Refractor focuser conumdrum..

[Whippy]

So I've finally managed to get a diagonal for my 'frac and it's clear so I thought I'll give it a proper test run tonight. Small problem though. As I was aligning the finder, I couldn't manage to get even close to bringing the star into focus. With the focuser all the way in, it was still a fairly big disc of light, nowhere near star-like. It's no problem focussing without the diagonal so I'm not quite sure what's going on really...

Any ideas?

Tony..

[John]

So I've finally managed to get a diagonal for my 'frac and it's clear so I thought I'll give it a proper test run tonight. Small problem though. As I was aligning the finder, I couldn't manage to get even close to bringing the star into focus. With the focuser all the way in, it was still a fairly big disc of light, nowhere near star-like. It's no problem focussing without the diagonal so I'm not quite sure what's going on really...

Any ideas?

Assuming it's the finder that you now find you can't focus (?) - If you have just added a diagonal to the back of what used to be a straight through finder you will need to cut a chunk off the back of the finder tube to get it to focus - the diagonal requires a few cm of light travel itself before the light reaches the eyepiece so you need to move the diagonal closer to the objective lens to allow for that.

If it is the scope that won't focus you might be running into the same problem. The TAL 100R that I used to own had a very short focusser travel and I stuck with the standard TAL mirror diagonal that came with it. Even then I found that some eyepieces (eg: Celestron Ultima 35mm) would not come to focus because of limited inward focusser travel.

I hope that helps.

John

[Whippy]

John, it is indeed the scope with it's limited inward travel. It didn't come with a diagonal at all so I picked up an Antares one fron S&S and I tried three EP's (Meade 4000 40mm, Celestron silver top 20mm and Antares W70 15mm) and I couldn't get any of them into focus. Short of shortnening the tube :shock:, I'm a bit stuck as to thinking up of a solution...

Tony..

[John]

Tony,

You should be able to focus these eyepieces with the TAL - it was only the Ultima 35mm that I could not get to focus. Assuming that the diagonal that your have bought is a 1.25 inch standard mirror diagonal then I cant see why it won't work - the TAL mirror diagonal was just a standard design as far as I could remember.

If you have gone for a 2 inch diagonal (although I don't think the standard TAL 100 focusser would take these) then you will have problems as they eat up much more inward travel.

If you could post a photo of the focusser end of the scope I could see how it compares with how mine used to look.

John

[SteelRat]

PM on the way matey!

[Whippy]

Tony,

You should be able to focus these eyepieces with the TAL - it was only the Ultima 35mm that I could not get to focus. Assuming that the diagonal that your have bought is a 1.25 inch standard mirror diagonal then I cant see why it won't work - the TAL mirror diagonal was just a standard design as far as I could remember.

If you have gone for a 2 inch diagonal (although I don't think the standard TAL 100 focusser would take these) then you will have problems as they eat up much more inward travel.

If you could post a photo of the focusser end of the scope I could see how it compares with how mine used to look.

John

John, it's just a bog standard Antares/GSO 1.25" diagonal from S&S for £20. Nothing special at all. Now, the focuser is where it gets 'interesting'. When I made initial enquiries about the TAL from it's previous owner, he told me that it's got an 80mm Celestron tube inside the TAL tube to improve the baffling. Fair enough, but the 'back end' is Celestron, including the focuser. As seen here...

Tony.

[barkis]

Without putting the diagonal into your scope. Place you highest power EP into your drawtube and focus on a star. Leave the drawtube in that position, and work out how far the drawtube is away from full travel into the scope. The distance measured should be less than the light path through your diagonal. If it isn't, then you will not get a focus with any eyepiece, with the diagonal in place.

Ron.

[Whippy]

Thanks for that Ron, how would I measure the light path through the diagonal? I'm guessing you measure from the side of the 'box' that contains the mirror (excuse my lack of terminology) that goes into the focuser to the back of the 'box', then up to the EP hole. I'm also guessing that you wouldn't need to measure the ring part that goes into the focuser as that would be covered by an EP if the diagonal wasn't there.

Tony..

[John]

John, it's just a bog standard Antares/GSO 1.25" diagonal from S&S for £20. Nothing special at all. Now, the focuser is where it gets 'interesting'. When I made initial enquiries about the TAL from it's previous owner, he told me that it's got an 80mm Celestron tube inside the TAL tube to improve the baffling. Fair enough, but the 'back end' is Celestron, including the focuser. As seen here...

Tony,

Well that's certanly not what the back end of my TAL100R looked like !.

