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Useful info about secondary mirror alignment

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#51 Jason D

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Posted 30 September 2009 - 02:19 PM

I have always had the issue of the secondary shadow being off to one side a little but the Center dot on the primary and the focuser tube reflection were dead center. In my chesire ep as well.

I also cut a template on the 4" diagonal and marked the dead center.
When attaching the diagonal, I used my laser collimator and positioned the diagonal so the laser hit my center mark on my template.


Dennis,

Your above setup ensures the optical (primary mirror) intercepts the geometric center of the secondary mirror. This is NOT an optimal alignment, why? Because if you cross section a code then the geometric center of the cross-section ellipse will not coincide with the cone axis, try it on a piece of paper.

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See attachment. What you have is similar to the left pic but what you need to strive for is what is shown in the right pic. Note how the secondary shadow (silhouette) is centered in the left pic but the secondary mirror itself is not centered under the focuser. Bottom line, you can’t center both the secondary mirror and its silhouette simultaneously. One has to be off-center and that should be the silhouette – not the secondary mirror itself.

Posted Image

One more data point: Squaring the focuser and centering the spider vanes is desirable to certain extent but doing so is NOT a requirement for accurate collimation.

Jason
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#52 wh48gs

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Posted 30 September 2009 - 02:57 PM

In another of your pics, the one with the overlays, it shows your primary mirror center spot just off center towards the OTA opening.
Is that the "OFFSET" everyone is always talking about?



No, offset is moving the diagonal from the position where its geometric center coincides with the point of intersection of the primary's and focuser's axis so that its *apparent center* comes to that point. It is accomplished by moving the diagonal towards primary and away from the focuser by the same specific increment; in effect, the diagonal slides along its surface plane away from the focuser.

It is benefitial to grasp the big picture of the whole setup; I tried to illustrate it on my site:

http://www.telescope...collimation.htm

Vla

#53 Jason D

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Posted 30 September 2009 - 04:13 PM

In another of your pics, the one with the overlays, it shows your primary mirror center spot just off center towards the OTA opening.
Is that the "OFFSET" everyone is always talking about?



No, offset is moving the diagonal from the position where its geometric center coincides with the point of intersection of the primary's and focuser's axis so that its *apparent center* comes to that point. It is accomplished by moving the diagonal towards primary and away from the focuser by the same specific increment; in effect, the diagonal slides along its surface plane away from the focuser.

It is benefitial to grasp the big picture of the whole setup; I tried to illustrate it on my site:

http://www.telescope...collimation.htm

Vla


You are describing only one type of secondary offset (classical offset). Dennis was referring to the observation where the primary center spot reflection was off-centered with respect to the secondary mirror shadow (silhouette) – that is indeed a typical outcome of a secondary offset.

Here is a reference to an excellent article written by Don Pensack click here

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Jason

#54 wh48gs

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Posted 30 September 2009 - 07:37 PM

You are describing only one type of secondary offset (classical offset). Dennis was referring to the observation where the primary center spot reflection was off-centered with respect to the secondary mirror shadow (silhouette) – that is indeed a typical outcome of a secondary offset.



No, I was simply answering the question by stating what the offset *is*. There is no "other types" of offset, simply because they do not do the same thing - they are not to be called the same. There is also nothing new about the arrangement you describe; if you look at the page I posted, it is described as one of the possibilities at which you arrive starting with geometrically centered mirrors/focuser (a sound way to start out) and taking the first step by centering the view of diagonal in the focuser by moving it axially toward primary. I'm sure many before me, you and Don were aware of such possibility.

By the way, it is not a secondary "shadow", nor "silhouette", it is an actual surface we are looking at; using these descriptives can be confusing.

If you start with nearly square mirror/focuser arrangement, and center the diagonal by moving it axially toward primary, the only way you can end up with the focuser axis orthogonal to the tube axis is to move it away from the primary in the process of collimation. That doesn't make it easier. The good side is that it eliminates image tilt that is otherwise unavoidable with the diagonal moved only axially. The tradeoff is that the disparity between primary's and tube/mechanical axes is even greater. We are talikng degrees. It induces significant tracking error, and can ruin go-to accuracy. Therefore, it is not for everyone.

