OK if you're willing to void the possibility of return to make it work.
Sunday, 26 June 2022 | James
NOTE: I bought the 8" F4 version, but since that's no longer listed and this scope is merely the same design scaled up, I think it's most appropriate to post the review here.
The positive points about this scope would be the GSO mirrors, which as expected are of very high quality. The scope also arrived with the secondary mirror nearly perfectly collimated (perfectly centered in my cocenter eyepiece, and the laser collimator showed it tilted nearly perfectly with the point appearing within the printed circle at the center of the primary mirror). However I'm struggling to think of other good things to say.
I immediately noticed that tilting the telescope shifted the primary mirror's collimation by a noticable amount. I was not anticipating, however, just how much more sensitive to perfect collimation F4 newtonians are compared to my 130P-DS, which is F5. Nearly any deviation results in very visible star deformation. This led to a lot of frustration, ultimately only solvable by returning this scope and buying a much more expensive one (Think 3-4x the cost), or voiding the possibility of return in the hope of fixing the design flaw with DIY. In this case the people on stargazerslounge pointed me towards neutral silicone sealant, and using it to glue the primary mirror to the cell and removing the retaining clips installed by default. Thankfully when I tried this it did solve the issue, but given as it was a £7.75 solution at screwfix, couldn't the manufacturer have done this themselves in the first place???
In addition, a similarly troubling issue is the design's distance from the secondary to the focal plane. While it was listed on here for the 8" F4, and not on the other PHOTON models, looking at the original manufacturer's website the stats are still listed for the 100% and 90% illumination zone. The manufacturer claims that you should have 100% illumination in a 25mm diameter at the focal plane. And 90% illumination at 38mm diameter. This means that a 31mm diagonal APS-C sensor should have better than 90% illumination at the corners, and 100% illumination at all 4 sides. However my flat frame tests and measures of background brightness in my images shows otherwise. With the TS-GPU coma corrector, I see close to 30% light loss towards the edge of the image, and if I defocus you can see why, as the defocused star shapes have big bites taken out of them, showing that the secondary mirror is not large enough or that the camera is too far away from the secondary mirror. Plugging the measurements I took into online newtonian designers, it seems rather hard to modify this design to actually achieve the manufacturer's claims. I tested it again with NO coma corrector and NO filters, just my RisingCam IMX571 camera with a no-lens M42-2" adapter. This did show less light loss, but still far outside of the manufacturer's claims.
I also had great issue with getting coma correctors and the plain camera and adapter to achieve focus without pulling the corrector out of the focuser, which puts the system at greater risk of focuser sag. I am not sure if my imaging system suffers from it or not as the focuser does seem to be sturdy enough to handle the weight of my camera, coma corrector and filter wheel, but when the focuser is all the way out already, and you still need to pull the coma corrector further out to achieve focus, there is something wrong.
Sadly I don't know what else I would recommend, I hear good things about the smaller mirror versions of these scopes (such as the 6", and my 130P-DS was also a good scope, but no doubt they are flawed in their own ways. 130P-DS needs the focuser barrel shortened, and has a small secondary mirror, and the 6" PHOTON may need the silicone treatment as well, but given as my 130P-DS did not, it may be that the larger primary mirror sizes need it while smaller ones do not.
These cheap newtonians (cheap being not the TS-ONTC design or the Orion optics equivalent, which I will say are PRICY) can be very good imaging setups I believe but only if you are willing to DIY it, which ultimately voids the warranty and puts you at risk of breaking it or ruining it somehow. A big pain for me when silicon-treating the primary mirror was getting the mirror cell back INTO the tube. Since rolled steel tubes deform very easily, and the cell was a tight fit. I still didn't get 2 of the 6 screws back in because of the shift.
This market segment surely needs some innovation from manufacturers, as for only £100-200 more they could deisgn these steel-tubed newts to be far far better for their advertised (imaging) purposes.