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Modifying TAL-1


bambuko

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When I got my TAL-1 (see: Astronomy on a (miniscule) budget) I soon discovered that mine was old and used 32mm (and very odd) eyepieces (see: TAL-1 eyepiece problem)

One of the solutions proposed involves using extra long collimating screws, but I didn't like it.

My preferred solution was to do what Russians have done when they switched to 1.25" focuser:

Adjusted tube for TAL-1

I found dire warnings :grin:  about doing that while searching through SGL, but...

It can't be that difficult?

I could of course make a jig/fixture and do the job on my milling machine, but this would not be in keeping with "miniscule budget" ethos and of no interest or help to majority here who haven't got engineering workshop at their disposal, so the following description "how I've done it" shows how the job can be done using just simple hand tools!

Start by preparing workstation:

IMG_7581.jpg

Clean the ube and prepare for marking:

IMG_7582.jpg

Use vernier calipers:

IMG_7583.jpg

To mark the cut line:

IMG_7585.jpg

Use metal hacksaw:

IMG_7587.jpg

If you any good at cutting to a scribed line there should be hardly any filing afterwards:

IMG_7589.jpg

The tool used to create a shoulder inside for rear backplate:

IMG_7590.jpg

Rotate the tube, don't linger at one place and judge the angle to keep the shoulder parralel to the tube wall:

IMG_7592.jpg

The fit achieved this way is actually better than original russian machining, which was (to use toolmaking term) proverbial in the bucket :grin:

IMG_7593.jpg

End result:

IMG_7595.jpg

It's not the end of the project, but after this it should be easy :tongue:

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:grin:  I thought you'd be the first one to reply.

Yes the result is better I could have dreamed about!

Russian paint just peeled off (they didn't seem to use any etch primers), so I will have to repaint the tube as well.

It will be all black!

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Put it back together yesterday just to test it:

IMG_7601.jpg

and ... than spent frustrating night dodging clouds to see at least something :grin:

With standard 25mm Skywatcher eyepiece fitted into original old TAL focuser (not really secure using 1.25" eyepiece in 32mm holder, but good enough, if careful, for testing) I needed to pull the focuser all the way out to get things sharp and to be honest on 10mm eyepiece could do with couple of mm more.

As it is, the edge of the original focuser comes out 78mm from the outside of the scope tube.

Which probably means that shortening scope tube by 33mm was probably too much - I guess now that 28mm would have been enough (funny enough - this is exactly the figure mentioned on the Russian TAL forum as mentioned in my other thread).

But it doesn't matter too much, since I was alway planning to make new Crayford focuser, so will have to design it around what I've got.

Question 1: how much focuser travel/adjustment would be desirable

Question 2; I didn't get the chance to test many eyepieces. Is it safe to assume that what has worked OK when testing 25mm piece will be the same for all other eyepieces? (ranging from 5mm to 40mm). What I mean - I needed 25mm eyepiece to be out by 78mm. Will it be the same for others? or should I have generous +/- 5mm for various eyepieces (on top of focusing range of say +/-10mm)

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No volunteers :grin:  so far to assist with my questions...

Will have to blunder on my own then :tongue:

Started by sketching current situation:

tal_project%2520Model%2520%25281%2529.jp

Looks a bit funny,  like a telescope attached to a focuser and eyepiece rather than other way round .... :rolleyes:

Original TAL eyepiece tube does not enter telescope (good thing as far as I am concerned, otherwise it would too much of an obstruction)

My new design will be the same, even if it means rather large focuser sticking on top of the telescope.

OK, I haven't got Tele Vue eyepiece, but maybe one day.

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There is quite a few of them going for silly money (depending on your location) right now.

I am after TAL-2 but am worried that I will like it much more than my Skywatcher 150p :grin:  and than I will real pillock for spending silly money on Chinese toy when I could have had Russian tank :shocked:

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Question 1: how much focuser travel/adjustment would be desirable

Question 2; I didn't get the chance to test many eyepieces. Is it safe to assume that what has worked OK when testing 25mm piece will be the same for all other eyepieces? (ranging from 5mm to 40mm). What I mean - I needed 25mm eyepiece to be out by 78mm. Will it be the same for others? or should I have generous +/- 5mm for various eyepieces (on top of focusing range of say +/-10mm)

 I've measured focus positions of my various eyepieces, using either my Generation II or Gen III type Tal plossl eyepiecs as the standard. The ones below are all I could find in my notes. I should have others, but where?

- = inside focus needed

+ = outside focus needed

Tal Kellner 42mm   -3.5

Tal Plossl 25mm Gen II & III   0

Meade M.A. 25mm    +9.5

Meade RG Efrle 20mm    -1

T.S. 20mm x-hair    -6.5

Bushnell 16.8mm ortho/RKE   0

Tal Kellner 15mm    +0.5

U.O. Konig 12mm   +10.5

K.K. Kellner 12mm   -2

Nikon WF 10mm   +1

TMB Stellar 10mm   +1.5

K.K. Kellner 9mm  -2

Bushnell Ortho 7mm    +1.5

K.K. Kellner 6mm     -2

Speers 5-8 zoom @ 8mm    -8.5

Speers 5-8 zoom @ 5mm   -11.5

Siebert S.S. 4.9mm   -2

So the minimum focuser travel I'd want, would be around 25-30mm. Preferably more, if possible depending on design.