There is no such problem with properly offset diagonal. There is no really other alternative to achieve visually near-centered elements when aligned and even field illumination, without tradeoff such as axial disparity. That makes it the best collimating alternative.

Vla

#55 Starman1

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Posted 30 September 2009 - 08:03 PM

Vlad,
The offset that requires movement of the secondary away from the focuser and down the tube by equal amounts (called Classical offset), which results in optical and mechanical axes coincident, is the best for digital setting circles, best for a 90 degree reflection angle at the secondary, and best for even illumination of the field from the entrance pupil (the tube opening).

But, it is not the simplest to achieve.
It requires attention to perpendicularity of the focuser to the tube, and accurate methods of measurement between secondary and tube walls on both sides.
It becomes even more difficult when, as is the case with my 12.5" f/5, the offset is literally 1/8". Measuring the offset to this degree of accuracy is a difficult proposition.

The new model method of collimation assumes the focuser axis is true and aligns to that. It definitely results in the optical axis tipping slightly toward the focuser, and it definitely can result in uneven illumination of the entrance pupil, and it can definitely result in inaccuracies in DSCs.

But, it is easier to achieve good collimation with this method because it is easier to measure to center, and because the focuser need never be adjusted. In fact, the same offset is achieved, but it is achieved with the primary optical axis tilted away from the mechanical axis.
What the error is in a DSC will be dependent on where around the circumference the focuser lies.

Here's the thing, though. I use a standard DSC and achieve excellent alignment anyway. The angular change cause by the miniscule tilt of the optical axis away from the mechanical center axis doesn't register in the DSC because its pointing accuracy is only +/- 5.4' anyway.
I've found, from experience, that not paying attention to leveling the scope base has a bigger impact on my pointing accuracy than the difference between Classical and New Model collimation. With my 12.5" f/5 scope, the angle difference is 8 arc minutes, not significant to my DSC accuracy.

Now I grant you that if my scope were f/4, the difference would be larger and so I probably shouldn't generalize about the superiority of the New Model collimation if a short f/ratio is used with DSCs.

There is definitely a place for classical offset collimation, but it really isn't critical for the smaller scopes.
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#56 Jason D

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Posted 30 September 2009 - 08:24 PM

There is also nothing new about the arrangement you describe


You are right. The two arrangements described including the titles came from Vic Menard.

By the way, it is not a secondary "shadow", nor "silhouette", it is an actual surface we are looking at; using these descriptives can be confusing.


These terms are commonly used around here. Again, I learned these terms from Vic Menard.

If you start with nearly square mirror/focuser arrangement, and center the diagonal by moving it axially toward primary,


Easy said – not too easy to do especially for beginners. It will either require the removal/reattachment of the secondary mirror – or readjustment of the spider vanes to move the secondary away from the focuser which goes against your recommendation to center the spider vanes.

The tradeoff is that the disparity between primary's and tube/mechanical axes is even greater. We are talikng degrees. It induces significant tracking error, and can ruin go-to accuracy. Therefore, it is not for everyone


Not all reflectors have go-to. Besides, this thread is about how to optimize for field illumination -- not about how to optimize for DSC and go-to

#57 wh48gs

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Posted 30 September 2009 - 09:00 PM

Don,

It was a few years ago - or little more - I was discussing this same collimation mode with someone on some forum - could have been CN - and he was talking the same thing about proper alignment being possible with any focuser position, and I was talking the same thing about axial disparity it can cause. Now you guys are comming with the "new" attribute. There's not too many new things under the Sun, and this isn't one of them.

I understand it simplifies the procedure if you assume the focuser is square. But that's kind of dicey, isn't it, if you need good axial parallelism? If possible significant axial disparity doesn't matter, that is just fine. But it should be mentioned, anyway.

Vla

#58 Jason D

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Posted 30 September 2009 - 09:36 PM

you guys are comming with the "new" attribute. There's not too many new things under the Sun, and this isn't one of them.

I am somewhat perplexed by your posts. No one is claiming anything “new”. This thread is not meant to introduce new concepts or terminology. Vic Menard who is well-respected and well-established expert in collimation published these concepts and terms long time ago.
Jason

#59 Starman1

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Posted 01 October 2009 - 12:40 AM

Don,

It was a few years ago - or little more - I was discussing this same collimation mode with someone on some forum - could have been CN - and he was talking the same thing about proper alignment being possible with any focuser position, and I was talking the same thing about axial disparity it can cause. Now you guys are coming with the "new" attribute. There's not too many new things under the Sun, and this isn't one of them.

I understand it simplifies the procedure if you assume the focuser is square. But that's kind of dicey, isn't it, if you need good axial parallelism? If possible significant axial disparity doesn't matter, that is just fine. But it should be mentioned, anyway.

Vla

I agree, which is why the next version of my collimation article will include instructions on accomplishing Classical Offset collimation. But I still think it isn't necessary with a lot of scopes, even those with computers attached.
But the f/3-f/4 scopes coming along will probably need to take classical offset into account if they plan to use digital setting circles. I've seen a few manufacturers build in the offset into their secondary holders in how they attach to the spider, and I've seen others glue the secondary on with the proper offset.
When I first started collimating scopes (in the early '60s), it was normal to use a completely centered secondary with no offset and collimate anyway. It didn't result in even illumination at the edge of the field, though no one ever complained, and the primary reflection always appeared off-center in the secondary.
That collimation can be achieved without offset is the point I'm making.
What is being referred to as the "New Model" is merely a way to achieve proper offset and uniform edge illumination without offsetting the secondary away from the focuser. It has visual cues that make it easy to accomplish, and so long as the deviation of optical and mechanical axes isn't important (and in a lot of cases it isn't), it's a viable collimation method to teach and discuss.
It differs from classical offset and no offset from having the optical and mechanical axes diverge. But the error is minor in any scope of f/5 and longer and tolerable in some cases at shorter f/ratios.

In fact, it is only one of several different collimation techniques, all of which work. Some just happen to be easier than others.

I'm glad to see a lot of people understanding a lot more about collimation.

#60 sixela

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Posted 01 October 2009 - 02:21 AM

and I was talking the same thing about axial disparity it can cause.


If you follow a sane collimation protocol, lack of squareness of the focuser cannot cause axial miscollimation (or do you mean an optical axis not along the tube axis when you say "axial disparity"? That's certainly a "new" term to me, so I'm unsure as to what exactly it means.)

That's because, in essence, you are aligning everything to the (non-reflected) focuser axis wherever it may point. First you ensure the reflected focuser axis points to the optical axis on the primary's centre (but you can achieve that by setting tilt on the secondary, regardless of where it's placed -- yes, if the intercept angle has to be 90° secondary placement isn't arbitrary, but nothing says the secondary has to be at 45° of both axes, and indeed in low rider Dobs it isn't), and then you make sure the reflected focuser axis crosses the non-reflected focuser axis at the focal plane, making the two axes (and their reflections) overlap everywhere.

#61 wh48gs

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Posted 03 October 2009 - 05:51 PM

No one is claiming anything “new”. This thread is not meant to introduce new concepts or terminology.



I don't know if anyone is claiming anything, but I do know that something that is not new is being called that. If we step back from talking in terms of focuser/primary axis, this collimation mode is essentially identical to the conventional 1-2-3 step procedure, with #1 being centering the diagonal in the focuser's circle by axially shifting it toward primary, #2 centering the primary's reflection in the diagonal by tilting the diagonal so that focuser's axis "reflected" from it hits the center of the primary, and #3, centering reflection of the diagonal in the primary by tilting the primary's axis toward focuser (so that it nearly coincides with focuser's axis "reflected" from the diagonal).

What is new is that I figured out why I couldn't get primary's axis to fall back onto the focuser's axis (didn't take into account that axial shift of the diagonal cancels most of the primary's reflection displacement in the diagonal). The result is that the magnitude of axial disparity is significantly lower than I thought. It is given by 57.3a^2/SH in degrees, with "a" being the diagonal minor semiaxis, S the diagonal-to-primary and H the diagonal-to-focus separation. For a 12" f/5 Newtonian, with a=1.5", H=10" and S=50", that comes to 0.26 degrees (assuming square focuser).

