Do your filters fit your filter wheel?

After purchasing my first monochrome camera 10 months ago I went on a shopping spree buying filters. In total I have four Optolong LRGB filters, two Astrodon Photometric filters, three Orion narrowband filters, and one Orion dark filter. My first filter wheel was an Orion 5-position manual model. A while later I purchased a ZWO 5-position Electronic Filter Wheel (ZWO MiniEFW).

I really liked the finish of the ZWO MiniEFW and that it was light-tight, the Orion FW not so much. I loaded the MiniEFW with my Optolong LRGB filters and the Orion dark filter. It worked great for many months until yesterday when I decided to swap out the LRGB filters with my Orion narrowband filters. I discovered quite unexpectedly at the telescope that the wheel would not turn. I scrambled to move the narrowband filters back into the manual wheel so I could continue imaging.

So here is the problem. The ZWO MiniEFW requires that the thickness of the filter cell, minus the threads, be less than 7mm. Unfortunately all of my Orion filters and Astrodon filters are 7.5mm. Only the Optolong filters meet the requirement at 5.5mm. For the astute reader you might have caught that I reported a fully functional wheel having four 5.5mm filters and one 7.5mm filter. The best I can figure is that the stepper motor could overcome the drag from that one 7.5mm filter but not four of them.

I thought this might be helpful for those of you who like me “mix and match” — beware!

It’s quite frustrating that there isn’t greater effort to agree on labeling standards among astronomy equipment manufacturers. I dabble in electronics and have always been impressed with the attention to detail in manufacturers’ datasheets. It’s as if they really want you to buy their products! Granted the electronics industry is much larger than the astronomy industry so maybe they have money to burn on good documentation.

Sadly, astronomy retailers don’t pick up the slack. They simply copy the specs that the manufacturers publish. Would you believe I had to dig down into the Q&A section of the ZWO site in order to find the 7mm clearance limit for the MiniEFW? And with regards to filter cell height, I can’t find a thing from the filter manufacturers.