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APM 140 f/7 2.5" focuser - extensions needed for visual?

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#1 Volvonium

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Posted 02 June 2020 - 10:29 PM

Hi all, I have an APM140 f/7 doublet that I obtained recently, and it has the big 3.7" focuser.    The 3.7" focuser is great, but as I'm a visual observer, it's likely way overkill, even with binoviewers.   I'm debating selling the 3.7, as I have an APM 2.5" focuser on hand, which should reduce the overall weight, albeit at the cost of making the scope a bit more front heavy.

 

With the 2.5" focuser I have, which comes with the tube mounting hardware (I believe it is the same between 130, 140, and 152?) and M95 threaded adapter; I notice the 2.5" focuser is notably shorter than the 3.7".  With the 3.7" I have no problems reaching focus for visual and binoviewing.   With the 2.5", I will definitely need some kind of extensions.   What are you all using to get enough back focus?    Are there any M68 drawtube extensions, or are most using eyepiece extensions with 2" barrels?

 

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#2 drd715

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Posted 03 June 2020 - 12:35 AM

I' ve ben told , but can't confirm that the 140ed tube length is slightly different for the 3.7 focuser compared to the 2.5 focuser. If the 2.5 is put on the scope previously using a 3.7, it actually could help when using binoviewers as they take up extra light path. Also the  position of the baffles may be a bit different as the 3.7 focuser draw tube may not retract fully due to the  baffles used in the 2.5 tube conflicting with the 3.7 draw tube. It  seems that the whole end bell tube to focuser plate may have to be replaced to exchange these two focusers. I  have a 140ed with 2.5 focuser I was thinking of trying to put the 3.7 focuser on  because my Ricardi needs the 82mm attachment threads on the 3.7, but not available on the 2.5. Was actually thinking of selling the 140ed 2.5 and buying a 3.7 version  as the focuser exchange needs the whole back plate and some baffle modes to swap. The scope optics are fantastic. I  also have the 152ed 3.7 and the 140ed is even sharper for imaging  with the better glass.


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#3 Volvonium

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Posted 03 June 2020 - 12:47 AM

Oh, very interesting and that does make sense.   I do have the end bell tube for the 2.5", so installation is no problem, but I will have to monitor the performance of the baffling between the two.  It would seem that a 140 owner with a 3.7 can downgrade to a 2.5, while likely needing some extensions, whereas a 140 owner with a 2.5 would have quite a few challenges ahead of them.   I'll go ahead and measure my tube, since this information could be useful for many owners who might be looking into something similar.

 

From the front of the lens cell to the rear of the end bell tube, i'm seeing about 25 1/4" length of the OTA, with dew shield retracted.


Edited by Volvonium, 03 June 2020 - 12:55 AM.


#4 drd715

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Posted 06 June 2020 - 08:03 PM

So this measurement I am about to post is from the front of the dust cover to the base of the 2.5 focuser flange so it might not be the same parameters as your measurement. I  may have to take the scope out of the box to get a bettrr definition. I  get 28-1/2 inches,  but this is probably not the face of the lens as it is currently in the box and the  distance from dust cap to lens face will require further measurements. Can do at a more convenient time.  Interesting to see the difference if any. 

 

The additional length of the 3.5 would not be a problem for me as I am not intending to use a diagonal. The only thing that I would have to do to convert would be to check the back baffle spacing. 



#5 drd715

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Posted 08 June 2020 - 09:35 PM

New measurement  - 27.5 inches from front of lens cell to the point where the white tube meets the black  rear flange.   Ps: I am interested in the 3.7 if you are selling it?




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