As usual, it's good for me to hang with you guys.

Vla

#62 wh48gs

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Posted 03 October 2009 - 06:00 PM

If you follow a sane collimation protocol, lack of squareness of the focuser cannot cause axial miscollimation (or do you mean an optical axis not along the tube axis when you say "axial disparity"? That's certainly a "new" term to me, so I'm unsure as to what exactly it means.)



That is true only if the "unsquarness" of the focuser is compensated by the diagonal alone, leaving axes of the primary and the tube nearly coinciding. If these two axes are at an angle, it induces pointing and tracking errors - assuming that the tube/structure axis is aligned with the mechanical axes of the mount. In all, there are four axes other than focuser's that need to be aligned for a perfect alignment.

Vla

#63 Starman1

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Posted 04 October 2009 - 12:23 AM

No one is claiming anything “new”. This thread is not meant to introduce new concepts or terminology.



I don't know if anyone is claiming anything, but I do know that something that is not new is being called that. If we step back from talking in terms of focuser/primary axis, this collimation mode is essentially identical to the conventional 1-2-3 step procedure, with #1 being centering the diagonal in the focuser's circle by axially shifting it toward primary, #2 centering the primary's reflection in the diagonal by tilting the diagonal so that focuser's axis "reflected" from it hits the center of the primary, and #3, centering reflection of the diagonal in the primary by tilting the primary's axis toward focuser (so that it nearly coincides with focuser's axis "reflected" from the diagonal).

What is new is that I figured out why I couldn't get primary's axis to fall back onto the focuser's axis (didn't take into account that axial shift of the diagonal cancels most of the primary's reflection displacement in the diagonal). The result is that the magnitude of axial disparity is significantly lower than I thought. It is given by 57.3a^2/SH in degrees, with "a" being the diagonal minor semiaxis, S the diagonal-to-primary and H the diagonal-to-focus separation. For a 12" f/5 Newtonian, with a=1.5", H=10" and S=50", that comes to 0.26 degrees (assuming square focuser).

As usual, it's good for me to hang with you guys.

Vla

The formula I use is that the displacement of the optical axis is equal to the offset in classical offset positioning of the secondary.
As such, you can then compute the angular error between the optical and mechanical axes as the inverse tangent of the offset divided by the distance from primary to secondary.
For a 12.5" f/5, that was about 1/8" offset and about 8' deviation between the optical and mechanical axes.
With a 2.6' resolution on my encoders, that could cause errors in pointing, but the object is always in the center 50% of the field of a 45' field eyepiece if I took the time to align using a reticle eyepiece. I've achieved 0.0 warp factors.
However, I should point out that my focuser is exactly on the side of the UTA, which means the deviation is only in azimuth, which it compensated for in alignment. If the focuser is at other angles, the axial deviation could lead to more problems in pointing.

Now, if offset is larger and the secondary-to-primary distance is less, the error grows. As I mentioned, it is probably wise to resurrect the classical offset techniques as scopes get shorter in f/ratio.

#64 wh48gs

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Posted 04 October 2009 - 07:53 AM

The formula I use is that the displacement of the optical axis is equal to the offset in classical offset positioning of the secondary.



Partial (axial) offset is not quite the same as a full offset, in that it needs larger value to have the diagonal apparently centered in the focuser's view (sinking the diagonal by the offset value has added effect of apparently shifting it toward primary, since the upper edge's apparent horizontal shift is greater). I got that the full offset value is given by (D-A)A/4S and axial offset alone by A^2/4H, where D is the mirror diameter, A the diagonal's minor axis, S the diagonal-to-primary separation and H the diagonal-to-focus separation. This means that the axial alone to full offset ratio is given by SA/(D-A)H. With the usual parameter values, axial shift alone is about 50% greater.

