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SCT Collimation- My Attitude Problem

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#1 Rustler46

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Posted 18 July 2019 - 01:11 AM

I'll have to admit that it is difficult for me to believe that for a SCT slightly less than perfect collimation can affect image quality over the entire FOV. Yes, I do believe perfect collimation is beneficial. The counsel of experienced users of SCTs shows the benefits of perfect collimation. But I must force myself to wholeheartedly accept this counsel.

 

My "attitude problem" comes from how a slight miss-collimation affects a Newtonian image. One method to see what collimation adjustment is needed is to place a slightly out of focus star image in the FOV in a location where the shadow of the secondary is centered in the disc. This is where the axis of the primary mirror is pointed. One then just adjusts the collimation screws to bring that out-of-focus image (with shadow centered) to the center of the FOV. When the "sweet spot" is not centered in the FOV, the image on-axis (i.e center of high power FOV) suffers from off-axis coma and perhaps other aberrations. But other than off axis aberrations, the consequence of slight miss-collimation isn't an overall degradation of the image. It just isn't putting the best part of the image on axis.

 

But if my understanding is correct, this simplistic view is not applicable to the Schmidt-Cassegrain system. The primary has no adjustment screws. It is the adjustment of the secondary which directs the optical axis of the primary back down the center of the baffle tube in the primary. And if it is off a bit, it isn't just a matter of the "sweet spot" not being in the center of the FOV. If I understand things correctly a miss-collimated secondary negatively affects the image over the entire FOV.

 

It has taken me understanding (and believing) that negative consequence, to be willing to check collimation on my Celestron-11, especially when seeing is good enough to benefit from perfect collimation. It seems like my C-11 doesn't hold perfect collimation very long. Just normal handling in moving from garage to permanent pier causes it to drift off a bit. Maybe the secondary housing is loose. I intend to replace Bob's Knobs with Allen head screws.

 

Yeah, I do believe perfect collimation is very beneficial. This is especially so when I fight my "attitude problem". So for experienced SCT users, is my thinking on this correct? Are SCT and Newtonian collimation issues fundamentally different as I described? Please help me with my attitude problem.

 

Russ


Edited by Rustler46, 18 July 2019 - 01:37 AM.

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#2 bbqediguana

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Posted 18 July 2019 - 06:09 AM

Hi Russ,

 

I can't really speak to the optical theory as I am pretty much a dullard in that arena. However, I can say that I did try Bob's Knobs and found the same thing you did - they didn't hold collimation for very long. I went back to the original Allen-head bolts that came with my Meade SCT and it is much better now. Perhaps I don't have the finger strength anymore to cinch down the Bob's Knobs sufficiently?

 

I think the fundamental difference between a Newtonian and a SCT is the shape of the secondary - a Newt it is flat, and on the SCT it is spherical (at least for my old beast). So perhaps the "sweet spot" is much, MUCH smaller on the SCT making accurate collimation critical.

 

Hopefully smarter folks than I will chime in and help us understand better. popcorn.gif

 

Cheers!

 

Rick in Canada (eh!)


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#3 Eddgie

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Posted 18 July 2019 - 06:27 AM

If you have any perceptible image shift when focusing or when doing your collimation, then it is very likely that your collimation is changing because you are changing the direction of the focuser knob you use to reach final focus.

 

If there is even a couple of arc minutes of mirror shift, then what happens is that if you approach final fucus using clockwise turns of the focuser, after you stop, the mirror will continue to move very slightly as it sags into the grease film on the baffle.

 

If you always approach final focus with counter clockwise motion, the focuser rod pushes the mirror up at the same angle all the time, and when you let go, there is no slop left.

 

Not saying this is your issue, saying it could be your issue.

 

Also, never collimate on an artificial star close to the ground. It is an almost automatic fail because the mirror will settle on the baffle at a different angle than when it is pointed towards the sky.


Edited by Eddgie, 18 July 2019 - 06:27 AM.

