TAL 1 Help

[Astro_Baby]

I am restoring an old TAL 1 scope. Unfortunately the focuser was well knackered and the TAL factory dont have any spares right now. The fitted focuser was one of the old cranky TAL ones that are not quite 1.25" and have a compression fit. So even if the TAL factory had some spares they wont be quite the same.

So I acquired a Skywatcher focuser from a Skywatcher 130 and after some shoehorning and adapating it fitted it to the TAL today for a test run. Now I wasnt so dopey as to not check the skywatcher focuser for travel and having determined that its was pretty close to the original TAL felt pretty sure it would work. Well it doesnt

Nothing will come to focus. Though near objects are closer to focus than objects at distance to infinity. Testing up to now was with a Celestron 25mm PLossl and a TAL Series 2 25mm Plossl.

So I thought okies maybe the primary needs to come up the tube - so I took the primary as far up the tube as its adjusters will allow (thats about 1/4 - 1/2 an inch). Nothing !! Still no go.

So I swapped the Skywatcher focuser over to another TAL focuser that came off a TAL 1 (at least as far as I know) but its a newer version of the TAL focuser. Still no go - not enough 'in' focus.

More twiddling to get the primary up the tube (and also the secondary down the tube and all combinations arising therefrom) Nope - nothing - scope cant pull focus at all.

It then ocurred to me to use some old TAL series 1 eyepieces I have which are from another TAL-1 and lo and behod - focus - hoorah !!!

But - I am damn sure before I took this scope to bits it would pull focus with a Vixen NPL and now it wont. In fact it wont work with anything except the old TAL 1 eyepieces.

Now as you know I am not exactly clueless about this stuff but I cant see for the life of me what could be wrong. I have tried adjusting mirrors up and down the tube but whatever I do theres not enough in focus for any eyepiece except the old ones.

Any TAL experts out there who can advise please do pitch in - I know from past reading that some of these old TAL 1 scopes can be perishers for this but until today I had no idea quite how big a perisher it can be on this scope.

[DIYASTRO]

Mel,

I have a feeling that some of the old TAL ep's had a lens arrangement which put quite simply is an integral Barlow lens. Even though the ep may say that it is a simple kellner or ploss I am sure that if you had a look that the ep has a double concave lens in train. Perhaps this is why you cannot use other ep's.

This is from a very distant memory and perhaps not quite correct but may be worth having a look. Some of the other ep's had a very simple single plano convex or bi convex of very short focal lenth.

Boyd

[yeti monster]

Mel, I couldn't get a focus with my TAL, until I used much longer screws to position the primary at least 3/4" further up the OTA, I can now gain focus and even go past the sweet spot (just), I recon that's the reason that mine was so cheap, all sorted now, just awaiting a new set of WAs and a decent focuser tube which will actually hold EPs securely.

[SpookyKatt]

Hmmm.....interesting. Did you have a look at the old Tal eyepiece

to see if it has a different optical arrangement as DIYASTRO suggested ?

All my Tal equipment is at least 1.25" so probably newer than this Tal 1.

[rawhead]

I had to put a new focuser on my Tal2 thus rendering the original EPs useless

(Got a good price for them though )

[Astro_Baby]

The old TAL EPS with this TAL 1 are the generation 1 types which are slightly bigger than 1.25"

I have had a look at the EPs and although I can see no obvious sign of a barlow style arrangement the optical elelemnts are quite close to the base of the EPs - like about an inch less. Tis is what I suspected the problem was (hence YMs movement up the tube of 3/4" would fix it). I am actually going to leave this one as is and repair the original TAL focuser.

Its been a real learning experience.

Hey YM where did you get replacement screws from and did you get replacement collimation lock screws as well - do you know the sizes etc ?

Would be handy to know.

Mel

ps thanks for the heads up guys.

[yeti monster]

Astro, Being a total shedologist, I found replacement screws in the shed! IIRC, they are M4 x 0.75 thread, B&Q/Focus etc, will probably stock such things, or your local engineering suppliers. I replaced both the collimation and locking screw sets, although I am lookinh around the workshops on the rig for an extra 1/4" in length, just to give me that wee bit extra.

[Astro_Baby]

I am looking into an ultra low profile focuser at the moment - see if I can sort it that way - I hve to say I dont like the idea of the primary being on super long screws - it seems a receipe for wobbly mirrors without being able to source very long springs to hold the whole thing in tension.

But thanks for the idea - I might just repaire the original focuser and only use the scope with its TAL series 1 EPs. Thats the other (least favourite) option.

[yeti monster]

Oh yes, I'd forgotten about the springs, I used some washers to step up the springs, thus retaining their tensioning effect, although springs are quite easy to get hold of, Lidi/Aldi often do useful assortment packs of springs, screws, washers etc.

[heathenwoods]

Quick thread resurrection. What did you do in the end Astrobaby? I'm facing the same dilemma but am hoping to shift the mirror up the tube. Before I do tho', I'm happy to hear of any other solutions (low profile focussers?)

[ubique]

I did move the primary up the OTA with some very useful direction and support from AndyH, but I was never happy with the result. I have had to let my Tal go and now use a Celestron Nexstar 6SE.