Collimation correct?

[tehmac]

Hi all,

I had a Cheshire and a laser collimator, but I just found out my laser collimator was way out and despite trying to get it collimated myself using the v-block etc, it's such a hassle I've given up on it. So I bought a collimation cap and will stick with that and the Cheshire.

I have attached a couple of photos below and I followed the Astro Baby guide and have watched and read just about every collimation thing online.

Basically, with collimation cap in, everything is nice and round, centered and all 3 clips from primary mirror are showing. I then put the Cheshire in and everything was out of whack, which I guess was to be expected. I needed to fiddle with the primary mirror's collimating screws pretty heavily to pull it all into alignment. Thing is, I have some extra movement from the Cheshire when it's in the focuser. I can push it from the side and it moves basically, so the crosshairs from the Cheshire can move a little, which makes things difficult.

When I look straight down through the Cheshire, I see the Cheshire's crosshairs in the foreground (albeit blurry) and past that I can see the secondary with the reflection from the primary with its black little circle looking back with the Cheshire's crosshair reflection. I tried to ignore the foreground crosshairs and just concentrate on what I could align in the secondary mirror. The little black circle from the primary was centered on the crosshairs reflection, and I think this is pretty much job done.

If I push the Cheshire from the side I can get the foreground blurry Cheshire crosshairs to pretty much align up with everything below in the reflections. I'm just wondering if I need to worry about the job I've done given the play in the focus tube? Does it look good?

Thanks.

Here is the image I pretty much see due to the movement of the Cheshire in the focuser. Crosshairs don't align perfectly with all the reflections below. 

But as you can also see, in the reflection below, the centre circle on the primary intersects perfectly with the crosshairs reflection. Is this all that matters?

 

[Peter Drew]

Getting there, bit to do yet though. Only one mirror clip showing instead of three. 

[tehmac]

When I use the collimation cap for the first part of aligning the secondary, all 3 clips show perfectly. As soon as I put in the Cheshire, the clips aren't as easily visible, but I know they should be aligned well enough. I thought I didn't need to worry about the clips once I had all that sorted in the early steps with the collimation cap.

The image is also not a great one due to the angle of the camera like when you move your eye off centre.

If I take a picture bang on centre, it can look like this, so as you can see, I wouldn't read into the primary's clips. Unless I'm vastly wrong here.

 

[andrew s]

From what you say I would guess the secondary is not accurately centered under the focuser - did you use a sight tube?. The collimation cap will have a wider filed of view than the Cheshire. Try pulling the Cheshire out until the secondary just fills the end of the Cheshire's tube is the secondary centered? When it is can you see all 3 clips.

Regards Andrew

[tehmac]

Hi Andrew,

In Astro-Baby's guide, I completed steps 1, 2 and 3 with the collimation cap as stated in the guide. All 3 primary mirror clips were perfectly visible and the secondary is bang on centred. In step 4 of Astro-Baby's guide, it then says to insert the Cheshire and proceed to adjust the primary mirror. There is no mention of the mirror clips needing to be visible in step 4 or when using the Cheshire (nor do you see the clips visible in her sample photos). Can someone confirm that the guide is correct or incorrect on this point?

Thanks.

[andrew s]

2 hours ago, tehmac said:

Hi Andrew,

In Astro-Baby's guide, I completed steps 1, 2 and 3 with the collimation cap as stated in the guide. All 3 primary mirror clips were perfectly visible and the secondary is bang on centred. In step 4 of Astro-Baby's guide, it then says to insert the Cheshire and proceed to adjust the primary mirror. There is no mention of the mirror clips needing to be visible in step 4 or when using the Cheshire (nor do you see the clips visible in her sample photos). Can someone confirm that the guide is correct or incorrect on this point?

Thanks.

Sorry to confuse you but I was just trying propose a method to check the alignment  of the secondary by seeing if it was concentric with the bottom of the Cheshire tube. It was not about the clips as such.

Regards Andrew

[Peter Drew]

The alignment in the latest photo looks a lot better, the camera angle previously probably gave the wrong impression. 

[tehmac]

Hi Peter,

Unfortunately as I was saying, the alignment in the second photo looks good, but it was only when I pushed and held the Cheshire so there was no slack with it in the focuser. So basically I'm worried that this is a false alignment because it only lines up like this when I move the Cheshire a certain way. Is this a concern and it's actually okay and aligned even though I have this slack?

