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  • Writer's pictureChad Leader

Back focus for the Edge HD 8 at f/7 and f/10 using a Celestron OAG

For this post, I want to share my spacing settings/diagrams for the Edge HD 8. I get a lot of questions about this. The Edge 8 can be tricky to achieve nice, round stars to the edges, particularly with the 0.7x reducer installed. Hopefully the included images will aid you in refining your setup, and give you an idea of what to expect in terms of field flatness with this setup.

It's important to point out a few things about this telescope that will greatly affect star shape and coma regardless of backspacing.

  1. Collimation. At this focal length, the field will be very sensitive to collimation errors. If your collimation isn't very good, your star shape will suffer, particularly on the outer edges. Confirm your collimation before tweaking back focus.

  2. Focus. I've noticed that if you are a little out of focus on this scope, the stars can appear slightly elongated and worsen around the edges. It's a good idea to confirm focus with a Bahtinov mask when analyzing your stars. These OTA's are sensitive to temperature changes, and suffer from mirror flop as the telescope tracks. Regular focusing is a necessity. I do it hourly during my sequences.

  3. Sensor size. The image circle is greatly reduced when using the 0.7x reducer. Without the reducer installed, the image circle is a whopping 42mm. With the reducer, it shrinks to 26.7mm. This technically isn't large enough to accommodate an APS-C-sized sensor with a diagonal of 28.2mm, though good flats may eliminate vignetting. I suspect the stars will not be perfect in the corners with APS-C sensors and larger (based on my experience with the smaller ASI294MM sensor).

  4. Seeing conditions / tube currents. Never attempt to collimate, focus, or adjust back focus during poor seeing conditions or while the OTA is equalizing to outdoor temperatures. These conditions create very inconsistent and funky-shaped stars. Generally speaking, if I'm adjusting my telescope optics, I let the scope equalize outside for about two hours. I also try to adjust optics on nights with at least average seeing. I know, it's frustrating to waste the clear sky time - but in the long run, once you get it tuned in you're good to go.

Failure to accept these four conditions will surely result in an exercise in futility! Trust me, I've been there.

I've saved my diagrams that have produced excellent results for me over the past couple of years. I use the Celestron OAG on my rig, which comes with several adapters. I've tried to be very specific in these diagrams so that you know exactly which adapters you need.

First up, f/7 mono (with the reducer). The reducer manual calls for 105mm of back focus. Here's my setup with the Celestron OAG and a ZWOASI294MM:

A couple of notes: the 11mm spacer in the diagram is not the camera ring - it is a standard 11mm m42 spacer (M to F). Since this configuration is a little short to spec, I added a 1mm "washer" spacer between the 11mm adapter and EFW. Honestly, I'm not sure if it made a difference, so that is certainly optional. It brings it to 105.3mm total back focus. The ASIair platesolves this at 1474mm focal length.

Here is a photo of the imaging train:

If you're looking for a OSC diagram, here's my OSC diagram of f/7 back focus before I switched to mono.

Here is the full FOV using this spacing (ASI294MM, H-alpha, 300s subs):

And a mosaic for a better look at the edges:

Below is my f/10 setup. I used this for galaxy season last year. You'll need the Celestron T-adapter (93644) for this configuration, and use the "shortened" version (50mm). The manual calls for 133mm of back focus at f/10. This configuration platesolved at 2094mm focal length:

A photo of the f/10 imaging train:

Here's a diagram of OSC at f/10. I've never actually used OSC with the f/10 configuration, but I suspect the result would be similar, and the extra 1mm of back focus won't affect the image given the large imaging circle of f/10 (42mm):

Here is a full FOV shot of M13 (R filter, 180s subs):

And a mosaic:

When configuring this, I wanted it to be easy to change between f/7 and f/10. To do this, you simply remove the reducer + SCT adapter and replace it with the shortened Celestron T-adapter attached to the thin female M42 adapter that came with the OAG. This configuration also means that your guide camera remains in focus with your imaging camera when you switch back and forth.

That's it - post questions in the comments if you have any!


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