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New Diagonal Issue

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#1 By-tor

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Posted 03 March 2021 - 08:30 PM

I purchased an Astro-Tech 1.25" diagonal to use on my Meade 125 Mak.  I have a 2" dia. x 2" long visual back with a 2" to 1.25" adapter to mount the diagonal.  I was using a old/cheap diagonal that work's with all my eyepieces on this scope, but the new Astro-Tech seems to be larger/further back and won't allow me to attain focus on anything in the sky.

 

Does anyone know if they make a shorter visual back (maybe 1.5") to pull my diagonal closer to the scope to allow me to attain focus?

 

If I'm doing something wrong please let me know.  Any advice would be appreciated.

 

Thanks for looking

Mark

 

 


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#2 By-tor

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Posted 04 March 2021 - 01:12 PM

I found this at Agena and think it will work for me.

 

https://agenaastro.c...-back-e-15.html


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#3 Starman1

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Posted 04 March 2021 - 03:24 PM

I found this at Agena and think it will work for me.

 

https://agenaastro.c...-back-e-15.html

Yeah, I was going to suggest a 1.25" Visual back.

Alternative: TeleVue In-Travel Adapter (reduces the height of the adapter), or Astrosystems In-Travel adapter (even lower height).


Edited by Starman1, 04 March 2021 - 03:25 PM.

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#4 By-tor

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Posted 04 March 2021 - 04:20 PM

This one is 1.57" long and I don't need the the 2" x 1.25" adapter that sticks out another 0.375".

 

Thanks



#5 BillP

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Posted 04 March 2021 - 04:51 PM

That AT 1.25" diagonal has a rather long light path too!  94mm!  Here's the measures I got when I testing it with others.

 

temp.gif


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#6 By-tor

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Posted 04 March 2021 - 08:53 PM

That AT 1.25" diagonal has a rather long light path too!  94mm!  Here's the measures I got when I testing it with others.

 

attachicon.giftemp.gif

Is that good or bad?



#7 Starman1

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Posted 05 March 2021 - 01:47 AM

Is that good or bad?

That long light path requires more in-travel of the focuser, which could bottom the focuser on a refractor or bottom the mirror on a Maksutov.

Either way, if using the 1.25" diagonal in a 2" visual back, the long light path only made the back focus length that much longer.

A shorter light path diagonal wouldn't have run into this problem.


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#8 By-tor

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Posted 08 March 2021 - 11:56 PM

I have this Celestron Star Diagonal that works, but has no brass clamp ring.  

 

https://optcorp.com/...usaAly4EALw_wcB

 

Any suggestions?



#9 Nippon

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Posted 09 March 2021 - 08:24 AM

Despite it's bad rap I have found the standard Celestron 1.25" prism diagonal quite good optically and mechanically strong enough to handle Plossls, Orthos, RKEs Etc. I've had several and the only issue has been alignment on some. That problem can usually be solved by loosening the set screws on each side of the diagonal body which allows the leaf spring under the prism to snap the prism back in place. Then just gently snug the set screws.


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#10 By-tor

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Posted 09 March 2021 - 09:46 AM

The Celestron doesn't seem as solid as others, but the view through it is fine.  It's the lack of the compression ring that drove me to replace it.  I went with the Astro-Tech diagonal that is nicely built, but won't work with my scope.

 

Thanks



#11 Starman1

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Posted 09 March 2021 - 09:59 AM

I have this Celestron Star Diagonal that works, but has no brass clamp ring.  

 

https://optcorp.com/...usaAly4EALw_wcB

 

Any suggestions?

That Celestron prism diagonal has a few flaws

--a plastic housing (though this works OK if the eyepieces used are small and light, but don't use Morpheus, Delos, XW type 1.25" eyepieces.

--2 thumb screws for the eyepiece (definitely overkill)

--(usually) not threaded for filters on the front end.

But,

--remove one screw

--replace the other with a nylon screw

and it's actually a pretty good diagonal.

