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100% Secondary off-set in Newtonians


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Following discussions on my mask for laser alignment, I've put together a couple of images ( From Suiter's great book " Star Testing Astronomical Telescopes") which may help to explain the concept of secondary off-sets in Newtonian telescopes.

The elliptical secondary mirror has to be positioned on the optical axis at 45 degrees to the intersection of the primary mirror axis and the focuser axis. To reflect the maximum amount of light to the eyepiece it must be sized and positioned correctly in the converging cone of light from the primary mirror; depending on the FOV required, you can use small or large secondary mirrors.

When the secondary is placed at 45 degree in this reflected cone of light the MECHANICAL centre must be off-set slightly down the tube ( ie towards the main mirror) and away for the focuser; this brings the OPTICAL centre onto the axis.

This is well shown in the first diagram. The red dot represents the OPTICAL centre and the green dot the MECHANICAL centre. You see the red dot in on the optical axis, and the green dot is lower.

When measured along the surface of the secondary mirror ( because it's sitting at 45 degrees, the physical difference between the two points is 1.7 x secondary offset.

Still with me.....??

When you align the optics using say a Collimating cap or a Cheshire eyepiece, you see the red dot central to the image of the secondary due to foreshorting, so if the secondary mirror appears circular and centred, it's close to the correct position.

This is shown in the second and third diagram

When the primary mirror is adjusted to get the reflection of the secondary central the red dot would also be in the centre.

Note that with the off-set applied the secondary mirror still appears central- ie on the optical axis.

The final "tuning" is shown in the last diagram where the centre spot on the main mirror is brought inline with the small hole in the Collimating cap/ Cheshire.

If you have a large fast newtonian ie F4 or so, you have to make allowance for the off-set of the diagonal. In these "fast" systems the diagonal is not placed on the optical axis but displaced (Off-set)by an amount = T/4F*F

where T is the distance from the centre of the tube to the focus and F is the focal ratio.

This formulae, for a 400mm f4 mirror, based on T= 320mm, gives a secondary mirror off-set of 5mm.

Hope this helps.

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"Merlin66"

You explain the "concept" of "secondary offset" clear enough - but how does this affect (or not) the physical act of collimating ones scope?

I thought collimating a scope simply involved centering the seconday mirror, then fine tuning the main utilising the paper circle stuck to the exact centre of the main mirror.

Regards,

philsail1

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Phil,

Centering the secondary in a Cheshire etc effectively compensates for the "towards the main mirror off-set" but doesn't show or confirm the radial ( away from eyepiece) off-set.

If this is not designed into the secondary mirror holder then the reflection of the secondary in the main mirror should NOT be central but slightly off-set ie the usual circular doughnut shape is not maintained and some vignetting will occur.

Remember we're talking about big fast newtonians, on smaller scopes the error is minimal.

I was attempting to show that when the secondary mirror is set up on the optical axis a laser beam can collimate the optics as good as if not better than the Cheshire/ Collimating cap.

Edited by Merlin66
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Am I right in thinking that the secondary is only likely to fall out of the optical light cone if it is the optimum size or smaller?

If it is larger than the optimum, then you will gain nothing by offsetting.

A useful piece of free software to check the optimum secondary size is Dale A Keller’s NEWT, available here.-

http://www.dalekeller.net/ATM/newtonians/newtsoft/newtsoft.htm

Paul

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Paul,

The secondary offset should not affect the sizing of the secondary; this is usually calculated on the assumption that the secondary is positioned correctly and based on the FOV required at the focal plane.

Certainly if the secondary is large enough the offset doesn't affect the efficiency of the system but makes collimating by using the mirror outlines more difficult. The ultimate star test will show any negative impact.

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Ah! "Merlin66,"

Your last sentence has clarified the collimation worries I was beginning to have with the secondary mirror "offset." When you say "The ultimate star test will show any negative impact".

So, you may colllimate in the usual way (using either a simple collimating cap, a "Cheshire," or laser collimater), then do a final check on a star (at fairly high magnification).

What I didn.t understand is how are the two spots (red and green) placed for you to obtain a reading if things are "out?"

