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wookie1965

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Bruce Leeroy

Bruce Leeroy

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I have the skywatcher ST 120/600 blue tube version and have been trying the tips from the links here, I've had this for around 2 years+ and never been satisfied with the views, using the cheshire I can see similar issues so I've aligned the focuser but can not see any collimation screws for the lens cell....I have a feeling the whole lens cell retainer is a little off and maybe cross threaded to the tube, going to try and loosen it...rotating only the doublet does not alter the reflected image through the cheshire.

Bingo ?......removed the whole lens cell cleaned up the threads of the scope end then I loosened the doublet and tapped it a few times and no more offset reflection through the cheshire...the open end of the tube where the thread starts was a little dented so I tapped it back into shape using the soft part of a screwdriver handle.

I had previously pointed the tube straight up and released the doublet and tapped but no difference.

I am confident the majority of the offset reflection was because of the thread of the tube wasn't right, thanks for the tips in this thread and good luck with yours Wookie!!

 

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jock1958

jock1958

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Steve Collingwood said the fist thing he would do was stand the OTA upright, loosen the lens retainer ring and gently tap around the top to centralise the lenses then nip up the retainer ring and adjusting screws.

As a matter of interest I assume it’s an air spaced doublet? If it is does it have 3 little foil spacers or an o ring?

Picture of my Tal lens with a displaced foil spacer and the start of all my woes ?

 

5101AF35-0C9D-4A5F-8BA4-1C625A357FFD.png

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Bruce Leeroy

Bruce Leeroy

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24 minutes ago, wookie1965 said:

Do I loosen the 3 screws and the retaining ring then tap to try and turn the glass or move the whole thing.?

I don't think you would need to rotate anything just SLIGHTLY release the retaining ring so in theory the lens cell will rattle around if you shook the whole ota then gently tap the outer of the cell all the way around then tighten the ring and check for progress, do this whilst the scope is pointing STRAIGHT up....I had to lower 1 tripod leg to get it pointing  STRAIGHT up but that was on an alt-az mount (careful the whole thing doesn't tip over if you too are using an alt-az mount). By tapping the lens cell you are hoping to shift the lens so all the lens elements are central to each other...as I said in my earlier post I rotated my doublet then rechecked and the reflection was exactly the same which lead my down the path that the entire lens cell was out of whack with the rest of the tube. I would let Peter look at it..

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Bruce Leeroy

Bruce Leeroy

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11 minutes ago, wookie1965 said:

Here is what the laser shows.

Do you still have the range of adjustment to center the laser to the dot....and are you sure that paper is accuratetely centered? I used the aperture hole in the ota cover for a smaller circle then made a bullseye.

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wookie1965

wookie1965

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I used the front cover to make the circle its as good as centre got the same gap all the way round it is slightly worse than when I started.

I have no idea how to centre it there is nothing on the focuser to do it.

It's much closer when in the diagonal.

 

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John

John

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59 minutes ago, wookie1965 said:

I used the front cover to make the circle its as good as centre got the same gap all the way round it is slightly worse than when I started.

I have no idea how to centre it there is nothing on the focuser to do it.

It's much closer when in the diagonal.

 

In the absence of collimation adjusting screws etc, the focuser and the objective cell can be tilted by using thin shims at the junctions with the scope tube.

I'd like to see that laser dot closer to the centre of the objective ideally. If you rotate the laser in the focuser, does the dot stay in position or does it rotate as well ?

The order is to sort out the focuser tilt first, then move onto the objective tilt, if there is one.

 

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jock1958

jock1958

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2 hours ago, wookie1965 said:

Here is what the laser shows.

 

2 hours ago, wookie1965 said:

I used the front cover to make the circle its as good as centre got the same gap all the way round it is slightly worse than when I started.

I have no idea how to centre it there is nothing on the focuser to do it.

It's much closer when in the diagonal.

