Collimation confusion

[Kokatha man]

Hi leinad - yes, it does become extremely confusing when one tries to fathom the absolute depths of collimation!

All the links suggested (including your own) are good to digest - but it can also cause added confusion which is often semantics as much as specifics. I've read (and heard) some heated arguement on these matters over the years: with people taking up positions that ultimately betray their own lack of understanding of certain aspects.

Which is to say pointedly that I'm no expert and many things still have me scratching my noodle: but I reckon that if you can get your scope to perform well with no (or very little of the) nasties like coma etc, and star-testing gives good diffraction ring images you are fairing way better than 90 odd percent of AA's!

Just a couple of things I've gleaned by re-reading the links provided in this thread and bouncing them off my own thoughts/experiences:

Bartels (imho) seems to be giving the methodology for "full" offset collimation whereas Carlin is opting for "partial" offset.

There "final View" diagrams are different, Bartels one being what you seem to be getting frustrated about you not achieving; whereas Carlin's equates with your attached image - figure where you stand there!

I repeat, these are non-expert opinions of mine but I've been led to believe that partial offset is preferable to full offset - the reason I thought was something to do with the amount of impedence the secondary creates for the primary (my presumption!)

In Bartels article there is the confusion of him stating that his "controversial advice to many amateurs" is not to square the focusser with the tube (ie mechanical collimation) giving one of his feasons as the difficulty of adjusting the focusser alignment - but in his "how-to" launched into shimming the focusser!

It would seem axiomatic to me, that Bartels' method of mechanically adjusting those 2 spider arms he mentions in his article, creates the full offset of the secondary and (possibly) creates the situation in his final collimation diagram wherein the reflection of the focusser tube is moved "up" towards the reflection of the secondary mirror (the thing you're getting frustrated about.)

Conversely, in Carlin's article (the first link Rajah's given you) he describes the partial offset methodology - in particular re-read paragraphs 4 & 5. This I presume produces the scenario in his "final collimation" diagram, which is similar to your own!

Finally, do what Rajah suggests and just get out and scope it out, if you're getting coma "comets" you'll know it and dong a star-test on a clear still night will give you more info.

I hope this hasn't been too long-winded: your problems are, I assure you, the same for many of us no matter how long we have been "at it" - it's just that most won't admit that collimation confuses them, and near-enough is good-enough.

Hopefully none of the above has made you more confused: I think it has helped me by articulating it - as I said, rajah's last bit is the most important

[ngcles]

Hi Leinad, Daniel & All,

Daniel wrote:

"yes, it does become extremely confusing when one tries to fathom the absolute depths of collimation!"

Too true, too true ...

If you want to observe the degree of angst the subject of collimation method (laser/cheshire etc etc) and offset (partial/full), why not join the Bigdob group of the Yahoo groups, pose a question on partial/full offset ... and stand well back -- ithe effect is akin to stirring a hornets nest with a large stick.

Both the partial and full offset camps have their devotees and for what it is worth, I don't think either of them are wrong per se -- they are just different methods for producing collimated optics with a centred and circular 100% field.

I have a different opinion. It is just my opinion and not an "answer".

Here it is: I don't do either. (Cries of heretic echo around).

Why?

Why collimate?

The purpose of collimation is to align the optical axis of the telescope's optics, with the optical axis of the eyepiece. If you don't do that, image quality will suffer.



Why offset?


To borrow a little from Nils site here (so you can read in full):

http://web.telia.com/~u41105032/kolli/kolli.html#Err2

But the salient part is reproduced here:

The secondary mirror has an elliptical surface with a major to minor axis ratio equal to the square root of 2, for 90 degree deflection. Depending on its size, it lets some of the focal plane be fully illuminated, that is any point within the area of full illumination sees the whole primary mirror reflected in the secondary. Outside of this, some light is lost.
Due to the 45 degree tilt, the elliptic surface appears circular when you see it with your eye centered on the optical axis near the focus. However, due to the perspective, the center of the circle you see is offset from the geometric center of the ellipse, towards the edge nearest to the focuser. To be optically centered, the secondary mirror must be offset both in the direction away from the focuser and towards the primary mirror. The offset in each direction can be calculated with very complex formulae, but the formula offset=minor axis/(4*focal ratio) is accurate enough for practical purposes (it is exact if just the center is fully illuminated - with a larger fully illuminated field, the error is insignificant anyway). The distance along the mirror face from the center of the ellipse to the optical center is the offset multiplied by 1.414 (the square root of 2).

