This needs collimation, right?

[greglloyd]

Hows it going szymon...... 10:52pm image looks close enough, In the areas I've numbered 3, there seems to be a gap (marked in red) between the blackness of the focuser/cheshire tube, and the secondary mirror marked as 4,  but I can't see an equal gap at the 3a marks?  This could just be the lighting angle, but this may also indicate the secondary mirror is further down the tube, and you should withdraw slightly with clockwise turns of the centre screw. Try to avoid rotating the secondary mirror as discussed. But it does look very close to an offset mirror, in which case things are looking good! as per the AB guide. Its very close! Try the Laser-Barlow shadow alignment, then a Star test, defocusing to see the airy disk. A pinpoint of light from a Star (Polaris wont move in the sky, so will give you some time to focus) can be imaged as a series of concentric rings when de-focused through the telescope. If the rings are concentric, you may have it spot on. If the rings are non-concentric, a little more adjustment required maybe.

What else can we see in this image that may help others to understand the reflections?  (1) is the centre hole in the cheshire target faceplate, (2) is the surface of the target/angled faceplate on the Cheshire itself. (3)is the darkness of the focuser tube. Take the Cheshire out and you should see your own eye reflected in-place of 1 & 2. (4) is the secondary mirror and that greyness is the reflection of the primary mirror, showing the spider arms  at (5) as a blurred images of the arms. The areas marked 'T' (tube) shows the inside of whatever your looking through, ie 35mm cap, Cheshire, focuser tube? Some folk may have to click the image to enlarge it somewhat to see my edits on syzmons image.

The areas marked "3" are the left hand edge of the secondary mirror (towards the front of the scope). You don't see anything at "3a" because that's the right hand side of the secondary and the beveled edge of the secondary on the right hand side is hidden from view (as it slopes to the left behind the coating).

The last collimation photo is pretty close, but without rotating the cheshire so that the reflected crosshairs are aligned with the spider vanes and actually seeing inside the donut, it's impossible to say if the collimation is spot on or not. The photo does not show the inside of the donut.

The black shadow of the secondary on the primary looks right to me for an F/5. It will not look like a circle. You need F/6 or more for it to look like a circle. 

What I would say from looking at szymon's photo is that the horizontal reflected cross hair looks just a little low. If he rotates the cross hairs to align with the reflected spider vanes, this will show the minor error. The cross hairs should exactly complete the + shape you are looking for. Once this is done, just make minor adjustments to the primary to re-centre the cheshire peep hole in the centre of the donut.

[greglloyd]

p.s. I'm not talking about the out of focus cross hairs. I'm talking about the ones that are in focus on the primary. You will find that once you get the in focus cross hairs in perfect alignment, the out of focus ones (which are actually the ones in the cheshire itself) will super-impose themselves over the reflected ones on the primary.

[szymon]

So, I haven't actually deliberately off-set the mirror.  But, if you look at the Astro-Baby guide, she gives explicit instrucctions for centring the secondary (i.e. not offset), but shows the following image as her final collimated setup:

Looking at the mirror, it "seems" to me that it isn't actually attached to the exact middle of the mirror, as if it has a "built-in" offset.  The side away from the focussing tube is much larger than the side towards the focussing tube.

With the scope set up as it is in the photos, the barlowed laser is off by about 2 inches -- so clearly there is *something* not right still.  I'll try again tomorrow, but for now...I just did a "star-test", and it looked concentric to me, so I'm going to claim victory for tonight ;-)

Charic, your annotated image is brilliant by the way, very helpful indeed!

-simon

[szymon]

p.s. I'm not talking about the out of focus cross hairs. I'm talking about the ones that are in focus on the primary. You will find that once you get the in focus cross hairs in perfect alignment, the out of focus ones (which are actually the ones in the cheshire itself) will super-impose themselves over the reflected ones on the primary.

Right, I noticed that I have the cheshire just slightly twisted out, so they're not in perfect alignment.  I'll try and take a "clean" picture tomorrow :-)

[Tiny Small]

With the scope set up as it is in the photos, the barlowed laser is off by about 2 inches -- so clearly there is *something* not right still.  I'll try again tomorrow, but for now...I just did a "star-test", and it looked concentric to me, so I'm going to claim victory for tonight ;-)

-simon

Don't use a barlow. That will throw it way off.

