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EdgeHD 1100 with Celestron OAG backfocus - 91mm?

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#1 deonb

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Posted 10 August 2020 - 10:34 PM

Can anybody else double-check a calculation/measurement I have from the back of the EdgeHD 1100's baffle lock nut down to the edge of the OAG's 48mm threads?

 

I've calculating / measured that the combined backfocus consumption of the OAG and 3.25"/2" adapter is 91 mm.

 

Does that sound right?

 

 

That includes the width of the 3.25" to 2" adapter, and the OAG itself up to (but not including) the 48mm threads. Since the next element of the optical train will consume those threads.

 

This also includes the fact that the OAG sticks around 2.5mm out from behind the adapter's 2" threads and doesn't go flush against it.

 

 



#2 deonb

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Posted 10 August 2020 - 10:43 PM

Wait - I just assumed the 3.25"/2" adapter screws flush against the baffle lock nut. It most likely doesn't.

 

Mmm. Ok, much backfocus does the back of the 3.25"/2" adapter consume when screwed against the baffle lock nut? I can't figure out an accurate way to measure it.


Edited by deonb, 10 August 2020 - 10:43 PM.


#3 rgsalinger

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Posted 10 August 2020 - 10:58 PM

You really don't need to be at some critical back focus when using an OAG on an Edge which has no flattener. SCT's do not have a fixed focal point so you can just move the mirror until the main camera is in focus. Then just focus the OAG to get it in focus as well. If you are using a flattener, then that's a different problem and you need an exact distance and the OAG and main camera also need to match.

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#4 MikiSJ

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Posted 11 August 2020 - 12:03 AM

I have a C11 EdgeHD with a Celestron OAG, Starizona Filter Holder and ASI294. I can also swap out the camera and throw an EP on the OAG and it moves the Celestron Focuser from a reference point of 19643 to 22400.



#5 GregsCNAccount

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Posted 11 August 2020 - 12:30 AM

I disagree with rgsalinger.  Backfocus is critical on the Edge family of scopes.  It is not a typical SCT.  See the attached file showing how the spot size of the EdgeHD 1100 varies with backfocus  distance.  146.05 mm is it, period.

Attached Thumbnails

  • Edge HD11 Backfocus vs. Spot Size Chart.jpg


#6 rgsalinger

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Posted 14 August 2020 - 10:16 AM

Thank you for correcting me! I've never seen that chart and I'm going to use it in the future.

 

I guess though that I would quibble a bit. Not that the on axis performance really doesn't change with back focus distance. You need to get to 13 mm off axis AND be 2.5" away from the sweet spot before you even get a 10 micron change which is (I think) about the size of an APS-C chip around 24x18. If I'm reading it correctly, that is. 

 

 

 

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#7 deonb

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Posted 14 August 2020 - 10:20 AM

Thank you for correcting me! I've never seen that chart and I'm going to use it in the future.

 

I guess though that I would quibble a bit. Not that the on axis performance really doesn't change with back focus distance. You need to get to 13 mm off axis AND be 2.5" away from the sweet spot before you even get a 10 micron change which is (I think) about the size of an APS-C chip around 24x18. If I'm reading it correctly, that is. 

 

 

 

Rgrds-Ross

Explain the 2.5" ?




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