And yet *another* collimation post!

[Jasonb]

Well, with the weather conditions being the kind of freezing foggy night that Holmes would have felt at home in, I decided as I couldn't observe anything, I may as well have a proper look at my new scope, and see what collimating is like.

I don't have a Cheshire/laser/etc. yet, but made myself a collimating cap and used the Astro Baby guide (Astro Babys Guide to Collimation - Newtonian Telescope - Reflector (astro-baby.com)) to see what it looked like.

The first stage, aligning the secondary mirror to the focus tube, looked fine to me, a nice circle framed in the centre. It might not have been perfect, but it definitely wasn't in a position that made me think it need to be fixed. So leaving well enough alone, I moved on to the next step, aligning the secondary mirror to the primary. Again, using the collimator cap, I had a look, and I could only see one of the primary mirror clips. If I took the cap off and looked straight down the focus tube, I could see all three by moving my head around a little (therefore not looking down the centre of the focus tube). So I got out an Allen key and slowly tried to work on the three screws. I was never able to get all three clips in view, at least not without potentially making a mess of it, and at one stage I was worried that I'd made it worse, but I eventually got it to the stage where I could see 2 clips through the cap, and the third if I took the cap off and barely moved my head, so it was close. 

I then decided to not try anything else with the secondary for fear of making it worse! My StarQuest 130P has a fixed Primary that can't be adjusted, so that was me done. At least I've tried it once now, and the next time I'll have the experience of having done it before.

I will be getting a Cheshire over the next few weeks, so I'll see then if that helps the process. As a newbie, all of this is very new to me, so hopefully over time it'll become second nature. The next time I'm observing (tomorrow night is *meant* to be clear?) I'll see if I can notice anything, either better, or more likely, worse! 

There's a question actually, can you tell from observing whether there's a clear collimation issue or not? I've read about Star Tests etc., but they seem to be done at 200x or more, on stars that are in the exact centre of the eyepiece and not moving. Is there anything obvious I should look for at either 25x or 65x (my only two options at the moment) that might show a collimation issue? Or is it just a more subtle 'not quite right' view and just looks better when it is collimated properly?

[NGC 1502]

Just do your best with what you have.  65x is not ideal to check collimation but should give you a fair idea. If your scope is not driven than viewing Polaris is most convenient, a bit out of focus with the star central in the field of view.  If collimation is ok then the shadow of the secondary should be concentric within the diffraction rings.

Surprised that the primary has no collimation adjustments.  I don’t know if this applies to your scope but some have a cover plate on the primary cell that hides the adjustments.

There’s 2 reasons for doing a star test. One is to check optical quality, the other is for collimation.  Don’t worry about optical quality too much, almost all Skywatcher reflectors I’ve looked through over the years have been fine, and you can’t change that by adjustments.  But collimation can be off and it’s well worth getting it right for the best views.

Ed.

 

 

[Jasonb]

Thanks for your reply!

I'll give it a go the next time I'm observing. I have noticed before when looking at stars that if I go well out of focus I can basically see the spider vanes, but didn't notice any concentric circles. But I'll check again soon to see how it looks...

[Tiny Clanger]

This is a nice simple and clear explanation I bookmarked for when I might need it  :

https://garyseronik.com/a-beginners-guide-to-collimation/

 

[Jasonb]

Thanks Heather! At this stage, I *think* I understand what I'm meant to do, it's just doing in in practice and getting used to it!

 

[popeye85]

One your Cheshire arrives you will get a better idea. Just adjust the secondary until the donut sits under the crosshairs. Unusual that you can't adjust the primary though.

[Zermelo]

11 minutes ago, popeye85 said:

Unusual that you can't adjust the primary though.

It will be one of the SW models with a fixed primary cell. Collimation is set in factory and shouldn't need touching afterwards. Secondary has the usual adjustments.
 

[Jasonb]

Thanks for your replies. Yep, the Sky-Watchers come with a fixed primary mirror. I had some concerns about it (based on my very limited knowledge) but anything I read online about them said that the primary was fine as was so not being able to adjust it wasn't an issue.

I reckon the crosshairs on the Cheshire will help alright, after Christmas!

[Jasonb]

Happy Christmas everyone!

So, good news, I now have a Cheshire, unwrapped from the FLO box this morning. This is the one I have...

https://www.firstlightoptics.com/other-collimation-tools/astro-essentials-cheshire-collimating-eyepiece.html

With 5 minutes to myself I put it in my SkyWatcher 130P and I've attached the view I get through the pinhole in the top of it, with the focuser all the way out.

