If you are the proud owner of a Lunt Ha Solar telescope and have noticed that the amount of detail you are seeing through the eyepiece or when imaging is not what it once was, never fear you aren’t going crazy, it may just be time to replace the blue-glass filter.

When I took my Lunt LS60 THa scope out of mothballs this year in readiness for a summer of solar observing I found that the views I was getting of the Sun were terrible, fine detail was completely absent and prominences were no longer visible. Luckily the issue could be tracked down to one component, the blue-glass filter that sits at the entry point to the diagonal containing the blocking filter.

Blocking filter with tarnished blue-glass

Over time (I’ve had my scope since 2014) humidity can affect the surface of the blue-glass filter causing the tarnishing seen in the images below, I was amazed I could see anything through such a compromised filter, it having turned almost completely opaque. I contacted Lunt through their website www.luntsolarsystems.com and they soon got back to me confirming that they would be able to send a replacement blue-glass filter free of charge via their European distributor Bresser GmbH. Within a few days the replacement filter arrived on my doormat, fantastic customer service, thank you Lunt and Bresser!

Old blue glass filter and the replacement - spot the difference

Replacing the filter couldn’t be easier. If the filter is being removed for the first time the you may have to rub off a small amount of silicone, applied by the manufacturer to ensure the filter remains in place during shipping. Next you can use either some fine pointed pliers or, as in my case, a set of compasses to unscrew the retaining ring, as shown in the image below. Once the ring has been unscrewed you can remove the old filter and pop in the new one. When screwing the retaining ring back on keep going until it is finger tight and then go back a quarter turn to allow for expansion as the filter heats up during a solar observing session.

You can now screw the tube back on to your blocking filter and mount it back in the scope. That’s it, you’re done, it really couldn’t be easier. I’m sure my issues were caused by storing my scope in a humid environment with no silica gel in its flight case. Now it sits in the house rather than the observatory and has a few bags of silica gel around the blocking filter to keep humidity to a minimum. Lesson learnt, but easy to fix thanks to the good people at Lunt and Bresser.