collimation tips on F4 ?

[Daniel-K]

after watching astonomy sheds advanced collimation were he use's a web cam to align his seconday got me thinking if i could do the same, i hadnt any web cams laying around so i tried the qhy5 and failed so then i got my point and shoot and placed it in the 1.25 holder tightened up the thumb screws and put the live view on and hey presto i can see all the tiny adjustments im making. then when i was happy put the cheshire in and aligned the primary. the view in the cheshire i think looks good but untill i get my farpoint kit i only have a cheshire, how do you guys think it looks?

Dan

[Moonshane]

as far as I can see it looks fine. there's nothing special about f4 collimation other than needing to be closer to 'perfect'.

[Daniel-K]

a quick star test on vega revels very good collimation out but a nats hair until i get the kit thats the best i will get, but im happy as it seemed a fair bit out the other night

[Mr Q]

From the operation manual of my 10" f4.5 newt, it says that the secondary mirror should be offset a few mm towards the primary and towards the focuser from an eyeball alignment. The manual says this is common for fast scopes but nothing more as to why. I'm thinking maybe in imaging respects? And now that I think of it, does anyone know why?

[Moonshane]

the offset on a secondary is set when the secondary is glued to the secondary holder - it's not offset up and down the tube but across the tube and ensures that all the light goes up the drawtube. It's not absolutely critical. Nils Olsen explains it better than I ever can. http://www.skyandtel...iy/3306996.html

[Mr Q]

Thanks Moonshane - Now I know why and how

[Jason D]

the offset on a secondary is set when the secondary is glued to the secondary holder

That is one type of offset which is sometimes referred to as away-from-focuser offset.

it's not offset up and down the tube

Actually, that is also an offset typically referred to as "towards the primary" offset.

and ensures that all the light goes up the drawtube.

That is a common misconception. The main reasons to mount a secondary mirror with an offset (away-from-focuser offset) are:

1- Improve DSC accuracy by getting both the mechanical and optical axes closer

2- Avoid front-end aperture for scopes with tight OTA/UTA opening.

It's not absolutely critical.

Correct. The away-from-focuser offset in not critical especially for scopes without DSC. But the "towards the primary" offset is desired.

Jason

[Moonshane]

I don't disagree Jason but surely if you slide the secondary laterally on the secondary stalk (as it's at 45 degrees) it moves it off centre and therefore across the tube (albeit also down the tube too). the same effect of course!

[laser_jock99]

How critical is getting the CCD bang in the centre of the focuser axis if you use this method?

[Daniel-K]

How critical is getting the CCD bang in the centre of the focuser axis if you use this method?

my theory is that if you tighten the thumb screws up evenly then the camera is going to in nearly the same postion as your imaging camera the star test i done last night looked great

[LeeB]

The best way to understand offset is to draw a light path diagram of an f2 newt , you will see that to catch the line cone the secondary is offset away from and down the tube , the away bit is normally fixed by the position the seconadry is glued and the down tube offset is accumplished automatically by centreing the secondary in the focus tube. As a rough guide its about 3mm both ways in an f4.8 system.

[LeeB]

While the comments about collimation are can be made without it are true a price has to be paid by either lossing some light or having an oversize seconadary. Getting it right also helps when collimating as centrering the secondary and getting it circular and poinmting in the right direct on the optic plane is easier to accieve. wow i must stop typing so fast the spelling does suffer......

[Jason D]

The best way to understand offset is to draw a light path diagram of an f2 newt , you will see that to catch the line cone the secondary is offset away from and down the tube , the away bit is normally fixed by the position the seconadry is glued and the down tube offset is accumplished automatically by centreing the secondary in the focus tube. As a rough guide its about 3mm both ways in an f4.8 system.

The away-from-focuser offset has very little to nothing to do with capturing the light cone. The same light cone is captured for those secondaries mounted without away-from-focuser offset. In the latter case the primary mirror is tilted towards the focuser to compensate.

Jason

[LeeB]

Why make it difficult , if the offsets are done correctly its the easiest way, I agree if not done then a tilt will solve the issue more easily then a secondary moved if glued,, one of my scopes is done that way, I was just trying to explain the offset, not give every remedy for iccorrect set up.

[LeeB]

As the sweet spot on an f4 system is so tight , i make it about 1.1mm error allowance from plane, so as not to induce optical inconsistancies.

On a fast system, I like to barlow a collimated laser as a quick check, as it removes a greater part of the slope in the focuser. However please remember that with such a tight collimation allowance it will likely cause the system to fall from perfection by using at different viewing angles. The barlowed technique allows a quick check in the dark.

[Jason D]

Why make it difficult , if the offsets are done correctly its the easiest way, I agree if not done then a tilt will solve the issue more easily then a secondary moved if glued,, one of my scopes is done that way, I was just trying to explain the offset, not give every remedy for iccorrect set up.

????

I am not trying to make it difficult. I was merely correcting a wrong statement that you made. You stated that unless the secondary mirror is mounted with an offset, part of the light cone will be lost. I corrected you explaining that when proper collimation steps are made the light cone will still be intercepted even when the secondary mirror is centered on its mount. Collimation steps and views are the same whether the secondary mirror is mounted with or without an offset. The tilt of the primary mirror and lowering of the secondary mirror is automatic if the typical collimation steps are followed.

Jason

[LeeB]

To tilt is more a quick fix than a total solution so I find myself in need of correcting you, to tilt a parabolic curve will induce its own errors, and move away from perfection. It would work on a spherical mirror but any tilt (as per an old english newt) will, however small cause a slight shift of the centre axis.

[LeeB]

I feel that we may continue to disagree, and get lost in detail, the guy starting the thread just asked for advice lets get back to giving it.

[Jason D]

To tilt is more a quick fix than a total solution so I find myself in need of correcting you, to tilt a parabolic curve will induce its own errors, and move away from perfection. It would work on a spherical mirror but any tilt (as per an old english newt) will, however small cause a slight shift of the centre axis.

We are so far apart. I do not believe you understood the main points I tried to explain. If you would like to discuss further, I will be happy to clarify my points; otherwise, it might be a good idea to stop.

Jason

[Mr Q]

O.K., let's see if I got this right. The secondary (in fast scopes) is offset slightly as a compromise of the brightest image using the smallest secondary (least obstruction)? In other words, the offset is done to enhance the light grasping performance of the scope?

[Jason D]

O.K., let's see if I got this right. The secondary (in fast scopes) is offset slightly as a compromise of the brightest image using the smallest secondary (least obstruction)? In other words, the offset is done to enhance the light grasping performance of the scope?

It is not about compromise.

1- The away-from-focuser offset (or sliding the secondary mirror on its mount away from the focuser) has to do with improving DSC accuracy and avoid potential front-aperture vignetting. This offset has nothing to do with brightness or capturing the light cone.

2- The offset towards the primary which is introduced automatically once the secondary mirror is centered/rounded under the focuser is meant for optimizing brightness and centering what is known as the 100% illumination field.

Jason

[Mr Q]

Thanks Jason. That's the answer I was looking for.

[LeeB]

Hope this helps, its from sky and telescope and explains it nicely.

Sky and telescope allow it to be emailed and forwarded, so there seems to be no copyright issues by copying it here or printing it out.

Offsetting Your Secondary Mirror

by Nils Olof Carlin

S&T illustrationThe secondary-mirror offset is no doubt the most misunderstood aspect of collimation. Luckily you don't need to understand it to collimate your instrument, but given the level of discussion the subject generates, it's worth delving into.

If you place the secondary centered in the telescope tube, with the primary mirror's optical axis at the center of the diagonal mirror's elliptic face, diagram A (greatly exaggerated for clarity) shows what will happen. The shaded area shows where light from the whole primary can be seen reflected in the secondary — at the focal plane, this defines the fully illuminated field. Inside it, you catch light from the whole mirror; outside of it, some light is lost. You would want the fully illuminated field to be centered in the eyepiece, but as you see here, it isn't — it is shifted away from the primary. This situation is known as nonoffset collimation. Although this condition doesn't cause any great problems, it is easily avoided.

In diagram B, the offset is achieved by sliding the secondary away from the focuser and toward the primary. Now the fully illuminated field is centered in the eyepiece, but the secondary is no longer centered in the telescope tube. This is known as fully offset collimation.

But what if you want the fully illuminated field centered in the eyepiece, but must leave the secondary mirror centered in the telescope tube? It can be done, as shown in diagram C, by slightly adjusting the tilt of both mirrors. Now the optical axis is slightly tilted within the telescope tube. In practice, this is not a problem because the tilt is never more than a small fraction of a degree. Since the secondary is offset down the tube, this is known as partially offset collimation. It is no doubt the most common situation, even among telescope owners who may not even realize that their scope's secondary is offset at all.

If you use a sight tube to center the secondary as described in Step 1, you have automatically offset the secondary toward the primary mirror, thus ensuring that the fully illuminated field is centered. (When using the sight tube, you make the near and far edges of the secondary appear to have the same angular size. This means that the distance from the far edge of the secondary to the optical axis is greater than from the near edge to the axis. This constitutes an offset.) Both partial and full offset conditions give good collimation.

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.

[Jason D]

Hello Lee,

That is a good article but it is unclear to me which point you are trying to support with the article?

If you look closely, figure “C" in the article is equivalent to 2nd figure I uploaded earlier in this thread. The point of the article is that you can intercept the whole light cone even when the secondary mirror is mounted centrally on its mount – that was my point.

By the way, that article is quite popular and I happened to know the author, Nils Olof Carlin – a true gentleman.

Jason

[LeeB]

I was trying to show I understand your comments and thought coming at an explanation from a slighly different angle would help, i think within this thread we got off on the wrong foot ., (my comment about light cone assumed a straight primary and central light path through focuser) and it was that situation (ie without the remedial tilt there would a price to pay). At the end of it all we likely both have very well collimated scopes.

Personally I have always found it easier to demontrate than to advise.