Fixing a secondary mirror in place

[Stu]

Quick question, I have a recoated secondary coming back to me shortly, what is best to use for fixing it onto the secondary support?

It seemed to be held with some form of silicon I guess? Any particular type I should get?

Cheers,

Stu

[Moonshane]

Decent quality clear is best. OOUK will usually glue on if you ask. Don't forget your offset if you do it :0)

[Stu]

Thanks Shane, just the sort of stuff you get from a DIY store?

Offset?? Can you explain?

Cheers!

[Moonshane]

Yeah as long as it is clear it is nothing special. Offset:

http://www.skyandtelescope.com/astronomy-equipment/offsetting-your-secondary-mirror/

[Stu]

Hmmmmm, must be being a bit daft, I need to get my

head around that in the morning!

Cheers Shane

[Moonshane]

I will try and dig out the calculations from Krieg and berry. Basically you squish it down the stalk a but towards the primary to ensure all the light is collected and directed to the right place. Not something to overly worry about.

[John]

I found this formula used on more than one source:

Offset = Diameter of Primary x Diameter of secondary divided by 4 x FL of primary

[Moonshane]

Looked in k&b and not much use as they start at 14"

I would use Johns formula (you know what I mean :0)

[Peter Drew]

Place a washer or a coin in the centre of the secondary holder then add 3 blobs of silicone at 120 degrees somewhat thicker than the washer/coin. Then press the secondary gently on to the silicone until it touches the washer/coin. This ensures the mirror seats squarely and gives a gap to facilitate eventual removal. 

[Stu]

Thanks all. Where is the offset measured from then? I can calculate the value but don't know what to do with it! :-)

[Moonshane]

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'.

 

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.

 

 

you then align the centre of the secondary with your drawtube and the offset is 'built in'.

[Stu]

Thanks Shane :-)

[John]

When I look down on my secondary it's offset exactly the way that Shane has demonstrated. Not a large amount though because my scope is F/5.3 so the offset should be 2.97mm for my 63mm diameter secondary.

[Moonshane]

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.

[Stu]

Thanks Shane, I get it now! Was reading the diagram incorrectly

Cheers

Stu

[Jason D]

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

[Jason D]

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

Jason

[Jason D]

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

[John]

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 ?

[Jason D]

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.

[Jason D]

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

[Moonshane]

LOL I give up!

:0)

Thank goodness it is something we rarely have to think about!

[Jason D]

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

[Moonshane]

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

[faulksy]

:confused: :grin: