I've recommended that beginners upgrade away from this correct image diagonal that comes bundled with beginner Celestron telescopes.
Most use it until the end when they quit the hobby.
Just did some comparison photo to show the difference, better than just verbally describing.
Due to changing sunlight and clouds I am unable to demo the expected 20% difference in brightness within reasonable effort of 1 afternoon.
I took 15-25 photos per configuration , compared each on computer screen to pick the best per batch in brightness and small detail clarity.
102mm Celestron achromat refractor with DIY mask 70mm F9.5 to simulate the common beginner 70mm F10
44x magnification with 15mm Celestron spotting scope eyepiece
Left: svbony sv188p 1.25" dielectric diagonal
Right: Celestron ball-shaped diagonal , correct image flipped with computer
Phone camera auto setting suggested 1/50 exposure with ISO 200 5400k so I manual set to these on all the photos, confirm manual focus with 4x digital zoom , 5second timer , separate tripod for phone , black A4 sunlight shade.
Photo digitally rotated by 11 to 17 degree to vertical with anti=aliasing off , resulting in minor sawtooth edges along the tree.
Comparison photos uploaded as .PNG file instead of .JPG because the image host does 25-30% filesize compression for .jpg which made the better performer look worse.
With Celestron x-cel 2X barlow to 88x magnification.
Left: svbony sv188p 1.25" dielectric diagonal
Right: Celestron ball-shaped diagonal , correct image flipped with computer
Difference in brightness could be due to clouds , I waited 1 hour for sunlight to get really bright again.
Below Right : same dielectric diagonal with cleaned window glass at same camera setting
Not posting the bigger size image as it got blurry at 88x mag.
Based on ad photos I estimate 5% of scopes in SG could be living room or office display pointed through window glass. It may work at low magnification.
It's available on Taobao for under $30 , about $32 delivered through Aliexpress , takes coupon patience and points to get it at $36 on Lazada