[Moonshane]

 

[N3ptune]

@tehmac

I noticed too the Cheshire has some play and it's generating errors, still, I like to use it to check if the secondary mirror is really concentric to the focuser tube, I found it good for that especially.

But usually I do the corrections on the secondary and primary with the CAP and a verification with the Cheshire to make sure everything is precise.

One thing that helped me was to dismount the secondary assembly and reinstall it from scratch, to learn how it works, to know the limits and the precision of the mechanic. I also cleaned the adjusting screws and add a slight amount of oil on the threads (not too much for it to run on the assembly body) now they work like a charm, they are much more precise with lubrication.

If this can help.

 

[Charic]

Discard the cap and use the Cheshire throughout! There is some conflict between the two tools, which you need to assess,address?

The cap is only a tool to align your eye with the focuser axis, and has a fairly wide field of view down the focuser tube.  You don't really need to use the cap and Cheshire, one or the other is sufficient! The Cheshire will have a longer tube, therefore cutting off  or limiting the view at the point your expecting to see the primary mirror clips?
If you withdraw the Cheshire slightly it will enlarge the field of view  to allow you to see the clips. I actually prefer the longer  Cheshire tool for the very reason you cant see the clips, the tool is longer, it  helps me align the secondary more accurately! 

If you feel the Cheshire is too loose, a single wrap of tape around its  waist may help?

Centre the secondary to the focuser axis ( has to appear round and concentric) then correct/tilt the  secondary mirror in order  to see the primary clips,  then align the primary last.

N3ptune thinks along the same lines? When I had my Celestron, I completely stripped it down the first day, cleaned it , re-assembled it  and it worked ok, I had a  much better understanding of   how it all works. It put me in the same frame of mind when my Skyliner arrived, infact I had to remove the spider to address an issue the day it arrived!  

Practice makes perfect, understanding helps, just keep reading, learning, trying. you'll get there in the end.

Note: You mentioned heavy adjustment to the primary? if it helps, loosen the lock nuts, then tighten the adjusters, not tight, but until no more movement! This places the mirror assembly flat against the mirror cell, a good starting point, and probably already aligned with the axis of the OTA. Then in reality, you can only adjust in one direction,  adjusting  with  small microscopic adjustments until the mirror is aligned. 

[tehmac]

Really baffled by this one. No matter what I did with the secondary looking perfectly round and centred, I couldn't get the donut and hole to line up in the crosshairs of the cheshire when adjusting the primary. I also couldn't get the secondary tilted using the three adjusters (I installed the bob's knobs adjusters on it) without turning them even further, but there was no more play left in the adjusters. They were all tightened up to the full, but it wasn't enough.

In the end I had to just fiddle with the rotation of the secondary by manually grabbing it and turning it back and forth (seemingly make no real change to the darn thing), even though it didn't look any rounder but obviously it was a difference of a microscopic amount of rotation that was making the difference, until everything sort of got close enough to align, then go back to the primary and adjust that etc and try to tweak everything out of order until I got it close. Eventually I got it all to line up, but it wasn't easy. I still don't know if it's good enough until I do a star test, but I do know the usual guides to follow weren't going to work for me straight out of the box, so to speak.

[Charic]

.......it really is just microscopic adjustments.

Collimation has to be done, its not easy at first, until its fully understood, but once you have mastered it you'll wonder what the fuss was all about, though some folk still have the fuss.

Actually being shown the technique helps, either in person or youtube videos, its finding one that you like. For me the Astro babe document suits me just fine.

http://www.forumskylive.it/Public/data/serastrof/201281510358_Astro Babys Guide to Collimation.pdf

Print it out, read it, re-read it, read it again if necessary, but read it  you must to understand it,  it will work, the instructions are simple once understood.

[Moonshane]

Did you read my collimation tip link above?

It's important to centre and 'round' the secondary down the drawtube, then centre the secondary in the primary mirror (centering the donut on the crosshairs using the three secondary adjusters), and only then centering the primary to the secondary (getting the dot inside the donut using the primary adjusters) and only in this order. Otherwise it won't work most effectively. The last step is the only one that needs doing regularly though.

In your later image above you cannot see the secondary mirror edge so need to bring out the cheshire a bit so you can align it to the end of the cheshire. You can still collimate without centering the secondary and aligning the secondary to the primary but you are likely to lose effective aperture and brightness.

