The next scope I used was a 60mm achro f:3.8 VersaScope from ScopeStuff. I like this little scope for really wide FoVs.
For these photos, I screwed the T2 diagonal directly to the back of the scope and used a 1.25" Clicklock eyepiece holder. A 1.25" nose with the 685nm filter was applied to the Mod 3C; this is native focal length: ISO 29, 1/2s, 8s stack
This second photo is the same arrangement but the 1.25" nose was replaced with a C to TT adapter and filter capture ring holding the 1.25"/ .5x reducer with 685nm filter. At this close proximity to the focal plane, there is visible reduction, but placing the reducer farther from the focal plane to work at .5x results in very severe vignetting. Vignetting was bad enough with this arrangement: ISO 29, 1/7s, 8s stack
As you can see, Arcturus was setting behind a tree so I didn't have time to do any further testing with the 40P in afocal. Maybe tomorrow.
Here's some of my thoughts about the test results. First, vignetting does not bother some observers and it is less obvious/annoying at the eyepiece than it is in photographs. For me, the level of vignetting caused by the addition of both the 1.25" and 2"/ .5x reducers in these photos is beyond my threshold of tolerance. Afocal with both the 55P and 40P does reduce the focal ratio without causing significant vignetting. But adding the .5x reducers or the .8x reducer did present more vignetting than I want to see, even visually, and in photos, it caused so much vignetting that the increased FoV was nearly cancelled by the vignetting. For me, there is no gain in adding the reducer to the afocal arrangement because I like taking photos. Visually, I saw the gain as being quite insignificant. I MUCH preferred the native FL images from my smaller scope over the highly vignetted image with similar FoV from the ST 120.
Having said this, and having taken photos to show what I saw, I will mitigate the results by adding that if I used a higher ISO or longer exposure, resulting in a brighter image, the vignetting would not look nearly as bad. Alas, I prefer a darker sky background, so even illumination of the sensor is key for me. If I'm able tomorrow, I'll take 3-4 more photos to show what this would look like, and I'll try to keep it on Arcturus for a fair comparison. I'll also test to see if the filter and afocal reflections which showed up at my heavily light polluted home, reappear at a darker site.
I'm hoping these images will help those who are wondering whether they should go with prime vs afocal. These photos demonstrate that problems (reflections, distortions, etc) can arise with either method, especially when dealing with bright, point light sources... like Arcturus.
Finally, different optical systems will react differently to the addition of reducers and different reducers, made for a system may have a dramatically different effect. These test images represent but two refractor telescope optical systems. So YMMV.
Here are 3 more images I took tonight, all of the Gamma Cygni H-a complex. All three were with the 60mm VersaScope achro at f:3.8 (about 8.4x). These were taken with 12, 7 & 5nm H-a filters. Settings were adjusted to keep the image at about the same brightness. Note how the star field shows so well with the 12nm, but fades with progressively narrower band pass filters, while the narrower filters show the H-a subject better. I think these photos clearly demonstrate why I do not like vignetting. Cropping the edges of these images with a dark halo just isn't appealing to me.
12nm H-a, ISO 3712, 1/8s, 8s stack
7nm H-a, ISO 1600, 1/4s, 8s stack
5nm H-a, ISO 2500, 1/5s, 8s stack
Thanks for sharing this with us. I will have to read your report couple of times to digest it but it will definitely help me with my "NV trial and errors" at the telescope.