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#4 Dynan

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Posted 18 July 2019 - 07:28 AM

Though pricey, the Hotech Advanced CT Laser Collimator showed me how bad my collimation was, even though the out of focus star test was adjusted as best I could.

 

The centering of the corrector plate was crucial to the process and there is no visual star test that showed me the error. I have an old model C11. So old that it didn't have corrector plate centering screws. I installed some #8-32 Delrin tipped allen screws and it made a world of difference.

 

The triple pass laser doesn't lie. After the HACT alignment, I still needed a final tweak of the secondary screws with a star test. But this time everything came into center, unlike the times I only adjusted the secondary. No more off center stars, dependent on seeing and tracking of course. The only thing it doesn't adjust is corrector rotation, but that can be reduced to a very small error by proper alignment of the plate, and secondary etchings/markings to the 3 o'clock position, facing the front of the scope. If you can guarantee (as I couldn't) the rotation of the secondary has changed from the factory, results could be even better than I achieved.

 

The results were outstanding, as is the quality of the instrument. The ACT does have a bit of a learning curve, but nothing unattainable. I was surprised that once I knew what to look for, I could use the ACT in the daytime, outside, under a 'picnic' type canopy, of course.

 

Admittedly the collimator is cost-prohibitive for the little use it gets. I wish there were an easy way that I could share the device with fellow CN'ers who can't quite afford the device, but I wouldn't want to impinge on Hotech's livelihood either. Dilemma...hmm.gif

 

Fellow Hotech ACT users, maybe chime in with your experience, here or in another thread.


Edited by Dynan, 18 July 2019 - 07:36 AM.


#5 treadmarks

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Posted 18 July 2019 - 09:10 AM

Another reason perfect collimation is critical with an SCT is the F/2 mirror. The collimation "sweet spot" scales with f-ratio and F/2 is a very low f-ratio. So the sweet spot for an SCT is very very small.


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#6 Eddgie

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Posted 18 July 2019 - 09:30 AM

The speed of the primary is not really an issue. The coma on the f/10 SCT is pretty much the same as the coma on an f/5 reflector, so the amount of error is the same for the amount of misalignment. 

 

The negative secondary though makes the tilting movement of the secondary much more sensitive.  Even the tineset movement of the secondary will make a meaningful change, vs the less sensitive tilt adjustment of an f/5 mirror. Now the SCT primary is not tiltable, so the adjustment has to be done with the more sensitive screw movements no the negative power secondary mirror but the actual amount of contrast loss for a given amount of miscollimation is about the same as the f/5 reflector.

 

5 Arc minutes of mis-alignment in either will show the same amount of movement of the secondary shadow and induce about the same amount of coma. 

 

The standard SCT has far worse field curvature and has more astigmatism, but these are edge of the field aberrations and are not really affected by collimation (at least the amounts typically encountered in these systems).  


Edited by Eddgie, 18 July 2019 - 09:38 AM.


#7 Asbytec

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Posted 20 July 2019 - 03:55 AM

Are SCT and Newtonian collimation issues fundamentally different as I described? Please help me with my attitude problem.

Russ

Yes, it's different. A parabola has a defined optical axis, a spherical SCT primary mirror does not. A parabolic mirror always has a sweet spot with zero coma on its optical axis, so we align the primary mirror axis to the center of the eyepiece field of view. We know its inherent sweet spot is centered because no coma is seen at center (or even near center) and the Airy disc is centered in the diffraction rings.

Aligning two spheres is fundamentally different in a way I cannot describe well enough. A spherical mirror does not have a defined optical axis or coma sweet spot, it has any number of them. Rather, collimation has to do with centering a star in the FOV so the Poisson spot caused by the obstruction (secondary mirror) diffraction is centered in the diffraction caused by the aperture. I presume the aperture is the corrector.