 

 

2 hours ago, wookie1965 said:

Here is what the laser shows.

20180926_145040.jpg

 

Sorry to say that looks out of adjustment. 

What I did was sort out focuser alignment first using a collimated laser with the OTA attached to my AZ mount pointing at the floor with a mirror so you can see the red dot when you adjust the focuser, this ensures the laser sits nicely and you can rotate it freely without any due loads.

Once I had the red dot centred I concentrated on the objective lens adjustment using the Cheshire.

Befiore and after pics after I adjusted the focuser tube.

 

113FA9C5-50F6-4495-B34F-8DDC8CCCB827.png

5B44084D-FA3E-4302-A5F8-54CF02736DAB.png

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jock1958

jock1958

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12 minutes ago, jock1958 said:

Sorry to say that looks out of adjustment. 

What I did was sort out focuser alignment first using a collimated laser with the OTA attached to my AZ mount pointing at the floor with a mirror so you can see the red dot when you adjust the focuser, this ensures the laser sits nicely and you can rotate it freely without any due loads.

Once I had the red dot centred I concentrated on the objective lens adjustment using the Cheshire.

Before and after pics after I adjusted the focuser tube.

113FA9C5-50F6-4495-B34F-8DDC8CCCB827.png

5B44084D-FA3E-4302-A5F8-54CF02736DAB.png

 

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John

John

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5 minutes ago, wookie1965 said:

....Thanks very much for the help really appreciate it.

No problem. Lets hope you and Peter can get things sorted. The laser position looks better in the last 2 photos you posted so maybe the focuser optical axis is reasonably well aligned with the objective after all.

It's annoying that some scopes have no provision to alter the tilt of the focuser and objective. For the precision made scopes this is OK because the accuracy of machining ensures that the optical train is well aligned but the lower cost scopes have enough variation in their manufacture to require that some form of adjustment facility is included. Meade and Skywatcher did do that with their refractors after a while. Having done a bit of research I now understand that initially the Meade AR5 LXD55 had no objective collimation whereas the AR5 LXD75 version did. After a while Meade just used the same objective cell arrangement on both scopes rather than manufacturer different designs. I guess yours is an earlier one ?

So shimming is the way to do it for both the focuser and the objective cell I feel.

 

 

 

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wookie1965

wookie1965

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2 minutes ago, John said:

No problem. Lets hope you and Peter can get things sorted. The laser position looks better in the last 2 photos you posted so maybe the focuser optical axis is reasonably well aligned with the objective after all.

It's annoying that some scopes have no provision to alter the tilt of the focuser and objective. For the precision made scopes this is OK because the accuracy of machining ensures that the optical train is well aligned but the lower cost scopes have enough variation in their manufacture to require that some form of adjustment facility is included. Meade and Skywatcher did do that with their refractors after a while. Having done a bit of research I now understand that initially the Meade AR5 LXD55 had no objective collimation whereas the AR5 LXD75 version did. After a while Meade just used the same objective cell arrangement on both scopes rather than manufacturer different designs. I guess yours is an earlier one ?

So shimming is the way to do it for both the focuser and the objective cell I feel.

 

 

 

Yes I read up on them mine must be an earlier version. I hope Peter can sort it out I am starting to dislike it, bought it for £140 the focuser was knackered got a Skywatcher 10:1 but had to have a adapter made that was another £135. So even now if I unload it I am going to make a big loss on it.

If Peter fixes it and I can get good views I will keep it.

like to say again thank you to all of you for your help.

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jock1958

jock1958

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2 hours ago, wookie1965 said:

How did you adjust the focuser tube to get the laser central.

I adjusted the focuser on my Tal by opening up the focuser screw holes with a needle file and adding a shim. It was a bit of a potch to be honest but it did improve things.

I had some great advice off @moriniboyon this forum about 2 years ago and I’m sure if you go through the Tal threads you will find what you are looking for.

Hope you get it sorted tommorow ?

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