Are you planning to install unmounted filters into your filter wheel, or attach your filter wheel to your camera? If so, this one is for you. I went through the process first hand with my new camera and filter wheel, and it tripped me up in a couple of areas, so here are three lessons I learned along the way.
My equipment:
- ZWO ASI 2600MM Pro (+ Antlia Sii, Ha, Oiii & LRGB-V Pro Filters)
- ZWO ASI 662MC
- William Optics Zenithstar 81 + Sky-Watcher HEQ5 Pro
- William Optics 50mm Slide-base Uniguide Scope + ZWO ASI 120mm Mini
- ZWO ASIAir Pro
The weather has been pretty bad lately so the new setup hasn't seen much action, though the week did bring one surprise: the Northern Lights, visible from southeast England during a big solar storm. Over two semi-clear nights you could make out not just the hue and glow but actual streaks in the upper atmosphere. I grabbed a couple of last-minute iPhone shots from the back garden at about 1am, and I'm pretty happy with the colours and definition that came through. But on to the filter wheel.
Mistake 1: not being prepared
Preparation is everything when it comes to installing filters, because they are so susceptible to dust. If you haven't cleaned down your work surface and you're not wearing gloves to keep grease, dirt and stray hairs off the filters, you're going to get into a pickle.
What I did: I completely vacuumed the surface I was working on, then wiped it down. I put on a box-fresh pair of latex gloves to create a barrier between my skin and the filters, and even with gloves on I tried to minimise touching them as much as possible.
The other thing I invested in was an air blower (bulb blower, air puffer, whatever the proper name is), very cheap on Amazon and probably one of the most useful pieces of kit in the whole job. I used it to clean the inside of the filter wheel before installing anything, then the cover, then both sides of each individual filter, and finally gave everything one more blast before closing it all up. It really does shift those tiny specks of dust. Highly recommended.
One more tip: once things are assembled, if there are any openings, like the main entry aperture of the filter wheel, cover them with something while you get on with other bits. It's surprising how quickly dust will settle in there.
Mistake 2: front or back? Which way round the filters go
Depending on the type of filters you have, they may have a reflective coating on one side. My Antlia filters each have one, designed to prevent, or at least reduce, star halos. That coated side needs to face the camera, which means with unmounted filters you have to work out which way round each one goes into the wheel.
I watched a few videos on this, and Glenn Martin's channel has a full walkthrough of installing filters that's well worth checking out: Glenn Martin Photography.
The trick is to look for the ghosting. Hold up a pen or any other object in front of the filter: on the coated side you'll see a subtle double reflection, and that side faces the camera. Flip the filter over and you get a single, perfectly crisp reflection, which is the side that faces the telescope. It's much more obvious in person than on camera, so don't worry if it's hard to spot in the video.
It also varies by filter. My luminance filter barely showed any ghosting at all and was really hard to judge even with the naked eye; that was the most challenging one. The others were no problem, and the blue filter in particular showed the double reflection very clearly. Take your time with it, there's no rush, but don't be too concerned either: it's a lot easier in practice than it sounds.
Mistake 3: letting light in
This one is a really rookie error: leaving open gaps where light can get in. I didn't notice it on the first test run with the camera and filters. It was only when I brought the rig back inside and started taking dark frames that a very distinctive shape showed up in the frames, caused by the positions of the holes I'd left open.
I had been careful in one place: I'd fitted the light-leak gasket ring between the camera and the filter wheel, and that was a perfect seal. The bit I overlooked was where the filter wheel connects to the telescope. The threaded connection itself is fine, but there are also several screw holes for mounting the wheel directly, and those holes I should, in hindsight, have filled with the screws or covered some other way.
In the end I covered them with black electrical tape rather than refitting the screws. I was wary of the screws poking all the way through the filter wheel with nothing to act as a buffer, and I didn't want to risk forgetting they were there, taking the filter carousel out later and having them scratch a filter. The tape did the trick.
So that's lesson three: check everything for light leaks, make sure it's all well sealed, no holes, no gaps, no cracks, use everything that's provided, and you should be golden.
If you'd like to see the whole job, the video above ends with a time lapse of the full installation, and subscribing on YouTube will catch the next imaging session when the clouds finally part.