It has been already three years since I have Samyang 135 as my travel astroimaging lens. On the other hand, I use Meade 10″ ACF telescope in my roll-off roof observatory that FL is over 1700mm. And I started to miss something between – with the focal length about 400-600mm. There are very many choices in this FL range, both refractors, but also Newtonian telescopes. I did not want to spend a lot of money at the beginning, so I decided to get a second-hand 80mm aperture scope, and the one that arrived at me was William Optics ZenithStar II ED 80/545.
This refractor was manufactured about 15 years ago. It has really nice blue gloss finish. The optics in this model is ED air-spaced doublet with FPL-51 glass. That is not the top-notch combination in terms of optics aberrations, but a well manufactured ED doublet may provide a really good quality image.
The rear part of the OTA with the focuser can rotate in the 360-degree range. The thing is, however, that this rotating connection does not seem to be straight, because when it is rotated, the collimation also changes. There is only one position where the collimation is correct. I had a similar situation with TS Photoline 102/714 ED refractor that I had some time ago. In both instruments, the front cell collimation is not possible, so I suspect that maybe this rotatable connection is not made straight on purpose – so the user can collimate the scope this way. But that is only my assumption.
The focuser in this instrument is a standard Crayford focuser with a 1:10 planetary microfocuser and 2 inches diameter drawtube. That is not the best option for very heavy camera setups, but for my 2kg combination of QHY247C camera and M63 adjustable flattener/reducer, it worked very well.
Talking about the flattener – I have received WO Flattener III 0.8x with this refractor, but it turned out that this is not the dedicated flattener for that model. I was able to adjust the flattener backfocus to the moment where about 3/4 of the APS-C frame was okay. So it may work for the 4/3 inch sensor size maximum – like in popular ASI1600 or QHY163 cameras.
Example images from the WO ZenithStar II ED80 and WO Flattener III 0.8x are presented below.
The images above are already cropped a little, but still, it is easy to notice elongated stars in the frame corners.
So the flattener upgrade was required and I decided to adjustable 0.8x flattener/reducer from the Teleskop Express offer. It has an M63 thread in the front and an M48 on the camera side. The backfocus distance is adjustable in the 13mm range, so no additional adjustable spacers are required.
The problem with that refractor model and the flattener I have chosen is, that the refractor backfocus is too short to focus on stars. There will be like 1-2 mm missing unless you have a threaded focuser drawtube that you can screw the flattener directly – but this was not the case. So I needed to order a custom adapter and make a thread on the focuser drawtube to move the flattener as close to the focuser drawtube as possible. All these actions went well and now I can reach the focus at 4-5mm position.
At the end of December, I was able to capture some test data. The setup contained: WO ZenithStar II ED80/545 refractor, TS 0.8x adjustable flattener, QHY247C camera, MC Sonnar 135/3.5 with ASI290MM as a guider, iOptron CEM26 mount, and AstroLink 4 Pi as imaging computer.
The conditions were barely okayish – some fog/smog present, Moon rising, and significant light pollution. So the processing of the images was mostly focused on color calibration and gradient removal. But results are somehow promising and the chromatic aberration seems to be under control – when we remember that is only an FPL-51 doublet.
M35 image is 30×2 minutes stack. There is some reflection in the bottom left that comes from the Christmas tree lights that I forgot to turn off. The double open cluster in Perseus is a 60×2 minutes stack. And the IC5146 Cocoon nebula is 100×2 minutes stack. M1 Crab nebula with Tianguan star (zeta Tauri) is 60×2 minutes stack.
Better resolution images can be opened with right-click and choose “open image in the new tab”.
The last image is a defocused star (omicron Gem) image – inside focus, in focus, and outside focus. Collimation looks good, but some chromatic aberration is visible in defocused images.
I am pretty happy with the results when you consider star shape across the APS-C frame. Also, chromatic aberration is controlled well, and no special actions were required to remove color fringing. That may however change for the longer exposures with very bright stars in the frame. Time will tell 🙂
Clear skies!
