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Equipment Discussions >> Reflectors

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Jon Isaacs
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Re: Another innovation from Howie Glatter? new [Re: davidmcgo]
      #5929162 - 06/19/13 12:52 PM

Quote:

Would going to a single tilt srew and rotation assume the spider is exactly perpendicular to the tube?

Dave




Dave:

I think it does work but from a practical viewpoint, it's not clear to me it's an advantage.

Jon


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mtb54703
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Re: Another innovation from Howie Glatter? new [Re: Starman1]
      #5929322 - 06/19/13 01:58 PM

Quote:

For each rotational angle of the secondary, there is a tilt angle that results in axial collimation. Once you realize that, you realize there are only a couple reasons to try to rotate the secondary to appear round under the focuser:
--it reduces tilt in the final focal plane to a level we won't notice
--it results in the best edge of field illumination for the short axis of the secondary's ellipse.




Question... assuming everything other than the secondary not being exactly round under the focuser is correct - how would this mis-adjustment of secondary manifest itself at the EP? I'm guessing it would show up more with longer EPs/lower power since those make the most use of the FIF.


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Starman1
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Re: Another innovation from Howie Glatter? new [Re: mtb54703]
      #5929385 - 06/19/13 02:20 PM

Quote:

Quote:

For each rotational angle of the secondary, there is a tilt angle that results in axial collimation. Once you realize that, you realize there are only a couple reasons to try to rotate the secondary to appear round under the focuser:
--it reduces tilt in the final focal plane to a level we won't notice
--it results in the best edge of field illumination for the short axis of the secondary's ellipse.




Question... assuming everything other than the secondary not being exactly round under the focuser is correct - how would this mis-adjustment of secondary manifest itself at the EP? I'm guessing it would show up more with longer EPs/lower power since those make the most use of the FIF.



Sure. Let's imagine a serious mis-rotation of the secondary, with the secondary rotated until it appears oval. You still can adjust the tilt to put a laser beam on the center of the primary (though the secondary holder won't be in line with the tube). The top of the image would be farther from the eye than the bottom of the image and there would be a substantial tilt to the focal plane. Focus would be a line across the field, with the top of the field needing additional infocus and the bottom of the field needing outfocus.
Larger fields of view would show it worse, you are correct.

Now, let's adjust the secondary until it's round to the eye and the reflected image of the primary is centered and concentric. We could still be a little off, but the angular error would be small. Tilt of the secondary to line up the focuser axis with the center of the primary would result in only a tiny amount of secondary movement. And the tilt in the final focal plane would be less than our eyes' ability to accommodate mis-focusing. There would be a range of rotation of the secondary where this would apply, but assuming the initial centering of the secondary using a sight tube and a concentric primary reflection, you would be within that range.

Which is why extremely fine rotation of the secondary in collimation is not necessary. But at least two axes of tilt will be necessary to adjust out residual errors with the autocollimator. So the idea of having only one axis of adjustment and rotation sounds reasonable, but in practice you need two axes of adjustment.

I'm a little more of a perfectionist, and I appreciate having 4 axes of adjustment + rotation. Even if I don't use all those adjustments all the time.


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Vic Menard
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Re: Another innovation from Howie Glatter? new [Re: Jon Isaacs]
      #5929425 - 06/19/13 02:38 PM

Quote:

Quote:

There are only two adjustments needed for a secondary - a rotational one to make it aligned flat with the focuser, and a one-axis tilt. Only a single tilt adjustment screw should exist. My secondary holder from Astrosystems has four screws, three too many (in a proper design).




I would like to see Vic, Jason and Nils comments, particularly in light of Don's comments. With a 2 dimensional tilt adjustment that we all use, I believe small rotations are part of the adjustment process.

Securing/locking the rotation of the secondary is mechanically more difficult than securing the tilt, as long as collimation adjustments represents a small variations, it's not clear to me that a secondary rotation adjustment is an advantage.




