Collimation required

[OK Apricot]

As above, decided to kill some time tonight and have a go at collimating the Atom Bomb. 

This is the view down the StellaLyra Cheshire pinhole...

 

Am I right in saying I need to adjust my  primary mirror only? Or does the secondary want a tweak as can't see one of the primary clips equally? Forgive me if these are silly questions or I'm missing something obvious, but I have been reading and watching videos but most have been done with a laser collimator. 

Thank you in advance for any tips and pointers 🙂👍

 

[OK Apricot]

I have got the marking on the primary moved as such... Any better? 

[dweller25]

Thats very good, you just need to check it on a star…..

[OK Apricot]

20 minutes ago, dweller25 said:

Thats very good, you just need to check it on a star…..

That'll be some time in a few weeks then 😅 

I believe the secondary may need a tiny adjustment to make the reflection of the primary more concentric to it? And to bring equally into view the 3 clips? 

[dweller25]

It does but it’s very close

[OK Apricot]

Tweaked the secondary, primary cell clips now equally visible, reflections concentric - overall spent about 45 mins trying to get this right 🤞

 

[bosun21]

It’s looking good and as has already been said a final star check would be ideal 

[bosun21]

You have moved the secondary 

[OK Apricot]

22 minutes ago, bosun21 said:

You have moved the secondary 

I did, is it looking worse then? 

[bosun21]

This is what you are aiming for 

 

[Spile]

I would work on your secondary in order to get a=b=c=d as per https://astro.catshill.com/collimation-guide/.

Seeing all the clips is a good check but only when the centre mark is actually centred. Again explained in the above link, a Cheshire eyepiece and sight tube combination tool will tell you if this is the case.

Then redo the primary alignment.

Edited February 15, 2022 by Spile

[OK Apricot]

So as I understand it:

1) position secondary so that it is concentric to focuser tube. 

2) align secondary so that reflection of primary is concentric with secondary. 

3) align primary so that all reflections and centre mark concentric. 

I will give it more time tomorrow. 

[bosun21]

In your part 2 forget about the reflection of the primary and concentrate on positioning your secondary mirror perfectly central and concentric in the focuser tube. Ensure it’s a perfect circle. Once done the crosshairs on the Cheshire/sight tube should be centre 

[OK Apricot]

Struggling with secondary placement within sight tube, looks like it's slightly closer to lower right as seen. Otherwise primary mark centred with sight tube cros

s hairs

22 hours ago, bosun21 said:

 

 

[Pixies]

HI.

The secondary looks twisted. It appears non-circular in the picture below (the red oval) within the yellow circle of the sight-tube.:

Another clue is the shape of the silhouette of the secondary in the reflection. In blue: the oval won't be central due to the 'offset' nature of the secondary when viewed from this angle, but it should be in line with the focuser and appear straight out as per:

I would suggest you first aim to get the outside edge of the secondary (red) central and concentric with the sight-tube (yellow). Firstly, loosen the secondary and rotate it until it appears perfectly circular through the sight-tube. Then adjust its position (up-down / left-right) so that it is then concentric. If you can get the sight-tube/cheshire inserted closer so that the gap between red and yellow is smaller, it makes it easier to see the concentricity.

Once you've done that, you can adjust the secondary tilt (3 screws) so that the doughnut marker is directly under the crosshairs. Then it will all be aligned and you can move on to the primary adjustment and get the eye-hole centred within the doughnut.

Edited February 16, 2022 by Pixies

[OK Apricot]

Thank you. The issue I think I'm having is that the secondary is slightly "low" in the sight tube. Slightly left and right is fine because I can use the centre screw to adjust that, but the "up and down" has to come front the spider vanes, right? Everything along the focuser is nice and tight, square, I get the same image when removing the Cheshire and reinserting it just to be sure I'm going from a single reference point. Do I need to adjust the spider vanes here? It seems to be the only way to get the secondary more central. Wish I never touched it now 😔

[Spile]

Move the secondary away from the primary in order to get a=b.

Then rotate secondary and adjust tilt to get c=d.

This will resolve the non circular primary reflection.

[Pixies]

Not the spider vanes - you adjust the up-and-down in your image using the 3 tilt adjusters. You can check the vanes and that  the secondary assembly is central, though. Measure the radius out from the central screw to the tube - it should be the same in all directions.

Then make sure the secondary is not twisted as I mentioned above. Trying to align the secondary when it is twisted and you end up with it tilted a bit, as it is. In Spile's guide, it's this adjustment: 

You want to make sure you start with it as straight as possible. This adjustment should only be a slight one.

[neil phillips]

First have you adjusted the spider? If yes. what i would do is start at the begining. Measure the spider vains so the secondary holder is dead centre. Use a ruler or tape measure. its not easy but it can be done. Try to get the secondary holder within a millimeter or two of centre. checking left right and up and down. When confident your spider is centered its done do not touch again..