[Moonshane]

Assuming the above astobaby pic is taken with the top of the scope left and primary to the right the secondary is incorrectly rotated and needs turning 'up'.

The most common secondary offset is done when the mirror is glued on and cannot be altered.

Based on the last photos with the notes I still stand by my earlier post as to what needs to be done.

[Moonshane]

Ps that said the secondary has the least effect on the visual image and small errors like this will hardly affect the view. Never miss an opportunity to observe through fiddling with collimation. Get your primary adjustment accurate and enjoy the views.

[wookie1965]

It was late last night and i marked the wrong circle sorry my bad.

[Moonshane]

The bottom left quarter of this image is more like it

http://firstlightoptics.com/user/content/prodimages/catseye_collimation_sgl.jpg

[Moonshane]

This may help. http://stargazerslounge.com/topic/197640-collimation-and-star-hopping/page-1

I take a more simplistic approach to collimation

[kirkster501]

Taking a Newtonian completely to bits for flocking the tube, modifications etc and reassembling it is a great way to learn all this.  Telescopes are remarkably simple devices if you leave out the accuracy of the optical surfaces themselves.

[szymon]

Don't use a barlow. That will throw it way off.

http://www.smartavtweaks.com/RVBL.html

-simon

[szymon]

So now I have two conflicting viewpoints.  Shane, you're saying that the pic Astrobaby provides of her 'perfectly collimated' F5 scope isn't actually perfectly collimated?  Greglloyd (and many others) do seem to imply that it's more or less right...

Assuming the above astobaby pic is taken with the top of the scope left and primary to the right the secondary is incorrectly rotated and needs turning 'up'.
The most common secondary offset is done when the mirror is glued on and cannot be altered.
Based on the last photos with the notes I still stand by my earlier post as to what needs to be done.

The black shadow of the secondary on the primary looks right to me for an F/5. It will not look like a circle. You need F/6 or more for it to look like a circle. 

[Moonshane]

Perfect collimation is possible to achieve but once you start using the scope it is likely to move slightly anyway with most scopes . The astrobaby image shows a well collimated scope but which needs a small rotation of the secondary to be 'perfect'. Basically the black lobe should point at the primary. It doesn't in that image. That said visually you would be hard pressed to see any effect as I said above.

[greglloyd]

Perfect collimation is possible to achieve but once you start using the scope it is likely to move slightly anyway with most scopes . The astrobaby image shows a well collimated scope but which needs a small rotation of the secondary to be 'perfect'. Basically the black lobe should point at the primary. It doesn't in that image. That said visually you would be hard pressed to see any effect as I said above.

Hi Shane,

Yes, I checked my 12" a short while ago and you are totally right that the black lobe should point at the primary (i.e. directly to the right) as mine currently does. The Astrobaby image shows it tilted down.

Also, you are totally correct to say that the collimation will go out slightly anyway as the scope is tilted and moved around. I also see this occur, and that is why I too agree with what you say in your PDF about performing the collimation at 45 degree angle so that the scope will hold it's collimation more at the angle you are most often using it. It will go out of collimation as you go towards the horizon of course, but this is unavoidable and you would ned to re-collimate at that angle in order to get the sharpest views for objects close to the horizon. Me? I cannot usually be bothered collimating more than once at the beginning of the session.

[Charic]

greglloyd.........Hi, I understand what your saying about the mirror edges, but their not important and if your seeing more of one side, then sureley your mirror is out of alignment. The secondary mirror is eliptical in shape and form, and when correctly positioned under the focuser tube, should show a tight concentric "circle" of light into the sight/focuser tube that your looking through, all chamfers and angles are ignored. You want as pure a circle as possible, with an equal gap all around ( or no gap depending on the length of your sighting tube/focuser dimensions, with some telescopes?) this image shows too much space at point 3. If I dont see anything at 3a then I can assume the mirror is not centred ( or possibly in this image, the lighting and camera angle is not quite right?) 

As for the spider vanes, ignore them, only use the sighting tool cross-hairs if fitted? The spider vanes are not always accuratley/ centrally  placed! My Skyliner has four arms joined to the central hub, on closer inspection, as each arm creates a 'quadrant' I have 4 screws (2-pair) in one quadrant, one quadrant with no screws, and the two other quadrants with 2 screws( if that makes sense) so the spider arms are not equaly positioned around the secondary  hub. This makes proper alignment with any other cross-hairs, impossible on my scope and possibly all Skyliners, unless the synta constructor was having a bad Day!

