Sharpstar SCA-310 update on first usage

A quick update on my experience so far with the Sharpstar SCA-310.

Overall, I’m impressed with the telescope itself. The optics appear to be very good, the mechanical build quality is excellent, and everything feels well thought out. From an overall usability standpoint, it’s been a very positive experience. I haven’t touched the collimation yet, and straight out of the box I’d say it was surprisingly good. I was able to lift it onto my pier myself that is holding the RST-300 mount without much problem (shoulder height lift).

I also wanted to share something that caught me completely off guard because I think it could save someone else a lot of frustration.

The Sony IMX461 medium format sensor itself does not appear to be the problem. In fact, it produced a remarkably flat field right out of the box. The issue turned out to be the filters.

I was using standard Chroma 3nm narrowband filters, and initially I was convinced I had installed one or more of them backwards. I pulled the filters, flipped them around, and tested again. Exactly the same result. Broadband imaging, however, looked perfectly acceptable, which was one of the biggest clues that this wasn’t an optical alignment issue or a problem with the camera itself.

The SCA-310 operates with a very steep converging light cone, and at these fast optical speeds the angle of incidence across the filter becomes significant. As light strikes the filter farther from normal incidence, the effective bandpass shifts toward shorter wavelengths. With an extremely narrow 3nm filter, that shift appears to be enough that a meaningful portion of the desired emission line is no longer transmitted efficiently. The result, at least in my case, was severe haze, diffuse reflections, and washed out images that had me questioning everything from the telescope’s baffling to filter orientation.

After a lot of testing, the problem disappeared when I switched to a ZWO ASI6200MM with standard 6nm filters.

I had reached out to Nico Carver to get his thoughts since he is the only person I've seen actually review the scope - and we both aligned that it's likely the issue. I also reached out to Sharpstar, and Agena astro, both are slow to respond and haven't really offered much advice. They did confirm I was using the extended baffle.

So here’s my biggest takeaway. If you’re planning to use this telescope with 3nm narrowband filters, I believe they really need to be the high speed versions that are specifically designed for these steep light cones. Even then, I’d still want to test before investing in a complete set. Based on what I’ve seen so far, I simply don’t think standard 3nm filters are well suited for this optical design.

The interesting part is that these exact same standard Chroma 3nm filters perform flawlessly on my other telescopes. They work beautifully on my Takahashi FSQ-106 reduced to roughly f/3.3, and I’ve also had excellent results on my Takahashi Epsilon. That really reinforces my belief that this isn’t a filter quality issue at all. It’s the angle at which the light reaches the filters in the SCA-310’s optical system that’s creating the problem.

For now, the 6200MM seems like an excellent match for the SCA-310, and the 461 is headed back to the FSQ where it continues to perform exceptionally well with the same standard 3nm Chroma filters.

Attached is a single raw sub captured with the SCA-310 and the ASI6200MM using standard ZWO filters. Those filters will eventually be replaced because I’ve seen haloing from them in the past, but they immediately eliminated the haze issue I was fighting. That alone was enough to convince me I was finally looking in the right place. (it's compressed of course for CN transport)

Hopefully this helps someone before they spend several nights chasing a problem that isn’t the telescope or the camera at all.

Here is the full resolution of the single sub -still JPG but much higher quality:

https://ozarkhillsobservatory.com/uploads/website_assets_178378053345259.jpg