Collimation woes

[knightrider]

So I just took ownership of a SW 10" Goto dobsonian. It's a step above my dad's old late 70's 130mm newt Tasco on an eq mount. The clarity difference is mind boggling, although the old scope has done well for it's age.

The tools I was using are a ProStar Laser, and a Farpoint Cheshire. I put everything together and started out on the alignment.

Been trying to get my head around the collimation process, even after much reading on it the scope wasn't adding up.

I initially did it with the laser, doing the secondary and primary was easy enough until I checked it with the chesire and it looked completely out.

So I thought I'll do the opposite and collimate with the cheshire and check with the laser. I thought it looked quite good when I did it with the cheshire, everything lined up. I popped the laser in and nothing lined up according to the laser, way off....

I couldn't understand what was happening. So I attempted to tweak the collimation of the laser, after messing it up for a while I got it to stay stationary when rotating it in the jig I made up for it @ 6m away.

I repeated what I did again, aligned with laser, checked with cheshire. Still didn't check out with the cheshire, The primary spot lined up but the secondary mirror didn't have half the mirror clips in there like it would if I initially collimated with the cheshire. Instead it had a fair portion of the OTA in view.

It didn't make sense...laser was straight...but doesn't marry up with the cheshire. If I collimated with the cheshire, it looked great...till I checked with the laser and the secondary was way off.

So, then I noticed the adjustments grub screws on the base of the focuser. I used the cheshire to visually get good secondary alignment. Then popped the laser in with it's own adapter and adjusted the focuser adjustment screws till the laser dot lined up with the primary mirror centre dot.

Swapped the cheshire back in, adjusted the primary to line up well with cheshire, then tested with the laser. Did some minor primary adjustments, then checked again with the cheshire and presto! Good alignment in both.

What got me was, everything I read on collimation in the manuals and on all the different threads and DIY's on the net did not even mention the focus tube alignment. It took me maybe 3-4 hours all up in my spare time playing with it till I had it figured out.

It didn't occur to me the focuser base would of bee off alignment. From this experience, it has shown me that just "trusting" what the laser says isn't always correct...even if it does shoot straight.

[barx1963 (Malcolm)]

Hi Knightrider.
The thing you need to remember is that the laser can only check the final alignment of the primary. So you should use the Cheshire first. In fact many suggest that Cheshire collimation is all you need.
There is a very good guide here http://www.astro-baby.com/collimatio...on%20guide.htm
which I think is an excellent intro to the joys of collimation.
I wouldn't be adjusting the focuser tilt unless it is way off, in a scope your size it is unlikely to make much difference.

Malcolm

[knightrider]

Yes I had a read of astrobaby's tutorial also prior. It's a very good guide.

I tried to align the secondary first with the cheshire, then the primary with the laser but that didn't work at all. After I checked it with the cheshire again it showed the spot no where near where it should've been.

I did do a cheshire collimation lastnight and did some viewing and it turned out quite fine, before i even touched the focuser this morning.

The thing that bugged me was that theoretically both laser and cheshire should return similar results. and although the cheshire collimation always seemed to look good, the laser would tell me it was drastically out, even after the laser itself was collimated.

I have better peace of mind now knowing what it was and I was able to fix it so both cheshire and laser agree.

I see the value in cheshire collimation. Laser can just be for a little convenience, providing everything lines up on the OTA.

[simmo]

Hello Knightrider

I've just experienced the same problem with my SW 10" dob's focuser. I bought some astrosystems collimating tools and in the handbook it referred to the focuser alignment.

A.S. instructed to find the half circumference oppposite the focuser and spot both primary and secondary mirrors. Once I had done this I tried to line everything up and this is where it all fell out of shape.

I had to pack the focuser using folded paper to what equalled 8 pieces of paper in one corner. It was that far out. ( I used calipers, etc. for measuring). Now with that done the secondary mirror centre would not line up and was so far back it touched the spider vanes and could not be shifted to align properly.

That was at 2am and that was it. Possible causes I could think of is focuser is really bad, optics are out of sync, the tube is not a perfect circle or the system is offset somehow slightly. Either way wasn't good.

Woke up the next day and tried again using Astrobabies guides as extra help (pictures are good). I drilled a hole (1.5mm) in the centre of the focuser dust cap to set the secondary as a circle in the focuser (disregarding secondary centre point, I checked the cp twice to make sure it was right).I followed the rest of the instructions. Checked against an autocollimator and it was perfect. First time fluke i'm sure!

Like you I also went outside last night and got very good viewing. I was able to make out NGC 2298,2455 and 2432 which are about mag 10 in a suburban dark site. Don't know if that's good or not but it seems all right to me.

If it's any consolation I think being anymore precise is for the photographic department of astronomy. I'm not sure if that's what your doing though? I suppose we have to take into account that a lot of errors can be introduced into a system. Usually human error is the biggest. If you look at the price of our scopes when compared to the pro's there's a big difference so it should be same to say that the quality is also the same. I hate to say that as I love my scope and to me it was relatively expensive.

