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TAL-2 collimation problem


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Dear astro friends,
I recently bought a TAL-2 Newton reflector telescope and tried to collimate it because it has not been used for over 20 years. I noticed that the secondary mirror could not come to the correct position, just below the focuser. See the picture ... When the secondary mirror is moved to the maximum to the left, it still needs to be moved a little more to be exactly in the middle below the focuser. In this position, the central screw is fully screwed on and can no longer be moved backwards ... Other components such as the secondary carrier (spider) or the focuser cannot be moved but are fixed. I wonder if any of you have had similar experiences with this telescope and how this can be solved?

Thanks in advance for the help,

SLIKA SEKUNDAR.jpg

Edited by godra
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  • godra changed the title to TAL-2 collimation problem

I don't know the specifics of this scope, but some scopes actually require the secondary to be off center so it may not be a problem as such

Worth a look at this guide - https://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

If you are still sure that it needs to come further up maybe carefully remove the center bolt and add a washer or two to give you the required extra travel?

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To move the secondary away from the primary (to the left) you would need to tighten the outer bolts so I do not understand what you mean by  "screwed on"?

Once the secondary is axially centred  I would correct the tilt/rotation error to make the primary mirror reflection circular.

 

Edited by Spile
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16 hours ago, Spile said:

To move the secondary away from the primary (to the left) you would need to tighten the outer bolts and therefore loosen the centre screw so I do not understand what you mean by  "screwed on"?

Once the secondary is axially centred  I would correct the tilt/rotation error to make the primary mirror reflection circular.

 

I can't bring the secondary holder more left (away from primary mirror) because it is maximum left position when I untighten tree outer bolts and when I fully screw central screw.  In opposite,when I untighten tree bolts and unscrew central screw,then secondary is moving toward primary mirror (to the right on picture)

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Looking at an image of a Tal-1 secondary (from Astro-baby's strip-down guide):

image.png.15a6b333ffb1e28f4498e23db4076127.png

Assuming the Tal2 is similar, it would be interesting to see whether the secondary holder is a far up against the vane assembly as it can go, or whether something is stopping it.

  • You have the 3 adjusters backed out so that they don't protrude out at all?
  • The adjuster bolt hasn't been replaced in the past, with something longer, which is fouling inside the holder when you tighten  it up?
  • The spring isn't incorrectly installed, stiffened-up, etc.?

I've never used a Tal in the flesh, so just guesses.

I would suggest removing the whole vane/secondary assembly to investigate, if you are happy doing something like that.

 

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11 hours ago, godra said:

I can't bring the secondary holder more left (away from primary mirror) because it is maximum left position when I untighten tree outer bolts and when I fully screw central screw.  In opposite,when I untighten tree bolts and unscrew central screw,then secondary is moving toward primary mirror (to the right on picture)

 

To move the secondary away from the primary secondary I loosen the three outer bolts whilst loosening the centre screw. When the secondary is axially aligned under the focuser I  correct and rotation error before tightening the centre screw. Then recheck the tilt and correct as necessary.

 As Pixies diagram shows, the springs would result in the opposite effect for you. As you tighten the outer bolts you are compressing the springs resulting in the secondary moving away from the primary. When you loosen the outer bolts the springs move the secondary towards the primary.

Do you agree Pixies?

Edited by Spile
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27 minutes ago, Spile said:

Do you agree Pixies?

Nope- sorry ;)

To bring the secondary away from the primary, loosen the 3 adjusters and tighten the central screw. This will pull the secondary back towards the vane assembly. When you hit the adjusters, back them off a bit more and tighten the central screw again. Keep going until the secondary is under the focuser. However, from my understanding of @godra's first post, this is what he is doing, but the secondary hits a limit before it is correctly under the focuser.

The spring is just to stop the secondary from flopping around when not being pulled against the adjusters.

I would be suspicious that the adjusters aren't fully backed-out or the spring is misplaced and stopping the secondary from being fully pulled back.

 

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9 hours ago, Pixies said:

To bring the secondary away from the primary, loosen the 3 adjusters and tighten the central screw. This will pull the secondary back towards the vane assembly. When you hit the adjusters, back them off a bit more and tighten the central screw again. Keep going until the secondary is under the focuser. However, from my understanding of @godra's first post, this is what he is doing, but the secondary hits a limit before it is correctly under the focuser.

Pixies,that's right.

 

9 hours ago, Pixies said:

I would be suspicious that the adjusters aren't fully backed-out or the spring is misplaced and stopping the secondary from being fully pulled back.

Nope....I even completely unscrewed adjusters to make sure they didn't block the secondary from fully retracting.

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Can you confirm that the secondary holder is able to be pulled right back on to the 'vane assembly' (not sure what this is actually called). If it really is as far as it can go, something is definitely wrong.

Any pictures?

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1 hour ago, Pixies said:

Can you confirm that the secondary holder is able to be pulled right back on to the 'vane assembly' (not sure what this is actually called). If it really is as far as it can go, something is definitely wrong.