Secondary mirror tilt that coincides solely with the major axis of the secondary mirror probably best defines what we are trying to do when assessing/correcting the secondary mirror tilt. Three adjustment screws complicate the process by requiring careful attention to the two screws not on the major axis. Four screws provides a more direct two screw tilt adjustment, allowing for the basic tilt/rotation adjustments you're considering. The other two screws in the four screw tilt models (Rick Singmaster's "forbidden screws") cause a tilt effect I refer to as "skew".

So why do we commonly see this additional alignment flexibility? I believe originally it was provided to accommodate "fixed" focusers. Remember buying the accessory skirt that attached to the bottom of the focuser so it would fit the round telescope tube with no gaps? With a fixed focuser, and pretty much a fixed spider (only basic centering adjustment), any residual misalignment of the focuser/spider geometry could still be accommodated.

Modern truss Dobsonians usually have flat focuser mounting boards. Configure the UTA with an adjustable, "leveling" focuser and you're one step closer to getting rid of the skew adjustment screws. Of course, there are several other considerations if you hope to achieve the best (not just the best "available") secondary mirror placement, all of them impacting the geometry associated with the elliptical flat.

In the last decade or so, it's become common practice to use plastic washers to facilitate manual rotation adjustments. This "unlocked" condition means assessing and correcting secondary mirror rotation is now part of the standard collimation process, especially with larger, more massive secondary mirrors. If you hope to use an autocollimator with one-dimensional tilt and rotation secondary mirror adjustment, you'll need fine adjustment capacity on both adjustments.

Getting back to the question at hand, if the geometries are accurate and robust enough, I see no reason not to limit the secondary mirror adjustments. Personally, I would prefer to "lock them out" myself (like Rick Singmaster), and still have them available when a truss tube gets bent or the focuser board gets damp (or overheated in the daytime sun) and suddenly torques.


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Vic Menard
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Re: Another innovation from Howie Glatter? new [Re: Starman1]
      #5929460 - 06/19/13 02:49 PM

Quote:

For each rotational angle of the secondary, there is a tilt angle that results in axial collimation. Once you realize that, you realize there are only a couple reasons to try to rotate the secondary to appear round under the focuser:
--it reduces tilt in the final focal plane to a level we won't notice...



I'm not sure what you mean here.
If the axial alignments are correct, the final focal plane won't be tilted.

Although I have seen seriously skewed (combined tilt/rotation error) secondary mirrors (usually the result of using a simple thin beam laser exclusively for collimation) that delivered anomalous star tests in spite of "good" axial alignment. If the secondary mirror is slightly undersized to minimize CO, this skew error can become visible in star image performance even sooner.


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Vic Menard
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Re: Another innovation from Howie Glatter? new [Re: Starman1]
      #5929501 - 06/19/13 03:10 PM

Quote:

Quote:

Question... assuming everything other than the secondary not being exactly round under the focuser is correct - how would this mis-adjustment of secondary manifest itself at the EP?



Sure. Let's imagine a serious mis-rotation of the secondary, with the secondary rotated until it appears oval. You still can adjust the tilt to put a laser beam on the center of the primary (though the secondary holder won't be in line with the tube). The top of the image would be farther from the eye than the bottom of the image and there would be a substantial tilt to the focal plane. Focus would be a line across the field, with the top of the field needing additional infocus and the bottom of the field needing outfocus.
Larger fields of view would show it worse, you are correct.



I haven't seen the focus change you've described when the axial collimation is correct. Although the secondary mirror can appear elliptical and tilted, the flat surface of the secondary mirror still coincides with the intercept plane. And while the optical axis is shifted as you described, the result is similar to that of the new model (except the field illumination can be more profoundly imbalanced).

Quote:

Now, let's adjust the secondary until it's round to the eye and the reflected image of the primary is centered and concentric. We could still be a little off, but the angular error would be small.



What angle are you talking about?

