Newtonian collimation woes

[BrendanC]

Hi all,

 

I'm having collimation woes with a Skywatcher 130PDS. I've just spent most of the day having read every guide online, using a collimation cap, Cheshire and laser, and just cannot get the laser alignment to agree with the cap alignment. It's a common problem, but one I just cannot seem to fix.

 

The laser's collimated, I've done everything by the book, several times, including Astro Baby's guide. I just cannot get them to agree.

 

So, I'm going to pay someone else to do it instead. I don't care.

 

So, does anyone know of anyone who can do this, in the Buckinghamshire area? Obviously if/when lockdown has ended. I've pinged my local scope dealer but they're not very responsive recently (probably very busy I'd imagine).

 

Btw, I know you might have a helpful suggestion as to what could be going wrong, but honestly, I really have tried everything and basically I cannot do it, so recommendations for people who can do it would be more welcome...!

Edited April 19, 2020 by BrendanC

[Timebandit]

 

Hello. This may or may not be of help? 

A lot of people cannot get a laser ,Cheshire/cap to agree. I know I have had this problem myself in the past . Astro baby's simple collimation guide is great. I would put the laser to one side. Follow astro baby's guide with the collimation cap/Cheshire once more. 

Then do a star test of the scope. If in clear good sky conditions you can get a good star test result, then you have successfully collimated. If this fails then I am sure there is an astro club or SGL member in the area that will have a look for you . Best of luck 

 

 

 

[BrendanC]

Thanks - I have come across the concept of caps and lasers not agreeing! There's actually an entire thread about it here (sorry, rival forum): http://www.astronomyforum.net/dobsonian-telescopes-forum/167415-laser-collimation-disagrees-collimation-cap-collimation-what-gives-3.html - but the guy does eventually get it to work. I was kind of hoping that would happen to me, but every time, starting from fresh, I get precisely the same result: seemingly perfectly aligned via the cap, then needs adjusting with the laser, look back into the cap and it's out in exactly the same way by the same amount.

Lasers get a bad rap but I'm pretty sure mine is collimated, and absolutely tightly held in the eyepiece holder. But, perhaps you're right: maybe I should stick with the cap, forget the laser, and do a proper star test.

One thing I still don't get however: whereas the collimator cap and end of the Cheshire perform the same task (to centre the eye), and the laser sends light through the optics which are bounced back into the target, I still don't quite understand how the target on the Cheshire works. I've tried shining a torch into the scope to see if anything appears on the Cheshire's target but nothing of any real note, also the other way ie down through the hole in the Cheshire's cap to see if anything appears. Nada. Any recommendations?

Edited April 19, 2020 by BrendanC

[CraigT82]

You dont need to look at the angled face of the Cheshire directly. Just at it's reflection in the secondary as you're peeping through the hole. 

To be honest ditch the laser. If you can reach the collimation screws whilst looking through the Cheshire then just stick with the cheshire. The only reason id use a laser is if I cant reach the screws whilst looking through the Cheshire. 

[BrendanC]

Happy to ditch the laser, but what would account for them being out of agreement with each other?

Also I just took another look at the Cheshire. So, the part that looks like a target on a laser, is actually really just another cross-hair to aid accuracy and also bounce some light into the Cheshire to help with this? Is that what it's for?

I normally try and find this stuff out myself but there are no Cheshire guides on YouTube that I can find!

Edited April 19, 2020 by BrendanC

[johninderby]

A Concentre is a great tool for setting up the secondary in perfect postion. No other method is as accurate or easy to use. Not a cheap tool but makes the whole business of collimating easier as when the secondaty is in perfect position adjusting the primary is so much easier as you are not compensating for a misadjusted secondary.

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p5506_TS-Concenter-2--colimation-eyepiece-for-Newtonian-Telescopes.html

 

Edited April 19, 2020 by johninderby

[CraigT82]

5 minutes ago, BrendanC said:

Happy to ditch the laser, but what would account for them being out of agreement with each other?

Focuser slop. Slightly different diameters on the cheshire and laser shafts probably. Causes the laser to be gripped slightly different to the cheshire and hence a disagreement. 

I had same issue with my 200p. A Howie glatter parallizer fixed it. 

[BrendanC]

Interesting stuff, thanks both.

So, now that I've been recommended to buy two new pieces of kit for around the £70 mark each, should I:

* Stick with the collimation cap/Cheshire (assuming I eventually understand what the target on the Cheshire is for) and ditch the laser?

* Buy a Concentre to set up the secondary?

* Buy a glatter so that I can use the laser as well as the Cheshire?

Votes please.

Edited April 19, 2020 by BrendanC

[CraigT82]

12 minutes ago, BrendanC said:

Stick with the collimation cap/Cheshire (assuming I eventually discover what the target on the Cheshire is for) and ditch the laser?

