ST80 - insufficient focus travel

[Commanderfish]

Now here's an unusual problem.  I'm trying to use a second hand ST80 for visual. The issue is it will only focus to infinity without a diagonal or prism in.  I have tried 1.25 and 2" diagonals and  1.25" prism, but none will focus to infinity.

 

This scope is one of three that Steve at ENS was/is selling. I am starting to wonder if it has been modified. Could someone have moved the lens cell away from the eyepiece end for example? I don't really want to unscrew the lens cell if I don't have to.

[SkyBound]

Hmmmm, I used to have one and it focused fine with a diagonal, with room to spare...

I wonder if the drawtube has been shortened for some reason

 

[John]

Sounds like it might have been shortened, perhaps to allow for a camera to focus or even for binoviewing (less likely) ?

Normally the ST80's need a diagonal in the path to reach focus for visual. As standard they are 1.25" fitting though - if you have been able to put a 2" diagonal in it, has the focuser been replaced ?

 

[andrew63]

As above, they should come to infinity focus with a 1.25" diagonal.  I have used a variety of different eyepieces up to uwa in mine also with a barlow.

 

andrew

[Stu]

The problem seems to be a lack of inwards focus if it won't function with a diagonal in it, so it won't be that the tube has been shortened surely?

I can't think of a sensible explanation why or how this is happening, perhaps a call to Steve in the morning?

[Commanderfish]

Just to clarify , the problem existed with the stock focuser but it's still there with the 2" GSO focuser I've just put on, which is slightly shorter/ lower profile. Would the scope have been lengthened or shortened to create this issue?

[Commanderfish]

Yes exactly Stu, I think it's been lengthened. I wonder if the lens cell has been moved outward, perhaps new holes have been drilled

[Stu]

1 minute ago, Commanderfish said:

Just to clarify , the problem existed with the stock focuser but it's still there with the 2" GSO focuser I've just put on, which is slightly shorter/ lower profile. Would the scope have been lengthened or shortened to create this issue?

If the focuser is all the way in with the diagonal and still won't come to focus then the implication is that the tube has been lengthened, but that sounds very unlikely?!

[SkyBound]

It won't work with a shorter focuser such as the GSO, but should be fine with the origonal...!

the way they are built with the lens cell screwing into the tube, I doubt it has been moved, and also very hard to lengthen it without there being some evidence of the DIY job, have you a picture of the scope you can post ?

[John]

Stu's logic is sound but I just couldn't conceive why such a scope would be lengthened so I jumped to the opposite conclusion

Sorry if that confused things !

Maybe the previous owner was Japanese ? - they tend to prefer viewing "straight through" I believe.

[brantuk]

It may have been lengthened to use a guide camera without a diagonal. I have to add an 80mm extension iirc to mine to get a Lodestar to come to focus. Didn't have to mod the scope though. Hth

[Commanderfish]

I took the focuser off and had a good look, no evidence of DIY, extra holes etc. Could this be a longer lens cell?

[SkyBound]

I might be way off here, but am wondering if the lens has been removed from the cell, maybe for cleaning or such like, could it have been put back, the wrong way round, would that cause this to happen, it is a doublet, and it is very easy to do that, as it looks the same both ways round when you take it off....

??

[John]

The proportions of your scope look pretty much the same as other examples you can see around the web so I guess that narrows it down to either the position of the objective or the objective itself, as SkyBound suggests

 

[nightfisher]

I think it needs to be returned

[alacant]

Hi. I think the focuser is too long. It's not the usual st80 focuser. You'll need to shorten the tube if you want to keep that focuser, or just look straight through. HTH.

[Commanderfish]

1 hour ago, alacant said:

Hi. I think the focuser is too long. It's not the usual st80 focuser. You'll need to shorten the tube if you want to keep that focuser, or just look straight through. HTH.

The scope came with the original focuser, which had the exact same problem, this focuser is actually shorter, but still doesn't come to focus.

[carastro]

Steve at ENS knows his stuff and has always been very helpful, I am sure if you give him a ring he will sort you out.  

Carole 

[SkyBound]

It is very easy to pop the objective out and turn round, and see if that is the problem, it is doublet, so two pieces of glass, a crown and flint, stuck together, I took mine out onece to blacken the edges, to help with light scatter.

if you do remove, make sure you don't re tighten the plastic retaining ring to tight, or you can pinch the optics, it only needs to be tight enough to stop the lens moving or rattling at all.

good luck

[Commanderfish]

3 hours ago, alacant said:

Hi. I think the focuser is too long. It's not the usual st80 focuser. You'll need to shorten the tube if you want to keep that focuser, or just look straight through. HTH.

The scope came with the original focuser, which had the exact same problem, this focuser is actually shorter, but still doesn't come to focus.

[Commanderfish]

I put the original focuser back on,  definitely doesn't come to focus with either a standard celestron 1.25" or a 1.25" amici prism. Focuses fine without a diagonal, with the focuser about 3cm out from its minimum height. The GSO focuser was better, focusing 42-44mm out from minimum depending on eyepiece - but that still didn't leave enough light path for a diagonal or prism. Maybe it is the objective being the wrong way round. Is it possible to mess up the collimation by removing the lens cell? Or is it OK as long as you don't mess with screws?

[alacant]

No. Flip the lens. Hold the tube vertically. As you tighten the ring, shake the tube until the rattle of the lens elements just disappears. With the emphasis on vertical. HTH.

Quote

 

 

[SkyBound]

Yes I think I may have got it correct.....

if it is focusing without a diagonal with only 3cm of tube showing then there is something seriously wrong, normally to get straight through focus with these you need an extension tube.

try what I suggested and flip the lens, I think you may be surprised, whoever sold the scope to the retailer, or the retailer themselves, may have removed it to clean,  and unknowingly put back the wrong way

[SkyBound]

It won't mess up collimation....just don't adjust any screws on the OTA, the lens just fits into a cell, but remember not to tighten the retaining ring too much as has been mentioned.....

[Dave In Vermont]

Oh for Heaven's Sake! The venerable ST80's have always worked 'out-of-box' with no qualms. This one sounds like a 'lemon.' Return it before you commit "major-surgey" on the scope - or yourself - trying every trick in the book.

No matter the cause, send it back?

Dave