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BeerMe

First time I've checked my collimation

14 posts in this topic

Bought a collimation cap and an FLO Cheshire over New Year, and got round to trying it out tonight to check the collimation.

I used the cap first and imo the secondary looks fine as you can see from the pic.  I then put the Cheshire in and it just isn't making any sense at all, which I think may be caused by the Cheshire sitting tilted in the focuser even though I have inserted it fully to the lip.  I'll post a pic of the Cheshire view but it kept focusing on the crosshairs in the Cheshire so it isn't great.  Getting annoyed a bit, I put the cap back in and, through the cap, it looks as if the collimation is fine (pic attached).

So...is my collimation ok, and what am I doing wrong with the Cheshire?

 

20170106_184813.jpg

20170106_185247.jpg

20170106_190242.jpg

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It definitely seems to be a problem with the focuser.  I put the Cheshire back in but never tightened the screws to secure it.  I was getting the same view (not clear in the above pic, but it's pointing in such a way that only the clips at 7 and 11 o'clock are visible.  Being loose in the focuser, if I push it slightly over to the left at the top/eye end of the Cheshire, all 3 clips come in to view.

I assume this means that my focuser is pointing my EPs at the secondary mirror at the wrong angle?  How big an issue is this and is it correctable?

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Hi Paul,

I'd like to see a closer snapshot, like this of my 150mm f/5 through a collimation-cap...

collimation1b.jpg

In any event, you want the scene to appear like that, close up.  Here's the same image but altered with a simulated secondary-stalk like yours, instead of a four-vane spider, and so to better relate...

collimation1b-stalk.jpg

Illuminate a white wall, then aim the telescope at it.  With the Cheshire inserted just so, rack the focusser to a position like this to the secondary mirror, and framed by the bottom opening of the focusser's drawtube...

secondary collimation2.jpg

Then insert the collimation-cap, zoom in a bit with a camera, and snap a shot of the scene as shown in the second image.  I can hardly make head or tails of the scene with it being that far away within your images.

I do know that those particular kits are very popular, and in the U.S. as well, the AWB OneSky(Synta makes both), but collimation can be more difficult with a secondary stalk instead of a spider.

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It looks to me that you need to align your secondary to the primary. Using the three secondary adjusters get the donut reflection centered on the cross hairs of the Cheshire.  then repeat step Obe and possibly two until both look good. Then adjust primary.  that bit's easy. My guide is a bit simpler than some

 

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I had to do a little adjusting with the collimation knobs, but this was the end result through the collimation cap which looks pretty spot on to me :-)

I'm going to get some plumbers tape for the focuser threads to see if that will cure the wobble before I try to collimate with the Cheshire again.

20170107_000937.jpg

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A little insulating tape around the body of the cheshire has been recommended by others to reduce movement of the Cheshire in the focuser.

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5 minutes ago, iapa said:

A little insulating tape around the body of the cheshire has been recommended by others to reduce movement of the Cheshire in the focuser.

The Cheshire actually seems ok in the focuser, but the focuser clearly rocks in the threads and the weight of the Cheshire seems to pull it to the right at the eyepiece end, off-centre.  I'm assuming that the same is true of any eyepiece I use, which you would think would cause a problem with viewing but it's been fine in that department  (seen M42 and M31 from my light-polluted back garden).

 

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59 minutes ago, BeerMe said:

The Cheshire actually seems ok in the focuser, but the focuser clearly rocks in the threads and the weight of the Cheshire seems to pull it to the right at the eyepiece end, off-centre.  I'm assuming that the same is true of any eyepiece I use, which you would think would cause a problem with viewing but it's been fine in that department  (seen M42 and M31 from my light-polluted back garden).

 

Sorry, cant think how this is working as I dont have that particular OTA.

Dunno if it's much use but I did find this How to fix a sloppy helical focuser? - ATM, Optics and DIY Forum ... tho' which may be worth look?

 

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Posted (edited)

A really useful tool is alscollimationaid it is free. You use it to check secondary outer circle and primary are concentric (if your view from the eyepeice is good then your probably don't need to fiddle anymore). Secondary inner bit should be offset as telescope is f5. See if I have an image of mine to compare.

A wind or two of ptfe tape on the focuser can help with slop.

Edited by happy-kat
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Posted (edited)

This is a good guide, on page 7 it shows what it looks like in an fast scope like ours at f5 far better than my poxy description.

link here

^ the link above is Astro Baby's guide

Edited by happy-kat
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Astrobaby's doc does seem to be a preferred reference doesn't it :)

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Hi Astro baby's collimation guide is great for collimation. If you Google it you will find it on the web.

But if you really want to check your collimation to see if the collimation cap and Cheshire has set the scope correct. Then I recommend you do a star test. A star test is basically putting a high power eyepiece in the Focuser , point it at a star and then defocus the image slightly. From the resulting circle alignment/shape you can get an accurate result on how well your scope collimation is set up. A star test is best done in good weather/atmosphere conditions as these can effect the clarity of the results.

I hope the above helps☺

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3 hours ago, happy-kat said:

This is a good guide, on page 7 it shows what it looks like in an fast scope like ours at f5 far better than my poxy description.

link here

^ the link above is Astro Baby's guide

Thanks kat, I had the guide downloaded then couldn't find it again!  I'll make sure I save it some place I can find it this time ;-)

1 hour ago, Timebandit said:

 

Hi Astro baby's collimation guide is great for collimation. If you Google it you will find it on the web.

But if you really want to check your collimation to see if the collimation cap and Cheshire has set the scope correct. Then I recommend you do a star test. A star test is basically putting a high power eyepiece in the Focuser , point it at a star and then defocus the image slightly. From the resulting circle alignment/shape you can get an accurate result on how well your scope collimation is set up. A star test is best done in good weather/atmosphere conditions as these can effect the clarity of the results.

I hope the above helps☺

A star test is definitely first thing on my to-do list on my next session! :-)

Just need some clear skies lol

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(Again, I'm no expert) but maybe if the focuser is a bit wobbly causing the chesire to point in different directions, maybe try rotating/positioning the scope so the chesire is pointing straight up so gravity is simply pulling is downwards into the focuser.  That way, it won't be tilting or twisting anywhere.

 

As well as Astrobaby's guide, the Astonomy Shed on youtube has a long series on collimation.  I found them most illumimating. 

 

I finally 'got' collimation (for dobs/newts) a few days ago.  Once the penny drops, I think it isn't so hard after all.  They key bit was placing some coloured card in the tube (one piece opposite the focuser and a different coloured bit to block off the view of the primary for the first step)

If I've not totally misunderstood, the goal is simple:  Get the centre of the secondary aimed at the centre of the primary then get the centre of the primary aimed at the centre of the secondary.

It seems you have to do three steps:

1) Get the secondary looking nice and circular and central in the focus tube as a starting point

2) Fine tune the secondary so it aims at the centre spot on the primary.

3) fiddle with the primary to get the reflection of the cheshire centred on the 'real' cross hairs.

Basically, the goal is simple:  Get the centre of the secondary aimed at the centre of the primary then get the centre of the primary aimed at the centre of the secondary.

 

Have I got it?  I followed those three steps and I noticed markedly better views: clearler, more contrasty.  I couldn't try a star test yet as the seeing hasn't been great.

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