Secondary Mirror Offset?

[glend]

Need a little advice here. My new 16" GSO dob was delivered with a loose secondary, how this happened I don't know but the secondary was jiggling around in the top section when I unpacked it. It may have happened in shipment, because I can see small scratches on the holder ring where the adjustment screws touch the surface, or it may have come that way from the factory (of course Andrews doesn't unpack these things to check them out before they ship them).

I'm not terribly fussed about it as I am pretty good at collimation but it might really cause a problem for some people.

My question is: Do 16" GSO dobs have any sort of secondary offset that I need to include when I reset the secondary?

I have checked the radial offset and there is none (all spider-vane arms are indentical in length). It's the axial offset that concerns me.

I have heard it said that in a scope (secondary)of this size that an axial secondary offset is not required. Anyone know for sure? Can any 16" GSO owners (glenc, for example) confirm their secondary setup?

I have it set (and locked) now, and the collimation appears to be perfect under the laser tool, and initial observations (start test); but I'd like to know for sure. ALso has anyone spotted their secondary - here I am assuming it gets spotted after offset is confirmed or not.

Thanks in advance.

glend

[Merlin66]

Glen,
It's the f ratio that impacts on the secondary offset....
Usually the secondary mirror is glued onto the support plate with the "correct" offset - so it just sits in the centred spider vanes.
The first image shows the extreme off-set in a f3.6 system
Spotting the secondary is a bit unusual....the mechanical centre is off-set up from the optical centre.....see second image

[Stardrifter_WA]

Good question Glen.

I too am confused by this offset on fast newts, which is probably why I have so much trouble collimating my 16" f/5.

How is this offset determined and why?

[Merlin66]

Look at the diagram...you'll see the intersection plane at 45 deg to the optical axis needs the elliptical mirror positioned with it's centre away from the axis to full reflect the conical beam coming from the primary mirror.
http://www.lcas-astronomy.org/articles/display.php?filename=newtonian_seco ndary_offset&category=telescopes

BTW all commercial secondary mirrors are NOT elliptical! They are a section of a cylinder cut at 45 deg - this can reduce the amount of light to the final image by up to 4%....they don't tell you that.

[glend]

I have found this forumla for calculating the Offset, which was in this link:

http://www.skyandtelescope.com/howto/diy/3306996.html

The formula is:

Offset = (secondary size)/(4*focal ratio).

So for my 16" f4.5 scope, which has a 88mm secondary (I think), the offset should be:

Offset = (88 )/(4*4.5) = 4.8mm

That would be offset towards the primary I assume (axial). So as all my reading is indicating the offset is small.

Does this make sense?

[Merlin66]

Glen,
As per the LCAS article in my previous post, the offset is applied towards the primary and away from the focuser.

[Satchmo]

Peter ,

The offset is really there on fast scopes to keep the illumination field nicely centered on the axis with a minimum size diagonal . If you have a large enough secondary you do not need to offset but you may waste a bit of light through having a slightly larger than necessary secondary.

At the end of the day if you have adequate size secondary and the secondary is catching all the on - axis light ( ie your extra focal star image is looking round inside focus and not clipped , having offset or not will not effect your ability to collimate .

This article will tell you everything you ever wanted to know !

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

[Stardrifter_WA]

Thanks Mark, now I understand.

Cheers Pete

[Steffen]

Isn't that what an ellipse is – a cylindrical section?

Cheers
Steffen.

[Merlin66]

No, an ellipse is a conical section by definition.

[Steffen]

Yes, but a cylinder is a cone, with the vertex at infinity. The section is still an ellipse.

Cheers
Steffen.

[Merlin66]

;)

Yeah but.....
If you draw a 45 degree ellipse (conical) say an f5 cone..and with the same minor diameter, draw a 45 deg "cylinderical section" when you overlay them you'll see the differences!!
Believe me, I've done it......

[ausastronomer]

Hi Glen,

Well in fact centre spotting the secondary mirror it not uncommon at all. It might be uncommon on "mass produced" telescopes which have everything CNC machined and everything just bolts into the correct position, but it's pretty common on hand built custom truss Newtonians/Dobsonians.

On page 284 of the "Dobsonian Telescope" book, Dave Kriege provides a template and explanation on exactly how to do this, both with and without the offset incorporated into the mark.

