Jason D

From visual observation perspective, there is no difference between a fully offset secondary mirror and a partially offset secondary mirror after the proper collimation steps have been followed.

The following diagram shows the difference between a fully offset secondary mirror mount (right diagram) and a partially offset secondary mirror mount (left diagram). In the partially offset secondary mirror mount, the secondary mirror is mounted centrally with respect to its stalk.

For a telescope with a secondary mirror mounted centrally on its stalk (partially offset), the primary mirror will end up getting tilted slightly towards the focuser to compensate as shown in the following diagram. The tilting will be taken care of automatically/implicitly once the proper collimation steps are followed.

The only cases where a fully offset secondary mirror mount would be needed:

1- The OTA opening is too restrictive and tilting the primary mirror towards the focuser will introduce obstructions into the light path as shown below (the obstruction is the vane clip in this case)

2- There is a corrective lens at the OTA opening. In this case, it is imperative to have the primary mirror axis being coincident with the corrective lens axis.

3- You are using DSC to get little more accuracy though the accuracy will be small -- not a major reason for the fully offset case.

Jason

Just follow the proper collimation steps and the "away-from-focuser" offset will be taken care of automatically. In other words, don't do anything special for the "away-from-focuser" offset.

Adjusting the spider vanes to move the secondary mirror away from the focuser might exacerbate diffraction spikes since you will end up with opposite spider vanes that are not inline/parallel with respect to each other.

The only valid reasons why you want to re-install the secondary mirror with the proper "away from focuser" offset are:

1- Your scope has a corrective lens at the OTA front opening

2- Improves setting circles accuracy but the improvement is minimal

3- Avoid front-end vignetting which happens when the OTA opening is almost as wide as the primary mirror diameter.

If the secondary mirror is centered in its stalk then following proper collimation steps will end up tilting the primary mirror towards the focuser as shown in the right diagram of the attachment.

Jason 

"single diagram is going to take away the simplicity I was trying to achieve"

My suggestion is to show a diagram without mentioning whether the secondary was mounted centrally or with an offset. Just don't mention how the secondary mirror was mounted since such an info will have no significance on collimation. That will keep it simple.

"But not if the secondary reflection (x2) were elliptical indicating a secondary  error?"

Well, the "error" has to be taken within context. Achieving axial alignment is the most important goal of collimation. Axial alignment is achieved when the eyepiece axis points directly at the primary mirror center and the optical axis of the primary mirror points back at the eyepiece center. The secondary shadow elliptical "error" you have referenced has nothing to do whatsoever with axial alignment. In that sense, the "error" impact could range from nothing to something. If the elliptical shape is minor and the scope is used for visual observation then the "error" can be safely ignored. If it is major and/or the scope is used for astrophotography then it needs to be corrected. The elliptical "error" only impacts the illumination of the FOV which impacts astrophotography more than visual observation.

But sometimes the elliptical "error" is not an indication of a secondary mirror rotation/tilt error but rather an indication of a slightly off-centered secondary mirror in the OTA and/or a non-squared focuser. In this case, achieving axial alignment will demand rotating/tilting the secondary mirror to achieve axial alignment which will give its shadow an elliptical shape. 

Check the attached animation. In each frame, my scope has achieved axial alignment yet only one frame includes the optimal placement of the secondary mirror. My scope happened to be mechanically aligned so the optimal placement will not show an elliptical shadowy shape of the secondary mirror. If I intentionally adjust the vanes of my scope to shift the whole secondary mirror assembly to the left or right perpendicular to the focuser axis then the optimal placement of the secondary mirror will show a slight elliptical shadowy shape.

Jason

You stated: a=b - That is my goal but IF a user had chosen a non-offset arrangement then it is right to say that IF  i=j THEN a≠b SHOULD be true? 

It is true if i=j then a≠b. But this statement is true regardless whether the secondary mirror was mounted centrally or with an offset. 

You stated: e=f - Incorrect labelling on my diagram. See below

We are in agreement. 

You stated: g=h  - Added to represent a non-centred secondary and therefore highlight an error.

