I was sent this Starboosa Variable Extension eyepiece adapter to review for free, but was not paid for this review and the opinions expressed are my own.
Summary: Does what it claims, but using eyepiece projection with the large image sensor camera (or even the APS-C sensor on my A6300) doesn't really make sense due to the amount of light needed. My 90mm telescope wasn't able to make it work....and if you have a much larger telescope, you will probably be buying a more expensive solution matched to your telescope.
Full review:
This kit includes a T-ring for your specific camera, a variable extension adapter with T2 threads that can optionally hold an eyepiece, and a 1.25" eyepiece to T-2 threads adapter. For prime focus, you can use just the T-ring with the 1.25" eyepiece adapter, or you can use the T-ring, the variable extension tube, and the 1.25" eyepiece adapter. The extension tubes slide past each other so you can adjust the amount of extension to use to adjust the magnification level a bit (1-2x). Obviously, the farther away you slide the camera, the less light hits the sensor [and your telescope needs enough back focus adjustability to make that work].
I tested it using a Meade ETX-90 telescope with a 1250mm focal length and 90mm aperture, with both a 26mm eyepiece and an 11mm eyepiece. The first picture of the moon (where you can see the entire moon) shows the plain Prime Focus configuration.
The 26mm eyepiece gave a little bit more magnification above the prime focus configuration, but was of almost no real benefit for magnifying the view. I could adjust the extension slide to adjust the field of view and magnification a little (in the 1-1.5x range). The 2nd photo where most of the moon is in view is using the 26mm eyepiece in eyepiece projection mode.
The final two images zoomed into the moon are using the 11mm eyepiece with a longer exposure time or a much higher ISO number (leading to graininess). The use of the 11mm eyepiece inside the variable extension adapter allowed me to get greatly magnified images that I would not have otherwise been able to do, but greatly reduced the available light, requiring much longer exposure times (risking image blur due to movement) or much higher ISO numbers (reducing image quality). Overall, I wasn't terribly happy with the image quality when using an eyepiece. [This was a TrueVue 11mm eyepiece, so the eyepiece was not the issue.]
I tried to image Saturn using the 11mm eyepiece, and I was never able to find it in my camera. I can't say for sure this wasn't a targeting issue, but I believe it was due to the fact that not enough light was getting to the sensor for it to display. [I was able to find/see it fine when using the 11mm eyepiece alone.] So in my case, eyepiece projection with large magnification was only useful on a very bright target like the moon. (I was able to image Saturn in prime focus mode, but it was very small.)
I've had better results with eyepiece projection to a cell phone camera (as the aperture size of a cell phone camera is a better match for the exit pupal size of most eyepieces.)
At the $40 price point, I think you would be better off buying a 1.25" eyepiece adapter and T-ring (skipping the variable extension tubes) and a cell phone eyepiece adapter bracket.
That being said, if and only if your telescope has enough back focus to allow you to use the variable extension tube in prime focus mode without an eyepiece, it does give you more flexibility for selecting the exact magnification and field of view you want for a specific target (within a small range). For example, my Celestron 102AZ requires a 42mm T2 extension tube, so replacing that extension tube with this variable extension tube gives flexibility in framing with no reduction in image quality.
Amazon Affiliate Links:
Kit as reviewed, $39: amzn.to/48wobh4
I'd suggest you buy the following items instead:
-A 1.25" eyepiece to T2 thread adapter and T-Ring appropriate for your camera: amzn.to/46h9HQM
You can also buy only the 1.25" eyepiece to T2 thread adapter if you already have a t-ring: amzn.to/3LGSwjh
That will allow you to do prime focus imaging (but not eyepiece projection).
For eyepiece projection, assuming you have a cell phone with a relatively good low-light camera, I'd suggest a cell phone to eyepiece adapter: amzn.to/3ZFuCui
All cell phone adapters require a bit of fiddly adjustment/alignment that a T-Ring and a camera just take care of, but in the case of eyepiece projection, I think it is a better option.