This would be my vote! Though I'm not sure what you mean by target? Do you mean the 45 degree shiny face? The purpose of that is to catch the light so you can see its reflection in the secondary as you look through peep hole. 

[johninderby]

You can also colimate the primary with the Concentre although I switch to a Hotec laser after adjusting the secondary.  Get a perfect star test afterwards.

 

Notice how we are always ready to help someone spend their money. 😁

Edited April 19, 2020 by johninderby

[John]

This is what I use for my 12 inch F/5.3 dobsonian:

I have tried laser collimators including the Baader and Hotech ones but the above has consistently worked and the scope star tests well after its been used so I'm happy with it  

[DaveL59]

I'd redo with the cheshire and then check against the stars before spending anything further, you may well find that it's accurate enough for what you are using the scope for. Are you finding that the cheshire/cap are giving consistent results after racking the focuser slightly in before and out after adjusting the collimation screws? If you see a difference there then it'll be the focuser that's rocking slightly and altering the angle in which you could consider trying a little tune-up on it, then perhaps re-try your laser and see if the results improve.

I guess one other question, are you using this for visual only or imaging? For visual you can probably get away with it being just slightly out than spending lots of time and cash trying to get absolute perfection  

Edited April 19, 2020 by DaveL59

[BrendanC]

Thanks all - and yes, I do appreciate the eagerness to help me spend more money...!

I'm going to stick with what I have for now. I actually cannot face collimating all over again so I'm going to see what sort of results I get tonight, maybe do a star test. If I have problems, I'm going to learn how to use the Cheshire properly and forget the laser. I've used the laser lots in the past but it was more out of curiosity than anything that I took a good hard look through the cap which I bought a while back, and the Cheshire was just thrown in when I got the 130PDS and frankly I didn't think more of it.

Good to know what else I can spend my hard-earned on later though. I like the look of the Concentre, I 'understand' it which is half the battle for my tiny addled little brain.

Post-edit having seen DaveL59's post - this is for imaging, but I'm just using a Skywatcher AZ Goto mount and EOS1000D camera so really, I'm not after any awards or anything. I've been satisfied with what I've done so far. I just felt deep down inside I should really address collimation at some point, and I'm rather wishing I hadn't now...!

Edited April 19, 2020 by BrendanC

[DaveL59]

some days its better to put things aside and come back to it the next day 🙂

I have a cheshire only but mine are slower scopes and for visual so more tolerance, in fact a couple are very roughly set at the moment as I'm planning to dismantle and overhaul but even at that they still give nice visuals. Will have a play with trying to get some pics one day but likely will use the Tal-100RS for that being its on a goto and has better focus travel. Need funds first tho, esp during the current times...

[almcl]

12 minutes ago, BrendanC said:

I like the look of the Concentre, I 'understand' it which is half the battle for my tiny addled little brain.

Just a word on the Concentre, I have one and a Cheshire and guess what?  They don't agree either!  I slightly prefer the Cheshire (maybe because I've used it for longer) but the scope star tests OK using its alignment.  Also have a cheap laser but hardly ever use it - it was rubbish from the outset.

[BrendanC]

Good grief! So, Cheshires, caps, lasers, concentres... and none of them agree with each other!

It's a bit like UK politics, albeit before the upcoming Great Realignment.

Thanks all though, lots of food for thought, and I've had an offer from a very kind person who lives not far from me to pop round with his scope and go through it all with me when all this nonsense has ended.

[Rob]

I will add this site for help. I like the images provided, they really help.

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

Best Rob

[prusling]

Another option which probably works on a similar basis to the Concenter is to use a camera with lens to image the secondary, in conjunction with free software showing overlaid concentric circles to help to circularise and align it.

This does require a camera so may not help the OP (e.g. a webcam with original lens not removed and a 1.25" fitting, or an add-on lens to an astro cam such as a wide angle meteor lens).

See the excellent YouTube video by AstronomyShed entitled "Advanced Newtonian Collimation".

[BrendanC]

Just thought I'd report back, after my second attempt at doing this seriously, that it worked! 

Followed Astro Baby's guide again - this time with a much clearer idea of what I was doing (yes, it really does help to leave these things a while) - and everything seemed to go ok.

Then, as an afterthought, I decided just to chuck the laser in and see what I could see.

Absolutely dead centre! Jubilations ensued.

Just a tiny little tweak needed to the primary to get the laser dead centre in the collimator target, checked again, still dead centre in the primary.

So it is possible. I think the difference was that I could just do it, in the right order, without tweaking this and changing that, puffing, panting, swearing (a lot), starting again, etc etc etc

I'll be doing a star test tonight, as well as trying out APT's collimation tool.

Edited May 13, 2020 by BrendanC