But considering that we both have enjoyed quality viewing in what seems to be flawed systems then maybe such precision isn't warranted and in the search for the perfect 'system' it is an imperfect system that is actually perfect if you get what I mean. I hear your frustration at having what seems as perfect logic fail though. I think the only way forward from here is to accept that and just enjoy the views.

Only help to give is maybe try what I did as well. Your eyes are actually quite good at judging alignment manually and I think you've come to that decision already from your last comment concerning the cheshire eyepiece.

As far as SW quality (or other mass big brand) is concerned I think for my next scope I'll build my own. At least then I'll know what's wrong with it before I use it!

Simmo

[Jason D (Jason)]

Knightrider,
I read your initial post and I have the impression most of your issues have to do with misunderstanding how certain collimation tools work. It is common for beginners to use laser collimators per given instructions to find out that the secondary mirror is not centered and rounded under the focuser and only part of the primary mirror reflection is visible. When that happens they get discouraged and assume the laser collimator is bad or assume they did not follow the instructions correctly. Bear in mind that the laser beam hits a tiny spot on the secondary mirror. Laser beams have no clue where the secondary mirror edge is located in reference to the focuser. Many assume that unless the secondary mirror is centered/rounded under the focuser they can never redirect the laser beam to the primary center and have the beam retrace its path all the way back. This is the wrong assumption.
Then you collimated with the Farpoint cheshire. Keep in mind that you can move your eye around until you can see the whole primary mirror and you assume the secondary mirror is collimated. But your eye axis is no longer in-line with the focuser axis.
Then you adjust your focuser by shifting the focuser axis to align it with your eye-axis. When that happens the laser agrees with the Cheshire.
I believe your collimation steps are not completely correct.
Check the attachment to explain what I meant.
Jason

[Jason D (Jason)]

Originally Posted by simmo

A.S. instructed to ... spot both primary and secondary mirrors.

Unfortunately, spotting the secondary mirror center is a bad idea but few manufacturers recommend it. You can’t center the secondary mirror with a center dot. If you do then the secondary mirror will be shifted up the OTA. Left diagram represents a collimated scope with a center dotted secondary mirror.

[knightrider]

Originally Posted by simmo

Hello Knightrider

I've just experienced the same problem with my SW 10" dob's focuser.....
...Simmo

Your focuser didn't have the adjustment grubs and locking screws? It works the same way as the primary mirror adjusters.

I'm not deterred about quality by having this minor issue, it just meant something needed to be adjusted. I know this scope will serve me well in the years to come

Originally Posted by Jason D

Knightrider,
I read your initial post and I have the impression most of your issues have to do with misunderstanding how certain collimation tools work. It is common for beginners to use laser collimators per given instructions to find out that the secondary mirror is not centered and rounded under the focuser and only part of the primary mirror reflection is visible. When that happens they get discouraged and assume the laser collimator is bad or assume they did not follow the instructions correctly. Bear in mind that the laser beam hits a tiny spot on the secondary mirror. Laser beams have no clue where the secondary mirror edge is located in reference to the focuser. Many assume that unless the secondary mirror is centered/rounded under the focuser they can never redirect the laser beam to the primary center and have the beam retrace its path all the way back. This is the wrong assumption.
Then you collimated with the Farpoint cheshire. Keep in mind that you can move your eye around until you can see the whole primary mirror and you assume the secondary mirror is collimated. But your eye axis is no longer in-line with the focuser axis.
Then you adjust your focuser by shifting the focuser axis to align it with your eye-axis. When that happens the laser agrees with the Cheshire.
I believe your collimation steps are not completely correct.
Check the attachment to explain what I meant.
Jason

That is very true.
I see what you mean also by the eye being off via the cheshire and then realigning the focuser to match when using the laser.

It was quite a challenge to be able to get all primary mirror clips to show through the cheshire, it was only a very minute window of opportunity i found to have them all in view equally.

Prior to changing the focuser alignment, I centred the secondary with the cheshire and aligned it with the primary. When I tested and adjusted the centred secondary with the laser, it decentered the secondary and I would no longer see the complete primary, but rather I would see the complete bottom clips and the matt black inner tube! The laser would've been useless to me completely if I didn't find what was happening.

Checking with the chesire, eye axial movement wouldn't go close to accepting this. Which has caused me to conclude even after having a straight laser it must be the focuser that is out, since it would not work any other way.

In saying all this, it seemed doing the cheshire collimation only prior to even adjusting the focuser alignment yielded good results at the eyepiece when observing.

Is it as simmo says and focuser alignment doesn't have to be that precise for it to even affect it after being collimated properly with just a cheshire?
Because if it wasn't for the laser, I wouldn't have even known.

I'm actually glad I've had this problem so early. It's given me some good practice (even if I was doing it wrong at the beginning) and it's making collimation a breeze for future occasions

[simmo]

Hello,

Knightrider. No not that I could see anyway I wasn't really looking. I'll go back and have another look though so thanks. But in any case things worked out fine anyway.

Jason. Thanks for the detail. I did end up with the right hand image after everything was finished. It was a bit of a mission but I feel like I've passed some kind of initiation ritual and have become a true fledgling astronomer.

