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Hocus focus BackFocus

Astrophotography Celestron Equipment Imaging Optics
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#1 astroEdK

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Posted 29 May 2023 - 10:02 AM

Scope is a Celestron Edge HD 11”.
I am trying to understand the focus system on this scope and have gone through the EDGEHD 24 page document from Celestron and numerous google and YouTube explanations, but still have questions.
OK, so the back focus is 5.75” and is described as the position of the telescope focal plane from the flange on the scope exit. But what does this really mean? 

 

 

​• Is this the ONLY axial station where the optics can possibly be in focus? 

• What happens to the focal plane, and its location, when the focuser knob is adjusted?
o The focal plane moves axially and has sharp images at other axial stations.
o The focal plane move axially, but with images that are not as sharp as when it is at the Backfocus location?
o There is no focus other than when the focus adjuster causes the focal plane to be at the Backfocus distance.

 

Other?

 

Also, How much does the focal plane move per one revolution of  the focuser knob? 

 

Thanks

 

Ed



#2 rgsalinger

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Posted 29 May 2023 - 10:11 AM

You can focus over a range of mirror positions but there is a sweet spot where the optics will produce small round stars. I set mine Edge 11 HD up haphazardly. I them just moved the mirror until I was able to focus the system. With my QHY533M camera I was getting FWHM betwee 3 and 3.5 arc second stars with short exposures. When I got to exactly the correct spacing (I use the reducer and an IR cut filter) my FWHM dropped to under 2 arc seconds. So, it's worth getting the back focus exactly correct for your system. 

 

Rgrds-Ross


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#3 MarMax

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Posted 29 May 2023 - 11:27 AM

This is an interesting calculator for SCT's and focal reducers. Not Edge specific but provides some understanding of the changing focal length with backfocus.

 

The 5.75" from Celestron is the "optimal" focal plane. If you run your focuser fully clockwise, moving the mirror to its full back position, and then fully counterclockwise, moving the mirror to its full forward position, and count the number of turns (full back to full forward) you should end up with around 37. At least this is the number of turns with the CPC 1100.

 

The "optimal" focal plane is at the mid-point of the primary mirror travel, or about 18 turns from fully back for the CPC 1100. There is a reasonable range of the "sweet spot", probably 14-22 focuser turns. The scope will still reach focus, depending on your image train, at 1 turn and 36 turns, but you are operating outside the "optimum" as designed by Celestron.

 

I'll add to the above that this is all IMO (and based on experience with a C11). If you search around you'll find a lot of opinions and not much agreement on the topic.


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#4 cneal

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Posted 31 May 2023 - 06:28 AM

Scope is a Celestron Edge HD 11”.
I am trying to understand the focus system on this scope and have gone through the EDGEHD 24 page document from Celestron and numerous google and YouTube explanations, but still have questions.
OK, so the back focus is 5.75” and is described as the position of the telescope focal plane from the flange on the scope exit. But what does this really mean? 
 
 
​• Is this the ONLY axial station where the optics can possibly be in focus? 
• What happens to the focal plane, and its location, when the focuser knob is adjusted?
o The focal plane moves axially and has sharp images at other axial stations.
o The focal plane move axially, but with images that are not as sharp as when it is at the Backfocus location?
o There is no focus other than when the focus adjuster causes the focal plane to be at the Backfocus distance.

 
Other?
 
Also, How much does the focal plane move per one revolution of  the focuser knob? 
 
Thanks
 
Ed

 
The specified distance is the designed distance where the focus should give the best image across the field, both the the sharpest stars and the least chromatic aberration. Since a cat focuses by moving the primairy mirror, focus can be reached at a wide range of distances. But the best should be at the specified distance.
 
Hope this helps.


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#5 ABQJeff

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Posted 31 May 2023 - 07:08 AM

Answers to your questions below…

Scope is a Celestron Edge HD 11”.
I am trying to understand the focus system on this scope and have gone through the EDGEHD 24 page document from Celestron and numerous google and Youtube explanations, but still have questions.
OK, so the back focus is 5.75” and is described as the position of the telescope focal plane from the flange on the scope exit. But what does this really mean? 
• Is this the ONLY axial station where the optics can possibly be in focus?
Ans: No.  With a moving mirror focus system, you can add filter wheels, reducers, 2” diagonals, etc. and still achieve focus.  However to accommodate longer backfocus (more stuff in optical train) the mirror will be moved forward.  If the optical train is significantly more (like 25mm or more) than the 146mm design spec this1) significantly increases focal length (by about 3 to 1 ratio) 2) can end up in reducing effective aperture if extreme 3) for Edge will cause non-flat field views and increased spherical aberration
• What happens to the focal plane, and its location, when the focuser knob is adjusted?
   o The focal plane moves axially and has sharp images at other axial stations.  
Ans: No if greatly off the 146mm spec (~25mm difference or less wouldn’t cause significant issues for visual, but imaging you want to keep it close to the146mm spec.)
   o The focal plane move axially, but with images that are not as sharp as when it is at the Backfocus location?  
Ans:  Correct (but not that sensitive for visual)
   o There is no focus other than when the focus adjuster causes the focal plane to be at the Backfocus distance. 
Ans; No, Not with moving mirror focuser.  With a moving mirror you can add and subtract optical train items and still achieve focus.  If you lock the mirror and use an external focuser then yes there is only one focus position.
 
Thanks 
 
Ed

Jeff


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