Hello, I think I did everything right but still the stars in the corners are very bad...
Which of the 55mm diagrams should I use?
2" efw +21mm + 16.5 mm + 0.8 flat/red this is okey ?
ZWO 533 MC PRO and M48 Rings back focus
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dereli_astro I think I did everything right but still the stars in the corners are very bad...
If the photo is correct, the reducer that you are using (FLAT 6AIII) is a generic one; it is not designed to perfectly match a specific telescope model.
Pull the reducer out, and you will find that it is adjustable to roughly work with refractors of different focal lengths. You need to first dial that part in so that it matches the focal length of your particular telescope.
See "adjust distance" in the following picture.
When you are done with that, consult this for the proper backfocus to use (it is not 55mm):
Chen
Now I am sharing the photos I took with the settings you see and I really don't understand this job, where should I measure 60.2 mm?
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dereli_astro where should I measure 60.2 mm?
The definition of back focus, as always. It is the distance between the metal back of the reducer and the sensor in the camera.
In the configuration that you are using (after removing the 11mm ring), the camera's flange focal distance is 6.5mm. Shown here:
https://astronomy-imaging-camera.com/product/asi533mc-pro-color
The filter wheel adds another 20mm. The interior filter glass adds another T/3 where T is the thickness of the filter glass.
See this reference for the optical distance for filter glass (the T/3 that I gave above is for a refractive index of 1.5):
If you want 60.2mm back focus, you measure out a distance of 60.2 - 6.5 - 20 - (T/3) between the camera end of the reducer (often called the "metal back" of the reducer) to the surface of the filter wheel that is closest to the reducer.
Any imperfection in the reducer manufacturing and the camera's flange focal distance (ZWO quotes about +/- 0.5mm for their cameras) is tuned out by looking at the pattern of the corner stars. See about 1/3 of the page down here for what the patterm looks like when the backfocus is too short or too long:
https://optcorp.com/blogs/deep-sky-imaging/how-to-set-the-correct-back-focus
Your star images show that you don't have sufficient back focus.
You can also see a section on where to measure back focus in the same OPT web page. If you are not familiar with the topic, just read that entire document.
See here for more explanation of back focus:
https://cloudbreakoptics.com/blogs/news/calculating-back-focus-metal-back-distance
Chen