Doing so defines "on axis" optical alignment with the eyepiece centered in the focuser (and primary baffle) at the geometric center of the primary mirror. I believe this also means the eyepiece optical axis is pointed at the spherical secondary mirror center of curvature and is normal to the secondary surface (so it points the eyepiece axis back onto itself) ensuring the secondary is centered on it. This is one reason we can align an SCT using the secondary only, as any axis on the spherical primary will do. And why we do not collimate on shadows of secondary mirrors, rather on diffraction close to or in focus. Centering the Poisson spot, in effect, centers and defines the sweet spot.

So it appears to be much like collimating a Newt primary, but it's fundamentally different. In the Newt we center its on axis coma free point on the eyepiece axis. This is axial alignment. In an SCT, without a specific optical axis, we center diffraction to remove coma from the center of the eyepiece FOV and define an axis pointed at that star along the line from the center of the spherical mirror (eyepiece) and its center of curvature and through that of the secondary.

 

When the primary mirror "flops" or is torqued a bit by focusing, the primary mirror's center of curvature shifts a little and collimation is out by a tiny bit. The baffle and focuser /may/ still point through the secondary center of curvature, but the primary mirror does not. This means paraxial rays parallel to the line through the geometric center and center of curvature, which come to focus about halfway along that line, deviate to some degree from the rest of the system (baffle and secondary). The primary mirror is 'looking' somewhere else. 

 

Maybe not the best explanation, but it's how I think about it.

 

https://physics.info/mirrors/


Edited by Asbytec, 20 July 2019 - 06:43 AM.

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#8 Orion68

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Posted 21 July 2019 - 06:59 PM

I am no optical expert but I do agree wholeheartedly with you and others who say that spot on collimation is vital for good views in an SCT. My C8 and C11 both have been a devil to collimate. I tried Bob's Knobs and could not seem to get a good collimation, my fault I'm sure. So, like you, I installed allen head screws and have a much easier time.

 

My collimation shifts from one side of the sky to another so I'm always trying to tweak it throughout the night. To make matters worse, seeing conditions never seem to allow for finding the Holy Grail of perfect collimation.

 

At times I would gladly trade my C11 in for a refactor and happily forget all about collimation. But, then again, to not be able to view M13 in the C11 anymore would break my heart.


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#9 Dynan

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Posted 21 July 2019 - 09:47 PM

I am no optical expert but I do agree wholeheartedly with you and others who say that spot on collimation is vital for good views in an SCT. My C8 and C11 both have been a devil to collimate.

As I found out, and said above, when I got the Hotech ACT, there's a lot more to collimating an SCT than the secondary screws...


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#10 freestar8n

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Posted 22 July 2019 - 02:58 AM

A C11 will show coma in a similar way to a Newtonian - with a sweet spot of no coma that you can move around the field - and collimation amounts to centering it.

 

Collimation is a particular type of optical alignment that uses limited adjustments to get the center of the field perfect - and even if the rest of the system is somehow not aligned, you can apply tilt to cancel out any residual coma.  The result may not be perfect, but the point of collimation is to make these limited final adjustments to compensate at least partially for alignment errors.

 

EdgeHD is very different when it comes to collimation because in theory once the center is perfect - everything is perfect.  That means you can't think in terms of centering the sweet spot - and instead you are just making adjustments until a star near the center is perfect.  But if everything is aligned well then the center will go perfect at the same time as stars toward the edge.  In practice, though, it's easier to use a star near the center.

 

The lack of coma in EdgeHD makes it less sensitive to small collimation errors - but at the same time the field is so well corrected that if it isn't collimated, the whole field will show aberrations.  And with a normal C11 you can see the sweet spot easily because the coma shows so clearly - and the adjustments provide more direct feedback.

 

If you are imaging with a large sensor then the adjustments amount to getting the overall field optimized - which I view as different from collimation per se.

 

One thing usually omitted from collimation discussions with sct's is the importance of loading the mirror properly - or else the optical system won't be in a well defined state.  You should pre-load the mirror against gravity by turning the primary focuser clockwise for 1/2 turn at least - then turn it back into focus.  If you don't do that before collimating then the collimation will change if the mirror shifts a bit.

 

Frank


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