Vla

#65 Starman1

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Posted 04 October 2009 - 10:33 AM

Hmm.
As I understand it, in classical offset the secondary is moved along the 45 degree plane of the secondary by an offset amount that is required to bring the opto-mechanical axis coincident with the geometric center of the secondary, and that that results in a centered secondary and even illumination.
Here's a calculator to figure than offset:
http://www.asnsw.com.../offsetcalc.asp
and
http://users.erols.c...s/diagonal.html

In the technique that results in tilting the optical path toward the focuser (what has been referred to as the "new model", rightly or wrongly), the *effective* offset amount for the secondary is the same.
However, the actual tilt of the optical axis away from the mechanical axis should be less than the offset on the secondary surface because it doesn't have to tilt sideways as much as the offset because the offset is along a 45 degree line, so the actual tilt of the optical axis in millimeters would be the square root of the quantity of the offset on the secondary surface squared divided by 2.
[If the calculator calculates the offset amount you need to move the secondary away from the focuser, ignore the formula and use this amount as the opposite side of the long triangle in the following paragraph.]
If you then use that offset amount as one side of a long triangle with the distance from secondary to primary as another side, the angular deviation between the optical and mechanical axes can be calculated as the inverse tangent of the lateral tilt in millimeters divided by the secondary to primary distance.
The question is how many minutes of angular deviation between optical and mechanical axes is significant for DSC accuracy? And that is something I can't answer.
The small amount of deviation on my 12.5" f/5 isn't significant. The large amount of deviation on Mike Lockwood's 14.5" f/2.55 scope would be. Where the dividing line is, I don't know.

Vlad would probably argue that if everyone used classical offset collimation this wouldn't be an issue, and he'd be right.
But accuracy in that offset could be hard to achieve and adds an additional dimension to collimation not important to telescopes that are not used with DSCs.

I intend to improve on the instructions for achieving Classical Offset secondary positioning in my next version of the collimation article CN just posted. As telescopes get shorter (and it seems they may be), this will become more critical.

#66 wh48gs

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Posted 04 October 2009 - 01:54 PM

Don,

As I understand it, in classical offset the secondary is moved along the 45 degree plane of the secondary by an offset amount that is required to bring the opto-mechanical axis coincident with the geometric center of the secondary, and that that results in a centered secondary and even illumination.



The geometric center of the secondary actually slides down bellow the primary's axis, and it is diagonal's apparent center that is brought to coincide with the point of intersection of the primary's and focuser's axis. This centers diagonal both in the focuser's view and with respect to the axial - as well as truncated full-field - converging cone.

The top link gives result identical to that with the formula I posted, and also with the Suiter's approximation (p330 1st ed). He calls it approximation even if it is exact for all practical purposes (the only thing that gets rounded off is the focal ratio number, as f/D, instead of the actual value given as (f minus mirror sagitta)/D).

The other calculator gives slightly lower (<0.1mm) value, but it won't matter.

There might be some exceptions, but as I can see the offset is commonly called the increment by which the diagonal moves axially toward primary, and away from the focuser. The actual travel of the diagonal's center is greater by a factor of sq.rt.2.

The secondary shifted only axially toward mirror needs more of a travel in order to have this accomplished. Thus it requires more of a compensatory tilt of the diagonal and primary than what the full offset value would indicate. It seems to me that you are trying to reason about this, but there is no substitute to putting it down on paper, and get the relations from the geometry. Assuming focuser square to the tube axis (or to the plane of the mechanical axis of the mount), the tilt angle is given by by (A^2)/4SH in radians (i.e. tangent), A being the diagonal's minor axis, S diagonal-to-primary and H diagonal-to-focus separation. For a 14.5" f/2.55 with A, S and H, say, 5", 26" and 11", respectively, that would come to 0.022, or 1.25 degrees. Whether it is acceptable or not, depends primarily on the telescope's intended use.

I don't think that the full diagonal offset is a must, especially after I fixed that faulty calculation that made me think that the required primary tilt angle is significantly larger than what it really is. One thing that shouldn't be left to chance is squarness of the focuser, because in the partial offset it translates to the primary tilt. If neglected, it is not too hard to generate up to a few degrees of pointing error.

