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Difficulty achieving focus, focus rings or new focuser?

Dob Equipment
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#1 Brenden

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Posted 20 September 2021 - 07:18 PM

Hi everyone,

 

Still pretty new and having a bit of an issue with achieving focus. I recently purchased and ES truss dob and some 100 deg eyepieces to go with it and a televue powermate (I spent too much on the eyepieces so I need to save some more to fill out the set, oops!). I have already added the two extension rings that came with my scope (I think 1.5 in total or so), and am still unable to achieve focus in some configurations. By gently pulling the eyepieces out while viewing I can tell I need about another inch of extension.

 

I am not in love with the focuser that the scope came with and have been considering buying a new one. Is it possible to get focusers that have more travel distance? Or will I run into the same issue and need to buy more focus rings/extension tubes anyway? Is it simple to install new focusers or would it most likely involve drilling new holes? Does anyone have suggestions for a midrange focuser that might help solve my problem?

 

If I go with more extension tubes am I able to use any that are available on the market? Or do I need to stick with the ones made by ES?

 

Thanks everyone!



#2 wrvond

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Posted 20 September 2021 - 10:05 PM

Yes, you can purchase an aftermarket focuser with a longer tube, and yes, you’ll probably have to drill new holes, but it’s not terribly difficult.



#3 Redbetter

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Posted 20 September 2021 - 11:21 PM

So I take it the ES design is to have the focal plane way outside the UTA.  I guess that helps for binoviewers and some of the ADC set ups, but it also means the light cone will be more vignetted by the focuser for the widest field eyepieces.  In addition it requires a larger secondary than would be necessary if the focal plane was closer to the UTA and secondary.

 

These ES focusers already seem tall to me, but my 20" Obsession has a low profile focuser, shorter draw tube and no extensions needed.   The Classic Obsessions were optimized this way to keep the secondary obstruction on the low side while still providing good illumination for 2" eyepieces.



#4 macdonjh

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Posted 21 September 2021 - 12:46 PM

Hi everyone,

 

Still pretty new and having a bit of an issue with achieving focus. I recently purchased and ES truss dob and some 100 deg eyepieces to go with it and a televue powermate (I spent too much on the eyepieces so I need to save some more to fill out the set, oops!). I have already added the two extension rings that came with my scope (I think 1.5 in total or so), and am still unable to achieve focus in some configurations. By gently pulling the eyepieces out while viewing I can tell I need about another inch of extension.

 

I am not in love with the focuser that the scope came with and have been considering buying a new one. Is it possible to get focusers that have more travel distance? Or will I run into the same issue and need to buy more focus rings/extension tubes anyway? Is it simple to install new focusers or would it most likely involve drilling new holes? Does anyone have suggestions for a midrange focuser that might help solve my problem?

 

If I go with more extension tubes am I able to use any that are available on the market? Or do I need to stick with the ones made by ES?

 

Thanks everyone!

Perhaps buy new truss pole material and make your truss poles 1" longer?



#5 SteveG

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Posted 21 September 2021 - 02:29 PM

Which ES truss dob? I looked up a couple of reviews but didn’t find anything about focus issues.



#6 Brenden

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Posted 22 September 2021 - 02:26 PM

I have the white version of the 10in truss dob. Its focuser is different than the dual speed on the normal model. I have a 20mm 100deg apm eyepiece that can achieve focus without issue used alone or with the powermates. I also have a 14mm 100deg ES eyepiece that has issues achieving focus in any configuration. It needs about 1cm more of outward travel. I already have one extender ring on (which allows the apm to reach focus), I am not noticing any vignette at all yet. I was considering moving my primary forward a bit, but I doubt it will be enough.

 

I think the easiest fix would be an extension ring since I don't want to drill holes at the moment.

 

Are all extension rings interchangeable? How can I tell which ones will or will not work? I'm having a terrible time finding info on this!

 

Thanks



#7 SteveG

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Posted 22 September 2021 - 03:41 PM

I have the white version of the 10in truss dob. Its focuser is different than the dual speed on the normal model. I have a 20mm 100deg apm eyepiece that can achieve focus without issue used alone or with the powermates. I also have a 14mm 100deg ES eyepiece that has issues achieving focus in any configuration. It needs about 1cm more of outward travel. I already have one extender ring on (which allows the apm to reach focus), I am not noticing any vignette at all yet. I was considering moving my primary forward a bit, but I doubt it will be enough.

 

I think the easiest fix would be an extension ring since I don't want to drill holes at the moment.

 

Are all extension rings interchangeable? How can I tell which ones will or will not work? I'm having a terrible time finding info on this!

 

Thanks

 

I would start with this solution. What you are looking for is a 2” extension tube. They start at 35 mm and get longer from there.

https://agenaastro.c...-tubes.html?p=2



#8 Brenden

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Posted 23 September 2021 - 02:16 PM

How would something like the extension tubes you linked compare to something like this? Are there any advantages/disadvantages to either?

 

https://www.amazon.c...YMCE1KQBF&psc=1




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