Quote:

Tilt of the secondary to line up the focuser axis with the center of the primary would result in only a tiny amount of secondary movement. And the tilt in the final focal plane would be less than our eyes' ability to accommodate mis-focusing.



Tilting the secondary to align the focuser axis to the primary mirror center spot eliminates focal plane tilt. Or are you suggesting misaligning the focuser axis to improve the secondary mirror placement?

Edited by Vic Menard (06/19/13 03:34 PM)


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Starman1
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Re: Another innovation from Howie Glatter? new [Re: Vic Menard]
      #5929711 - 06/19/13 05:14 PM

OK,
I might be missing something here.
If so, I need to be "illuminated".

But axial collimation seems to me to be independent of focal plane tilt.

I'll explain.

Picture a secondary mirror rotated off the correct angle by 30 degrees, with the "top" rotated away from the observer, and the bottom rotated closer.
A laser hitting the secondary would reflect upward, above the primary mirror. The light cone from the primary would reflect to hit a point above the focuser.
But, the secondary could be tilted down to put the laser beam dead center on the primary and still have the 30 degree rotational error.

The return beam of the laser would return to its source.
A cheshire would show the primary's center marker in the center of the tool.

YET, the cone of light from the primary, hitting the secondary, would be reflected into the focuser properly. From the primary's center marker position, I believe the secondary would not appear round, but I am not sure about that.

BUT, the part of the light cone hitting the top of the secondary would have to travel farther to reach the same place above the focuser's drawtube than the part of the light cone hitting the bottom of the secondary. In essence, though the axial beam is perfectly aligned, and collimation can be achieved, the final focal plane would be tilted at the focuser since the focal length of the mirror is identical from all points on the mirror.

Would the axial beam still reflect from a point on the long axis of the elliptical secondary? I don't know, but the focal plane would not be perpendicular to the axis of the focuser. It would be similar in shape to how the secondary appears from the focuser, and tilted to the observer.

If what I just described is not correct, then what's the point of trying to get rotation correct for the secondary mirror? To make the secondary's shroud colinear to the centerline of the tube just to get a round shadow?

It seems to me that as we rotate the secondary mirror we are making all the rays reflecting from the primary mirror come to a focal plane that is perpendicular to the focuser's axis, or as close as we can to it.

But I may be missing something.


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careysub
Carpal Tunnel
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Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Another innovation from Howie Glatter? new [Re: Jon Isaacs]
      #5929714 - 06/19/13 05:15 PM

Quote:

...
A rotation adjustment could be done in a similar fashion, a lever with two antagonistic screws.. But locking a rotation generally requires clamping and that can induce rotation.

Jon




But many rotation locking schemes can be imagined where this isn't a problem.

Consider rotation locking using a plate at the upper end of the rotation axle (or a ring attached along it somewhere).

A cam lever when flipped down can provide vertical pressure on the plate or ring that prevents rotation, without creating any torque.

Even a simple screw with a pivoting head could do this, or a screw depressing a leaf spring clamp, or...


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Vic Menard
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Reged: 07/21/04

Loc: Bradenton, FL
Re: Another innovation from Howie Glatter? new [Re: Starman1]
      #5929774 - 06/19/13 05:50 PM

Quote:

...Picture a secondary mirror rotated off the correct angle by 30 degrees, with the "top" rotated away from the observer, and the bottom rotated closer.
A laser hitting the secondary would reflect upward, above the primary mirror. The light cone from the primary would reflect to hit a point above the focuser.
But, the secondary could be tilted down to put the laser beam dead center on the primary and still have the 30 degree rotational error.



Yep.

Quote:

The return beam of the laser would return to its source.
A cheshire would show the primary's center marker in the center of the tool.



So far, all true.

Quote:

YET, the cone of light from the primary, hitting the secondary, would be reflected into the focuser properly.



Well, the part of the cone that the secondary intercepts will be reflected into the focuser properly. The part of the cone the secondary misses, will of course, not be reflected.