Vla

#67 Starman1

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Posted 04 October 2009 - 03:25 PM

Vlad,
Thanks. I always thought offset, in millimeters, was the amount the secondary had to slide along the plane of its surface.
Since offset is, as you indicate, the amount the secondary has to move away from the focuser and down the tube, then your calculations hold true.
I did graph it on paper, and I thank you for pointing to that as a better way to visualize what's going on. I see the centered diagonal having the opto-mechanical axis hit the secondary below the geometric center of the elliptical surface (toward the end closer to the primary). I also see that if the secondary is slid down along the plane of its surface until the geometric center is coincident with the opto-mechanical axis there will still be a little more secondary outside the cone on the end farthest away from the primary.
As you indicate, the secondary actually has to slide down a little more than the geometric center to get uniform edge illumination all the way around. Fascinating.
Ergo, the tilt of the optical axis away from the mechanical axis is a little more than I calculated.

#68 sixela

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Posted 04 October 2009 - 03:38 PM

and #3, centering reflection of the diagonal in the primary by tilting the primary's axis toward focuser (so that it nearly coincides with focuser's axis "reflected" from the diagonal).

You've got it slightly wrong. #3 uses a Cheshire, and *perfectly* overlaps the optical axis and focuser axis and their reflections (even though the intercept angle on the secondary is not 90° and the optical axis is not along the tube axis), because you make the reflected optical axis cross the focuser axis half way between the Cheshire ring and the pupil.

Using the reflected secondary's silhouette is a bad idea, and no sane protocol would use it as a reference.

#69 sixela

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Posted 04 October 2009 - 03:44 PM

If these two axes [the mechanical tube axis and the optical axis] are at an angle, it induces pointing and tracking errors


Yes, if you care about them (and have a GEM with GoTo or use digital setting circles).

But it doesn't introduce axial miscollimation, i.e. aberrations or defocus in the field.

BTW, it doesn't introduce tracking errors on my scope. I have an equatorial platform and its polar axis is quite independent from the axes of the Dobsonian mount and their squareness.

It also doesn't introduce tracking errors on mounts that don't track at all, obviously, and as I star hop and do not use DSCs, it also doesn't introduce pointing errors on my scope either ;).

#70 wh48gs

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Posted 05 October 2009 - 08:47 AM

Don,

It is an unvolved procedure to grasp. The main reason I got into this thread is to make sure the page on Newtonian collimation that I added just recently got it right. Good move for me, since Jason's illustrations and Alexis's comment made me obvious an inconsistency that led me to find and correct an oversight. Seems that we all got something from this (except Alexis, who doesn't sound all that happy ;) ).

Vla

#71 wh48gs

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Posted 05 October 2009 - 09:10 AM

You've got it slightly wrong. #3 uses a Cheshire, and *perfectly* overlaps the optical axis and focuser axis and their reflections (even though the intercept angle on the secondary is not 90° and the optical axis is not along the tube axis), because you make the reflected optical axis cross the focuser axis half way between the Cheshire ring and the pupil.



How do they cross if they perfectly overlap? Nothing gets really perfect, except on our illustrations. One of the reasons is that actual mirror surfaces, when tilted, do not rotate around the point of their intersections with the axis, but around a point effectivelly displaced from it (particularly diagonal). As a result, any tilt introduces decenter, which makes the surfaces and reflections less than perfectly concentric.

Using the reflected secondary's silhouette is a bad idea, and no sane protocol would use it as a reference.



Lots of insanity around: Sidgwick (p204), Thompson (p120), Lecleire (p262), Norton's Star Atlas (p72)...

Vla

#72 wh48gs

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Posted 05 October 2009 - 09:15 AM

It also doesn't introduce tracking errors on mounts that don't track at all, obviously, and as I star hop and do not use DSCs, it also doesn't introduce pointing errors on my scope either.



That's nothing. I have a small Tal Newtonian that never needs collimating at all, because I never use it. Isn't that as good as it gets? :cool:

Vla

#73 Vic Menard

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Posted 05 October 2009 - 10:02 AM

If these two axes [the mechanical tube axis and the optical axis] are at an angle, it induces pointing and tracking errors


Yes, if you care about them (and have a GEM with GoTo or use digital setting circles).