Also at present I feel your description of the cross-hairs may be back to front? The clearest cross-hair is from the sighting device, the blurred ones are the spider arms, and should be ignored!

[Charic]

Tiny Small........Hi there. Using a lasered Barlow takes out any errors that the laser has? and is usefull for final primary mirror  collimation only!

[Charic]

syzmon......Once you think its correct (and i feel you are almost satisfied, despite any contradictions?)  you need to (when the clouds have gone) carry out that Star test. Its possible to collimate just by using a star alone without all these tools. Concentric circles of  Starlight confirms proper collimation. They say use a bright star for this and high magnification. Sirius being the brightest when visible  is  always on the move due to the Earths rotation. Select Polaris as your target star.

[szymon]

Sorry I should have mentioned, I did a star test last night and the rings appeared to me to be perfectly concentric, so I'm "more or less" happy with it. I am however going to swap out the screws on the secondary for some thumbscrews -- one if the screws I have seems to be worn and is hard to turn, so I want to replace it before it becomes a problem, and if I'm going to replace anyway I may as well replace with thumbscrews.

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[greglloyd]

greglloyd.........Hi, I understand what your saying about the mirror edges, but their not important and if your seeing more of one side, then sureley your mirror is out of alignment. The secondary mirror is eliptical in shape and form, and when correctly positioned under the focuser tube, should show a tight concentric "circle" of light into the sight/focuser tube that your looking through, all chamfers and angles are ignored. You want as pure a circle as possible, with an equal gap all around ( or no gap depending on the length of your sighting tube/focuser dimensions, with some telescopes?) this image shows too much space at point 3. If I dont see anything at 3a then I can assume the mirror is not centred ( or possibly in this image, the lighting and camera angle is not quite right?) 

As for the spider vanes, ignore them, only use the sighting tool cross-hairs if fitted? The spider vanes are not always accuratley/ centrally  placed! My Skyliner has four arms joined to the central hub, on closer inspection, as each arm creates a 'quadrant' I have 4 screws (2-pair) in one quadrant, one quadrant with no screws, and the two other quadrants with 2 screws( if that makes sense) so the spider arms are not equaly positioned around the secondary  hub. This makes proper alignment with any other cross-hairs, impossible on my scope and possibly all Skyliners, unless the synta constructor was having a bad Day!

Also at present I feel your description of the cross-hairs may be back to front? The clearest cross-hair is from the sighting device, the blurred ones are the spider arms, and should be ignored!

Hi Charic,

We are not talking about the bright circle of the secondary. We are talking about the black shadow of the secondary that is reflected on the primary. It's the bevelled edge that you can see to the left hand side of your secondary mirror when you simply look down the focuser tube. It's the grey coloured edge you see in the focuser which from the primary's point of view will be a black shadow. You are correct that the bright circle image coming from the secondary should be perfectly circular. I agree that these bevelled edges have nothing to do with the collimation. We are just pointing them out on the collimation images as some people may be confused as to why the shadow exists and sticks out the way it does.

re: The cross hairs. As I said before - it's the smaller reflected ones in the primary that you concentrate on - not the blurred ones. This is what I said before :-

"p.s. I'm not talking about the out of focus cross hairs. I'm talking about the ones that are in focus on the primary. You will find that once you get the in focus cross hairs in perfect alignment, the out of focus ones (which are actually the ones in the cheshire itself) will super-impose themselves over the reflected ones on the primary."

Now, maybe they don't super-impose exactly for everyone's scope. But they do in my F/5. I fully understand this coincidence has nothing to do with the actual collimation itself, but it's just an observation.

I guess the problem with explaining collimation is that there are various different elements to discuss and it can become confusing when we all talk about it in sightly different ways!

[Charic]

greglloyd........as you say........."I guess the problem with explaining collimation is that there are various different elements to discuss and it can become confusing when we all talk about it in sightly different ways!"

I totally agree, but on the issue of confusion, my point was, that areas '3' indicated by the red arrows, in my mind, are showing a gap between the edge of the sight tube marked 'T' and the secondary mirrors 'circle'  marked 4. I do agree with you that the gap between those two points is actually the mirrors bevelled edge! But if there is a gap on one side, at any intersection between points 3&4 regardless of the beveled edge, there must be a gap on the opposite side if the mirror is correctly positioned.There must be a concentric circle from the secondary mirror with an equal space all around. 

