Barlowed laser collimation by Nils Olof Carlin?

[turboscrew]

I wonder if anybody could bend the method for me from iron wire?

I tried to figure out how it's done from Carlin's web page, but I still didn't quite get it.

And why pretty much only the primary mirror collimation affects the result?

Does anyone know where to find the S&T article?

 

 

Edited August 10, 2020 by turboscrew

[Ricochet]

It basically turns the laser into a torch which illuminates a patch in the centre of the primary. So long as the secondary collimation is close enough that the doughnut is illuminated, you will see the shadow and be able to use it to collimate. I think because the beam is diverging, small changes in secondary collimation will not alter the angle of the rays hitting the centre of the primary and so the reflected image is the same. 

[AstroMuni]

Over and above what Richochet has mentioned, it helps compensate for any wobble in the focusser. This figure taken from book Astronomy hacks may help explain it better...

[turboscrew]

3 hours ago, AstroMuni said:

Over and above what Richochet has mentioned, it helps compensate for any wobble in the focusser. This figure taken from book Astronomy hacks may help explain it better...

Thanks.! It definitely did!

Still curious, though, about what makes it work such that alignment errors of secondary and focuser don't mess up the results.

 

[turboscrew]

10 hours ago, Ricochet said:

It basically turns the laser into a torch which illuminates a patch in the centre of the primary. So long as the secondary collimation is close enough that the doughnut is illuminated, you will see the shadow and be able to use it to collimate. I think because the beam is diverging, small changes in secondary collimation will not alter the angle of the rays hitting the centre of the primary and so the reflected image is the same. 

I understood that moving the secondary mirror or focuser a bit, moves the lit spot, but the doughnut reflection stays put around the laser output (as long as the spot covers the doughnut).

Edited August 11, 2020 by turboscrew

[turboscrew]

BTW, is it important that the laser output hole is in the very middle of the target? I assume that it doesn't have to be that precise.

[Ricochet]

1 hour ago, turboscrew said:

BTW, is it important that the laser output hole is in the very middle of the target? I assume that it doesn't have to be that precise.

If you are doing it as in the picture above with the hole at the bottom of the focuser then I think you want it as precise as possible as you will be using the hole to centre the reflection. However, if you have a laser collimator with a 45° angled face there is no need for the cut out. Just angle the face so that you can see it from the primary end and the reflection will show on the face of the collimator, and you can centre on the hole in the centre of the face. This is shown in the image below. In the flesh the shadow is seen as an even circle, but for some reason it did not come out properly in the photo. 

[AstroMuni]

11 hours ago, Ricochet said:

However, if you have a laser collimator with a 45° angled face there is no need for the cut out

Did you mean you see the image on the flat face which has the reticle like markings??

I think there was a logic behind NOT doing it this way....this would mean the light returning from secondary would go via the barlow (again) and thus blow up the image giving less accuracy.

Edited August 12, 2020 by AstroMuni

[John]

I use this approach when I'm using the barlowled laser collimation method. It works well for me

http://www.smartavtweaks.com/RVBL.html

Mind you, the collimation adjustments that my F/5.3 12 inch dobsonian requires are generally very small so usually I just use a cheshire eyepiece and then a quick star test to check.

 

[Ricochet]

3 hours ago, AstroMuni said:

Did you mean you see the image on the flat face which has the reticle like markings??

I think there was a logic behind NOT doing it this way....this would mean the light returning from secondary would go via the barlow (again) and thus blow up the image giving less accuracy.

I suppose if you have a particularly sloppy focuser it might be a problem, but in that case you could never collimate accurately. In my dob the results this way are indistinguishable from the result of using a star, and you can see the collimation while you make adjustments. 

[turboscrew]

I'd still be interested on how the barlowed laser method works - optically.

 

[John]

9 minutes ago, turboscrew said:

I'd still be interested on how the barlowed laser method works - optically.

 

There is a section in this piece by Howie Glatter titled "BARLOWED LASER PRIMARY ADJUSTMENT" which goes into the optical principles:

http://www.collimator.com/collimation

[turboscrew]

12 minutes ago, John said:

There is a section in this piece by Howie Glatter titled "BARLOWED LASER PRIMARY ADJUSTMENT" which goes into the optical principles:

http://www.collimator.com/collimation

Loads of thanks! I have to ponder that for a while, but it's just the right language for me, and it seems to make sense to me. I actually think, what I'm looking for, is there. I just need to get that info organized in my head. I think I'm getting close...

Edited August 12, 2020 by turboscrew

[AstroMuni]

11 hours ago, turboscrew said:

Loads of thanks! I have to ponder that for a while, but it's just the right language for me, and it seems to make sense to me. I actually think, what I'm looking for, is there. I just need to get that info organized in my head. I think I'm getting close...

Just drawing the rays on a piece of paper helps for me 🙂

[turboscrew]

On 13/08/2020 at 11:41, AstroMuni said:

Just drawing the rays on a piece of paper helps for me 🙂

Did that yesterday to figure out the secondary offset.