How Do I Use a Focal Reducer with the ASI183MC ?

rjl

Is there a special provision for using a focal reducer with the ASI183MC ?

I am imaging the Leo Triplet which requires reducing the field-width to accommodate three objects in the same frame.

I am seeing radial star streaks near the outside of the frame (coma ?) and circumferential streaks toward the middle of the frame. (These are not the usual star trails related to guiding). I only saw these streaks when using a focal reducer. The brand is SVBONY 1.25" 0.5X Focal Reducer. The telescope is a 8" reflector, f/5.

[https://rlassoc.org/AP_Image/Leo_Triplet.jpg]

rjl

The second sentence which states, ".. which requires reducing the field-width to accommodate .."

should read,

" .. which requires reducing the magnification (thereby widening the field-width) to accommodate .."

WalterT

Going by the stars near the center your first issue is that you are way out of focus. You need sharp focus before you can tackled the other problems. It also appears that your back spacing is off and/or you have severe sensor tilt.

rjl

It looks like the camera sensor was too close to the focal reducer. I backed-off the focal reducer by several millimeters and it changed the result. A Bahtinov mask was used for focusing. The radial aberration (radial star streaks) is most likely a side-effect that will always be present from the focal reducer. Backing-off the focal reducer from the sensor will eventually place the radial streaks outside the field-of-interest (Leo Triplet). Then by cropping the picture the radial streaks can be eliminated: [https://rlassoc.org/AP_Image/Leo_Triplet2.JPG]

rjl

Side Note: The radial aberration introduced by the focal reducer is possibly corrected by a field flattener. Sometimes a focal reducer and field flattener are combined into one. (About a $100 -$200 item).