tips please on how to check my focuser is in my refractor straight using a laser

[sonnymoon]

It's an ED80 with the stock focuser. I think there is some astigmatism somewhere as my stars can be a little stretched and so I want to see if I can use a collimating laser to check if the focuser is in straight. (I'm also going to make a determined effort to get the spacing right between the FF//FR and the sensor, but that's another story). I can't see any threads that get into the detail of lining up the focuser with a laser. 

If I put the laser in with the target bit facing up, then the red dot that bounces back off of the lens is slightly off centre on the target on the collimator. If I loosen the screws, turn through 90deg, retighten and look again, then the red dot is in a different place and so on all the way through 360 deg. I'm thinking that the red dot should be central (as when collimating a Dob that I used to have). Is that right?

It may be that the laser needs collimating, but I'm not sure how to do that.

Any advice on whether this approach is going to get me anywhere would be appreciated.

 

[John]

The way that I do it is to remove the diagonal, put the laser collimator (which has itself been well collimated) in the drawtube and turn it on. At the other end of the scope, see where the laser beam exits the objective lens. If it is right in the center, the focuser optical axis is aligned with that of the objective, if the laser exits off centre, the focuser is tilted to some extent and it's opical axis is not aligned with that of the objective lens.

I don't use the angled target face of the laser collimator at all for this test.

To test the collimation of the laser collimator itself (which is importan as thay can often be out, especially with low cost units) I use this method:

http://www.stark-labs.com/craig/llcc/llcc.html

 

[johninderby]

Assuming your laser colimator is collimated it's self yes fit it into the drawtube without the diagonal. An easy way to tell if the beam is centered is to print out a pistol target on thin paper and place it over the objective making sure it is centred. Then you can see the red dot on the target.. 

[sonnymoon]

Thanks for both of your replies, very speedy and helpful.

Cheers

[jock1958]

I had a number of collimation issues with my Tal 100RS - alignment of the focuser draw tube to lens and the doublet lens cells itself.

Firstly I inserted a laser collimator in the draw tube and target on lens cell then pointed the tripod mounted OTA at the floor where I rotated collimator and saw the red dot fluctuate confirming that the draw tube was incorrectly aligned with lens cell. I used the wife's make up big mirror on the floor to see the red dot on the target.

Having the OTA pointed at the floor reduces sideways movement and backlash between collimator and draw tube (which I had) you also have the ability to rack the focuser in and out whilst checking if the red dot is moving away from the bulls eye, this checks for any misalignment / wear issues in your draw tube (which I also had!)

Once I sorted the focuser to lens cell alignment I then moved onto checking the lens cell alignment with a Cheshire eyepiece. I faffed around for ages and didn't fully resolve the problem but it is much better. 

A few pics below before and after adjustment of the focuser draw tube and double doughnuts of my misaligned lens cells through the Cheshire. Note: you should have the one doughnut when lens cells are correctly adjusted, then confirm all is well with a star test. Good luck.

Edited September 25, 2019 by jock1958

[sonnymoon]

Thanks Jock for the tips. There’s no adjustment of the lens cell on my ed80 as far as I know so lining up the focuser in the draw tube and getting the correct distance between the FF and the sensor is the best that can be done.

all the best 

Chris 

[sonnymoon]

One more thought. If I line up the focuser and then shine the laser through the scope and there are still two points of light that hit the wall (or the floor), overlapping or otherwise, does that mean the lens cells needs collimating?

[John]

2 hours ago, sonnymoon said:

One more thought. If I line up the focuser and then shine the laser through the scope and there are still two points of light that hit the wall (or the floor), overlapping or otherwise, does that mean the lens cells needs collimating?

What matters is where the laser exits the objective. Don't worry about what it looks like when it hits the wall / floor / whatever.

The tilt of the objective (stage 2 of the checks) is done with a cheshire collimating eyepiece, not a laser collimator. The last 2 photos posted by jock1958 show the the view through the cheshire collimating eyepiece when the objective tilt needs adjusting. You should see a single illiuminated disk rather than the 2 partially overlapped ones.

Just to re-iterate, there are 2 stages which need to be carried out in sequence and 2 tools involved:

1. Check and adjust (if needed) the focuser alignment with the objective optical axis. This is done with a collimated laser collimator.

2. Check and adjust (if needed) the tilt of the objective lens. This is done with the cheshire collimating eyepiece.

Where there are no collimation adjusting screws (very common), you can apply some tilt to the focuser by loosening the screws that hold it onto the scope tube and adding a shim or two to one side to tilt it slightly. Then tighten up the holding screws. If the objective tilt is out then things are more tricky but there are some tricks which can help.

 

[jock1958]

5 hours ago, sonnymoon said:

One more thought. If I line up the focuser and then shine the laser through the scope and there are still two points of light that hit the wall (or the floor), overlapping or otherwise, does that mean the lens cells needs collimating?

@John put it very well and exactly what I did.  

I fear I might have confused things by mentioning pointing the OTA at the floor, I will take some pics and post them to clear up any misunderstanding. 

 

[sonnymoon]

Thanks for clarifying , John. That makes sense.

