New to C8 Astrophotography

[Jaspreet164]

Good day everyone. I have been in this hobby for about three years now. Recently upgraded to Celestron C8 XLT telescope. Really for galaxy imaging only. With .63 reducer I'm struggling to achieve proper back focus. Image train consist of 50mm Celestron adaptor 16.5 spacer and 21mm filter wheel attached directly to ZWO ASI294MM camera. Any help with solving this issue will be appreciated.

[Graeme1858]

Good day everyone. I have been in this hobby for about three years now. Recently upgraded to Celestron C8 XLT telescope. Really for galaxy imaging only. With .63 reducer I'm struggling to achieve proper back focus. Image train consist of 50mm Celestron adaptor 16.5 spacer and 21mm filter wheel attached directly to ZWO ASI294MM camera. Any help with solving this issue will be appreciated.

Hello Jaspreet

You have probably seen this diagram. It's from the Zwo website for your camera. You need to have 55mm from the back of the focal reducer to your camera sensor. That includes your filter wheel.

Welcome to the forum.

Regards

Graeme

[JayTee]

Put another way, you need 105mm of spacing between your .63 corrector and your camera sensor.

Hope that helps.

[AstroBee]

As JT mentioned. The backspacing for the Celestron .63 reducer is 105mm.
Remember, the necessary backfocus is reducer dependant, not camera, so different reducers have different distances. The most common distance is 55mm but that varies. I have a 1.0x TS optics flattener that actually varies depending on the focal length of the telescope from 60mm for a 300mm fl telescope up to 108mm for my scope that is 1220mm fl.

You mentioned the filter wheel is attached "directly" to the camera. Is that with or without the 11mm ring as shown in the photo above by Graeme? I'm not familiar with the 294 so I don't know if the filter wheel can be directly bolted to the camera without that 11mm ring. Either way, with the 11mm ring you have 17.5mm to start with and without the 11mm ring you have 6.5mm to start with.
Fortunately, or unfortunately, depending on how you look at it, even with the reducer you are shooting at f6.3 so your backfocus distance is not as critical as if you were shooting at F2 on a RASA. If you can get within +-2mm you will probably be fine.
You will want to get between 103-107mm to start with. Then take an image and look at the corner stars. If the edge stars appear to be radiating outward like you are going into warp speed then your sensor is too close to the correcter and you need to add a bit of space. If the stars appear to be wrapping around the edge of the frame then you are too far from the corrector and you need to remove space. See this image for the best example of the effect I'm talking about.

3D printers are great for making tiny spacers because the retailers charge a ridiculous amount of money for a sliver of plastic!

[Graeme1858]

Image train consist of 50mm Celestron adaptor 16.5 spacer and 21mm filter wheel attached directly to ZWO ASI294MM camera.

So if your filter wheel is connected to the camera using the 11mm ring, you have the correct 105mm back focus. If your filter wheel is connected to your camera body directly, you will be 11mm short.