Any pictures?

Yes,I can confirm that. 

I will post pictures later this day!

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Here's the pictures of vane assembly and secondary holder. As you can see...nearly no space between two holders,just top of the adjusters can be barely seen.

 

 

20210624_213848.thumb.jpg.7a24468f1320e0e04e9937e86e9a5ad2.jpg

20210624_214417.thumb.jpg.ca8278f960a530e66c60312649bd76d8.jpg

 

:

20210624_213742.thumb.jpg.b259d27be1b017e71ff8a18f57f59522.jpg

 

 

20210624_214223.thumb.jpg.2acc5861aadadc1ca45d61d4d25a2202.jpg

 

 

 

20210624_213504.jpg

Edited by godra
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Could the spider vane have been changed resulting in the misplacement of the secondary? Would a workaround be to offset the fixing of the vanes in respect to the holder or OTA? That would seem easier than moving the focus tube.

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Just a couple of thoughts from me. Firstly, is the focuser straight? If it is off line this will affect the secondary position. Also, looking at the design, would it be possible to rotate the spider vanes by 180 degrees as this looks like this might move the mirror out slightly?

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Spile,Clarkey thanks for the suggestions! Yes,it is technicaly possibble to rotate spider vanes and maybe this could help! I also have an idea to move focuser base(whole focuser) few millimeters to the right but that involves drilling wider holes (under focuser) on OTA.  

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Looking at the pictures I suspect rotating the vanes 180 degrees is going to be the fix

If I was a betting man I'd say someone has previously removed the vanes and secondary and reinstalled them the other way round, the two screws seem slightly off to one side but that may just be my eyes...

Maybe worth slackening off the screws slightly and see if you can move it all towards the end of the tube? Sometimes with these things there is a little play which may be just enough

I really, really need to own one of these one day too, there's definitely something about that over engineered look that I like

 

Stupid question left till last, have you tried collimating and star testing as is? With no TAL2 owners jumping in so far, maybe it is by design...

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42 minutes ago, doublevodka said:

Looking at the pictures I suspect rotating the vanes 180 degrees is going to be the fix

If I was a betting man I'd say someone has previously removed the vanes and secondary and reinstalled them the other way round, the two screws seem slightly off to one side but that may just be my eyes...

Maybe worth slackening off the screws slightly and see if you can move it all towards the end of the tube? Sometimes with these things there is a little play which may be just enough

I really, really need to own one of these one day too, there's definitely something about that over engineered look that I like

 

Stupid question left till last, have you tried collimating and star testing as is? With no TAL2 owners jumping in so far, maybe it is by design...

Yes, beautifull scope despite old fashion,industrial look🙂 Build like a tank!

Acctualy,I went trough all phases of collimation and star testing. Stars don't snap to focus and they are pretty blury.

What I found while doing star testing is that on outside side of focus, stars showing images like on those when pinched optics is present.

Something like this: pinch.jpg.a509963e034481d9885206f4d09bd95a.jpg I attributed it to bad collimation though...

 

Inside side of focus looks much better-nice concentric diffraction rings but atmospheric condition were not so good for definitive conclusions.

 

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Ooooh the plot thickens...

So I'm a bit of a tinkerer and managed to pinch the optics on a refractor after giving it an overdue clean, backing off the tension a little made a huge difference!

While you are taking things apart to check it would be worth carefully removing the whole primary assembly assuming it's possible and checking that the mirror clips are not over tightened, need to be just tight enough to hold the mirror in place

Counter intuitive to most things which is why people get it wrong, I'm really starting to suspect this scope has been taken apart and reassembled at some point

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Maybe your Tal was built in a Friday- it all looks present and correct just a bit off. Maybe if you loosened the spider vane tensioning screws, loosened the vane to hub screws and then pulled the secondary assembly upwards whilst retightening the hub screws you could get enough movement to centre the secondary under the focuser? 

Mark

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  • 3 weeks later...

Many,many thanks to Spile,Doublevodka,Clarkey,Mark and others who suggested to rotate spider vanes by 180deg!! Guys,you are great!!

With the "little" help of my friend I finally managed to do it! Tho whole secondary cage is now moved around 5mm toward primary mirror. Doing so,it was easy than to center secondary under focuser(and complete a collimation). Also,we cleaned the secondary mirror.

Unfortunately I haven't had a chance yet (due to bad weather) to check the collimation on stars.

Before modification:

20210624_213504.thumb.jpg.03880b51315d7efd5abdcffd3e403cf0.jpg

After:

20210714_070729.thumb.jpg.77d861fbd646e391bbb47210d876955d.jpgA few more pictures during the operation:

20210703_172641.thumb.jpg.440c6dde15939e038ad547dccc807150.jpg

20210703_174424.thumb.jpg.e2c6981a372e60eaa612583c6988a8c2.jpg

 

20210703_173552.thumb.jpg.e3bf8e20589f60c4c2c6d5f0899f4193.jpg

Edited by godra
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