Above statement is true regardless whether the secondary mirror was mounted centrally or with an offset.

For the most part, there is no difference in the collimation view between a scope with its secondary mirror mounted centrally or with an offset. Interestingly, the only difference is the apparent relative placement of one set of the vanes. For a scope with a secondary mirror mounted centrally, the vanes will appear centered with respect to the secondary shadow (figure 2 below). For a scope with a secondary mirror mounted with an offset, the vanes will appear centered with respect to the secondary mirror (figure 1 below). For example, I can tell the secondary mirror was mounted with an offset in your illustration.

Below is the view through my scope with the secondary mirror mounted centrally:

Whether the secondary mirror mounted centrally or with an offset, the optical axis of the primary mirror will intersect the secondary mirror at the same spot, hence, the view (with the exception of the vanes) will look the same.

"New Model" figure corresponds to a scope with a secondary mirror mounted centrally

"Classical Offset" figure corresponds to a scope with a secondary mirror mounted with an offset.

On 30/07/2021 at 01:54, Spile said:

For a well-aligned secondary, I am looking for 

a=b  (or b>a for a centred alignment with no offset)* 
c=d  
e=f   
g=h  
i < j    IF a=b

or i=j  IF *b>a by the correct offset amount

In terms of the view - https://astro.catshill.com/through-the-eyepiece-tube/

a=b  (or b>a for a centred alignment with no offset)*     [[ a=b should be the goal regardless whether the secondary mirror was mounted centrally or with an offset ]]
c=d  
e=f                                                                                     [[ This will always be true no matter what you do. ]]
g=h                                                                                    [[ This should be ignored. If collimation is done correctly for a mechanically aligned scope then g=h will be true. That is, there is no collimation step specifically for g=h. It is an automatic result of a good collimation ]]
i < j    IF a=b                                                                      [[ a=b should always be the target. i<j will always be true for a good collimated scope regardless whether the secondary was mounted with an offset or centrally.]]

or i=j  IF *b>a by the correct offset amount                     [[ This statement should be dropped. It is an incorrect target ]]

Jason

22 hours ago, ZiHao said:

Pointed my telescope to the sky through the window, it shows the center spot but not the focuser edge. Second picture was how I attach the cap, is it correct? 

 

Your collimation as shown on the above photo looks good

Jason

51 minutes ago, David Levi said:

Whether that black circle is central or not depends on the telescope. 

Provided those cross hairs in the picture are centred on the primary mirror centre dot 

Secondary mirror silhouette will always appear skewed towards the primary mirror. It is just more paramount for fast scopes.

The cross hairs in the photo are the spider vanes. These should be ignored when collimating. Refer to my photo in the pre reply. My scope's spider vanes intersect slightly to the right of the center spot. 

1 hour ago, Demonperformer said:

but I think the black circle is supposed to be perfectly central and it looks offset to the right to me.

It should look like that (with an offset). Here is a photo of my collimated scope

2 hours ago, ZiHao said:

I notice that one of the primary mirror clip looks 'smaller' than the others, means I have to adjust my secondary mirror? If yes, how? I don't have either a Cheshire or a laser collimator. Thanks. 

That does not mean much with respect to collimation. I would ignore that observation if I were in your shoes.

Your photo does not provide enough information to evaluate your collimation. The primary center spot is not showing and the focuser edge is also not showing. 

A good collimation cap will have a reflective (or at leave a white) underneath surface.

I reckon Jason, you are a Lone Ranger in understanding all this.   I cannot argue with anything you say, and appreciate your advice and experience, but it's not something I delve into too deeply - I have lots of other things to learn already

Hi Shane,

Below are the two main takeaways I tried to convey:

1- Mounting secondary mirrors with an offset has limited benefits that do not apply to all. There is no need to detach/remount secondaries with an offset. Meanwhile, if someone is mounting a new secondary mirror for other reasons then mounting it with an offset is a good idea unless there is a mechanical reason to prevent it.

