3 replies to this topic

[arijeet]

Hi everyone,

 

I know this question has been asked quite a bit, but I’m having trouble figuring out how to properly calculate the backfocus for my Celestron EdgeHD 8 while imaging at the native f/10 focal length.

Here’s my current imaging train:

• Moonlite CHL 2.5" Crayford focuser attached directly to the rear cell
• Between the focuser and OAG, I’m using the Moonlite 2.5" 68mm thread to M48 adapter
• That connects to the Celestron OAG via the included female M42 adapter
• On the camera side of the OAG, I have the COAG male M42 adapter
• Then that connects to the ZWO M42 female adapter, which is attached to my ASI533MC

When I measure from the flat front surface of the Moonlite focuser to the sensor plane, I get about 146.14mm, which obviously exceeds the required 133.35mm backfocus for the EdgeHD 8.

I’ve seen others successfully use this exact combination, so I’m wondering:

• Am I measuring from the correct point?
• Do I need to shorten my spacing, and if so, which components should I swap or remove?
• Or is there something I’m misunderstanding about how to factor in the Moonlite's optical path length?
• I’d really appreciate any clarification or suggestions—this has been a bit of a rabbit hole for me. Thank you!

 

Clear skies,

Arijeet

[arijeet]

This is what I calculated

Attached Thumbnails

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[Notdarkenough]

The back focus requirement is a design spec for specific optical components. It is usually determined using the "last glass" method. For example, the back focus requirement spec for the 8HD is different when using the focal reducer, as the reducer is the "last glass" in the optical system. 

 

So, regarding your case, I have two thoughts. First, the 133 number is the design spec, not the actual required number for your specific OTA. I use the design spec as a starting point, and then use the images to determine the need for more, or less, backfocus. Have you tried your pictured train? If not, hook it up and see what the images look like. As you already no-doubt know, 10mm is a lot, whucb leads me to my Second thought; try a different OAG. The Celestron OAG is very good, but it does consume a ton of backfocus. I have used both the ZWO, and Askar large prism OAGs in the past year and they take up much less space. Maybe it will make up the difference? I know my ZWO OAG-L takes up at least 10mm less backfocus than the COAG. How you connect what to where will also make some difference, so it is hard to predict what you will actual need, and what your choices are. See how your camera is attached to the rear of the COAG? That can be much less depending on different choices of OAG.

[arijeet]

I still haven't taken the setup out to know what it looks like. I'll do that soon. I know the COAG is big, but I have seen people make it work here in CN. Just had a hard time understanding what exact spacers they used to achieve the proper backfocus.