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C8 XLT with f/6.3 reducer and TEC turret - question

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#1 vkhastro1

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Posted 14 January 2019 - 07:32 PM

Just purchased a used orange Celestron C8SE XLT optical tube SCT (got to love this new orange rendition of the C8 - I prefer this color it to my first SCT - long gone 1981 orange fork mounted C8). It's been 7-8 years since I have had a standard C8).

This C8 has absolutely no image shift - smooth !

 

I attached my Celestron f/6.3 (Japan) focal reducer and Baader C8 clicklock adapter.

The inserted the TEC turret. Extremely secure!

Inserted a TV 26 Plossl eyepiece- easily reaches focus on a nearby (.5 km) tower.

 

A note on the TEC turret:

It has a non removable 2" barrel and only accepts 1.25" eyepieces.

The 2" barrel length is 27mm.

 

A note on the Baader 2" C8 clicklock adapter).

It has a working length of 42mm.

 

Therefore this combination is wasting about 15mm of light path (say 12mm to give clearance 

bewteen the optical surface of the f/6.3 focal reducer and the front of diagonal 2" insertion barrel).

 

Should I be concerned with this excess 12mm light path distance?

 

There are alternatives (albeit limited) to possibly reduce this 12mm light path.

The Televue 2" short SCT adapter is discontinued, any other possible solutions?

 



#2 AxelB

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Posted 14 January 2019 - 07:53 PM

There’s an optimal distance to respect between the focal reducer and the focal plane (be it the camera or the field stop of an eyepiece). If I remember correctly it’s about 105mm

#3 vkhastro1

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Posted 14 January 2019 - 08:13 PM

I have done an exhaustive search on the TEC turret.

No info available on the actual light path.

I may need to contact Yuri @ TEC for this information.


Edited by vkhastro1, 14 January 2019 - 08:14 PM.


#4 junomike

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Posted 14 January 2019 - 08:46 PM

Based on the measurements shown here, I'd hazard a guess of around 100mm.



#5 vkhastro1

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Posted 14 January 2019 - 09:10 PM

Mike - Thanks for the TEC schematic photo.

If everything is to scale, I measured the following:

 

Optical path of Clicklock C8 adapter is 42mm.

 

Distance from the light path entering the 2" nosepiece [removing the barrel length since this distance is already determined (measured) that the Baader C8 clicklock is 42mm] is 28mm to centre of diagonal mirror.

 

Distance from centre of diagonal mirror to point of exit of 1.25" eyepiece adapter = 59.4mm

 

Total light path (from back of Celestron f/6.3 focal reducer = TEC turret (59.4mm + 28mm) = 88.4mm plus 42mm Baader clicklock = 129.4mm  

 

If the idea light path length is ~ 105mm then we are 24.4mm longer than the ideal value.

How much is this extra 24.4mm light path affecting the TFOV (true field of view)?


Edited by vkhastro1, 14 January 2019 - 09:11 PM.



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