Having shot exclusively with a Sony a6100 and using a Rokinon 135mm F2 ED UMC for the past year, I have finally made the jump to my first dedicated astro-camera, the ZWO ASI 585MC Pro. I was mostly sold by the great comments from Cuiv and Lukomatico. On the other hand, I really liked the idea of getting a tighter FOV and a resolution bump with the 2.9 micron pixels. Yes, it's also the cheapest cooled camera.
As some of you might already know, adapting a Sony E mount lens with a Flange Focal Distance of 18mm to a ZWO camera with a back-focus of 17.5mm doesn't seem like a doable idea at first glance. I read and re-read multiple times the following CN thread: https://www.cloudynights.com/topic/740245-sony-e-mount-lens-adapter-to-use-with-asi-pro-cameras/
... and then I found the following 3D printed adapter, made by "ThikableCreations":
https://www.etsy.com/listing/1550779704/m42-adapter-for-color-astronomy-cameras?click_key=f289402f917fa89852200ebd743eb626bec06be0%3A1550779704&click_sum=77df4799&ref=shop_home_active_9&pro=1&sts=1
At first I really didn't like the idea of disassembling my lens, then I started to consider it more seriously. Having a $40 3D printed part shipped to Argentina wasn't a good plan for me, and I don't fully trust the ability of any 3D printer to make perfectly round parts and threads (I already have a filter adapter that barely works...).
I tried to copy the idea and then got in contact with an old friend from school, which happens to be a mechanical engineer and has some amazing tools at home, including a beautiful lathe. "We" lathed a Delrin adapter, improving the design from ThinkableCreations... while also completely missing the required back-focus distance
:
The adapter shows as that tiny white ring in between the filter drawer and the back of the "severed" lens.
The idea was to include a 21mm filter drawer, matching one of the solutions proposed by ThinkableCreations. All the mounting hardware was 3D printed from designs I found in Thingiverse, mixing and poorly matching 3 different pieces.
Last Saturday was the first night under the stars, and then the big back-focus problems showed their ugly face. Summarizing: I was getting infinity focus waaaay before the end of the focus travel. Since I didn't knew what was wrong and the skies were very clear, I still completed a first round of shooting, using the Svbony SV220 2" dual narrowband filter and the lens fully opened at F/2. I was blown away by the amount of data captured by the 585MC Pro...
This is a quick PI edit of a single 3-minute exposure at gain 252 and -10° C, not calibrated:
(1:3 scale).
BX did an outstanding job correcting the awful stars I was getting on almost half of the frame. I would have never believed that I could get that amount of data in 3 minutes from my Bortle 7-8 backyard. I'm also not supposed to be using the SV220 filter at F/2... who cares.
Here a 2-hour (40x3min) integration:
I didn't use any calibration frames, I have yet to build a library of darks, bias and flats.
Today, after a lot of thinking and reading, mostly about back-focus and flange focal distance, I set up the rig for a few tests. I was able to validate that I'm missing back-focus, somewhere between 5mm and 16.5mm. A digital caliper is in the way and a well-measured version of this picture will provide me the correct answer:
Luckily, I still have Delrin leftovers for two new pieces. It's now time to get back into lathing... hopefully with the right numbers this time.
Cloudy nights are ahead.... I'll keep you posted with all future updates!
