Help with Collimation

[ThomasMK]

New comer here.   So I just got my first telescope Nexstar 130SLT.  After the first use, due to my OCD I wasn't happy with the excess play in the focuser so I took it apart and added teflon tape and adjusted it and got rid of all the slack.  I also noticed that from the 

manufacturer the collimation looked off so I collimated it using just a collimation cap and got it pretty bang on as seen in the bottom pic.  I decided to confirm this with a laser to be sure.  Checked with the laser and was apparently significantly off.

So next I checked the collimation of the laser and the laser was out.  Used a v-block and got the laser dead on, re-checked and no change.   The top picture was collimated with a laser and as you can see the secondary mirror looks like its 

more upward after adjustment and isn't centered and the primary looks off too.  The picture on the bottom is collimated manually with just a collimation cap and the secondary and primary look centered.  I've tried this multiple times with the same result.  Not really sure why it appears so far out when

collimating with the laser.  Please note the eyepiece holders are perfectly centered with zero play.  I made sure the laser is sitting center in the focuser.  Any suggestions?

 

Edited January 9, 2021 by ThomasMK

[Pixies]

Hi,

Forgetting about the alignment of the mirrors just now, have a look at the position of the secondary under the focuser. from your first image, it's quite a bit off:

Ignoring the reflection of the primary in the secondary - the circular outside edge of the secondary is too far up in the picture.

Now, look at your second image:

This one looks a lot better. Did you move the mirror significantly between these two pictures, there's quite a difference?

One way of checking the collimation of the laser is to rotate it in the focuser and see whether the dot moves. Also see if it moves when tightening the thumbscrew to hold it in place.

I would suggest getting a Cheshire and collimate the scope with that. It's a lot easier to get a good alignment of the secondary with that, IMHO. Then you can see how the laser compares. 

[Ricochet]

Setting the secondary requires a cheshire/sight tube. This allows you to set the secondary under the focuser properly, and then when you are aligning the secondary with the primary, allows you to see both the position under the focuser (edge of secondary compared to edge of sight tube) and alignment of secondary to primary (crosshairs over doughnut) simultaneously. When you just use a laser, you don't see where on the secondary that laser is hitting, just where it ends in relation to the primary doughnut, which can result in the laser being reflected off the secondary mirror edge to to the primary and back. The laser might hit the right spot on the primary and the laser return target, but only a single point on the secondary is collimated and the rest of the secondary can be way out as you have seen. Also, using a collimation cap for setting the secondary position is wrong, because as Pixies has demonstrated with the overlays above, we are not very good at judging concentricity when the gap between two objects (focuser edge and secondary edge) is so large.  

I would recommend that you buy either the FLO Premium Cheshire or a Concenter to collimate your telescope. I would only recommend using the laser via the barlowed laser technique to collimate the primary after the other tool has been used for the secondary.

[ThomasMK]

Yes so I mentioned the position of the secondary required to be adjusted really far up when using the laser to get it center on the primary dot.   When doing it manually I got a much better result and the secondary look much more centered. 

Thumbscrews are tight on the 2 to 1.25 adapter and I'm using just tape around the laser to make it center in the eyepiece holder.  There is no play anywhere and I've rotated the laser in the holder and its true.  I also just tweaked the vanes a little as they also seemed off from the manufacturer.  

I already ordered a cheshire and will report back.  Thanks! 

Edited January 9, 2021 by ThomasMK

[Spile]

On 09/01/2021 at 10:16, Ricochet said:

Setting the secondary requires a cheshire/sight tube.

Just checking your option of https://garyseronik.com/a-beginners-guide-to-collimation/ which is the (no laser, no cheshire) method I have been following?

The same method - 

 

[Ricochet]

1 hour ago, Spile said:

Just checking your option of https://garyseronik.com/a-beginners-guide-to-collimation/ which is the (no laser, no cheshire) method I have been following?

I agree with the following sentiment:

Quote

So what are we trying to do when we collimate our scopes? Just this: ensure that the eyepiece is aimed at the center of the primary mirror, and that the primary is aimed at the centre of the eyepiece. That’s it. End of story.