Regards,

philsail1

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Phil,

The green and red spots diagramatically show the relative postion of the secondary mirror.

On a fast system if the secondary is not off-set radially to bring the optical centre on axis, the main mirror MAY end up being slightly tilted off-axis during the normal collimating process to make it "align" with the outline of the secondary... this will compromise the collimation and add to the coma in the field.

Unless the primary is aligned with the optical axis and the optical centre of the secondary, the system is not properly collimated.

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Sorry for sounding a bit thick "merlin66," but what I don't understand about the "green and red dots" is:- "how are they placed on the mirrors with accuracy in the first place?"

With regard to the rest of your answer (if I have understood you correctly!)

Is it the case that if during collimating, one accidently allows (through several collimating adjustments over a period of time) the scopes secondary mirror to move substantially off the optical centre of the tube, and then (in all innocence) re-align the main mirror (using a collimating tool) with the (by now) off centered secondary mirror, the whole system - although appearing successfully colimated - will actually be "out of true" and show excessive "coma" - which of course a star test will not show - (if the star is viewed at the centre of the scopes field of view).

I ask this because my Orion Optics 8" f4.5 Newtonian does indeed show some coma at the outer edges of its field of view, whereas I didn't get this with my old Skywatcher 8" Newt.

I'm wondering now if this coma is indeed through me inadvertantly allowing the scopes secondary mirror to drift "off the optical" centre - or is it an inherrant fact due to the shortness (850mm) of the scopes focal length? (which tends to show "coma" when using the cheaper eyepieces).

Regards,

philsail1

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Phil,

1. The green and red dots represent the mechanical and optical centres of the secondary mirror. The green dot is physically positioned on the measured centre of the mirror. The red dot is then positioned x1.7 of the design "off-set" towards the top of the mirror.

These are "virtual points" and only highlighted to show the impact of mis-aligning the secondary mirror.

If a mask is placed over the secondary with a 3mm hole at the red dot position, a laser beam can be used to centre and collimate the optics, as per my other post.

2. The star test will show any residual coma etc. The mis-alignment of the primary only aggravates the inherent coma in fast newtonians.

Any f5 ( or faster) newtonian system WILL have coma; this can be corrected with the use of "coma correcting lenses" ie Ross lens etc.

The bottom line is: unless the secondary off-set is taken into account on fast newtonian, the collimation will be compromised.

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great thread albeit slightly over my head. when i collimate my F4.8 newt, I am aware of the offset only becasue it is visible whne I centre the primary using a cheshire. However, i don't take the offset into account when centring the secondary (but then when I come to centre the primary the offset seems "there" anyway). is the offset then somehow built into the scope to the effect that it withstands my foolish tinkering and/or have I misunderstood?

Again, really interesting thread. Thanks.

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Thanks for your reply "Merlin66."

Could I now ask - "how would one go about checking for any possible excessive "offset" on a particular scope?"

Would I be correct in assuming that one could obtain (from the scopes manufacturer) the figure for the amount of designed "offset" a secondary mirror should be set at? If this figure cannot be obtained, how would I find out what the designed offset position should be?

Regards,

philsail1

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Kniclander,

By centering the secondary mirror in the outline of the focuser tube, as per diagram 2, the axial "off-set" is applied automatically.

When you centre the reflection of the main mirror in the secondary, you are basically "compromising" to get the best fit. If the secondary mirror is radial well out of position, you will see a slight brightening of the edge of a star image close to focus. Unless the secondary is re-positioned, this cannot be removed by further "collimating"

Phil,

As per a previous post, you can easily calculate the off-set:

If you have a large fast newtonian ie F4 or so, you have to make allowance for the off-set of the diagonal. In these "fast" systems the diagonal is not placed on the optical axis but displaced (Off-set)by an amount = T/4F*F

where T is the distance from the centre of the tube to the focus and F is the focal ratio.

This formulae, for a 400mm f4 mirror, based on T= 320mm, gives a secondary mirror off-set of 5mm

To show this even more clearly I attach a couple of images of my 150mm f3.6 Cometseeker... you can see the central black boss which holds the secondary mirror, and the outline of the secondary well off-set away from focuser.