Block the reflection of the primary off. by putting paper. Or a cloth inside the newt tube. so you can not see the primary anymore. Next use a cheshire  and maybe even some kind of added extension. rack your focusser out. the idea is, when you look down your focuser you can now see the edge of the focuser and the edge of the secondary easier. Because you made the gap between the secondary. and your focuser drawtube smaller. With a smaller gap its much easier to see when the secondary is to far back or too far foward. And if up and down is centered. Put a white bit of paper behind the secondary when you do this. its easier again to see. When confident that your secondary looks perfectly circular under the focuser drawtube ( switch back and forth with a colli cap too if you must ) to get a feel that your secondary does now look bang centered right under your focuser. All you need to do now. is remove the paper or cloth that blocked the reflection of the primary from earlier. And center the primary exactly inside the secondary. Again by racking your focuser out, you should be able to see the edge of the primary inside the edge of the secondary ( keep the white paper behind it. it helps ) I can do this myself at night with a torch by shining it at certain angles untill i see the primary edge. inside the secondary. You Do this by using the 3 secondary tilt screws. you do not touch the primary yet

its easy to see if the primary is dead center inside the secondary. if you rack the focuser in and out. when doing this. you should see the edge of the primary inside the secondary edge. Getting bigger and smaller. Once your eye can see all this. its often a wow moment. because you will know what to do. The primary Mirror clips can also be used for this. try to get them all looking even. Again if you rack the focuser out. you can get all three mirror clips right on the edge. Again its easier to see. I prefer to see the primary edge. inside the secondary. But both will work. When your confident the primary now looks exactly inside the secondary. By using the primarys edge against the inside of the secondary edge. Just tweak the primary mirror so your centre sticker is in the centre. . if all that has worked. your collimated quite good. Its what i did recently when i rebuilt my 10" Orion. And my images speak for themselves i hope. 

Hope you have understood most of this. it sounds complicated but is actually not. 

Edited February 17, 2022 by neil phillips

[Spile]

This is where the secondary (green) and primary reflection (red) will appear when you have aligned your secondary (after checking the vanes and focuser as explained in my guide).

Edited February 17, 2022 by Spile

[alacant]

On 15/02/2022 at 22:04, OK Apricot said:

spent about 45 mins trying to get this right

Hi

Be sure to read both seronik and telia's collimation myths;)

If you are simply using the telescope with an eyepiece to look at stuff, get the secondary somewhere near, then leave it; other than reflection, it has no optical properties. Then go out and observe.

Cheers

Edited February 18, 2022 by alacant

[Captain Scarlet]

4 hours ago, alacant said:

Hi

Be sure to read both seronik and telia's collimation myths;)

If you are simply using the telescope with an eyepiece to look at stuff, get the secondary somewhere near, then leave it; other than reflection, it has no optical properties. Then go out and observe.

Cheers

Indeed. In my book, the positioning of the secondary mirror is not strictly part of collimation. It's merely a prelude to it, and getting the positioning wrong only affects illumination. As alacant says, get it somewhere near.

To add some detail to that: when you place your eye at approximately the point of main focus (with a collimation cap or cheshire pinhole), if the secondary looks roughly circular, and you can see the whole primary mirror and some of its surroundings, you're good to start the "collimation proper" (i.e. lining up the optical axes of the focus-tube and the primary mirror).

Put another way: if (from the focal point) you can see the whole primary mirror, the focal point (your eye) is receiving all on-axis rays reaching the primary (rather like "if you can't see my wing-mirrors, I can't see you!"). Seeing the whole primary mirror means the focal point is "fully illuminated". If, in addition, you can also see (from the focal point) a roughly circular area around the outside of the primary mirror's reflection, then the full converging light-cones from some off-axis rays also get fully reflected by the secondary, thereby providing an area rather than a point of full illumination at the centre of the image.

If your view of the secondary is not perfectly circular, and/or the reflection of the primary and its surround are not concentric, (and the collimation proper has been done correctly), the worst you'll suffer is uneven illumination across the field which you simply won't notice unless the mis-positioning is gross.

Collimation proper then simply involves using the Cheshire as a "gunsight", pinhole at one end, crosshairs (out of focus) at the other end, with fuzzy cross-hairs aimed at the central doughnut (aim by using the tilt screws on the secondary). Then adjust the primary to get the reflection of the bright rear of the cheshire to also coincide with the same point. Those last 2 steps, the collimation proper, are extremely easy and the only really important parts of the whole process.

Cheers, Magnus

Edited February 18, 2022 by Captain Scarlet

[neil phillips]

Good to see Gary Seronik explain in this part what i was taking great pains to explain

 

Last, you adjust the tilt until the outer edge of the primary is concentric with the outer edge of the secondary. Usually this involves a small Allen key (hex wrench) or a screwdriver. Again, if your secondary mirror holder has three adjustments (and most of them do), try working with only two. Start with the focuser racked all the way in, and as you get close to perfect, rack out until you see the edge of the primary just about touching the edge of the secondary. This little trick will help you more accurately judge the effects of your adjustments as you approach (hopefully) perfection.

[OK Apricot]

Thanks guys - I have read your replies and I'm working through it as I type this. I went to the van to get some digital verniers and  have made sure the secondary is central in the tube by adjusting the spider vanes, with a max deviation tolerance of 0.4mm (close enough, right?) 

 

Through the Cheshire tool the secondary does appear in better position. "a" looks more or less equal to "b", but "c" doesn't look equal to "d". I'm not sure the 0.4mm deviation is responsible here, so I'm checking the focuser is square. First is with focuser backed out, second fully in. 

 

I will check back shortly with an update. 

Thanks for your patience ladies and gents, I appreciate this question does come up a lot. 

 

 

[OK Apricot]

Reflection in secondary now looking like this with paper removed. 

Edit: Focusser seems to be square as well. 

 

Edited February 18, 2022 by OK Apricot