Peter Read at SDM Telescopes centre spots the secondary on every telescope he builds, and he might well be the best truss tube dobsonian telescope builder on the planet. Many of the other "boutique" builders of truss dobsonians also centre spot the secondary. My 10"/F5.3, 14"/F4.5 SDM's and my 18"/F4.5 Obsession all have centre spotted secondary mirrors, with the offset incorporated into the centre spot position.



Mark is 100% correct here on the optical theory behind secondary offset.

In addition to the optical theory there are mechanical considerations. Having the correct secondary offset makes the optical axis and the mechanical axes of the telescope coincidental. This is fairly important in having accurate tracking for long exposure astrophotography, particularly as the aperture of the telescope increases and the F-Ratio gets faster. In addition it will make tracking and pointing with digital setting circles like Argo Navis, "fractionally" more accurate. However the effect on tracking and pointing accuracy for non astrophotographic purposes is very minimal and something not really worth considering IMO.

All the above having been said, the secondary mirror size on the GSO and Synta mass produced newtonian telescopes tends to be fairly generous and I don't know that there would be any benefit in offsetting the secondary at all with the 16"/F5 GSO dob. Further as Mark Suchting mentioned, it doesn't affect your ability to collimate the telescope. If it were me. I wouldn't worry about offsetting with that telescope. My 3 truss dobs all have the secondary offset incorporated into the centre spot, as they all use minimally sized secondary mirrors, which is a topic for another thread.

Cheers,
John B

[Merlin66]

John,
I can only quote the most knowledgeable expert I know on collimation, Jason D.


Spotting the secondary mirror was never the norm and it should not be the norm. Following the collimation steps using a cheshire eyepiece will get the job done without the need to spot the secondary mirror.
Jason

[multiweb]

+1 for JasonD. I can't understand the need to spot the secondary either. There are two many variables involved in positioning the secondary: twist, offset, centering under the focuser and on the tube axis. Also a mark on the secondary would interfere with the various reflections of the primary center spot. :question:

[ausastronomer]

Hi Mark/Ken,

At no point in time did I say you used the center spot on the secondary mirror to collimate the telescope. I just said a center spotted secondary was more common than you think, particularly with the builders of premium truss tube dobsonians. Further, at no point in time did I suggest anyone collimate a telescope in this manner.

In fact the center spot on the secondary mirror is used by "some" of the builders of premium truss tube dobsonians to correctly position the spider vanes and secondary holder in relation to the focuser, prior to the telescope ever being assembled for the first time. ie. When you have the Upper Cage Assembly (UCA) sitting on a bench and the primary mirror and mirror box not even connected. It's good to use this method because it allows you to make the first cut on the length of the truss poles a lot more accurately than just guessing where the secondary is going to finish up. You can use a sight tube, but many telescope builders prefer to use a center spotted secondary, as opposed to a sight tube. A Cheshire under these circumstances is pretty useless as you are missing a few reflections. Using a laser and the center spotted secondary method allows you to properly square and position the focuser on the UCA prior to positioning the secondary and secondary holder.

Once this is done it is highly unlikely you will ever use the center spot on the secondary mirror, ever again. You certainly don't use it to collimate the telescope when it is assembled.

When you subsequently use a Cheshire to collimate the telescope, or a laser, or a barlowed laser for that matter, you don't even see the center spot on the secondary mirror because of its position in relation to the focal plane.

While Jason D is very knowledgeable on collimation, there are plenty of people that know just as much about collimating a telescope as Jason D. Some of them were accurately collimating telescopes long before Jason D was born. The difference is they don't spend hours of their spare time writing about it and posting on the internet. A couple of them in fact are members of this forum.

Cheers,
John B

[Jason D]

Originally Posted by Satchmo
If you have a large enough secondary you do not need to offset but you may waste a bit of light through having a slightly larger than necessary secondary.
This is a common misconception. If you do not offset the secondary mirror away from the focuser, then following proper collimation steps will automatically tilt the primary mirror forward towards the focuser to compensate. The end result will be the same. That is, a scope with a minimal sized secondary mirror that is mounted without an offset will be collimated as well as another scope with a secondary mirror mounted with an offset.