From what I can see, the crosshairs are fairly close, if not perfect, on the primary donut. As a reminder, the Primary is not adjustable on this scope, just the secondary. The donut doesn't look centred on the larger black circle of the secondary. And the 'white' circle with the black dot is from the hole at the side of the Cheshire, I don't know if that's meant to be centred or not? One of the three spider vanes looks straight on, but the other two look like I can see the side of them a little?

So, what do people think, is it terrible/ok/pretty close/too hard to tell from this photo? I'm going to have a proper look at it when things have quietened down today or tomorrow, but just wanted to put a pic up for opinions!

Thanks...

 

[Zermelo]

Hi Jason (and Happy Christmas - not sure how many will be tuning in today)

I hope you'll get an answer from someone who does a lot of collimation, but for what it's worth I posted a similar query when I first received my 150i (which also has the same, sealed primary): https://stargazerslounge.com/topic/357345-how-far-off-collimation-is-this/

You'll see that my collimation wasn't a lot better than yours, but the Great and the Good didn't think it was that significant. I did a star test at around 200x and it seemed OK.   At the time, I was a bit nervous of adjusting the secondary and possibly knocking it even further out, so I left it. A few weeks ago I plucked up the courage and dug out the metric allen keys. It wasn't difficult at all, just a bit fiddly (don't attempt it with the tube vertical!). If you play with one screw at a time, you can see which direction the mirror is moving in - I found it very like adjusting a finder. It didn't take me long to get the spot inside the doughnut.  

[Jasonb]

Happy Christmas to you Zermelo! It did occur to me that there was nothing more 'newbie' than posting about Collimation on Christmas Day! 🤣

Yeah, I've tinkered a bit with it over the last few weeks, but was also wary of making it worse, or buggering it up completely! Now that I have a Cheshire I'll definitely dedicate some time to it and see what I can do. I just want to make sure that the scope is optimally set up as much as possible.

 

[Tiny Clanger]

Merry Christmas both ! The burning question is ... did Santa bring Jasonb a BST, or were they out of stock even for magical beings ?

Heather

[malc-c]

To be honest whilst the tools like a Hotech laser and a Cheshire are not required to collimate a scope, having them makes the job so much easier.  Also with this being one of the more frequently discussed topics on the forum, collimating a scope should now no longer be the dark science it is always claimed to be !

[Jasonb]

7 minutes ago, Tiny Clanger said:

Merry Christmas both ! The burning question is ... did Santa bring Jasonb a BST, or were they out of stock even for magical beings ?

Heather

Ha! And a happy christmas to you too Heather! Yourself and Zermelo (and everyone else, to be fair) have been so helpful and welcoming since I joined, so do appreciate it!.

I'm afraid that a BST, and a Barlow, were beyond even Santa's capabilities. But that's ok, a Cheshire was always first on the list, no point having better eyepieces and magnification if the scope isn't optimised in the first place. I'll be adding a Barlow next.

I was also lucky to get a copy of Turn Left at Orion which turned out to be much more detailed and bigger than I thought. Have already started reading it, it's fascinating!

Collimation does feel like the dark arts Malcolm, hopefully it'll become more natural over time!

[Davey-T]

5 minutes ago, malc-c said:

To be honest whilst the tools like a Hotech laser and a Cheshire are not required to collimate a scope, having them makes the job so much easier.  Also with this being one of the more frequently discussed topics on the forum, collimating a scope should now no longer be the dark science it is always claimed to be !

Depends how "fast" the scope is, I've been messing with the SharpStar 150HNT f/2.8 for a while and acquired an array of collimation tools, unfortunately the adjusters on the scope are too clunky to achieve the perfect collimation needed.

Dave

[Jasonb]

Ok, I now have a new plan for collimation. I spent a while at it yesterday, and in the end I couldn't tell if I was making it better or worse!

It just seems a bit too vague and inexact. Even with the cheshire, I could change the position of the crosshairs etc. by barely touching the focuser tube (the part that moves in and out, not the part that's mounted to the OTA). There was enough give in the tube to make getting it all centred guesswork at best. I got different results if I had the tube extended fully or not extended at all. And every time I adjusted one of the secondary mirror screws that act of just putting pressure on the Allen Key would move the view in the Cheshire, never mind actually moving the screw itself. And then the guides say that some fast telescopes (mine's an f/5) might have a built-in offset, so the secondary might be offset from the centre, and that's correct. So it's hard to know if an offset secondary is right for your scope, or wrong!

So, in the end, I have a new, two part plan. Firstly, when I get a Barlow (when they are back in stock) and I can get much higher magnification, I'll try a star test and see how it goes. And the longer term plan is to get someone who knows what they're doing to have a look at it, once we're actually allowed to travel/meet people again.

Until then I'll read through Turn Left at Orion and wait for some clear skies, whenever that might be! 😀

Edited December 27, 2020 by Jasonb