And

An error of type 2 causes the fully illuminated field to be offset relative to focus, and will cause an uneven light loss near the edge of the low power field. For wide-field photo, the secondary mirror should be large enough to let the whole film frame be fully illuminated, but for visual use, a secondary size of no more than 20-25 % of the primary mirror diameter is commonly preferred, in order to minimize unwanted diffraction effects. This means there is usually some light loss by the edge of the field, but at least the focus should always be fully illuminated - the tolerance should not be larger than the radius of the fully illuminated field. At least for short focus instruments, light loss is very gradual outside the fully illuminated field, and an offset of a few millimeters should have little effect visually. Sufficient accuracy is easily achieved with suitable tools.

So, we offset the secondary in order to circularise and perfectly centre the 100% illuminated within field within the eyepiece field. If you don't offset, you get a 100% field that is ever so slightly offset from the centre of the eyepiece FOV and is also slightly elliptical. In a visual only newtonian, is this so very, very objectionable?

Personally, I don't think so. I think it hardly matters in a visual use newtonian that has a good sized secondary because you will never notice its practical effect. As an example, my old 31cm f/5.3 Newtonian with a 56mm secondary has a 100%illuminated field 24 arc-mins diameter and a 75% field 69 arc-mins diameter. With my Plossls, the largest low power field was 57 arc-mins diameter whcih was about 84% illuminated at the very edge. Could I (visually) see a difference in brightness of a star from the centre to the edge?

Nope.

In practice a 20% drop-off at the edge can't be visually noticed. If it can't be noticed visually, what is the point in making it circular? None that I can see. If you (anyone) can, let me know please!

The alternative then is to do away with offsets and centre everything -- which is what I do. The stars are just a sharp, the 'scope is just as well collimated it is just that the 100% field is slightly oval.

Centration has the decided advantage of simplicity and ease of collimation. I've been through the whole offset deal and find it offers no practical advantage for a visual use newtonain with a good-sized secondary.

Things change however for photographic/imaging applications -- particularly wide field imaging where any vignetting toward the edges of the frame will not be centred within the frame. Then, by all means offset ...

Use either method (partial/full) and I think that for most practical purposes they will produce a similar result.

Only my view for what it is worth.

Best,

Les D
Contributing Editor
AS&T

[ngcles]

Hi All,

Oh, I forgot. laser -v- cheshire/sight tube?

Both have advantages and disadvantages. I purchased a laser about 6 years ago when they first became very popular. I liked it and it produced perfectly good results. A friend of mine on his 20" truss dob uses a barlowed laser and it too works fine and is a pretty simple method.

However, about 12 months ago I went back to a cheshire/sight tube. The laser I had was a pain in the neck to collimate itself and had a habit of falling very slightly out of collimation itself when I when through the operation of changing its batteries. Also the batteries unless very fresh were unable to produce an easily seen (bright enough) dot for daytime setup/collimation. One time I found the batteries were dead out in the field at night and it was totally useless (so I collimated on a star image)

The old-fashioned cheshire never needs batteries, unless seriously abused does not fall out of collimation itself and can be used day or night (provided you have a weak light source at night) and makes errors in secondary colimation more easily seen and fixed.

So, that is my own personal preferrence -- my 2c worth.

Best,

Les D

[leinad (Dan)]

Thanks heaps guys.

Whilst I'm not entirely up to speed with the technicalities etc, I think for the meantime I can live with this until I learn a bit more.

Viewing seems fine at the moment, and until I get more hours under my belt Im pretty satisfied with the collimating so far and its results.

I guess as they say you can only learn from experience, unfortunately it often comes the hard way.

I thank you all again for going out of your way to assist a beginner and sharing your knowledge.

[ngcles]

Hi Leinad,

Good on you -- enjoy the 'scope which seems to be producing good images and don't obsess over it till later (much) on. Observing (particularly for the beginner) should be a joy and not filled with worrying over whether the 'scope is adjusted so perfectly that you spend no time actuallly enjoying it. In fact that is one of the reasons I like to keep my collmation aims as simple as possible.

Best,

Les D