resulted in the reflection of the primary being offset downward

toward the lower end of the secondary mirror.

 

What that all means is that the mark on the secondary must and should be ignored in order to attain good collimation on this scope.

--center the secondary under the focuser

--adjust the tilt until the laser is in the center of the primary's center marker

--adjust the primary until the Hotspot is in the center of the Cheshire's dark center

--If you want to follow up and eliminate some tiny residual collimation errors, use an autocollimator on the secondary, alternating with the Cheshire on the primary until both agree at the same time.

 

Sorry the marker on the secondary is wrongly positioned.  I can imagine how frustrating that must have been.

Thanks Don, I am after 14 months ready to now accept that that dot is wrong. I will do as suggested next round. If the correct offset should be 5mm, do I then drop the secondary mirror 5 mm from the corrector or 3.54mm. I take it how far down the SM is dropped on its central bolt towards the primary is also important. I will then encircle it by moving the focuser and start from there. Holding thumbs for the weekend.



#25 Starman1

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Posted 28 October 2018 - 05:11 PM

Depends how the offset is calculated.

Some calculators calculate the offset MOVEMENT the secondary has to have to put it in the right place (meaning away from the focuser and down the tube),

but most offset calculators show the offset along the major axis of the ellipse ON the surface of the secondary.

As for figuring out the distance the secondary is moved toward the primary mirror, the process of centering the reflective surface of the secondary under the 

focuser using a sight tube is more than accurate enough.

As for the other dimension, away from the focuser, can I assume your scope automatically has that built into the secondary holder?




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