C6R (150mm F8 Synta lens):
The spacing mod on the lens:
Update, my previous data with the C6R respaced to 2.13mm which worked out well. I then masked it to 120mm F10 and used a 1.62x amplifier and took it to F16 with the ASI290MM camera (2.9um pixels).
AR2824 & AR2825 from May 29th:
Seeing was good with 0.7 to 0.8 arc-second peaks.
Omni XLT 120 (120mm F8.3 Synta lens):
The mod with a 1.94mm ring between the lenses:
Update on the synta 120mm F8.3 lens (the common lens found in scopes like the Celestron Omni XLT 120 OTA). I respaced the air gap to 1.94mm as advised by Mike Jones. I had bad weather the past few days, lots of storms, but this morning have be a small window between clouds and good enough seeing to explore the diffraction limited image scales of the system. I masked it to 100mm F10 for the moderate resolution scales, needing around 1 arc-second (0.8~0.9 arc-second ideally) to sample its resolution best, the image scale is 0.37"/pixel with a 1.62x amplifier (F16.2) and 2.9um pixels of the IMX290 sensor. For the full disc, I masked the same refractor to 60mm F16.7 and used an IMX253 sensor's 3.45um pixels for CaK and used 395nm for a photosphere disc as well. Imaged earlier in this thread of the re-spacing of the lens.
This morning's results:
Astrobin for more info & full resolution:
It was cloudy, so my graph is bad, shows the cloud gaps, peaks were sub-arc-second, so I was within the realm of being able to assess diffraction limited of 100mm at near critical sampling.
Synta 120mm F8.3 (air gap respaced to 1.94mm) operating at 100mm F10 with 1.62x amplifier (F16.2) with Lunt CaK B1200 module and ASI290MM sensor (2.9um pixels)
Synta 120mm F8.3 (air gap respaced to 1.94mm) operating at 60mm F16.7 with Lunt CaK B1200 module and IMX253 sensor (3.45um pixels)
Synta 120mm F8.3 (air gap respaced to 1.94mm) operating at 60mm F16.7 with Baader Blue CCD-IR Block, Baader ND 3.0, GSO Polarizer & Altair Astro 395nm 7nm filter and IMX253 sensor (3.45um pixels)
Both the C6R & Omni XLT 120 are good for this. The spacing tested so far seems to work well and is a significant improvement over stock air-gap spacing at least on these two lenses. I opted to move to the Synta 120mm F8.3 lens because I can keep my C6R for red and green wavelength imaging. The Synta 120 F8.3 allows me to operate at 120mm for high res imaging (needing 0.8~0.9 arc-seconds to really benefit, better is better of course), but also mask it down to 60mm and do a full disc with naturally long focal-ratio with a bigger sensor, so this works for me to have both moderate to high res and full disc imaging on this scope optimal for near UV wavelengths and leave my C6R for longer wavelengths. Both are inexpensive lenses and this re-spacing is such an easy to do modification and inexpensive to perform to get diffraction limited results. The 120/8.3 is of course easier for most to mount and masking it to 100mm F10 or 80mm F12 makes it a much more realistic solution for anyone in worser seeing conditions more commonly.