Collimating with Paracorr

[Scooot]

I don't ever hear of anyone collimating their scope with a paracorr in place so I thought I'd post a few pics of the way I do my new Truss Dob. It's not usually too far out when I reattach the secondary cage but I'd be surprised if it doesn't normally need some tweaking.

I've found in the past that if I collimate without my paracorr in the focuser, it's normally not quite right when I put it in. Presumably because it's not sitting exactly in the same place as the 2" to 1.25" adapter I'd otherwise have to use. With the paracorr in place I have to use a laser because a cheshire would touch the paracorr lens.

This is the laser dot on the primary as seen through the paracorr before adjustment. It's taken in daylight so not as clear as at night of course.

I only needed to use two of the secondary collimation knobs to position the beam on the central doughnut in a matter of seconds. The beam fits nicely over the whole doughnut as opposed to the view of a small dot without the paracorr.

This is the view of the reflection back from the primary before I've adjusted the main mirror. The doughnut is reflected making it easy to center it on the laser display disc.

And finally after the primary is adjusted. The ring is now clearly shown in the centre of the disc.

This works very well for me, it's a two step process that's quick and easy to do in the dark. To leave the paracorr in place means one less thing to throw out the collimation by changing bits of equipment.

I hope this of some interest.

[Moonshane]

Interesting. I generally collimate with the paracorr out and have never really thought about it. I suppose your method is a bit like the narrowed laser method.

[andrew s]

Hi Richard, collimating this way will not ensure the focuser axis and the optic axis are parallel only that they cross at the laser collimator viewing screen. This may be why you see a small difference between the with and without Parcorr adjustments. This also means that any focus adjustment will result in a small de-collimation.

I don't know how to square on the focuser just with a laser collimator but you could try looking at the results at the extremes for the focuser range.

Regards Andrew 

[Owmuchonomy]

Nice presentation and interesting approach. I've always been wary of lasers due to the risk of having one not collimated but looks like it's needed with the Parracor in.

[Scooot]

Hi Richard, collimating this way will not ensure the focuser axis and the optic axis are parallel only that they cross at the laser collimator viewing screen. This may be why you see a small difference between the with and without Parcorr adjustments. This also means that any focus adjustment will result in a small de-collimation.

I don't know how to square on the focuser just with a laser collimator but you could try looking at the results at the extremes for the focuser range.

Regards Andrew

Not sure I follow Andrew. It's no different to collimating normally with a laser. I'd previously used a Cheshire and collimation cap to ensure the focuser is square and the secondary is central and round. I use this method for fine tuning it, not for setting it up eg after the mirrors had been installed. If I rack the focuser in and out there's no movement in the laser dot so nothing changes whilst focusing.

If I'm missing something else I'd be curious to know.

[Moonshane]

I think that the suggestion is that you cannot align the secondary to the drawtube of the focused using a laser method which of course is correct. You need a holographic widget for that.

[Scooot]

Ah ok, if the focuser wasn't quite straight I don't think the cheshire would fully encircle the secondary when you look through it.

I think there's a slight difference between the way my 2" to 1.25" adapter and the paracorr tightens in the focuser, and putting them back in exactly the same position is unlikely.

I'd be interested if someone else could test whether there's a difference, ie collimate without the paracorr and check again after its inserted. Maybe this difference is one reason why a star test sometimes leads to slight tweaking?

[John]

Maybe Andrew is referring to the 3 stage collimation process ?:

1. Centre secondary in focuser drawtube

2. Adjust tilt of secondary

3. Adjust tilt of primary

My understanding is that the barlowed, or in your case Paracorr, laser approach is used for the 3rd stage only.

Apologies if I've misunderstood anything though !

[Scooot]

Thanks John, why not stage 2 as well though? Laser instructions tell you to use it to tilt the secondary until the dot is on the centre of the primary. I understand you wouldn't use this method to start from scratch but once it's more or less aligned, why would shining it through the paracorr be any different?

[Moonshane]

It is stage one that the laser cannot do

[Scooot]

Okey doke thanks everyone, I wasn't trying to start a thread about the benefits of using a laser or not, I know it can be controversial. If I was starting from scratch I'd use both a collimation cap and cheshire for the first alignment.

I find the laser very useful for fine tuning which I can do with the paracorr in place so I thought I'd share it in case anyone else wants to try it.

[Moonshane]

Absolutely and you did a great job. I think the intention was to explain the with and without paracorr anomaly.

[John]

It is an interesting thread and interesting feedback that the Paracorr combined with the laser produces a useful collimation tool

The projection of the primary ring on the face of the collimator is clearer with your Paracorr approach than I've managed to obtain with a conventional barlow.

I'm happy to try out all sorts of methods to assist with collimation. I've just acquired a set of the collimation tools that are on Astroboot at the moment and I'm seeing what those are like to use. They are supposed to be exact copies of the Tectron collimation tools which have been out of production for quite a while.