See attached illustration. Left illustration is what typically comes to mind but the one on the right is less intuitive. It shows how the primary mirror is tilted towards the focuser. The end result is about the same -- assuming no vignetting is introduced. Typically, the tilt is in around 0.5 degrees.

Interestingly, in the article referred to by Merlin
http://www.lcas-astronomy.org/articl...ory=telescopes

Bryan Greer was quoted stating:
“When you get to the final collimation step of tilt adjusting the primary, you are pointing the primary back at the optical center of the secondary. Once again, the light cone will be intercepted perfectly symmetrically. The only consequence of not offsetting is that the primary mirror is no longer pointed exactly down the middle of the tube.”
Bryan who is knowledgeable about scopes is 100% accurate. However, the author of the article, Jack Kramer, who quoted Bryan did not understand what Bryan meant. Jack goes on to reinforce the common misconception by stating:
“One exception is when the size of the secondary is at the absolute minimum, in which case a non-offset secondary would not intercept the entire cone of light coming from the primary mirror.”

As others have stated, the away-from-focuser offset will bring the optical and mechanical axes closer. That helps DSC and tracking unless the DSC computer has the intelligence to compensate for this error.

To make a long story short, collimate per proper steps and do NOT worry about the secondary mirror offset and do NOT worry about how the secondary mirror is mounted.

Jason

[Jason D]

You are right. 45 degree cross sections of a cone with typical telescope F-ratios versus a cylinder are different; however, both are ellipses but with different aspect ratio. As Steffen stated, you can think of a cylinder as a cone with its vertex at infinity. Bear in mind that a conical 45 degree cross section will look perfectly circular only if viewed from its cone's vertex. For a cylinder, that would be infinity. Having said that, there is nothing sacred about the secondary mirror looking "perfectly" circular. If the secondary mirror is perceived to be centered and round then it is good enough.

See attachments. The first illustration is for a cylinder. The ellipse will have an aspect ratio of sqrt(2) (~1.414) between its major and minor axes. The second illustration is for a cone. The ellipse will have an aspect ratio greater than sqrt(2) between its major and minor axes due to offset. Since the cylinder has an "F-ratio" equal to infinity, its offset is zero. The third illustration shows the different but that is for an F1.5 scope. For a typical F5 scope the different will be less than 1%. The difference is too small to consider. It is in the shadow of other potential collimation errors. I would not worry too much about the cone/cylinder cross section impact.

Jason

EDIT: Updated the illustrations

[Jason D]

John, you seem to agree that there is no need to spot the secondary mirror to perform periodic collimation. That is the point many of us are stating.
Even during a telescope construction, I do not see the need to spot the secondary mirror. There are better methods to figure out scope measurements.

David Kriege recommends spotting the secondary mirror to perform collimation and many disagree with him.

Jason

[multiweb]

Hi John, while I'm sure you are very knowledgable in the topic there is nothing left to chance or interpretation in Jason's raytrace diagrams and I doubt his age is a factor with his expertise in the matter. My 2c. :)

[ausastronomer]

Hi Jason,

While that is fine in theory; and in fact in practice, in terms of telescope performance, there are other things to consider. The main one being that most people that spend somewhere between $10,000 and $100,000 on a custom built truss dob don't understand the optical theory behind collimation and they don't look at collimation ray trace diagrams. They just expect everything to be squared, properly symmetrical and properly aligned and positioned. The fact that it doesn't matter in practice in terms of telescope performance, is totally academic to them. In this regard a well collimated laser does a good job of squaring the focuser and positioning the secondary when the Upper Cage Assembly is initially being set up for the first time.

Cheers,
John B

[Merlin66]

We seem to have drifted away from the needs of the original post and into the realms of multi thousand dollar telescopes.....

[Jason D]

John, it is the other way around. In my last post, I was advocating for a simple and a practical collimation method.
A laser does do a good job is squaring the focuser but it does not do a good job in positioning the secondary mirror under the focuser -- unless it has a holographic attachment.
Jason

[ausastronomer]

Hi Jason,

You don't use the laser and center spotted secondary for anything other than the vertical positioning of the secondary holder in the UCA; and it does an excellent job of this task. You don't use the laser or center spot for any of the other secondary adjustments like tilt, or axial rotation.

Cheers,
John B