The small Lunt LS50 solar scope is provided with a pressure tuning system as several scopes of the same brand are. Tuning the etalon requires the piston to be perfectly sealed against the cylinder otherwise pressure is lost and the etalon can’t work at its best. As a result the solar disc becomes progressively featureless, chromosphere details as plages and filaments are lost first then even prominences refuse to show.
After some use it may become necessary to replace the o-rings that seal the chamber and allow to vary the pressure inside. Replacement o-rings are provided for free directly from Lunt or from its area representatives. In Europe they shall be requested to Bresser by filling in the form that can be found on the website.
I replaced the o-ring of my Lunt LS50 after three years of intensive use as I am imaging the Sun every clear day, but a less frequent use may lead the rings to last for longer. I received from Bresser a set of 4 o-rings, two black and two red rings of a slightly different diameter being the black rings larger than the others. Black rings are not for Lunt 50 as they are too large to be inserted in the threaded cylinder, so only the red rings shall be used.
Part of the replacement procedure is explained in a short video that can be accessed from Lunt website, however it lacks of some important details that I am giving here below.
First step is to completely unscrew the black knob and put it on a table.
Unscrewing the small cover with Lunt logo will give access to a screw that can be removed by using of a 2.5 mm Allen key.
Removing the screw allows to remove the pressure piston. It has two grooves that are the place where the o-rings shall be inserted. The piston arrives from the factory with only one ring mounted in the rear groove, I suppose because many users may find difficult to engage the cylinder thread with both rings mounted.
However I suggest to use both the replacement red rings as it makes the pressure system more effective
Before reinserting the piston in the knob the rings shall be slightly greased with a silicone grease the can be found in any hardware store.
Once the piston has been remounted and secured with its screw, the black knob shall be threaded again on the cylinder. At first it will be tricky because the front ring will resist to be inserted in the cylinder, however with some patience the knob can be remounted successfully.
Be very (very!) careful not to ruin the thread by using of the brute force, just approach the thread and press gently while rotating the knob until you feel that it engages in the right way and can be rotated by exerting only a modest force. If the knob tends to block after one or two turns unscrew it and try again.
If after many attempts the knob refuses to return in place, remove the front ring and use only of the rear one, this will make the procedure easier but tuning will require the knob to be inserted until all thread has been covered and less margin will be left for further adjustments.