[Piero]

Whilst it is true that a miscollimated telescope can show astigmatism, in this specific case astigmatism dominates over coma when the strehl is below the diffraction limited threshold (about 0.80). At that point the images are quite bad.

If your focuser is miscollimated, you should detect coma on axis. That is the predominant aberration.

If you are certain you see astigmatism, it could be in the eyepiece or the optics are retained too tightly. Also, make sure the telescope has cooled down properly.

For test collimation I use the Glatter square attachment with my Glatter's laser. As others said, the diagonal must be removed.

[Merlin66]

As per the above.....

I use an Ronchi grating as a final check ( on any telescope) tosee if there is any residual issues/ problems.

point to a fairly bright star and replace the eyepiece with the grating, refocus to only show 5 or 6 lines and look for straight, even and un distorted lines.

 

[jock1958]

@sonnymoon first set of photos are how I check the focuser to lens alignment with mirror on floor.

Second set of photos is with my Cheshire fitted to check the adjustment of the objective lenses.

These photos are of my Tak 100DL and as you can see this is an exceptionally well corrected scope, with the laser beam going through the bulls-eye and no double circles (doughnuts) through the Cheshire.

Hope this helps.

 

[sonnymoon]

Much appreciated Jock1958, thanks for taking the time to post the photos, makes your approach much clearer. 

[Piero]

2 hours ago, jock1958 said:

@sonnymoon first set of photos are how I check the focuser to lens alignment with mirror on floor.

Second set of photos is with my Cheshire fitted to check the adjustment of the objective lenses.

These photos are of my Tak 100DL and as you can see this is an exceptionally well corrected scope, with the laser beam going through the bulls-eye and no double circles (doughnuts) through the Cheshire.

Hope this helps.

 

I am not sure you can rely on the mirror on the floor. The returning beam will depend on the angle of that mirror.

If the beam goes through the hole of your template, which is supposed to be at the centre, that's a good indication that the focuser is collimated. 

In any case the tolerance for the focuser axial alignment is 0.03xD, where D is the aperture and here no coma corrector is used. That is 3mm for a 100mm refractor.

For checking collimation of the lens, see Suiters' book.

[jock1958]

2 hours ago, sonnymoon said:

Much appreciated Jock1958, thanks for taking the time to post the photos, makes your approach much clearer. 

No worries glad to be of help, had a lot of advice off fellow SGL’ers,  tinternet and lots of trial and error. 

Edited September 26, 2019 by jock1958

[John]

I think jock1958 is simply using the mirror on the floor so that he can see where the laser is exiting the objective and the paper target from the focuser end of the scope ?

I'm not sure that he is using the mirror on the floor as part of the collimation system apart from the above ?

[jock1958]

@Piero thanks for additional info, I shall have a look at Suiters book.

A couple of reasons I had my Tal 100RS pointing at the floor was my laser collimator wasn't a good fit in the focuser tube and I couldn't confirm the accuracy of where the red dot was pointing, also the draw tube was slipping and drooping when racking out (later confirmed to be poor machining tolerances).  Having the OTA pointing at the floor alleviated some of these problems. 

 

[jock1958]

15 minutes ago, John said:

I think jock1958 is simply using the mirror on the floor so that he can see where the laser is exiting the objective and the paper target from the focuser end of the scope ?

I'm not sure that he is using the mirror on the floor as part of the collimation system apart from the above ?

Exactly you took the words right out of my mouth

[Piero]

Good to hear that!

Anyway, I would also suggest to rack out the drawtube as when a normal eyepiece is in focus. A fully retracted or extended drawtube is not in a good position to check the focuser collimation, as this can be quite different.

Also, why should this be done with the telescope pointing to the floor or ceiling? The laser is expected to be quite aligned in those positions and does not stress the focuser off axis.

Focuser collimation with a refractor should be done with the tube put horizontally (and drawtube as said above). With your template, you can check that the laser beam hits the centre. If it doesn't and remains within tolerances, it's up to you. Otherwise it should be collimated.

 

[sonnymoon]

So I've got my laser collimated and my paper circle with the middle marked on the objective lens and the red dot is a little bit off centre.

Probably worth getting dead centre with a little adjustment however I can't work out how to adjust this. The focuser seems to be a very tight fit in the tube. I have read the Tommy Nawratil guide which seems to me to be about adjusting the grip strength of the focuser rather than having it exactly centred in the shaft.

Do I have to take the focuser out and use a spacer of some kind?

 

 

[Piero]

Could you post a photo of your focuser, please?

Edited September 27, 2019 by Piero

[sonnymoon]

Hi Piero,

its this one - stock ed80 . It’s got a sesto senso attached at the moment . Hope you can see the Allen key heads....

[Piero]

Thanks. I need to do some research as I don't have that telescope.

From preliminary search, it seems that the focuser can be collimated.

https://www.cloudynights.com/topic/140083-sky-watcher-100ed-collimation/

Check jrbarnett's comment.

I'll have a proper look at this this weekend.

Edited September 27, 2019 by Piero

[Piero]

Keep in mind that some of the Allen screws are for regulating the tension of the focuser - making smoother or firmer. What you will need is to adjust the drawtube axis instead.