2- The idea that some light is lost for scopes with centrally mounted secondaries is incorrect as I have explained.

Jason 

Jason,

I have just measured the aperture at the top of my Orion Optics 300mm F/5.3 optical tube and is is just 7mm wider than the primary mirror diameter. 

Wow!!!! That is way too tight. 

In reality, secondary mirrors in  Newtonians should be pear shape rather than ellipses as the lower edge of the mirror intercepts a wider part of the light cone than the top edge. 

Based on "conic section" geometry, it is elliptical. I know it is not that intuitive but you can't argue with math

http://en.wikipedia.org/wiki/Conic_section

Actually, the secondary mirrors we use are a 45 degree cross section of a cylinder -- not a cone. Both are ellipses but only the latter will appear perfectly circular at 45 degrees angle but only when observed from the cone vertex -- mathematically speaking.

Jason

On my old Orion Optics 10" newt the tube opening was so tight that if I didn't use the offset then one side of the tube opening was always more visible that the other when collimating.  It was clearly visible in my flat frames as a darker area, so with narrow tubes you really should offset the secondary.

Are you stating that the OTA opening diameter is not greater  than ~5 millimters compared to the diameter of your primary mirror? Typically, scopes have larger diameter.

Jason

Shurely the need to offset the secondary applies only when you are using a minimum size secondary ( as is shown in all the above diagrams ). As most Newtonians are fitted with oversized secondaries to increase the size of the fully illuminated field, the extra size of the secondary prevents the problems mentioned. The only effect of having the oversized secondary centrally placed is a fully illuminated field that is not concentric with the optical axis. This is hardly noticeable in visual ( and only at low powers ) and will only affect photographic applications if the sensor size is so large that it's boundaries exceeds the boundaries of the fully illuminated image area.

Nigel

Hello Nigel,

Please refer to my earlier post in this thread -- referring to post#16. It will clarify why I disagree with the statements you have made.

Jason

Jason, a question. If offsetting is a relatively simple process, and assuming your secondary mirror covers the holder still, is there any reason not to do it?

There is one reason when offsetting the secondary mirror away from the focuser could be a problem -- which happened to be the reason why I had to mount my secondary mirror centrally.

When you offset the secondary mirror away from the focuser, the attached stalk will have to move up as shown in the attachment. It  means you need to ensure there is enough clearance between the stalk and the spider vanes structure. In my case, when I upgraded my secondary mirror, the new one was thicker and used up most of that clearance. I did not have much room left to accommodate offsetting the mirror. I could have loosened/tightened appropriate spider vanes to move the secondary mirror away from the focuser but that would have meant getting thicker diffraction spikes since the vanes will no longer be parallel as shown in the 2nd attachment.

Jason

But if the calx is secondary divided by 4x focal ratio, surely 2mm offset is correct?

The formula gives you the offset away from the focuser and towards the primary mirror. It is always the same. In this case, it happened to be 2mm.

I was responding to your statement "then you would slide it in the direction of the arrow by 2mm". The sliding distance needs to be 2mm*sqrt(2)

If you want to calculate the offset, use this simple formula:

Offset = (secondary size)/(4*focal ratio).

This is how much the secondary is offset toward the primary mirror and also how much it should be offset away from the focuser for fully offset collimation.

If you have a secondary of 50mm for your f6 12" then the calc says

 

50/24 = about 2mm so assuming the current position of the secondary mirror is the centre of the secondary is aligned with the centre of the stalk then you would slide it in the direction of the arrow by 2mm (obviously when putting it onto the silicone blobs.

Hi Shane, you forgot about the sqrt(2) multiplier -- as in 2mm*sqrt(2)

Jsaon

Interesting stuff Jason

I might be wrong but I think Stu (the original poster) is talking about an Orion Optics scope. Their tubes and their top openings tend to be narrower than many so I wonder if, in this case, the offset approach is more appropriate ?

The primary mirror tilt is small -- less then 0.2 degrees. That translates to only few millimeters shift at the OTA opening. Check my last uploaded photo. Even with the shift my scope still has a nice margin from the OTA edge.

Jason

The following photo is off my scope. I inserted a laser with holographic attachment in the focuser -- light rays working in reverse. Note the parallel light reflected off the primary then projecting on the paper placed at the OTA opening. The circles are shifted towards the focuser.

if you look at the (exaggerated) S&T illustration 

A shows the secondary glued on the stalk with the centre of the flat glued at the centre of the stalk. This results in part of the light from the mirror not going into the eyepiece. B shows the offset as I described above and the effect is that all the light is then in the right path. It's a set once and forget issue.

Sorry to pick on you again, Shane

That diagram comes from an article written by NIls Olof Carlin (The inventor of the barlowed laser technique). You have misinterpreted it. Diagram C shows a scope with a centrally mounted secondary mirror. Note the green arrows at the primary. The primary mirror is tilted towards the focuser. The whole light is captured in this case.

Jason

The tilting of the primary mirror does not tilt the focal plane at the eyepiece.

Jason

I've sent you a pm Stu but copied it below in case it helps anyone.

Offset is not that big a concern. The main issue would be that if you just put it in the middle (i.e. centre of secondary aligned with centre of drawtube/stalk), a portion of the light from the mirrors will miss the focuser. Offset allows the whole of the light gathered to be 'sent up the pipe'.

 

 

Hi Shane,

What you have described is a common misconception. See attachment.

Left diagram has the secondary mirror mounted with an offset. This is the diagram that comes to mind for most when thinking of incoming light and reflected cone. I included it for reference.

Middle diagram has the secondary mirror mounted centrally. This is what you have described as "portion of the light misses the secondary mirror" -- denoted in shadow black. But this is not what happens when collimating a scope with its secondary mirror mounted centrally.

Right diagram shows what really happens when the secondary mirror is mounted centrally. After collimation is done, both the secondary and primary mirror are tilted. There is no loss in light. 

Main disadvantages for mounting the secondary mirror centrally are:

1- The tilted primary can introduce intrusions in the light path as I have shown with the yellow arrow. This happens only when the OTA/UTA opening is too tight -- most scopes will not have this issue.

2- DSC accuracy might be impacted but the impact will be small.

3- If the scope comes with a front corrective lens then both primary mirror and corrective lens axes will be misaligned.

By the way, I have my secondary mirror mounted centrally.

Jason

Lasers in my opinion cause beginners more problems than they solve - its perfectly possible to get a good laser response when in fact the mirror are not aligned.  Solid lasers like the Hotech or the Blug are a lot better in this regard but also a lot more expensive (I have never used a blug but some people who know about collimation swear they are the bomb).

Hello Mel,

Unfortunately laser collimators get a bad rap due to a basic misunderstanding about their intended use. I ran into countless frustrated beginners who would use laser collimators correctly but get frustrated because they end up with only partial view of their primary mirrors -- not being not able to see all primary mirror clips. No matter what they do, they just can't get the whole primary reflection into their secondary mirrors. Then out of frustration they would seek assistance in astro forums. Most of the time, they get the wrong typical responses about checking the accuracy of their laser collimators using V-blocks and about wrapping them to reduce slop. Many beginners give up and use their laser collimators as paper weights. 

Many miss the fact that there are two independent alignments for secondary mirrors. The first alignment has to do with rounding/centering the secondary mirror under the focuser for optimal FOV illumination. The second alignment has to do with eliminating the focal plane tilt between the eyepiece and the primary mirror by redirecting the laser beam to the primary center. A typical laser collimator handles the second alignment -- not the first one unless it comes with a holographic attachment. The first alignment is what ensures the whole primary mirror reflection is visible through the secondary mirror The laser beam does not interact with the secondary mirror edge and it can't tell you if the secondary mirror is placed optimally under the focuser -- again, unless a holographic attachment is used which spreads the laser beam to interact with the secondary mirror edge. 

In summary, it would be helpful if collimation guides explain the above. A sight-tube or a holographic attachment is needed in addition to a typical laser collimator to complete the task of collimation.

Jason