Optolong and ZWO Filters Transmission Report

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:

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.

ZWO H-alpha 36mm filter from 2017 – we sell it currently with very good discount as mark I. At normal settings there is no visible leakage outside of the intended transmission band.
ZWO H-alpha 36mm filter from 2017 – Same image as above, but I’ve heavily (really heavily) stretched the right hand side of the image to see if there is really any leakage. As you can see there is a little bit of leakage around the green-blue part of the spectrum. Bear in mind, I suspect that the reddish glow around the original H-alpha part is not leakage, it is simply a result of the heavy stretching that you can create in any image, where there is data. Plus don’t forget, I used my mobile phone to take these images. One could argue whether the leakage that we see here would make huge or any difference to the resulting images. I think this filter is still a good option for those first buyers, who cannot afford to go for more expensive filters.
ZWO H-alpha 36mm filter from 2018 and onward – we sell it currently as mark II. At normal settings there is no visible leakage outside of the intended transmission band.
ZWO H-alpha 36mm filter from 2018 and onward – same as above, but right hand side really badly stretched. Quite easy to understand that there is absolutely none of any leakage outside of the intended part of the spectrum…
Now I wanted to see the similar Optolong 7nm H-alpha filter. We used a 2″ version from 2018. Looks pretty good… lets see what happens if I stretch the image… see below…
Same as above original image, but now heavily stretched the right hand side. Absolutely no leakage.
Optolong 7nm H-alpha 2″ filter – 2018 version and onward
Optolong 6.5nm OIII filter 2″ version. This is the version from 2018 and onward. No leakage visible.
Same as above, Optolong 6.5nm OIII filter 2″ version (2018 version and onward). Even after really heavy stretching there is no sign of leakage outside of the intended part of the spectrum… pretty good. (Not sure if that tiny bit around the green part is leakage or just the result of stretching as this was taken with a mobile phone… only a more quantitative analysis with some more serious equipment would tell it apart…
Optolong 6.5nm SII filter, 2″ “under the microscope”. We didn’t expect a surprise here as there have never been issues with the SII filters, so Optolong only had to make some minor improvements here during the last few years, as far as we heard.
Here is to prove the above statement. Pretty good result.
Although there have been no inquiries about the quality of the famous Optolong L-PRO filters, once we were in the process, I thought why not to take a spectrum of the L-Pro as well…
Here it is, slightly stretched… As this is a very wide band filter, it didn’t really make much sense to stretch it any harder…

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 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

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Posted on March 31st, 2019.