Quote:

From the primary's center marker position, I believe the secondary would not appear round, but I am not sure about that.



I agree--it will not appear round.

Quote:

BUT, the part of the light cone hitting the top of the secondary would have to travel farther to reach the same place above the focuser's drawtube than the part of the light cone hitting the bottom of the secondary.



No. It may be easier to visualize if you look at the secondary mirror from the center of the focal plane. The mirror surface looks like a skewed ellipse and you can clearly see one side of the secondary mirror holder. If you use a simple thin beam laser, you can see where the laser bounces off the surface of the secondary mirror (it will likely not be in the mechanical center, or the optical center, on the secondary mirror surface). If you were to mark that point and return to the focuser, you would see the point somewhere on the skewed ellipse. You could in fact, mask off a smaller ellipse that is properly aligned to the focuser so that the smaller ellipse appears centered and round. That's because the mirror surface is part of the new intercept plane (defined in part by the change in position of the intersection of the focuser and optical axes).

Quote:

If what I just described is not correct, then what's the point of trying to get rotation correct for the secondary mirror?



To get the maximum balanced illumination with the least diffraction contribution from the CO.


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Starman1
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Re: Another innovation from Howie Glatter? new [Re: Vic Menard]
      #5929784 - 06/19/13 05:54 PM

So the only common way to get a focuser plane tilt is to have the focuser axis not correspond to the optical axis?

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Vic Menard
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Re: Another innovation from Howie Glatter? new [Re: Starman1]
      #5929895 - 06/19/13 07:08 PM

Quote:

So the only common way to get a focuser plane tilt is to have the focuser axis not correspond to the optical axis?



Yes.
The focuser axial alignment defines the focal plane perpendicularity.
And the primary mirror axial alignment defines the centering of the coma "free" field diameter in the focal plane.

Optimal secondary mirror alignment ensures the maximum usage of the light cone.


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Jason D
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Re: Another innovation from Howie Glatter? new [Re: Starman1]
      #5930156 - 06/19/13 10:02 PM

Quote:

But axial collimation seems to me to be independent of focal plane tilt.







http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/3901098/...


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Nils Olof Carlin
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Re: Another innovation from Howie Glatter? new [Re: Starman1]
      #5930579 - 06/20/13 03:25 AM

Clearly, the focal plane of the primary (it isn't exactly a plane, but...) is perpendicular to the mirror's optical axis. The focal plane of the EP or detector is (hopefully) perpendicular to the focuser axis.
So the focal plane(s!) are not tilted if (and only if) the focuser and primary axes are indeed parallel - and of course preferrably coincident. This is established by the usual methods of axial collimation. This holds even if the secondary is "skewed".

But checking the shape and placement of the actual fully illuminated field is not difficult.
(Put a piece of transparent foil over the focuser opening, at the level of the focal plane. Mark with a Sharpie the line of sight where the primary and secondary seem to touch, at a couple of points to form an ellipse or circle).

Nils Olof


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Jon Isaacs
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Re: Another innovation from Howie Glatter? new [Re: careysub]
      #5930659 - 06/20/13 06:26 AM

Quote:

A cam lever when flipped down can provide vertical pressure on the plate or ring that prevents rotation, without creating any torque.




It would probably apply a bending moment though.

Jon


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Jon Isaacs
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Re: Another innovation from Howie Glatter? new [Re: Vic Menard]
      #5930661 - 06/20/13 06:29 AM

Quote:

Optimal secondary mirror alignment ensures the maximum usage of the light cone.




Which implies that if the secondary is on the smaller size, this is most important, if it is on the larger size.. not so much???

Jon


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Chucky
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Re: Another innovation from Howie Glatter? new [Re: FirstSight]
      #5930742 - 06/20/13 07:58 AM

<< I have a Protostar secondary holder, and don't seem to have the sorts of torque/rotation problems people describe above. >>

Ditto for me. Never had one bit of a problem. Simply works just fine.