However, I think the photo already posted in this thread is evidence of my opinion that a collimation cap simply isn't an accurate enough tool to achieve this because the visual gap between the focuser edge and the secondary edge makes it too hard to judge when the secondary is centred. With a long tube from a cheshire/sight tube you can adjust the focuser so that the edge of the tube is almost the same size as the secondary mirror appears and then it is so much easier to accurately get your secondary under the focuser. With the crosshairs at the bottom of the tube you can see exactly where the primary doughnut should be and you can then tweak the secondary so that they are aligned (this is the important step of secondary collimation). With a collimation cap there are no crosshairs and so it is very difficult to know is the doughnut is precisely in the centre.

For primary collimation you want to get the reflection of the eye hole inside the doughnut. Here the crosshairs get in the way so it is time to remove the cheshire and use the collimation cap. Alternatively, I prefer to use a short cheshire I bought cheaply on ebay, and simply unscrewed the crosshairs as this can be angled to give a brighter reflection than a collimation cap. 

Also, it is not obvious to me why the linked article suggests to collimate the primary, then the secondary and then tweak the primary. The first step doesn't seem to do anything, you only have to collimate the secondary and then the primary. 

 

[Pixies]

6 hours ago, Ricochet said:

I agree with the following sentiment:

However, I think the photo already posted in this thread is evidence of my opinion that a collimation cap simply isn't an accurate enough tool to achieve this because the visual gap between the focuser edge and the secondary edge makes it too hard to judge when the secondary is centred. With a long tube from a cheshire/sight tube you can adjust the focuser so that the edge of the tube is almost the same size as the secondary mirror appears and then it is so much easier to accurately get your secondary under the focuser. With the crosshairs at the bottom of the tube you can see exactly where the primary doughnut should be and you can then tweak the secondary so that they are aligned (this is the important step of secondary collimation). With a collimation cap there are no crosshairs and so it is very difficult to know is the doughnut is precisely in the centre.

For primary collimation you want to get the reflection of the eye hole inside the doughnut. Here the crosshairs get in the way so it is time to remove the cheshire and use the collimation cap. Alternatively, I prefer to use a short cheshire I bought cheaply on ebay, and simply unscrewed the crosshairs as this can be angled to give a brighter reflection than a collimation cap. 

Also, it is not obvious to me why the linked article suggests to collimate the primary, then the secondary and then tweak the primary. The first step doesn't seem to do anything, you only have to collimate the secondary and then the primary. 

 

The Seronik guide was the first one I used. I think the reason he suggests a quick alignment of the primary to start with, is that as you are going to be aligning the secondary using the outside edge of the primary (mirror clips), you need to make sure it isn't way out of alignment first.

I agree with your comments on the advantages of the Cheshire. but I also think that its greatest advantage is that it's so much quicker!

[Spile]

Is the proof of the pudding in the eating?

If the star test shows that alignment with a cap has worked then what is the problem?

Has anyone carried out a survey of Newtonian owners to find out which method they use? I would find the results interesting but then I probably need to get out more.

I am currently going with the cap method as that involves less expenditure.

[Tiago Ferreira]

Sooner or later people realize that lasers aren't good to collimate (even if they are collimated). Just have to align all and then mess with the focuser screws or move around the laser to see all messed up again.

Fortunately i quickly realized that and since then i just use my own system that i can assure you that works like a charm and i just check collimation before observations and have it almost all the time collimated or moved 1 or 2 mm that i adjust quickly. And guess what? I also use my crap laser to collimate, yes. If you have a crap laser please check my video before throw it to the bin. You will need a concenter and any laser. A concenter is a wonderful european tool that you can buy easily on Europe and super efficient and easy to use.

This is not a regular method that use a concenter then use a laser like 2 independent tools. This is a method that use a concenter secundary collimation and then you MANUALLY ADJUST a laser dot at the primary mirror TO THE CONCENTER JOB in order to obtain the full collimation as a hole. It's a concenter and laser INTERACTION that allow a cheap laser to be used with an accuracy it doesn't have by itself alone because it relies on the concenter accuracy.

 

Edited February 1, 2021 by astrocanito