All I'm trying to say here, regardless of anything else in the centre of the image, the secondary mirror appears slightly off centre from the photo supplied (especially if it was for my f/6) but syzmon has the f/5 and the guide he is using covers this fact. Further more, as the Star test has now  been completed to syzmon's  satisfaction, I think collimation is completed, however we look at it, Until he changes the secondary's  adjusters with his no-tool  thumb-bolt adjusters! and off-tweaks that secondary again, but he's getting to  be quite a professional now?

This is the best bit about forums. We all learn, re-learn, and sometimes stand corrected, as we progress with our ability to learn and understand this technical subject. 

[greglloyd]

Ah, I'm with you now! You are simply talking about the gap between the bright mirror image and the edge of the focuser. Regardless of whether there is a bevelled mirror edge to be seen or not. Got you!

But yes, this gap must be the same all around the bright mirror image at F/6 and above. But possibly slightly offset at F/5 and below. Agreed.

Certainly seems that szymon's scope is in much better collimation after this exercise! Hope you get great views szymon!

[Charic]

greglloyd......you beat me with your reply (I`ll still respond with what I typed, as others can also learn from us, even though we are seeing the same now - there's always two sides to every  story)........as you say........."I guess the problem with explaining collimation is that there are various different elements to discuss and it can become confusing when we all talk about it in sightly different ways!"

I totally agree, but on the issue of confusion, my point is that areas '3' indicated by the red arrows, in my mind, are showing a gap between the edge of the sight tube marked 'T' and the secondary mirrors 'circle'  marked 4. I do agree with you that the gap between those two points is actually the mirrors bevelled edge! But if there is a gap on one side, at any intersection between points 3&4 regardless of the beveled edge, there must be a gap on the opposite side, if the mirror is correctly positioned.There must be a concentric circle from the secondary mirror with an equal space all around. 

All I'm trying to say here, regardless of anything else in the centre of the image, the secondary mirror appears slightly off (especially if it was for my f/6) but syzmon has the f/5 and the guide he is using covers this fact. Further more, as the Star test has now  been completed to syzmon's  satisfaction, I think collimation is complete, however we look at it!

This is the best bit about forums. We all learn, re-learn, and sometimes stand corrected, as we progress with our ability to learn and understand this technical subject. 

[Charic]

syzmon.............just musing now and looking back through the thread. Your image 05 Sep 11:04pm looks almost perfect,  a white circle concentric to the "frame" ie. the darker area of the image, with equal distance at all points. However my onscreen measurements  still show that a slight adjustment could be required. But this is how the secondary mirror should look down your focuser tube. Your second picture with Cheshire shows that the circle appears to be slightly off? There appears to be a bigger gap between the 10 oclock and 4 oclock positions. This tells me  that the Cheshire may not be totaly centered, although you have told me you have a centring device. So a slight adjustment to the secondary and/or any correction to the fit of the Cheshire, would at some stage correct for a perfectly aligned mirror! people always assume the poor fit of a laser is its downfall, but if lasers wobble in the focuser, give a thought that a Cheshire may move too? You mention under the "Close-up" " ....it looks quite round to me......" and it does, but look again, its not totally concentric with the edge of the Cheshire tube. The gap is wider at the top?

The fact you have Star tested, should prevent the need to further tweak. There are two axis within a telescope. The Mirror's axis, and the eyepiece axis. Once they are aligned, and the star test has showed a positive, there might be nothing else you can achieve.  Its still possible for the Mirror's axis to be slightly offset to the OTA and offset to the Eyepiece axis, and  yet still produce acceptable results.  Once your happy with the final result, job done!

[szymon]

My centring device may not be perfect ;-) Although it's better (as in more consistent) than my other 2"-1.25" adapters, it still has to go into the 2" focus tube and be held on by two thumbscrews and these wobble a little.

That said, I found a "trick" with the Cheshire which helps me centre it. It seems to me that the Cheshire is correctly centred if and only if the black dot in the middle of the reflection of the Cheshire (which is basically the reflection of the dot you look through) is exactly in the centre of the cross-hairs. So if on that reflection the black dot is slightly to the left or right of centre, the Cheshire is not straight.

Now, is that always true and correct? Or is this just misleading me and I should be correcting something else to move the black dot into the middle?

-simon

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