I have a confession (and a question)

[dharma66]

Ok, I have a dark secret in my cupboard. Well, my loft actually...

In my posting in my welcome thread, I implied that I don't have a scope, and am thinking about buying my first.

It's not true.

At the time of posting, I kind of considered it true, but now I no longer do.

In 1995 I bought Tal-1.

I used it maybe three times, before it got put into the loft to make space for something else...and there it stayed until I remembered it last night!

So today, I brought the OTA down from the loft, and had a look. The outside is filthy, but the object cap and eyepiece cap were on. I thought, though that mirror would be a of a mess by now. So I stripped down the cell, and was delighted to find it is still very bright, if a little grubby. Half an hour with tepid water, tiny drops of Fairy and some cotton wool, and it's sparkling!

So I've stripped the whole thing down and am going to give it a bit of a makeover.

In the meanwhile, I cannot for the life of me find the eyepieces anywhere.

Which brings me to the point of the post. I thought that the Tal-1 had a proprietary eyepiece size, but measuring the inside diameter of the draw tube, it looks awfully close to 1.25" to me?

If it is 1.25", then I want to seriously consider getting a decent eyepiece. I will almost certainly be getting a new, larger, scope in the mid-term, and it would be nice to not have to spend money on an eyepiece that will be useless for the new scope. So, I'm willing to have over-specified eyepieces for the Tal, if the same ones can then be transferred to the new scope (which looks like being a 10" f5 dob).

Is this feasible? Sensible? Daft?

Thanks.

[pvaz]

If it's indeed 1.25" then any EP of that format will work on the new scope.

For f/5 you'll need to spend some on EPs to get sharp edge to edge views, but if you're not too fussy about edge correction you can spend less. Televue plossls will give you the edge to edge sharpness and they got a 20% discount running 'till April so you may want to consider them. Bellow 50 you have the TMB clones and for around 100 the baader hyperions, just to name a few.

Link for all TV plossls (the price shown is full price, you need to remove 20%):

TeleVue Plossl Eyepieces

PS-> You should have a look at the EP stickys and research a bit (or ask here) to know what you're shopping for. Also try reading some reviews before buying.

http://stargazerslounge.com/primers-tutorials/63184-primer-understanding-choosing-eyepieces.html

http://stargazerslounge.com/beginners-help-advice/80772-eyepieces-very-least-you-need.html

[dharma66]

Excellent. Thanks Paulo.

I'll do some reading in between fettling the 'scope!

[Astro_Baby]

The eyepiece size might well 'seem' to be 1.25" but I;d bet they arent on a TAL 1 from 1995. I have one of its sisters here.

On the good news front if it has the old style compression focuser which looks like a shiny silver collar the chances are 1.25" EP will fit it ok. The TAL 1.25" EPS wont fint anything else as they are just a smidge oversized.

[dharma66]

Thanks Astro Baby.

Your Tal pages have been a HUGE help in my stripping the OTA, and I'm sure will be even more helpful for the mount!

It has the spring compression draw tube. I was considering replace the focuser with a Sky Watcher r&p focuser, after stripping this one down, it's in such good shape, it seems a pity to not use it!

I guess I'll risk getting an ep or two, and if they don't fit well, I'll replace the focuser then.

Still a way to go with the rebuild, though!

[Astro_Baby]

Blimey - you must be the first person to have ever have used those pages - I am still miles away from finishing them because my own TAL keeps getting pushed to the back of the list of projects

[Astro_Baby]

Be wary by the way - the TAL focuser sits quite low and to get many eyepieces to work you also need to lift the mirror up the tube - I still dont have a way of doing that yet thats REALLY elegant. The Skywatcher focusers sit too high and as a result NOTHING will come to focus without the mirror moving a VERY long way up the tube - far further than the factory collimation bolts will allow.

Longer bolts mean the mirror becomes very unstable and wobbly on very long spindly bolts. I am still working on some ideas for that

The problem really stems from the fact that tha TAL Series 1 eyepieces which were supplied with the TAL 1 have their glass elements quite close to the base of the eyepiece barrel whereas more modern eyepieces tend to have the glass further up the barrel of the eyepiece. On the TAL eyepieces the glass is almost flsuh with the bottom of the eyepiece barrel.

