EdgeHD 800 -- How long is the threaded tube on the back of the telescope?

📷 Edge HD BF.jpgHi John.

I am not sure if this will help or not. This is from the Celestron Edge HD scope white paper showing how the back focus is measured on these scopes. If I understand your question correctly, none of the threads on the back of the scope are included in that measurement. It starts at the top of the threaded flange away from the back of the scope. I am guessing you would have to see how that adapter fits your scope in relation to that flange and only parts above that flange (camera side) would matter. I don’t think the focuser tube itself is going to fit past that flange. Also, with the primary mirror moving to initially focus the scope, that distance will never be exact. Only a close approximation that should fit within the travel of the focuser you are adding on the back of the scope. Your initial setup should be to try and get that 133.35mm distance to fall near the center of the focuser travel. At least that is how I configured it setting up my EdgeHD 11 and everything worked out just fine with a similar style focuser.

CS,

Jeff

John Stone · Sep 17, 2025, 08:48 PM

I understand that screwing on an adapter won’t change the backfocus of the telescope, but without knowing the thread depth or how far down the primalucelab adapter will go there is no way for me to find out if my imaging train will fit within the backfocus of the telescope before actually buying everything, screwing it together, and measuring.

i can’t place my order for all the equipment until I know for sure whether or not it’s going to fit ahead of time.

This why you buy adapters that have a fixed size. They screw down onto the thread until they reach a lip in the adapter. The length of the adapter is measured from that lip. If you don’t want to buy a fixed adapter, there is another way to do it. Simply screw your threaded tube all the way onto the baffle nut until it is snug. Then take a digital caliper and measure the precise distance from the rear of the baffle nut to the rear lip of your adapter. Use that dimension to reference the spacing of everything that you want to mount to that part. You can also use a lock ring(s) to set the spacing as well. It helps a lot to make a scaled mechanical drawing in CAD if you have that capability.

John

I can see one glaring problem here. Primaluce is confusing the terms “mechanical distance” and “optical distance” in their description, which makes their entire explanation super confusing. So, let’s first get one thing straightened out here. The backfocus (called the back working distance [BWD] in optics) is defined by the distance from the rear of the baffle nut to the focal plane. That is simply a mechanical distance between two planes. The term “optical distance” simply refers to the equivalent mechanical thickness needed for the required light path including internal optical components. This term appears to be most specific to amateur astronomy and I personally wish that everyone in the industry would dump it. It is simply too confusing for almost everyone and it can be easy replaced with some very minor adjustments due to optical widows. It is very simple to say that the BWD of the reducer is 105 mm but if we include a 3 mm filter and (say) a 3 mm camera window, the required spacing will be 107 mm. That’s the distance that will be required from the rear of the baffle nut to the sensor, taking into account the two optical windows in the system. How hard is that?

Be that as it may, the problem that Primaluce has is that when they stack up all of their gear, the “optical thickness” of the assembled stack is greater than the 105 mm mechanical BWD they have with the reducer—by 10.8 mm. Note, that has nothing to do with any notion of “adding backfocus” by screwing things together. That concept is even more confusing than computing optical thicknesses!

They are solving the problem by simply screwing the rear of the baffle nut far enough inside the focuser until the sensor is in the correct position. They can do that because of the depth of the threads on the baffle nut and the size of the aperture through the focuser. They are simply overlapping parts to get it all squeezed into the required space.

At the end of the day, I personally never trust any of this stuff until I make a scaled drawing of all of the components showing the virtual and real focal points relative to where the sensor sits. Here’s an example from a CDK17 project I did a while back—and this particular drawing doesn’t even show all of the true ray paths through the window! They are simply represented by the offset center line and the displaced focal point. This is the best way to avoid mistakes, which is really important if you have to order expensive custom machined parts.

📷 image.pngGood luck with it!

John

Hi, John.

Sounds like you are looking to build a rig similar to mine. I went through this exercise at the beginning of the year. I opted to use an OAG, so there was no way to use the reducer.

📷 edgehd8.png📷 Screenshot 2025-09-18 at 2.46.30 PM.pngI have no issues whatsoever with focus. I could even lose the spacers and move the focuser tube in more, but… ain’t broke, don’t fix.

Hope this helps.

I should have been more specific. If you are adding the LP focuser, rotator, and also want to use an EFW, you won’t be able to. There isn’t enough back focus. The only way I could draw it up to have the reducer, rotator, and the EFW was to lose the LP focuser and use the SESTO SENSO 2 focuser, which would mean not being able to lock the primary mirror and having to deal with mirror flop.

I would love to know if you figure out a way to get it all to fit.

Good luck.

I started with the same article. But if you notice, there isn’t an EFW in that imaging train. It’s a filter drawer. I even emailed Phillipo and asked. If you aren’t using an OAG, which I am, you may be able to make it work.

Post your results when you have it all worked out.