Andy.

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AndyH, you are gentleman and a scholar in my book :grin:

Thank you very much indeed!!

+/- 15mm (ie total 30mm) travel should be easily achievable (I will sketch in a day or two - probably not in the next wo days, because weather looks like rare couple of days for getting the scopes out! :laugh: )

Only minor difficulty is that my baseline is Skywather 25mm eyepiece rather than TAL one that you have used, but will have to experiment with that?

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Whilst I've never had any skywatcher 25mm plossl to test(or any Skywatchers actually), I'd imagine they won't be very far away from, if at all, from the Tal 25mm.

The Speers zoom is an eyepiece that is pretty extreme as to inside focus travel needed. I think only the Speers 18mm exceeds it?

Andy.

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I am noy keen on zoom pieces anyway, so can safely discount this extreme.

Thank you for suggestion about TAL 25mm plossi - I will use it as a baseline! and even if it doesn't work perfectly it's not a big deal - whatever I'll make will be easy to change anyway.

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There's a cracking newish model on fleabay at the moment, with full modern accessories. ie: the type that are produced today, with the steel tube and braised on flanges at front and rear.

sorry for the hijacking, but is the TAL 1 a good choice for occasional use

meant to add, they will be pretty good on lunar and a bit of planetary, how do they fair on some dso work

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sorry for the hijacking, but is the TAL 1 a good choice for occasional use

Oh God yes.

Every astonomer should add one to their collection, in my biased opinion. If it was the only scope I had, I would be a happy chap. Hell, you can buy one 2nd hand for the price of a new TeleVue plossl. Madness.

SOLID mount. Virtually zero vibrations at the eyepiece. The scope is a perfect match to the mount. This newt was what the mount(MT-1) was originally designed for.

In theory if you can source one produced from roughly '98/99, then the eyepieces/focuser should be industry std 1.25" . Older ones can be used with the supplied accessories or can be modified, various ways, depending on available cash/DIY skills.

Mount too bulky for your grab n go use? Then buy a set of tube rings(if it's the older model) and stick it on your alt/az. Perfecto.

Andy.

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Oh God yes.

Every astonomer should add one to their collection, in my biased opinion. If it was the only scope I had, I would be a happy chap. Hell, you can buy one 2nd hand for the price of a new TeleVue plossl. Madness.

SOLID mount. Virtually zero vibrations at the eyepiece. The scope is a perfect match to the mount. This newt was what the mount(MT-1) was originally designed for.

In theory if you can source one produced from roughly '98/99, then the eyepieces/focuser should be industry std 1.25" . Older ones can be used with the supplied accessories or can be modified, various ways, depending on available cash/DIY skills.

Mount too bulky for your grab n go use? Then buy a set of tube rings(if it's the older model) and stick it on your alt/az. Perfecto.

Andy.

the mount is good for at lass`s place, the plan is to get one keep it at her place then if we move into a place together i can sell it if i need space.......cheers Andy

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I am noy keen on zoom pieces anyway, so can safely discount this extreme.

Thank you for suggestion about TAL 25mm plossi - I will use it as a baseline! and even if it doesn't work perfectly it's not a big deal - whatever I'll make will be easy to change anyway.

The Speers 5-8mm(82deg AFOV) is no ordinary zoom. They are superb. Different, but as good as the TeleVue type. Rare to find now.

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The Speers 5-8mm(82deg AFOV) is no ordinary zoom. They are superb. Different, but as good as the TeleVue type. Rare to find now.

I take your word for it :grin:  - never heard of Speers (but that's just my ignorance).

My comments about zoom pieces were general and not specific to Speers.

I suppose it's just theoretical prejudice on my part... :rolleyes:

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I have laid out the focuser travel etc and must admit that my earlier statement that to move main mirror by 33mm was probably too much and 28mm would have been enough is probably wrong.

tal_project%2520Model%2520%25281%2529.jp

I suppose it would be easier if I accepted focuser entering into the telescope tube at the extremes of travel, but I think any obstructions are bad idea.

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  • 1 month later...

It's been ages since I reported anything on the TAL-1, but that doesn't mean nothing is happening :grin:

In my spare moments I have started gashing out lumps of aluminium to make new focuser and this is the progress so far:

IMG_7745.jpg

Crayford 1.25", big and chunky as befits Soviet tank :tongue:

Still fair bit of work, so it will be some time, but it should make the TAL very usable :smiley:

This is btw the focuser on it's base (which still needs to be machined to follow the diameter of OTA)

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