Not necessarily. If the plane defined by the two lines (optical axis and OTA axis) is perpendicular to the axis of rotation (declination or altitude), then the axial offset will have no impact on DSC performance.

Even when the plane is skewed, the error is often small enough to fall within the resolution of most DSC displays (+/-0.1-degree). The Argo Navis (used with 8000 or 10000 count encoders) pointing accuracy does have a higher resolution, but the AN can also model the pointing error and correct it on the fly...

That said, I've used an f/4.1 with a Sky Commander and an f/4.0 with a Argo Navis, both aligned using the "New Model" (full offset with centered secondary mirror) and, selecting good alignment pairs, have found the pointing accuracy normally places the selected object within 0.1- or 0.2-degrees from the center of the fov. The worst case scenario is when the selected object is within a few degrees of the zenith--the possible azimuth error can be very large, and still fall within the fov of a widefield eyepiece! Remarkably, once the object has been centered, tracking is largely unimpaired.

FWIW--I think the first time I used the phrase, "the new model", was in 1998 (although back then I suggested mechanically tilting the focuser to ensure a 90-degree intercept). I was getting quite a bit of opposition from those who felt collimation was not accurate unless it was either centered or offset. Choosing a model that was both was probably enough to start some debate--but I didn't stop there, going on to assure those who would follow the procedure that the focuser need not be precisely perpendicular to the OTA, the intercept angle need not be exactly 90-degrees, and the primary mirror need not be perfectly centered in the OTA. The goal, of course, was precise axial (focuser and primary mirror) alignment with an optimal secondary mirror alignment (best available offset and minimal skew).

Considering lightweight, open truss OTAs might have small eccentricities or tilt errors from the lower mirror section to the upper focuser section, and the potential for small geometry errors in the upper focuser section that might impact roundness, squareness, and centration, and then adding that the secondary mirror might not be a true sqrt2 major axis ellipse...it seems prudent to focus on the optical alignment first, and then address the mechanical alignment issues as/if they arise.

#74 Starman1

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Posted 05 October 2009 - 10:21 AM

You've got it slightly wrong. #3 uses a Cheshire, and *perfectly* overlaps the optical axis and focuser axis and their reflections (even though the intercept angle on the secondary is not 90° and the optical axis is not along the tube axis), because you make the reflected optical axis cross the focuser axis half way between the Cheshire ring and the pupil.



How do they cross if they perfectly overlap? Nothing gets really perfect, except on our illustrations. One of the reasons is that actual mirror surfaces, when tilted, do not rotate around the point of their intersections with the axis, but around a point effectivelly displaced from it (particularly diagonal). As a result, any tilt introduces decenter, which makes the surfaces and reflections less than perfectly concentric.

Using the reflected secondary's silhouette is a bad idea, and no sane protocol would use it as a reference.



Lots of insanity around: Sidgwick (p204), Thompson (p120), Lecleire (p262), Norton's Star Atlas (p72)...

Vla

It should be pointed out that the silhouette (reflected) image of the secondary grows more concentric with all other images as the f/ratio gets longer. The illustration that Jason posted is slightly exaggerated for f/6 and longer scopes. Many of the sources you mention were written back when most reflectors were f/8 to f/12.
It might also be pointed out that standard collimation techniques of the '50s and '60s did not always call for even edge-of-field illumination. As late as 1967-1968, books by popular authors such as Sam Brown of Edmund Scientific called for a centered secondary with all images concentric except for the reflected primary, which appeared dropped toward the lower end of the secondary.
The interesting thing is that there are so many different mechanical alignments that can result in good optical collimation. It is when we aim to optimize field illumination and maintain coincidence between optical and mechanical axes that things get more complicated.

#75 Jason D

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Posted 05 October 2009 - 11:29 AM

Not necessarily. If the plane defined by the two lines (optical axis and OTA axis) is perpendicular to the axis of rotation (declination or altitude), then the axial offset will have no impact on DSC performance.

Hi Vic, I believe we discussed this specific point looooong time ago and the end result of the discussion was that the above error can be corrected but it will have to be by the DSC computer. However, if the DSC computer does not have the proper software to make this kind of correction, the error will impact accuracy.
Jason


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