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Collimation query


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I treated myself to a Concenter with my Christmas FLO vouchers. It made centering the position of my SW200dps secondary mirror to the focus tube pretty easy, but when I then look through my Aline Collimation Cap I can't get the secondary aligned perfectly with the primary mirror. Does this mean that the focus tube needs adjustment?

Attached are handheld camera snaps (a) through the concenter; (b) through the collimation cap & (c) the view through a Cheshire.

I've modified the scope with a plastic ring to hide the primary mirror clips & added a bit of masking tape to the ring to show their position during collimation. The collimation cap image has been adjusted to make them visible & you can see that the one at the top extends much further into the view than the other two. The image through the Cheshire also shows quite a bit of crud that's accumulated on the primary.

Is the collimation good enough, or should I prioritise getting the position of the 3 mirror clips equally visible, over making the secondary nice & round?

I've also attached a examples of my most recent images, prior to attempting the collimation. I think I may have other issues with data on some of the nights as the stars in the Spider/Fly are worse than in the other two. All were taken around the same time just over a week ago.

Any advice much appreciated!
Ivor

concenter.JPG

aline cap.JPG

cheshire.JPG

IC417_NGC1931_v2.jpg

M66_Group-v1.jpg

NGC3079.jpg

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Not a Newt owner, unfamiliar with the Concenter. But I can say that an inexpensive laser can be a big help -- won't do you any good for centering, but will get the tip/tilts correct. With a cheap one like that, you'll want to check the laser's collimation, but that can be done easily by perching it on an L-shaped support (pair of angle brackets, miter box, whatever), noting where the spot falls on a distant wall, and rotating it. If your mirror's index donut is in its optical center, you can repeatably ensure that your primary,  secondary, and focuser are correctly aligned. AFAIK if the laser beam exits the focuser, hits the secondary, hits the center of the primary, bounces back up  off the secondary and returns to its source, the focuser has to be at 90° to the primary and the secondary at 45° to both of them.

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Getting the secondary centered is essential.

What I do after getting the secondary perfectly centered is to level the primary mirror. Simply loosen the locking screws then using a little steel rule or similar adjust the primary screws to exactly the same length. You should then find the scope is very close to being collimated. With a perfectly set secondary achieving collimation should only require minor tweaking of the primary screws.  

For final collimation I use the locking screws. When the screws are tightened you will find they throw collimation slightly out but just adjust the tightness of the locking screws until perfect collimation is achieved. This way your scope will stay in collimation for ages. For some reason some seem to leave the locking screws loose and then wonder why the scope needs more frequent collimation. They are called LOCKING screws for a reason as they lock in collimation.

I use a laser for final collimation but as mentioned cheap lasers themselves need collimating. A Hotech laser will be sppt on but of course is £££££.

Edited by johninderby
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On 03/04/2022 at 18:57, Spile said:

This is my interpretation of your alignment. As you can see (Ref https://astro.catshill.com/collimation-guide), there is rotation/tilt error that I would work on. First I'd work on the less severe offset error. The dark secondary reflection should be "pointing" at 9 rather than 10 o'clock.  

2-4-22.jpg

That’s a nice link! I hadn’t read about the correct rotation of the secondary offset before… Why do you say 9 o’clock? Isn’t the OP’s primary mirror at 3 o’clock in the above pic?

Edited by Anunnaki
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On 01/04/2022 at 15:27, Aramcheck said:

advice much appreciated

Hi

Preparation

So that any collimation holds, you may want to fit a long Losmandy dovetail to set the rings further apart, a rigid top rail to tie them, up-rate and augment the number of mirror springs and seal the primary to the cell to prevent lateral movement. 

Collimation

We'd recommend losing the concentre and go instead with just the Cheshire and collimation cap.
Don't stress over the secondary; apart from reflection it has no optical properties. Get it somewhere close and leave it. 

It's always a good idea to revise the common collimation myths and then put it all together using seronik's plain English guide.

Cheers and good luck.

Edited by alacant
Translation from es.
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8 hours ago, Anunnaki said:

That’s a nice link! I hadn’t read about the correct rotation of the secondary offset before… Why do you say 9 o’clock? Isn’t the OP’s primary mirror at 3 o’clock in the above pic?