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That's clarified it for me "merlin66." (especially the excellent photo). I fully understand the "off set" set up now.

I will use the info to double check my Orion Newt.

Thank you very much for your help - and patience with my ignorance of technical matters.

Kind regards,

philsail1

P.S. glad you got some help too "nickcandler!"

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Aye "phisalis", as they say, I am none the wiser but at least i am better informed :D. My knowledge of colimation is a bit like collimation - when I think I have got one bit, I lose another bit :).

Merlin66, your posts are a pleasure to read - thanks for taking the time :). Unfortunately, you are fighting vast hordes of ignorance on my part but I am doing my best.

K n i c l a n d e r

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  • 1 month later...

I'm wondering if you do house calls.. I've posted elsewhere on SGL some Q's on correct collimation on my (new to me) LB 16" F4.5 In essence, the previous owner always used a simple cap on this scope (he is of 20+ years experience with fast newtonians). Over the period of a couple of hours playing with the scope, he clearly demonstrated how it's easy for a laser and the cheshire to "get it wrong" but the hole in the cap (with focuser fully extended) got it right every time.

However, that was on Sunday - and I may well have forotten by the time I go to recollimate my scope.. I've got to now learn all about this secondary offset in more detail.. I think Meade did this with unequal length vanes on the spider but I cannot be sure. However, I do recall seeing the offset clear as night and day

You ought to post a youtube tutorial on this subject merlin 66 :icon_rolleyes:

Edited by albedo0.39
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I'm in Surrey, M25/ A3 intersection; always available to help!!

The good news I think, is the previous owner's experience with fast systems and I'd hazard a guess that the secondary off-set has already be addressed. To an experienced eye, the collicap can do the initial collimation very well. You still need that final "tweek" under the stars to get 100%

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You still need that final "tweek" under the stars to get 100%

Hi Merlin and thanks for that. From your experience, is the final tweek most likely to be on primary or secondary (or is there no "rule" )

?

Thanks

Edited by albedo0.39
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Secondary partial offset is when you have adjusted the tilt and rotation of the secondary so that the elliptical face of the mirror looks round from your eyes perspective and have centered that 'round' mirror face under the sight-tube/colli cap. The secondary is thus partially offset towards the primary.

Full secondary offset is not really necessary except in 1 instance. With partial offset the optical axis is not aligned with the actual scope tube axis. This was no bearing on the quality of the collimation nor on the views through the eyepiece. What it does have a bearing on though are DSC's. So if you don't have DSC's fitted to the scope you don't have to worry about full offset.

Which brings me to my point. With your LB16 you definately don't have to worry because you already have full offset, (assuming you centre your secondary under the sight-tube.) When the collimation experts talk about adjusting the spider to move the secondary away from the focusser to achieve full offset, what is never clarified and which is something that confused me for months, was that it is assumed by them that you have a secondary mirror holder and not a stalk mounted secondary. With the holders the mirror is smack bang directly underneath the centre of the spiders. To move the mirror away from the focuser with a 'holder' on does of course need to adjust the vanes. However a lot of chinese dobs have stalk mounted secondaries. Our LB16's and my old Orion XT12i have stalk mounted secondaries. Guess what. Meade and Orion already incorporated the 'away' from focuser offset when gluing the mirror to the stalk. See the way there is about 12mm between stalk and the edge of the mirror on one end but about 25mm between the stalk and mirror edge on the other end. Thats the 'away from focuser offset'.

So you don't need to adjust the spider at all, save to make sure it is back dead centre in the tube if you already messed with it.

With my Orion when I thought I had to adjust the spider to move the mirror away from the focuser, what I was actually doing was adding way to much offset and I wondered why A. My Intelliscope DSC warps got worse not better and why I was suddenly having a harder time centering the laser in the donut while maintaining a round mirror face in the sight tube.

To confuse things even more some Meade Lightbridges like the 12" 's have a holder.

To save all that head scratching, there should be small print after all collimation advice. "This advice assumes a secondary holder...except for those with stalks like the Meade LB16.........but excluding the LB12.......doesn't matter unless you use DSC's..... depends whether one set of spider vanes is parallel with focuser axis.....

