Fixing a secondary mirror in place

[Stu]

Crikey, I didn't expect to generate this level of debate! [emoji6][emoji6]

Thanks all for the input, I have a far greater understanding of it now.

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?

[Moonshane]

You can tell the weather is rubbish 

[Jason D]

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)

[Jason D]

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

[Astrobits]

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

[Starflyer]

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.

[Peter Drew]

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. 

[Jason D]

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 D]

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

[Jason D]

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

[John]

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

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. 

[Moonshane]

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

[Jason D]

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. 

[Jason D]

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 

[John]

Wow!!!! That is way too tight. 

I'd better drop a line to Orion Optics then - they have been making them like that for the past 30 years

[Stu]

My 300mm is 323mm diameter at the top of the tube so a little more generous than yours John

[John]

The tube on mine is the OO standard of 326mm. It's the top ring around the tube and it's lip that reduce the actual aperture 308mm +/- a mm. That the same as the VX range use now.

[HD-AP]

I hadn't

realised that orion optics uk ota diameters to primary were so tight .

My old fullerscopes have an inch at least i.e. 10 inch primary mirror dia 12 inch ota id .

Given this tight clearance i would have thought that image degrading tube currents would be an image degrading problem ,potentially !  ?

My planetary newtonian with optical window is 12.5 inches ota 10 inch mirror a 1.25 inch clearance alround .

And alloy tubes don't mitigate or eliminate this effect .

Surprised at that ,be interested to know the reasoning for it .  

[Stu]

The tube on mine is the OO standard of 326mm. It's the top ring around the tube and it's lip that reduce the actual aperture 308mm +/- a mm. That the same as the VX range use now.

Ah yes, I've got my top ring removed at the moment so was just measuring the actual tube internal diameter. I forgot there was such a lip on the ring.

[mdstuart]

My goodness what a thread. I am about to re-attach my secondary. The main mirror is 500mm and the focal length is 2000mm and the secondary is 95mm wide. I will shift the secondary 6mm towards the main mirror and as long as I can see the secondary as a nice circle through the focuser then I will be happy.

Mark

Edited June 24, 2020 by mdstuart
Update figures

[fozzybear]

23 hours ago, mdstuart said:

My goodness what a thread. I am about to re-attach my secondary. The main mirror is 500mm and the focal length is 2000mm and the secondary is 95mm wide. I will shift the secondary 6mm towards the main mirror and as long as I can see the secondary as a nice circle through the focuser then I will be happy.

Mark

have a look at an old app newt I've attached a copy for you to play with to calculate offset

 

newt25.zip

[Merlin66]

Just a comment:

The secondary mirrors we buy are not elliptical! They are a cylindrical section ( easier to make)

 

[mdstuart]

Thank you everyone.

As long as my epoxy and silicon hold and my mirror does not fall and break later today I will be happy!

Mark