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Sbvony Eyepiece Won't Focus


Art McConnell
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I bought a Sbvony illuminated double reticle 12.5mm Plossl eyepiece and I can't get  the star to focus. Eyepiece all the way in, focuser all the way down, and with the reticle focused clearly and I have a star image that is useable for centering but still quite out of focus. If I then run the reticle focuser in, the star image comes into clearer focus, but never sharp.  At some point the reticle becomes useless, and I try to split the difference. I get this same behavior with this eyepiece in both my BT-70 binoculars and my 8" Newtonian. Is it me or the eyepice? I'd like to use this eyepiece for viewing also, but it needs to focus.

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The reticle is at the focal plane of the eyepiece and, obviously, fairly high in the eyepiece, so extra in travel of the focuser is necessary.

this is not unusual for reticle eyepieces.

If your scope has a 2" focuser (your 8" probably does), then the answer is simple.

The average 2" to 1.25" adapter is about 9.5-10.5mm tall above the focuser.

There are lower height adapters.

The Howie Glatter Parallizer adapter is 0mm tall, which would gain you about 10mm of in focus.

TeleVue has an In-Travel adapter that is -1.5mm tall, which would gain you even more.

And Astrosystems has an adapter that is -12.7mm tall, where you would get so much in-focus you might need to move the focuser out from its normal position.

The lower adapters would solve the problem in the 8", but it won't solve the problem in the bino-scope since it only has 1.25" eyepiece capability.

Lower height adapters solve many in-travel problems.

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On 25/10/2021 at 13:42, Don Pensack said:

The reticle is at the focal plane of the eyepiece and, obviously, fairly high in the eyepiece, so extra in travel of the focuser is necessary.

this is not unusual for reticle eyepieces.

If your scope has a 2" focuser (your 8" probably does), then the answer is simple.

The average 2" to 1.25" adapter is about 9.5-10.5mm tall above the focuser.

There are lower height adapters.

The Howie Glatter Parallizer adapter is 0mm tall, which would gain you about 10mm of in focus.

TeleVue has an In-Travel adapter that is -1.5mm tall, which would gain you even more.

And Astrosystems has an adapter that is -12.7mm tall, where you would get so much in-focus you might need to move the focuser out from its normal position.

The lower adapters would solve the problem in the 8", but it won't solve the problem in the bino-scope since it only has 1.25" eyepiece capability.

Lower height adapters solve many in-travel problems.

Thanks for that info. Didn't know about in-travel adapters. I now know my 2" to 1.25" adapter is a High Hat, which is supposed to give a 16.5mm rise, but my dial calipers show me a full 19mm rise in my own adapter.

So that should fix me up with my Newtonian, which has a 2", low profile helical focuser. I built that scope, and chose that focuser because I wanted a wide(er) field of view for objects such as Andromeda galaxy. I was not disappointed....

But as for my BT-70s, might I find a different (brand? magnification?) reticled eyepiece that will perform better in them? Otherwise, it's to the machine shop with my Sbvony.....

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5 hours ago, Art McConnell said:

Thanks for that info. Didn't know about in-travel adapters. I now know my 2" to 1.25" adapter is a High Hat, which is supposed to give a 16.5mm rise, but my dial calipers show me a full 19mm rise in my own adapter.

So that should fix me up with my Newtonian, which has a 2", low profile helical focuser. I built that scope, and chose that focuser because I wanted a wide(er) field of view for objects such as Andromeda galaxy. I was not disappointed....

But as for my BT-70s, might I find a different (brand? magnification?) reticled eyepiece that will perform better in them? Otherwise, it's to the machine shop with my Sbvony.....

Reticle eyepieces have to have their illuminators at the focal plane of the eyepiece.  That means the focal plane cannot be located at the shoulder, where it is often located on non-reticle eyepieces, but must be placed higher in the eyepiece.

Ergo, just about every eyepiece with a reticle will require at least 8-10mm more in focus than other 1.25" eyepieces.  I don't see a solution for a 1.25" focuser that doesn't have sufficient in-travel.

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