Of course I jest, thats not practical, but really collimation advisors should always clarify what scope the questioner has before starting to advise. Its something now do.

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Hello "Calibos"

I've just returned to this thread, and I note your comments with regards to

"Secondary partial offset is when you have adjusted the tilt and rotation of the secondary so that the elliptical face of the mirror looks round from your eyes perspective and have centered that 'round' mirror face under the sight-tube/colli cap. The secondary is thus partially offset towards the primary".

So are you saying that telescopes are manufactured with a secondary mirror offset (auto) adjustment inbuilt into the mechanical design of the seconday mirror holder? Once one has adjusted and centred the seconday so that it appears as near as round one can discern, then the seconday is automatically "offset?"

If this is the case, then the whole matter of ensuring the amount of secondary offset becomes very easy to achieve!

Regards,

philsail1

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As Calibos says:

Meade and Orion already incorporated the 'away' from focuser offset when gluing the mirror to the stalk. See the way there is about 12mm between stalk and the edge of the mirror on one end but about 25mm between the stalk and mirror edge on the other end. Thats the 'away from focuser offset'.

If this is true of all the larger scopes, then there's no need to worry about this aspect of the collimation. Sighting with a Cheshire/ Collicap will confirm the axial alignment and that's it done!

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Thanks to this fascinating thread-I went back and had another look at my collimation.

I noted in one of the links I found myself following re the Cometraker ( F3.6 :icon_rolleyes:) and the Celestron "Cometron", someone had mentioned that sometimes collimation can look spot-on but only when coming almost (and I mean almost !) to focus, then and only then, do the otherwise concentric airy rings appear eccentric !

Sounded familiar and sure enough it was- pretty sure the scope is NOW collimated to within a micron of it's life.

PS I feel abit like Phil .

PPS no laser insight !! -I'll never use one again, all done using a sight tube, collicap (Hme made out of an AE ally Barlow cap !) and threads such as these.

Karlo

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Hello "Calibos"

I've just returned to this thread, and I note your comments with regards to

"Secondary partial offset is when you have adjusted the tilt and rotation of the secondary so that the elliptical face of the mirror looks round from your eyes perspective and have centered that 'round' mirror face under the sight-tube/colli cap. The secondary is thus partially offset towards the primary".

So are you saying that telescopes are manufactured with a secondary mirror offset (auto) adjustment inbuilt into the mechanical design of the seconday mirror holder? Once one has adjusted and centred the seconday so that it appears as near as round one can discern, then the seconday is automatically "offset?"

If this is the case, then the whole matter of ensuring the amount of secondary offset becomes very easy to achieve!

Regards,

philsail1

Just to clarify.

There are two offset parameters. There is the 'towards the primary offset' and the other is the 'Away from the focuser offset'. When 'towards the primary offset' is done but not the other you have done a 'partial offset' collimation. When both offset adjustments are done, you have Full offset collimation.

The partial offset 'Towards the primary' is just a way of saying that you have adjusted the centre screw of your secondary holder to move the holder and mirror further down the scope tube in the direction of the Primary below..'towards the primary'. Picture it like this. Imagine you took the secondary out of the scope and measured along the major and minor axis of the elliptical shaped secondary mirror and then marked the exact centre of the mirror. If you then centered your secondary centre mark under your cheshire/sightube/collicap you would in fact have no offset at all. You could still adjust the tilt and rotation of the secondary to centre the reflection of the primary but without offset you could never achieve a perfectly round view of the secondary. I have seen all the diagrams showing the tilting of the lightcone and all the terminology about PAE, FAE etc. I can't claim to understand it all but the consensus seems to be that even with no offset one can still get decent views, its just with no offset your FOV in the eyepiece is unevenly illuminated or parts are vignetted by the scope tube. Not so much a problem when looking at planets or small galaxies which you would have centred in your EP but on extended objects or objects viewed at higher mag which fill the FOV more, they are extending into areas of your FOV that are not optimised because these areas are not illuminated fully with your Zero offset collimation. This is why everyone should at least achieve the partial offset 'towards the primary' collimation.

