Hello! I would like to purchase in the near future a ZWO ASI178MC USB 3.0 Color Astronomy Camera (the uncooled version) and i know this camera doesn't come with adapters for reaching 55 mm back focal length. I want to pair this camera with a red cat 51. So my question is will i be able to reach focus with only what's provided in the box or do i have to buy some adapters? And if so, what adapters do you recommend for this particular setup?
ZWO ASI178MC USB 3.0 Color Astronomy Camera
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Alexandru i can show you the backfocus on my SpaceCat 51 (same as RedCat 51, just silver finish instead of red) and an ASI 1600MM Pro. In the attached image, you can see the 55 mm backfocus, which is achieved by summing up the widths of the 11 mm and 16.5 mm adapters (both came with the 1600MM Pro), the 20 mm filter wheel and the 1mm adapter (came with the EFW) and the 6.5 mm sensor plane distance relative to the edge of the camera.
The 178MC (which i do own as well, but use only for planetary on an SCT with about 2000 mm focal length) has an M42 thread (the thread inside the red ring of the camera body) and also comes with a T2 nosepiece. The nosepiece has the same diameter as the guide camera on top of my SpaceCat 51 (see the arrow pointing to the helical focuser).
Just on a side note: the EFW comes with a T2 adapter that allows you to attach any camera with a T2 diameter to it, just like the helical focuser in the image.
So in general, it is possible to attach the 178MC to that scope, with some additional adapter rings in between.
But – i would highly recommend, that you tell us, for what type of imaging you want to use the 178MC and the RedCat together. From my personal point of view, those two do not fit well together because the 178MC is not the best camera for guiding but rather for planetary imaging or an allsky camera usage. The RedCat is a wide-field scope, that is well suited for larger nebulas, but you won't be happy to shoot planetary or galaxies with it. But well… just my 2 cents.
Olli
Thank you for the answers! I would like to use this camera for planetary imaging in the future when i will buy a SCT. Meanwhile, i want to pair it with the red cat (not using it for guiding) to have a narrower field of view and maybe shoot a galaxy, a star cluster and other small DSO (the pixel size of this camera matches the small focal lenght of the cat). Other than that, i use the cat for wide-field imaging. Do you think is a bad ideea? My reasoning was that given the small sensor size of the 178MC it would take the 250 focal lenght of the cat to 1215, giving me a closer look on the object. I'm a beginner so please bear with me if i don't make any sense, i'm on a steep learning path 
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Alexandru the camera should be quite well suited for planetary on an SCT. Again, my personal opinion is, that it’s different in regards to the RedCat. Have a look at the attached image – it is a screenshot from SkySafari on iOS and shows the FOV of the 178MC on a Celestron EdgeHD 8“ (2032mm f/10) at the inner rectangle and on a RedCat 51 at the outer rectangle. The target here is M101 which is a rather big galaxy. You can see that the RedCat still produces a very wide field image even though the camera has a small sensor. I would recommend to play with some scope/camera combinations at https://astronomy.tools/calculators/field_of_view/ where you can actually see the results. This will help you make the correct decision which gear to buy.
Olli
Edit: you can actually get the same result from my screenshot in astrotools. Saturn may make the FOV even more clear: https://astronomy.tools/calculators/field_of_view/?fov[]=4549||267||1|1|0&fov[]=122||267||1|1|0&solar_system=saturn
Thank you so much for the help! I have some food for thought now . I wish you clear skies!
Alexandru you are very welcome i hope you will find the right camera that fits your needs. Clear skies!
Olli