I can understand wanting to replace the original TAL focusser but it look as if the way it's been achieved in this case is to stick an ST80 tube (minus the objective, les cell and dew cap presumably) up the back end of the TAL :shock:

I think your problem is that the replacement focusser that your scope is fitted with is not as low profile as the orginal TAL one was so to fit it the scope tube should have been shortened by a few cm. From what I can see that does not seem to have been done, which is why you are having trouble bringing eyepieces to focus. If it's possible I would contact the previous owner and ask if he had the same issue - I'd be very interested if he has managed to get around this.

John

[barkis]

Thanks for that Ron, how would I measure the light path through the diagonal? I'm guessing you measure from the side of the 'box' that contains the mirror (excuse my lack of terminology) that goes into the focuser to the back of the 'box', then up to the EP hole. I'm also guessing that you wouldn't need to measure the ring part that goes into the focuser as that would be covered by an EP if the diagonal wasn't there.

Tony..

correct Tony. Measure from the front of the housing containing the mirror. Measure to the centre of the mirror, and again from the centre to the exit end of the mirror cell. It seems to me that that distance will be longer than you can accommodate, when the focuser is racked right in to the stop position.

I have a Meade 90 degree diagonal, and I have just measured the distance involved. The light path through the diag. to the focal point just in front of the field lens of my 32 mm Plossl EP, Is just about 2.5 inches. I would think that if the distance of your focuser after you have focused your highest power is 3 inches, I think you are going to struggle. This without your diagonal in of course. I hope this makes sense to you.

Ron.

[Whippy]

John, the guy I bought it from explained to me that he inserted the Celestron tube not only for the focuser but also the baffling issue with TAL's which I was also aware of. I'll email him and ask him what he did (why didn't I think of that in the first place?). I'll let you know what he says.

Ron, I'll hopefully give that a bash tonight. The skies are clear ATM so fingers crossed....

I'm hoping there is a simple solution to this because from what I've seen so far through it (minus diagonal obviously!) has been great and I don't want to get rid of it (it does look 'well used' so it needs a little TLC), but I don't have a tripod that can extend to oooh about 6/7 feet up!

Tony..

[John]

John, the guy I bought it from explained to me that he inserted the Celestron tube not only for the focuser but also the baffling issue with TAL's which I was also aware of. I'll email him and ask him what he did (why didn't I think of that in the first place?). I'll let you know what he says.

I would certainly have a word with him. I'm a little dubious about his claim TBH - the baffling in the TAL 100 that I had was basic but fine - certainly no worse than the Celestron / Skywatcher scopes. Also you have to be quite careful when changing baffling in a scope to ensure that you don't vignette the light path. Inserting one OTA inside another seems a rather crude way of doing it IMHO.

It maybe that the previous owner was using the scope straight through as a big finder / guide scope in which case he might never have noticed that it would not focus with a diagonal of course.

It might be worth asking him if he has the original visual back and focusser for the scope - then at least you could put that back on.

John.

[Whippy]

Just an update really, I did email the seller asking about using a diagonal and he didn't reply. I left it a few days and noticed the tube inside wasn't fixed in and the focuser basically fell off the back. I reckon he knew that it wouldn't work with a diagonal and still sold it to me knowing that I was going to use one on it. I emailed again telling how disappointed I was with the scope and he didn't reply to that either.

So with some discussion I decided to sort it out myself and shorten both tubes and fix it all in properly. Which I've done and now waiting for the first available opportunity to test it out on some objects. We'll see what happens.

I'm pretty annoyed at the seller as he does do a lot of business on Ebay and astro buy & sell and I would have thought he would have at least replied to either of my mails. Needless to say I won't be dealing with him again. Lesson learnt and another notch on my cynical side.

Tony..

[SteveL]

Its looking fairly good for a clear night tonight Tony

[Whippy]

Had a quick thrash just now. At low mags it seemed to be ok, but using anything shorter than 8mm, I couldn't get a decent image. Just blurry and not sharp at all. This could be any of the following. Dirty optics (most definately. But they're not great anyway in terms of general condition), out of collimation (probably) or rubbish seeing (probably). I'll give the optics a clean up and try and collimate it, but it's always going to be at the back of my mind that it's just not 'right', even if it probably is. Bah. You live and learn I suppose.

Tony..

[GazOC]

If its dodgy at high mags it might just be seeing?

[Whippy]

Might be Gaz, but I have tried it out on a few terrestrial objects a couple of times on different occasions too and it was the same. But the lenses are filthy for astro use, so they need a good clean. I did put my laser collimator through it and it wasn't miles from the centre, so I might have a bash at that. We'll see..

Tony..