By moving the secondary further down the tube to centre the round face of the secondary we are evenly illuminating the whole field.

So basically for partial offset its a simple matter of adjusting the centre screw to move the secondary further down the tube and adjusting the tilt so that you see a perfectly round face of the secondary centered within the round edge of the bottom of your sighttube. A way to make this easier without other reflections confusing you is to block off the primary. Some people use a piece of card below the secondary. With our LB16's just leave on your dustbin lid on the primary while adjusting your partial offset. Then put a piece of graph paper or maths copy book paper along the inside of the scope tube behind the secondary opposite the focuser. That way you can count the graph paper squares around the round secondary to ensure you have it perfectly centred under your focuser/sight-tube. This adjustment only needs to be done once unless you take out your secondary in the future for some reason.

Full offset like I said is not important unless you have DSC's. Not having full offset does not affect the views in anyway unlike not having partial offset does.

But it turns out that owners of chinese scopes with a secondary glued to a stalk have full offset built in from the factory anyway whether they need it or not unlike owners of scopes with a secondary that is held within a hollow stalk with clips. Like I said it gets a little confusing giving advice unless you know which type of secondary stalk the person has. My advise that full offset is built in at the factory doesn't even apply across the same brand of scope. EG, our LB16's have a secondary glued to a stalk with the 'away from focuser for full offset' built in. The Meade LB12's however have a secondary 'holder' which by its design cannot have the away from focuser offset built in. So its the MEade LB12 owners and most premium dob owners that have to adjust their spider vanes to move there secondary away from the focuser towards the other side of the tube if they want to achieve full offset, while us LB16 and Orion XT owners just have to make sure our spiders are exactly centered because our away from focuser offset is already built in by the factory gluiing the secondary to the stalk...offset already

Just another thing I thought I might clarify because its something that confused me for ages. A lot of the simple collimation diagrams show concentric round....everything :icon_rolleyes: ie. Cheshire/collicap eyehole centred within the primary donut, centred within a round secondary face which is dead centre under the spidervane reflections dead centred within the edges of the bottom of the sighttube. Trying to achieve that will mean you adjusted out all your offset!!

If you have collimated correctly with full offset, ie towards the primary offset and away from focuser offset (built in already in our LB16's) you should see the following.

A round face of your secondary centred within the boundaries of the end of your sight tube with the reflection of the primary centered within it, with all three primary mirror clips visible. Now just focus your eye on the reflection of the secondary that you can see. Your peep hole should be centred in the primary donut and this should be centred in the round reflective surface of the secondary but that round reflective surface wont be smack bang centred with respect to the reflections of the spider vanes. It will look like its off centre with respect to the spider vanes to the left towards the open end of the tube. On the right hand spider vane towards the direction of the primary end of the scope you should see the non reflective edge of the elliptical secondary. Of course on scopes that don't have one of the spider vanes parallel with the focusser axis, this offset shift visible in the collicap/cheshire wont be along a spider vane to the left and right but will still be towards the open end of the tube and towards the primary end of the tube

I'll try and dig up some diagrams.

Once it all clicks into space in ones head collimation suddenly becomes the easiest thing in the world. The problem is that for most people there is confusing information out there, overly simplyfied diagrams, advice based on the wrong type of secondary holder which confuses the offset advise. even the fact that most US scopes have the focuser on the other side of the scope confuses one when someone says, offset to the right, which would mean to the left in our shinese scopes with the focusser on the other side. Trying to incorporate all this advice on ones head and not getting the expected results because of all the confusions and assumptions made on the part of the person giving advice means that one thinks that this collimation lark is not inuitive at all and one worries one will never get their head around it. . When in actual fact it is very intuitive and simple when you know what type of secondary mount you have or who's left they are talking about...."No your 'other' left" ;)

[Edit], just realised that I quoted Phil but he doesn't have an LB16 and also realised how wordy the post is and I am starting to confuse even myself. :) Collimation is one of those things that sounds confusing written down, remains confusing even when one has diagrams to look at, but were I standing beside you at the scope and you could view through your cheshire while I described what you should see and how to adjust everything. It would all click in 2 minutes flat.

Edited by calibos
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