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Vic Menard
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Re: Another innovation from Howie Glatter? new [Re: Jon Isaacs]
      #5930940 - 06/20/13 09:49 AM

Quote:

Quote:

Optimal secondary mirror alignment ensures the maximum usage of the light cone.




Which implies that if the secondary is on the smaller size, this is most important, if it is on the larger size.. not so much???



For visual applications, a "smaller" secondary may be selected to illuminate a very small field diameter (~0.1-inch) to minimize CO for high magnification performance. In this kind of application, optimal placement of the secondary mirror may be necessary to achieve the desired illuminated field and to minimize anomalous diffraction effects (even in the center of the fov).

Given a slightly oversized secondary mirror, the same effects are still possible, even if they are less visually detectable.


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auriga
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Re: Another innovation from Howie Glatter? new [Re: Nils Olof Carlin]
      #5930976 - 06/20/13 10:08 AM

This is a great discussion and I even understand some of it.

I use a tuBlug and it works very well.

My only problem with collimation is rotation of the diagonal; there is some nut or bolt that governs the rotation, that I need to figure out how to tighten.

But even tightened, I can see how adjusting the collimation screws produces a rotation of the diagonal.

I view the current designs for diagonal adjustment as weird. Three screws with relationships to each other such that tightening one requires loosening the others. The direction of movement of the red laser dot by tightening one screw is orthogonal to the center of the diagonal. Weird.

I have full confidence in Howie Glatter and am impatiently awaiting his invention.

Innovation of any magnitude is often difficult even in amateur astronomy.

There is always resistance.

For years were were told that mass was important in telescope mountings, and people were using massive airplane propeller housings for equatorial mounts. Finally an engineer wrote an article in Sky & Telescope showing mathematically that mass in a mount exerts a bending moment and also torque.

John Dobson submitted an article on his mounting to Sky & Telescope, and the editor rejected it, telling him it would never work.

For years we were told that focal ratios as short as f/6 were a bad compromise, and 8 inch reflectors were manufactured with f/7 and f/8 ratios, and 12.5 inch reflectors were manufactured with f/7 ratios. We were told that very short focal ratios would never work.

There is always resistance. But I am betting on Howie.

Bill Meyers


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Pinbout
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Re: Another innovation from Howie Glatter? new [Re: Chucky]
      #5930986 - 06/20/13 10:12 AM

Quote:

<< I have a Protostar secondary holder, and don't seem to have the sorts of torque/rotation problems people describe above. >>

Ditto for me. Never had one bit of a problem. Simply works just fine.




the center bolt and stalk are one piece, and the bolt is used as a spring cause it bends as you adj the tilt so there is always a constant force applied to keep it from rotating.


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Vic Menard
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Re: Another innovation from Howie Glatter? new [Re: auriga]
      #5931024 - 06/20/13 10:34 AM

Quote:

Innovation of any magnitude is often difficult even in amateur astronomy...

There is always resistance. But I am betting on Howie.



I'm equally amazed at how much innovation we've seen in the past twenty years--computer controlled Dobsonian mounts, slumped, thin blank mirror technology, breaking the f/5, then f/4, and even the f/3 barrier, coma correctors and 100+ degree highly corrected eyepieces, star charts on your cell phone (that use m-theory mathematics to display Near Earth Asteroid motions on the charts), real-time video astronomy and high resolution planetary imaging with DSLRs...the list goes on and on.

Last year I spent some time in an email discussion with Mike Zammit prototyping an upgraded spider hub that provided precise control of the secondary rotation and offset--both away from the focuser and toward the primary mirror! I believe a working prototype exists, although I only saw pictures. I'm not sure why Mike hasn't brought the product to market (probably tooling and the variety of parts required to upgrade the various UTA/spider sizes--while keeping the price competitive)...

Don't get me wrong--Howie's products rock! We're fortunate to have his vision and attention to detail in this era of innovation.


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