The cap view shows the spider assembly at 3, so I the offset secondary should be pointing to the primary at 9.

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  • 2 weeks later...

Thanks for all the advice. We were away for a week (without scope), so I've not had chance to look into it properly yet.

I think the Cheshire pic may have been taken off centre, but as I've made further adjustments since, I need to start from scratch. I've bought a cheap Svbony laser & am attempting to collimate that. Also looking at Astronomyshed's youtube series I think I'll invest in a length of M5 stud bar to check the focus tube angle. (pt 1 of his series: https://youtu.be/zd-fl9SEYHw )

Thanks again!

Ivor

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  • 3 weeks later...

Yesterday I spent pretty much all day going around in circles, getting good central alignment of the secondary to the focus tube, & then messing everying up at subsequent steps...

This morning however I think the penny may have dropped. Here's what I did:-

i) Adjusted the three primary mirror screws so that distance from the OTA base to mirror cell was approx equal (measured with digital caliper)

ii) Used the Concenter to position the secondary central to the focus tube. I then took a picture through the Aline collimation cap, measured the pixel co-ordinates of the focus tube edges & used Pixelmath to draw the centre lines, and photoshop to overlay concentric circles to verify the alignment.

iii) Adjusted the secondary mirror tilt in small adjustments to bring the primary into view, taking as much care as I could to avoid rotating the secondary & only loosening one screw slightly / tightening another at any one time. Once my markers on the Primary mirror ring were visible in the Aline cap, I replaced it with the Cheshire & made final adjustment to make the Primary centre dot coincide with the out of focus cheshire cross-hairs.

iv) Adjusted the primary mirror tilt to bring the centre spot reflection concentric with the primary mirror dot.

As a sense check I then took another picture through the Aline cap & added the focus tube centre / concentric circles to check. The result is far from perfect, but I think it will have to do for the time being.

I've not had much success with the cheap laser however. I bought a Bader ClickLock Reducer to help centre it (and also the Cheshire), but haven't manged to get the laser collimated sufficiently. Best I can manage is about a 0.17 deg error & whilst the laser reflection appears close to the primary mirror spot, it's not good enough...

Looks like we might be getting a bit of clear skies tonight, so I'm eager to see what difference this makes (if any!).

Cheers
Ivor

PS: Over the last few weeks I've also adjusted the focus tube, but now I'm not convinced that was needed. In the end I used an old surveying Laser Distometer and a heath-robinson jig to hold it in position & then measured the distance fom the top of the focus tube to marks I'd made on some pvc tape stuck to the OTA, at a set distance from the focus tube base. I'm not sure of the accuracy of the distometer at these short lengths, but the measurements suggest it's aligned to the OTA to within 0.2mm in either axis.

A_lo_res.jpg

B_lo_res.jpg

C_lo_res.jpg

D_ht_lo_res.jpg

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Posted (edited)

That looks good enough to me! A little tilt rotation error if you feel obsessive.

Edited by Spile
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Posted (edited)

I've used my concenter once since buying it, worked a treat in getting the secondary perfectly centred. 

Now I just use a collimation cap ever now and then and rarely have to adjust anything and when I do it is minor tweaks.

I don't rate cheap lasers. Never tried an expensive one but a collimation cap does the job nicely. 

I also collimate with the scope pointing straight up as the primary mirror can move slightly when you have it at 90°

Edited by Jamgood
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Looks pretty good to me. Do a star test.

if you are finding that you are having trouble keeping the rotation square when you tighten up the 3 adjusters, make sure they haven't been overtightened in the past and cut some 'gouges' into the base of the secondary mirror holder. (Not that I've done that when starting out! 🙄 ). It means that you can never fine-tune the rotation of the secondary mirror as it will always settle back into the same position.

An easy fix - inserting a steel washer between screws and mirror base. A milk-carton washer improves adjustability, too.

 

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22 hours ago, Spile said:

That looks good enough to me! A little tilt rotation error if you feel obsessive.

Thanks @Spile. I'll no doubt get obsessive & also take up @Pixies suggestion of the steel & ptfe washers. The secondary looks like it could do with a wash in any case!

I haven't checked the collimation 'in the field' yet, but the stars are now a lot better than they were.

Here's a very rough process from last night (NGC3198).

Thanks folks!
Ivor

NGC3198_v1.jpg

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