Simmo

[simmo]

Uh! Bingo on those adjusters. Man I feel stupid bloody small though and camoflaged black. Maybe I should take back that last bit from the last post! Doh! opened my mouth to quick didn't I.

Jason (or anyone else for that matter) here's another question or few as well as knightriders.

In a perfect scenario wouldn't the secondary mirror have a plane running through the centre of the face of said mirror (shortest distance across mirror) where the centroid of the ellipse is? Wouldn't it be that both sides should then be equal to distibute the light evenly?
I would have thought that the light still travels the same distance to the focuser no matter where it comes from when coming from the primary? Then are we stacking the light to one side by making the secondary mirror a circle (through the focuser) and moving the centroid further forward of the focuser axis point?
Or is it due to the fact that we are tensioning/adjusting from behind the secondary away from the central plane of the secondary, therefore setting another centroid further back, which creates a spherical/arcing movement to the secondary mirror and that is why that we see the picture on the right as you have shown? If so wouldn't it be better for manufacturers to attach the mirror with spiders (or some other mechanical device) at the central plane of the secondary mirror face to get a better alignment? I think this might be over complicating things and bring its own set of problems but it seems logical.

Or is this why we adjust the primary last to compensate for that shift forward in the centroid of the secondary mirror?

Last question I promise! Or is the centroid of the ellipse negated because what is "seen" by the light as it travels towards it is actually a circle and its centroid is what we are ligning up on? To me it would seem that the circle viewed from the primary would have a different centroid to the one viewed from the focuser in regards to where that falls on the elliptical secondary mirrors face ie. longitudinal axis.

I hope that all makes sense!

Simmo

[Jason D (Jason)]

See attachment. When the secondary mirror is properly positioned to appear centered and rounded under the focuser, the focuser and primary mirror axes will intersect above the geometric center of the secondary mirror face. That is why the secondary mirror shadow appears shifted. And that is why it is a bad idea to center the secondary mirror.
Jason

[Jason D (Jason)]

When the laser beam hits the primary mirror center spot and retraces its path all the way to the source, the primary mirror reflection is centered under the focuser -- however, not necessarily the secondary mirror appearance. Read the previous sentence few times before proceeding.

The secondary mirror acts as a “window” to the primary mirror. After centering the primary mirror reflection under the focuser using the laser collimator, now we can assess the position of the secondary mirror with respect to the primary mirror reflection. Should the secondary mirror moving lower? Should it move higher? Should it move to the left or right? Make the judgment and adjust. You can use the central bolt of the spider vanes to move the secondary mirror lower or higher with respect to the OTA. You can use the spider vanes thumb knobs to move the secondary mirror left with respect to the OTA. But before you make any of these assessments or movements make sure your focuser is reasonably squared with the OTA.

Important: Do not assess the position of the secondary mirror with respect to the primary mirror reflection unless you have completed the laser collimation steps. That is, when you start moving the secondary mirror ignore the primary mirror reflection. Make the movement then realign with the laser then assess the secondary position – keep re-iterating.

In the first attachment, the laser beam hits the primary mirror then retraces it path to the source in each frame. Note how the primary mirror reflection remains centered under the focuser.

The second attachment is an animation that explains how the secondary mirror can be viewed as a window to the primary mirror reflection. Again, each frame assumes the laser hits the center and retraces its path to the source.

[simmo]

Hello,

Thanks again for the illustrations Jason. I can see using the 1st picture from the left where the mirror is angled at 45 degrees is why the centroid of the mirror moves forward to catch the light at the edges. To have the centroid in the middle both mirror faces would have to face one another. ie. cats and casses right., otherwise it would be as you say and the mirror would go back up the ota and some of the secondary mirror closest edge (when viewed from the focuser) would not catch any light.

But I'm not all that convinced yet and am still out on this collimation thing. Your picture is nice but is maybe a little over exaggerated for basics. I would like to see a more detailed drawing with something like Autocad with a bit more realistic measurements. I have access soon so maybe I'll do a bit of research myself.

I guess that the light cone would also shrink as you go further away from the primary and as long as the light remained within the boundary of the edge of the secondary on the far side (when looking from the focuser) and that it did not pass the focuser (which it can't if I'm not mistaken if you want to view anything) no light would be lost correct?

My only concern then would be the axis that the light then travels into the focuser but judging by the first illustrations you sent (one on the left) it shows the axis as still in line and the primary fully shown.???

My only problem with that set up is I don't have enough mechanical room to move the mirror back to get to the center point with some ability to adjust it.

Therefore it seems as if both cases are plausible as long as I could get enough movement with the secondary.

Is another problem that by using a smaller cone/area (when moving the secondary back up the ota) of light that something is changed or diminished in anyway or will this only affect the usable range of the focuser?

By the way I'm not trying to attack you in anyway just trying to understand it fully and would appreciate any comments that clarify any of these points. I also encourage anybody else to comment as well. Thoughts please.

Regards
Simmo

[knightrider]

There's some very good information here, we're fortunate to have members like Jason to take the time out to explain how this stuff works and why.

There's nothing like pictures to make it easier to understand

Thanks Jason