collimator for my TAL-1

[bambuko]

When I recently acquired an old TAL-1 I needed to collimate it after taking it all apart for a bit of tidy-up.

Unfortunately my TAL-1 is 1994 model, so the focuser takes eyepieces with 32mm barrel diameter.

I could have bodged my 1.25" Cheshire to make it fit, but even then it would only be a compromise affair because of it's length ve ID ratio being more suited to f5 telescope, rather than f7,3 that is TAL-1.

So I set out to make my own Cheshire, following advice from Nils Olof Carlin and his excellent web page

I started with two pieces main body to the right with 20mm ID (made btw from old bicycle seat post) and front end turned to locate in 32mm focuser and prepared to accept TAL cross-hair reticle:

Here, front end with original TAL cross-hair:

and the cross-hair in place (retained by springs typical of TAL eyepieces)

First test fit of the main parts:

and first try in the telescope:

Next - the actual Cheshire part, finishing off (polishing and anodising) and Bob (or Igor) is your uncle

[JamesF]

Very neat

Now you need to polish up the main body.  It doesn't look right stuck in that lovely shiny adaptor you've turned

James

[bambuko]

Cheshire 45deg piece:

The diameter of the centre hole is a bit of a guesswork (at least for me).

You are supposed to "...make it narrower than your main mirror center spot, but if you want the combination tube, not so narrow that you can not see the inner end of the tube through it!..."
I made mine 5mm and we will see ...

[bambuko]

Very neat

Now you need to polish up the main body.  It doesn't look right stuck in that lovely shiny adaptor you've turned

James

Yes boss

Of course you are right and I will polish it all, but for now I wanted to test it and finish the cheshire part.

btw - the Russian cross-hairs look lovely down the tube reflected in the secondary - lot better than what I got from the shop for my Skywatcher...

[LeeB]

once its collimated the tal1 stays done, I colli all my scopes except the Tal each time I use them . It however never needs doing even after a 200 mile drive. Russian design......

[bambuko]

Yes, I've heard about their robustness and I must say I like what I see.

Is your TAL-1 modern version with 1.25" focuser? or the older one? (like mine)

[bambuko]

Finished job (on the right), next to the shop bought one (already "experimented with", hence missing blackening)

My only concern is the small diameter of Russian cross-hair piece (on the left), giving in effect field stop that might cause some issues?

I will test it fully tomorrow

[LeeB]

Yes, I've heard about their robustness and I must say I like what I see.

Is your TAL-1 modern version with 1.25" focuser? or the older one? (like mine)

Mine is from 1998 3046 and has the 1.25 focuser Rack and pinion, the mirror is still original and is in good nick. I sprayed it red to fit with its origin and the mount slow motions have been stripped and greased, higher tension springs have been used to make the action even smoother.

[AndyH]

Lovely machining job.

Looking forward to your report, after the test.

Andy.

[AndyH]

Mine is from 1998 3046 and has the 1.25 focuser Rack and pinion, the mirror is still original and is in good nick. I sprayed it red to fit with its origin and the mount slow motions have been stripped and greased, higher tension springs have been used to make the action even smoother.

A '98 with the 1.25" focuser?  Good info. That must have been right at the cusp of the change over to std 1.25".

Mine is a year later. Although it came with the older MT-1 mount, the tube rings as one complete casting with the top plate.

I do like the soviet red, you finished yours in.  Tasty

Andy.

[bambuko]

How did the Russians solve the conversion to 1.25" ?

It is still rack and pinion and I believe they didn't shorten the distance between the primary and the focuser? (or did they?)

[LeeB]

the mount on mine is the later one with the different alt adjuster, the finder on the tal is a real gem and a fine little scope in its own right. lovely cross hairs to

[AndyH]

How did the Russians solve the conversion to 1.25" ?

It is still rack and pinion and I believe they didn't shorten the distance between the primary and the focuser? (or did they?)

Hi.

