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Jason D    233
On 4/20/2018 at 04:01, Buzzard75 said:

A collimation cap or a cheshire is the only way to ensure there are no (minimal) rotational errors in your secondary/focuser alignment. 

Just curious. Can you clarify what you meant by secondary mirror rotational errors? And how do you use the collimation cap to minimize this error?

Jason

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Buzzard75    159
28 minutes ago, Jason D said:

Just curious. Can you clarify what you meant by secondary mirror rotational errors? And how do you use the collimation cap to minimize this error?

Jason

One of the first steps in collimating is lining up the secondary mirror to the focuser. You need to make sure that the secondary mirror isn't rotated one way or the other and is perfectly straight with the focuser. If it's not, it will be impossible to collimate correctly. You can't use a laser for that part of the collimation. You have to look at it visually though either a collimation cap or a Cheshire. You could probably eyeball it and get it pretty close without, but those two devices make it much easier to tell if it's out of alignment (i.e. rotated one way or the other or in or out of the tube too far). If you just try and collimate with a laser and don't check this alignment with a cap or a Cheshire, it's possible you could introduce some rotation, by the tightening and loosening of the set screws on the secondary post, when trying to align the secondary to the center spot on the primary.

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Jason D    233

Suppose I start off with a perfectly collimated reflector then I follow these steps:

1- I loosen the secondary mirror set screws by a tiny bit

2- I rotate the secondary mirror by a small amount

3- I insert a qualify laser collimator then adjust the secondary mirror to redirect the laser beam back to the primary mirror center but by only using the set screws -- I do not hold the secondary mirror stalk and rotate it whatsoever. Adjustments of this step are only carried out by touching the secondary mirror set screws.

4- I finally readjust the primary mirror to redirect the returning laser beam to its source.

 

In your opinion, can you describe the negative impact of the above secondary rotational error on the view at the EP? For example, will coma get worse?

 

Jason

 

 

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Buzzard75    159
5 hours ago, Jason D said:

Suppose I start off with a perfectly collimated reflector then I follow these steps:

1- I loosen the secondary mirror set screws by a tiny bit

2- I rotate the secondary mirror by a small amount

3- I insert a qualify laser collimator then adjust the secondary mirror to redirect the laser beam back to the primary mirror center but by only using the set screws -- I do not hold the secondary mirror stalk and rotate it whatsoever. Adjustments of this step are only carried out by touching the secondary mirror set screws.

4- I finally readjust the primary mirror to redirect the returning laser beam to its source.

 

In your opinion, can you describe the negative impact of the above secondary rotational error on the view at the EP? For example, will coma get worse?

 

Jason

 

 

You rotated the secondary mirror so you're not going to be perfectly collimated. When the light bounces off of the secondary mirror, it won't be on a straight path to your eye, it will be toward one side of the focuser/eyepiece. When that light gets bent by your eyepiece, your stars will most likely appear to be misshapen. It will be much more evident when out of focus. On top of that, trying to adjust for that misalignment of the secondary with the primary mirror is going to make your stars look even worse and will be difficult, if not impossible. A hair off here or there in the secondary isn't going to make a huge difference and won't be noticeable or at least not distracting enough to correct it. If it's off by too much though, you'll have difficulty with the rest of your collimation, if you can get it to collimate at all, and you'll have terrible looking stars. You simply can't align a secondary with the focuser using a laser unless there's some method I'm not aware of. It has to be done visually.

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Geoff Lister    234

If your 'scope came with a blanking/dust cap protecting the eyepiece holder, a 1mm hole drilled in its centre makes a very simple collimation cap.

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Jason D    233

There are several misconceptions about collimation. One of these misconceptions is that secondary mirror adjustments via the set screws are completely independent and orthogonal to secondary mirror rotation. That is, starting off with a perfectly collimated scope, once the secondary mirror is physically rotated by a small amount there is nothing that can be done to undo the impact of the rotation -- incoming star light will never be perfectly aligned with the EP.

Mathematically speaking, adjusting the secondary mirror via the set screws does have a "rotation" component and therefore it is possible to undo the impact of the secondary rotation and restore perfect axial alignment -- without physically rotating the secondary mirror back to its original position. I know this does not make sense. Here is a hypothetical example: Assume the secondary mirror stalk has a hinge. Start off with a perfectly collimated scope. Now flip the secondary mirror to face the opposite side of the focuser via the hinge. The mirror is still at 45 degrees but facing the opposite direction. This setup is equivalent to physically rotating the secondary mirror 180 degrees. Similarly, by adjusting the secondary mirror using the set screws we cab achieve a an equivalent effect to small amount of rotation. The rotation component is small; therefore, it can only undo small amount of physical rotation to the secondary mirror. If the secondary mirror is physical rotated by a large amount then we will need a huge amount of tilt adjustment.

You might wonder if I start off with a perfectly collimated scope, rotate the secondary mirror by a small amount, recollimate then the final result can't be exactly the same!!!! You are correct. Even though axial alignment is exactly the same (incoming star light is perfectly aligned with the EP), the secondary mirror will not appear perfectly circular under the focuser. It will be slightly oval but that has no impact on the central area of the FOV and a minute impact only on the illumination of the stars around the FOV edge -- which might be noticeable via astrophotography but not visually.

Jason

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