Back focus

[Bozzy]

Anybody got a rough idea of the back focus distance when using a 183C on a 200 DP Newtonian.. I’ve tried it directly in and through and extender and can’t see a thing, absolutely nothing!

cheers

[Merlin66]

The ZWO website shows 6.5mm camera back focus....this means that the front of the camera should be that distance in front of the prime focus.

https://astronomy-imaging-camera.com/product/asi183mc-pro-color

An image of your set-up would help.....

[adoldesa]

Are you using a coma corrector? In this case the corrector will have a fixed back-focus distance (usually 55mm), in this case the ZWO guide at this link helps: https://astronomy-imaging-camera.com/tutorials/best-back-focus-length-solutions-55mm.html

If not using the coma corrector you should work out where is the focal point of your Newton, usually this is inside the focuser barrel for standard focusers (like SkyWatcher) and you may struggle to reach it (although I was able to reach focus with a DSLR and a a standard crayford on my old SW200P). If the focus is too deep in the focuser you either add a coma corrector or replace the focuser with a low-profile one.

Hope this helps.

[bobro]

23 hours ago, Bozzy said:

200 DP

What make is a 200 DP? (Sky-Watcher do 200P and 200 P-DS. with different focusing requirements.)