They simply made a new internal drawtube. Going by the pics I've seen of stipped down early Tal 1 focusers, it certainly looks that way. I've often thought that if someone had access to a lathe or mill, they could make one up, and re-use the rack on the old drawtube. 

The mirror cell looks exactly the same thoughout all ages of the scope. So has the tube been shortened? If you want I'll measure the tube length and the positioning of the spider vein setscrews, just in case they just moved them down the tube by an inch or so.

 How is the action on your focuser? On all the R&P Tal focusers I have, after a stripdown and re grease, they are as smooth as butter. You've probably seen the stripdown/rebuild pdf, but I'll attach it on here just in case, or for anyone reading this thread.

Andy.

TalScopes Tal focuser rebuild.pdf

[AndyH]

the mount on mine is the later one with the different alt adjuster, the finder on the tal is a real gem and a fine little scope in its own right. lovely cross hairs to

The one with the small worm and gear ?

I have that on my Stepper driven Tal MT-3S. Quite brilliant, along with the longditude adjuster and inbuilt super polar finder, it was a marked stepup in user friendliness.

Andy.

[bambuko]

Hi Andy,

I have a copy of focuser overhaul .pdf

The issue with using current focuser (I understand) is that when converted to standard eyepieces it doesn't go close enough to the secondary to permit focusing.

I have lathe and mill and can do the mods, just need to know what I am doing

Assuming I can do the mod - would the draw tube entering the scope obstruct the light enough to cause issues?

My focuser is a bit sloppy - still waiting for me to take it apart.

I am sure it can be easily sorted out.

[AndyH]

Yeah.

 Converting the focuser to a 1.25" drawtube won't fix the infocus problem. If anything it pushes the eyepiece back out/away from the tube, due to the flange at the top.

 That'll have to fixed, either by

a/ raising the mirror up in the cell, by using longer setscrews and stronger, longer springs.

b/ cutting about 25mm, perhaps a bit more, off the cell end of the tube. This leaves the problem of the machined I/D of the tube, that the cell fits perfectly in. Another possible (easier?) option, though I've not checked the cell for thickness, is to machine the O/D of the cell to fit into the tube.

edit:  From what I remember, the drawtube on mine, doesn't protrude by much, at all, into the tube. But as you can see from the above pic, it only has a racking length of 25mm. Not sure if that's common to all. I'll check when I get home & do some measurements.

[bambuko]

Here, for reference a photo of my 1994 focuser body:

If you could confirm overall length of telescope tube on later model and how the Russians solved the problem it would be great!

[bambuko]

I will knock up some simple friction 1.25" adapter to slide my "regular" eyepieces up and down the focuser to see where they should be to make it work.

[AndyH]

Will do.

[AndyH]

Some sizes. Taken with primary mirror cell off.

Tube length = 811mm

Length to centre of spider screws = 732mm

Length from centre of spider screws to top of tube = 79mm

Height of focuser body inc baseplate = 57mm

Drawtube flange width/height = 7mm

Length of drawtube, inc flange = 55mm

The drawtube still has a good bit to go before  it enters the tube. Around 12mm.

[bambuko]

Tube length = 811mm (old one 844mm)

Length to centre of spider screws = 732mm (old one 765mm)

Length from centre of spider screws to top of tube = 79mm (old one 79mm)

Height of focuser body inc baseplate = 57mm (old one 57mm)

Thanks Andy,

So it looks like they have shortened rear end by 33mm exactly

Which is almost exactly the figure quoted on TAL forum: ...Переносить зеркало нужно на 28-30 мм...

Oh well... at least I know now my options

[AndyH]

Whilst I had an inkling that that's what they'd done, it's nice to have had it confirmed.

I look forward to any updates on your work on the scope.

Cheers,

Andy.

[bambuko]

And going back to my collimator - tried it and wasn't happy.

Although sighting tube was 20mm ID, the cross-hair piece at the front was only 15mm dia and my cheshire at 146mm was too long.

Shortened it to about 110mm and it is just perfect - it frames primary very neatly and can be used either with or without cross-hair (as is your preference).