Some of the ones I've seen have had multi-coatings on the prism and gave sharp, high contrast, images.

They have usually been preferable to the mirror diagonals at the same price.

 

But few diagonals will have a light path as short, which can cause issues, as we have seen from Bill's data.


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#12 By-tor

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Posted 09 March 2021 - 10:27 AM

I now mainly use an Astro-Tech Paradigm eyepiece and have a SvBony 10-30mm zoom and both have a bit of heft to them.  The zoom is a bit wide at the bottom and hits the short thumb screws when trying to install it.

 

I'll have to find out what the threads are to replace the screws with nylon ones.

 

This one seems to work fine with the 1.25" and 2" backs I have where the Astro-Tech and it's long light path won't work with either and was a waste of money.

 

Thanks


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#13 Starman1

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Posted 09 March 2021 - 01:13 PM

I now mainly use an Astro-Tech Paradigm eyepiece and have a SvBony 10-30mm zoom and both have a bit of heft to them.  The zoom is a bit wide at the bottom and hits the short thumb screws when trying to install it.

 

I'll have to find out what the threads are to replace the screws with nylon ones.

 

This one seems to work fine with the 1.25" and 2" backs I have where the Astro-Tech and it's long light path won't work with either and was a waste of money.

 

Thanks

You can also buy nylon thumb screws long enough to put the heads outside the eyepiece on that larger diameter one.

https://www.mcmaster...n-thumb-screws/


Edited by Starman1, 09 March 2021 - 01:14 PM.

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#14 By-tor

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Posted 09 March 2021 - 02:08 PM

You can also buy nylon thumb screws long enough to put the heads outside the eyepiece on that larger diameter one.

https://www.mcmaster...n-thumb-screws/

That's what I was thinking, but will have to find out what the thread size is before ordering.  

 

Do you think I should just stick with this Celestron diagonal or replace it?

 

What would be a good replacement?

 

Thank you


Edited by By-tor, 09 March 2021 - 02:10 PM.


#15 Starman1

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Posted 09 March 2021 - 02:28 PM

It looks like the TeleVue is not much longer, and it is built strongly enough for very heavy eyepieces.


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#16 SteveG

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Posted 09 March 2021 - 02:56 PM

That's what I was thinking, but will have to find out what the thread size is before ordering.  

 

Do you think I should just stick with this Celestron diagonal or replace it?

 

What would be a good replacement?

 

Thank you

I would stick with it and add a long nylon screws as suggested. I have one, and it easily outperforms my 2” Dielectric diagonals on planets. I didn’t like it when I first received it years ago, but later I found the prism was knocked out of collimation, likely after a previous owner cleaned it. 


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#17 By-tor

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Posted 09 March 2021 - 02:59 PM

Ok TY.

 

I have thought about getting a SCT diagonal that would screw right into the back of the scope.



#18 Nippon

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Posted 09 March 2021 - 03:10 PM

All the Celestron prisms I have had in recent years are threaded for filters.


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#19 By-tor

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Posted 09 March 2021 - 03:25 PM

All the Celestron prisms I have had in recent years are threaded for filters.

The one I have can take filters...


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#20 Steve Cox

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Posted 09 March 2021 - 07:24 PM

I would stick with it and add a long nylon screws as suggested. I have one, and it easily outperforms my 2” Dielectric diagonals on planets. I didn’t like it when I first received it years ago, but later I found the prism was knocked out of collimation, likely after a previous owner cleaned it. 

I'll agree with this finding.  The little "cheap" Celestron 94115 prism is a really good diagonal, and you'd be hard pressed to find a better 1.25" without spending a lot more money for either a Tak or Baader 1.25" prism.  Just make sure it's collimated as Steve says.

 

As for your focus travel issue, it's because a prism actually shortens the focus path compared to a mirror, so you need more out-focus travel for a prism and more in-focus travel for a mirror; a 2" mirror would push the focal plane even closer to the focuser..


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