First of all, the 8SE is not a reflector, it is a Schmidt-Cassegrain Telescope, or SCT for short. It has a really long focal length, making it suitable only for smaller DSOs.
Contrary to popular beliefs held on this site, SCTs are NOT bad scopes for imaging, they are bad to LEARN with. You will find that refractors are MUCH easier to learn with, and you will get better quality images as a result.
Now, let's just say you decide to continue with the C8. If you are serious about doing any sort of DSO AP, you'll need a lot of equipment, and the cost can get really high really fast. Be sure you are making informed purchases! I'm not the kind of guy to go around recommending a bunch of stuff because it costs money, so take what I'm about to say with a grain of salt.
If you are using the mount that came out of the box, you need to change that ASAP. With imaging, the single most important piece of equipment in your toolkit is the mount. If you plan on doing any serious deep sky imaging, you need an equatorial mount. A regular Alt-Az mount will limit you to extremely short exposures (usually 30 seconds or less), which is no good for some of the fainter DSOs. By the looks of things, you already have an EQ6-R Pro. This is the exact mount that I have, and it is great!
Now that's settled, next up is your imaging train. So far it looks OK, F/6.3 reducer is definitely a priority, obviously something to fit your camera, etc. You'll need an autoguiding solution to go with all this as well. I like OAGs, they fit nicely onto the C8 and pair well with the ASI174MM-Mini. Highly recommend. You'll need to bring your PC along with you and hook it up to PHD2 or some other guiding software.
OK, now after that rant, I'll get to your main question: the extenders. Let's assume for a moment that you're going with the f/6.3 reducer, OAG, and ASI224MC. The thing you need to look out for is backfocus, which is the distance between the camera sensor and the reducer. There is no formula for this, It's a process that takes time to learn. However, you can get a good estimate. Generally speaking, the backfocus distance for the f/6.3 reducer is 105mm. The OAG's body length is 29mm, and the distance between the sensor and the lens of the ASI224MC is 12.5mm, meaning that somehow you're going to have to come up with 63.5mm of spacing. Celestron's OAG comes with some spacers so you can play around with those and the extension tubes to get yourself to 105mm.
Clear skies and good luck!
Edited by Mr. Pepap, 19 April 2021 - 06:29 AM.