If your TAL focuser looks like the one in my strip down guide you can simply slip a 1.25" EP into it - I dont like that much as the focuser has a propensity to scratch the eyepiece barrels. Call me fussy I'm sure

[Whippy]

For f/5 you'll need to spend some on EPs to get sharp edge to edge views, but if you're not too fussy about edge correction you can spend less. Televue plossls will give you the edge to edge sharpness and they got a 20% discount running 'till April so you may want to consider them. Bellow 50 you have the TMB clones and for around 100 the baader hyperions, just to name a few.

FWIW, the TAL1 is f7.3 so it's a bit more forgiving on cheaper eyepieces.

If the scope has been stripped down and rebuilt, have you collimated it? Might be worth investing in something to help out with that first .

Tony..

[dharma66]

@Whippy: It's still very much in the stripped down state. It's going to be a good while yet before it gets put back together. Haven't even started the mount and pillar yet! But yes, I'll be investing in something to help collimation.

@Astro_Baby: hm. If the secondary mirror has to move that far down the tube, is it possible to get a truly good collimation? Or does the mirror move a long way off the centreline of the ep? Would I be better maybe filling the hole for the eyepiece and cutting a new one a little further down the tube? I could then cut it, say, 135 degrees from the old one, and drill new holes for the spider at 45 degrees from the original, bu also further up the tube?

What do you think?

[Astro_Baby]

It would be easier to skim around 1" off the back of the tube and remachine the rebate for the primary back into the tube in my opinion BUT I am not doing that because I want a more elegant solution that leaves things as they are.

I have an idea (a cunning plany my lord ) which should work out but I am so far behind with other struff you may have to wait.

Certainly if your going to shorten the tube do it from the parmary mirror end - much easier to do.

[Astro_Baby]

and dont do anything until you are totally sure its right because once you cut bits off you cant stick them back on again.

[dharma66]

What's the thinking behind removing from the primary end?

I'd be worried about shortening the tube from that end, as the cut would have to be quite precise in order to ensure the cell sits square enough in the tube that the collimation bolts can offer enough adjustment. Given I don't know anyone with decent tooling, I'd basically be using a hacksaw to shorten the tube!

I would have thought that repositioning the ep and spider would be the easier option? Then any slight faults in the spider alignment can be trimmed out in the collimation. What am I missing?

[Astro_Baby]

Well if you take it off the front end you have to deal with the spider, filling in the focuser hole and cutting another one, none of which would be easy.

On the other hand removing it from the back end means so long as its reasonably square it can all be adjusted on the collimation bolts of the primary BUT (and theres always a but), on the TAL the rear end is rebated slightly.

To be honest I dont have a satisfactory solution yet other than an experiment which is based on very long collimation bolts which are routed tjhrough some custom made spacers with washers on top - the spaces act to make the bolts more rigid but as yet I havent found the spacers I need.

In a nut sehll it would look like this

BOLT HEAD - BACK PLATE - SPACER////WASHER///SPRING ///BOLT THREAD/// PRIMARY CELL

I cant draw you a pciture but hopefully you get the gist - the washer acts as a flat surface for the collimation springs to push against. The space fits into the holes on the TAL backplate.

[AndyH]

Regarding the mirror move - As long as you use strong springs, you'll find the mirror and cell are very stable. I tried this method on my Tal M and tested it by throwing the ota from one sofa to the other a few times and the collimation never moved !

I think I've still got some longer cheesehead screws(M4x40) and locking grub screws(M4x30), but may have run out of the longer springs(0.8x6.7x30).

If you wrap a single layer of pvc electrical tape around the eyepiece barrel, you'll get a good fit on these older focusers. A decent short term fix? **edit: and it'll protect the eyepiece from any potential scrapes.**

These links may be helpful - Stargazers Lounge - Fixes & hints for old or newer Tal 1's.

Cheers,

Andy.

[dharma66]

D'Oh!

Push the mirror up the tube!

So darned obvious once it's mentioned!

Could you let me know where you found your bits and pieces?

[AndyH]

I've sent you a pm.

I got the bits from a fastener store on fleabay.

Cheers,

Andy.

[dharma66]

Cheers Andy