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Laser collimator or Cheshire or both?


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Just got my first reflector that definitely will require collimation from time to time, a Skywatcher 130pds.

Ploughing my way through all the reviews and videos I'm even more conflicted. I'm drawn towards getting a mid priced laser as it seems more intuitive and quick to me but so many reviews seem to rubbish them particularly from a quality perspective.

I know I could get both, one for each mirror, but we're watching the pennies at the moment.

I'm only using the scope for observing at the moment but imaging is on the horizon, comments appreciated.

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For an F5 130 PDS you could collimate with a collimation cap and not a lot else. If you have the choice of a cheshire or laser, I would opt for the cheshire. Cheap lasers do work (I have used them) but they do need collimating themselves.

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  • 2 weeks later...

A laser came with my scope and yes I had to collimate it before I could correctly do the scope.  It's really all I use and thus far I am quite satisfied with the results.

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I bought a laser for my 130PDS when I started out and didn't know any better. Even after collimating the laser, I found it next to useless. The cats love it though, it's one of their favourite toys.

A Concentre and a collimation cap are all I use now. Quick and easy, when needed. 

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4 minutes ago, kev100 said:

Hi,

I use a combination approach: Baader laser collimator; and an Aline collimator cap - I find that any discrepancy tends to be minimal, and usually down to slack in the focuser adapter.

Kev

This is the path I am now taking. I bought a Cheshire and to my shame I simply couldn't understand it, I found the various bits of documentation confusing, put it down to my somewhat less than agile 74 year old brain 😅.

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45 minutes ago, LaurenceT said:

This is the path I am now taking. I bought a Cheshire and to my shame I simply couldn't understand it,

Firstly , for visual your collimation should be good but doesnt need to be absolutely perfect , unless of course you have perfect vision . And as written previously a star test is the best way to check your collimation . I used to favour Laser collimators as ,and this is very shallow, they are "techie " but, i am now a committed Cheshire user . In truth , the Cheshire doesnt lie ( as long as its placed squarly in the focuser )  and whilst like you , Laurence , i struggled with it at first , it makes perfect sense in the end.  The problem with laser collimators which has been well documented is the fact that they need collimating . But i understand why people use them.  

Edited by Stu1smartcookie
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I bought a Svbony laser to use with my Heritage 150P but a) the laser dot was oval even outside of scope-use and b) it's too loose in the focuser to be in any way reliable, as a slight change in angle massively impacts where the laser hits.

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I only use my laser to get my secondary pointed back at the primary's center after loosening everything up and starting over when something seems amiss, but I can't quite nail down where.  I have to be very careful not to lase my own eyeball while looking down the front of the tube to get the laser off the tube wall and back to the mirror.

After that, I mostly use a cheshire sight tube with crosshairs and an Aline cap to finalize everything.

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On 28/12/2021 at 07:54, Mr Spock said:

No collimator gets it exactly right. You always need to finish with a star test. I use a laser as it gets it close enough, then finish with a star test.

Not really an option for Dob users with no view of Polaris and no EQ platform like myself.  Unless the star is perfectly centered, you can't easily distinguish decentering from bad collimation; and since it drifts off axis fairly quickly, it doesn't leave much time to adjust collimation.  It's mostly useful as a quick confirmation of decent collimation before the star drifts off axis.

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3 minutes ago, Louis D said:

I only use my laser to get my secondary pointed back at the primary's center after loosening everything up and starting over when something seems amiss, but I can't quite nail down where.  I have to be very careful not to lase my own eyeball while looking down the front of the tube to get the laser off the tube wall and back to the mirror.

After that, I mostly use a cheshire sight tube with crosshairs and an Aline cap to finalize everything.

I do exactly the same (laser for secondary, Cheshire for primary), works like a charm.

Nicolàs

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I seem to have got into an almighty mess in adjusting the position of the secondary mirror in my 130pds Newtonian. Initially when I thought I had the secondary adjustment spot on the laser beam reflection was missing the mirror and could be seen on the opposite wall. I then tried to alter the positioning of the secondary mirror by loosening the centre screw and now the image as seen through the focus tube is tilted to the left and for the life of me I cannot rotate it back to perpendicular.

20220110_143632_DxO.thumb.jpg.b982d0de7d00160f814e3bb323197517.jpg

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17 minutes ago, LaurenceT said:

I seem to have got into an almighty mess in adjusting the position of the secondary mirror in my 130pds Newtonian. Initially when I thought I had the secondary adjustment spot on the laser beam reflection was missing the mirror and could be seen on the opposite wall. I then tried to alter the positioning of the secondary mirror by loosening the centre screw and now the image as seen through the focus tube is tilted to the left and for the life of me I cannot rotate it back to perpendicular.

20220110_143632_DxO.thumb.jpg.b982d0de7d00160f814e3bb323197517.jpg

From the reflection in the primary, you can see that the secondary is twisted away from the focuser.

Red: secondary centre axis

Green: focuser centre axis

image.png.2a742c763e00d7dd50285a88a3d4453b.png

You'll need to twist the secondary back. What tool are you using to adjust the secondary, a Cheshire?

Edited by Pixies
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When I first tackled the collimation procedure I found it an immense help if I placed a yellow card in the tube blocking the reflection from the primary mirror and a sheet of white paper behind the focuser. This allowed me to focus on the secondary alone without (at that time) the confusion of reflections in both mirrors. This will allow you to centre the secondary and get the tilt close by getting a nice perfect circle. Try this to get a better handle on your wayward secondary mirror 

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This is where a long Cheshire (acting as a 'sight-tube') comes into action. You rotate the secondary after loosening the centre bolt and making sure it is under the focuser. Twist the loose secondary so that it appears as a perfect circle. You can do this with a collimation cap, but it's harder to be accurate. With the coloured paper trick:

With a Cheshire: 

image.png.d416522f59b55c52b9281488bb3d1d04.png

With a collimation cap:

image.png.7c4fb94701e4c12ff158d5f35aca03a2.png

You want everything concentric:

image.png.5dd8b740d3fec424edb155fc9b43491b.png

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6 hours ago, LaurenceT said:

I'm using a collimation cap, I'm not sure how to rotate the secondary on it's axis.

I explain it here - Correcting rotation/tilt error by loosening the central screw (coarse adjustment). https://astro.catshill.com/collimation-guide/
 

I also do not recommend “hiding” the reflections in the primary. The sight tube cross hairs need to align with the centre mark and also the “direction” of the offset secondary (r2) should be toward the primary as outlined above.

 

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2 hours ago, Spile said:

I also do not recommend “hiding” the reflections in the primary. The sight tube cross hairs need to align with the centre mark and also the “direction” of the offset secondary (r2) should be toward the primary as outlined above.

He was using a collimation cap not a Cheshire/ Sight tube and he was asking how to position the secondary back into place.

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