[tehmac]

Hi guys,

Yes I followed all the guides listed and videos. Regarding your "It's important to centre and 'round' the secondary down the drawtube", I diid do this, but it seems my judgement of a perfectly round mirror was out by a fraction. I had get everything lined up with what I assumed was 'round' enough, then get all the things lined up as close as I could, then go back to the secondary and turn it just a little back and forth a fraction to what is apparently perfectly round, and then dial it all in.

Surely this is an okay method since it was the only way for me to do it?

[Moonshane]

Seems reasonable but in your photo above you cannot be sure the mirror is rounded and centred as you cannot see the edge of the secondary. In my tip as above, I used an extension tube and pulled out the cheshire a little to get the position correct, then adjusted the position of the secondary. The image below is quite good and shows after the first step I described above. Purple is the cheshire, pink the secondary edge and green the primary edge (of a badly collimated scope of course!). You want to aim for the cheshire looking like this and then you'd get the green centre donut aligned with the purple cross hairs using the three secondary adjusters. then the easy bit with the primary

 

[tehmac]

Thanks, yes the second image aligning wise with the crosshairs is what it now also looks like at this point too since I seem to have got it correctly collimated. When I pull the Cheshire out some distance it still shows secondary as perfectly centred also. The tip of wrapping a single layer of cellotape around the Cheshire made it a nice snug fit and aligning much easier. I just pray the star test goes well.

Thanks for all the help folks!

[Moonshane]

The main thing to do is enjoy the scope. You'll get better with collimation over time so don't worry too much and do your best. Better to enjoy views with collimation slightly off than to miss out on a session due to frustration!

[tehmac]

Well the other day I set it all up, sub zero temperatures, and for the second time now, I got it all set up to put Polaris in the right position and looked through the Polar Scope and saw nothing. It was a little cloudy out and Polaris was getting fainter by the minute. I checked the cap was off the Polar Scope on the top of the mount. It's off. I adjust the Polar Scope LED brightness up and down. Still can't see it. I move the whole mount a little to the side in case my point north is off. Still can't see Polaris. Getting more faint by the minute. Eventually I give up. I just manually slew and look around at some things. At the end of the session as I'm packing up, I remember the OTA needs to be in correct position otherwise it blocks the Polar Scope hole view through the mount. ARGH! Live and learn. I shall not make the same mistake 3 times let me tell you!

 

[Charic]

Moonshanes image is  quite detailed.
From the image provided, the secondary alignment (Pink) is achieved by adjusting the secondary mirror center bolt +/- and if required, with  a small rotation (by hand - don't touch the mirror surface, the edge is fine!)  to get the secondary perfectly circular and equidistant   with the  Cheshire (Blue).

Then the secondary adjusters ( three of them) are turned (one at a time - no specific order )  until you can see the three primary clips on the main mirror. Gently (GENTLY) tighten these adjusters until their just snug, recheck the mirror is still centred and the primary clips are  still visible.

Then the primary mirror is adjusted, loosen the locking nuts/grub-screws/Bobs knobs? and adjust the primary adjusters until the centre spot coincides with the centre axis/cross hair. Nip up the locking nuts, re-check everything, probably good to go at this stage.

Keep at it, your getting there.
 

[tehmac]

I should also note that manually trying to find things when you don't have the Finder Scope aligned yet is a pain in the ass. However, while trying to find Vega as that was still bright in the sky, I thought I had found it, but I realised later I had found The Double Double! Was very fun to have stumbled upon this and watch it for a while. Felt like I got something from the session.

[Charic]

Best to setup the finder during the day, but can be done at night.
Just lock your telescope onto a fixed point, Polaris or a Tower miles away, once in focus, adjust the finder to match, job done.

If only we lived less than an hours drive, I would love to  help you with your setup!, unless you wan't to drive up to Scotland and experience some dark skies.

[tehmac]

Yep. Setting up with Polaris is the plan as I don't have anything far enough I can see without drawing eyes from the neighbours

[Charic]

....any bright Star will do, if your quick?
Polaris is almost stationary in the finder, hence its the Star most recommended, but for many folk, its still not the brightest. infact if it were not for the  surrounding  space, it  could be hard to find for some folk!

 

[Moonshane]

I ignore the clips (not all mirrors have them). . centre the donut on the cross hairs. it amounts to the same thing really but it's more accurate to centre the donut on the cross hairs than gauge whether the clips are centred.