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Backfocus Meade 10" ACF F10 (not F8)

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#1 StefanLunar

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Posted 29 February 2020 - 12:35 AM

Hi there,
Mirror systems with primary mirror focusing have the best image at a certain distance between the primary mirror and secondary mirror.
This results in a certain value of the focus behind the 2 "SC thread of the back wall. Does anyone know how many mm it is?

 

ACF F10 NOT F8

 

Thanks Stefan

 

backfocus.jpg


Edited by StefanLunar, 29 February 2020 - 11:11 AM.


#2 sonny486

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Posted 29 February 2020 - 12:36 AM

10" ACF IS 115.8MM OR 4.56"

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#3 StefanLunar

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Posted 29 February 2020 - 12:52 AM

Yes, I found 4,56" for the F8 Tube, I have a F10.



#4 StefanLunar

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Posted 29 February 2020 - 12:53 AM

Yes, I found 4,56" for the F8 Tube, I have a F10.


Edited by StefanLunar, 29 February 2020 - 12:53 AM.


#5 carolinaskies

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Posted 29 February 2020 - 06:31 PM

The number is pretty much the same between the F/8 and F/10.  Unlike the Edge which requires a specific spacing, the Meade SCTs have a relative range because the focus point varies as the primary is moved in relation to the secondary mirror.  If you are using a focal reducer or other external glass attached to the back of the SCT that dictates the critical focal distance.   I'm not sure why so many people get it in their head the backfocus is extremely critical.  In the film days we used the camera adapter that attached to the housing regardless whether we used Canon/Nikon/Pentax/Minolta... all having different film plane depths.  



#6 StefanLunar

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Posted 01 March 2020 - 01:49 AM

Hi there,
extremely critical, probably not, but I would like to assume the optimal conditions.

Thank you, Stefan



#7 carolinaskies

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Posted 01 March 2020 - 05:23 PM

Hi there,
extremely critical, probably not, but I would like to assume the optimal conditions.

Thank you, Stefan

Optimal conditions?  Every SCT OTA has variant optics so that if you put 10 samples side by side and measured the actual distance between primary and secondary and the 'optimal conditions' they will vary in seperation distance and hence, backfocus as measured from the rear of the OTA.  

Further, I'll cite the excellent optics documentation from https://www.telescop...cs.net/SCT2.htm

"The relation implies that, for nominal primary mirror shift ∆, error induced by extending back focus .... depends on secondary magnification and primary's focal ratio. For F1=2, m=5, and K2=0, every mm of reduction in mirror separation (∆=1), or nearly for every inch of focus extension, induces ~1/23 wave P-V wavefront error of over-correction, and as much of under-correction for widening the separation."

So, for every 25.4mm(1 inch) change between the primary and secondary(this is a huge value in seperation) the relative effect is 1/23 wave error.  That is not significant error when talking P-V when high P-V values are 1/10, and average values for SCTs falls between at worst 1/4 and 1/10th.  

In real world effect, be aware, if you were to measure a particular set of optics, you may find that the optimal correction therefore is NOT some magic number dictated by the manufacturer, but a number above or below that figure.     

By the way, the actual distance a typical SCT mirror travels for focusing near infinity is well below that 1" change threshold so therefore the actual error induced is even smaller than 1/23.  




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