I’ve come up with this test report of Optolong and ZWO filters not only because we are the main Optolong importers and ZWO distributor in the UK, but because I myself also wanted to see what we sell and whether we should expect any problems, which is of course very important if one wanted happy customers…

It was also a response to an article I read on the Cloudy Nights forum:

https://www.cloudynights.com/topic/602155-testing-optolong-and-baader-narrow-band-ha-sii-filter/

As you will see this is not a quantitative report, but it will show the tendency and that Optolong and ZWO had good intentions and made positive improvements and that it is safe to buy their filters that were manufactured comparatively recently (i.e. from about 2018). I found it unfair that many users were only reporting issues, but they didn’t state when exactly they purchased the filter they used.

For this test I’ve used a Paton Hawksley made Benchtop Spectroscope and an universal mobile phone adapter that one would use for attaching a smartphone to an eyepiece, however in this case I’ve replaced the eyepiece with the Benchtop Spectroscope. I took full spectrum images whilst the spectroscope was pointed at a full spectrum white light LED bulb and then I took images of the same bulb whilst I held various filters in front of the spectroscope. Then I montaged together the images with the full spectrum image of the bulb for comparison. So on the left hand side is the full spectrum and on the right hand side is the spectrum of the light through a filter.

At normal levels practically no filter would show a leakage in other than the intended part of the spectrum, however as we would normally stretch images during image processing in astro photography, I found it fair to stretch the spectrum of light through the various filters to see if there is really any leakage. I could only find one filter that we have been already selling with some extra discount, although we anticipate that in real life use there isn’t huge difference between these filters or it would possibly heavily depend on the length of exposure, imaged object, level of stretching etc. I can say the above confidently, because whilst we sold lots of these filters in 2016-2017, we only received few complaints, so we assume that most users were happy with the performance of their filters.

I should also mention that whilst i understand that one should aim for the use of perfect components, imaging is a complex process and there are huge number of surfaces in most optical systems that will play their role in the quality of the final image (not even talking about image processing). We have seen a wide variety of results with all kinds of optical components, cheaper or more expensive and more expensive filters didn’t always bring closer to a perfect image if other components were causing reflections and other kind of artifacts. Above is merely my personal humble opinion, however based on many years of imaging with the addition of hundreds of cases from customers that we studied.

So the conclusion is that current stocks of ZWO and Optolong filters are safe to buy. The only filter that we currently sell that showed a little bit of leakage in the green-blue part of the spectrum is the ZWO H-alpha 7nm that we sell with heavy discount as Mark I version. That is still a very good opportunity for those who look for a very good filter at a budget price. So most of the filters that we sell through our website at 365astronomy.com are safe to buy or when it is an older stock, you will see that we mention that in either the product title or description (or both) and those few filters would usually come with a very heavy promotion, but don’t be confused by just the promotions as currently we are running very good promotions on many Optolong filters, even though those are from the latest stock…

Review by Zoltan Trenovszki, 365Astronomy