Pass Through Plunger with Integral Hose Stem for Remote Pressure Tuning of a Lunt Solar Telescope
This is a replacement plunger for a Lunt pressure-tuned Ha solar telescope that allows me to introduce air pressure from an external source, passing through the plunger to the etalon for the purpose of remote pressure tuning to optimize the detail visible on the Sun. Remote tuning allows me to tune the etalon pressure while seated in front of a computer monitor instead of being at the telescope turning the pressure knobs and introducing vibrations. For the external pressure source, I can use either a hand pump (Ralston Instruments DP0V) or a compressor with miniature pressure regulator (0-15 PSI Dwyer MPR1-1) . In either case, the pressure level is monitored and caution is used to avoid exceeding 15 PSI to prevent damaging the solar telescope. When using regulated pressure, I use an oil-free compressor with an inline desiccant filter to remove contaminants and moisture along with a 0-15 PSI pressure regulator.
After printing, the pass-through plunger was coated with 30-minute epoxy (see photos) and allowed 24 hours to cure in order to reduce porosity which causes slow pressure leaks. The original O-rings from the Lunt plunger were transferred to the O-ring grooves in the replacement plunger. I removed the original Lunt plunger from the pressure tuning knob by unscrewing the outer cap, removing the small button-head screw along with the retainer ring and roller bearing to leave about a half inch hole for the tubing to exit. A 1/4" inner diameter clear vinyl hose was routed through the outer hole in the knob and through a 3/8" hose clamp before pushing it onto the the hose stem inside the back of the replacement plunger. I inserted a 1/4" nut driver through the hole in the side of the replacement plunger to tighten the hose clamp around the hose on the stem, being careful not to crush the plastic stem.
To install the replacement plunger, I lubricated the O-rings with MOLYKOTE High Vacuum Grease before screwing the knob with replacement plunger inside onto the telescope. I could feel the knob resistance increase as the first O-ring and then the second O-ring were pushed inside the cylinder on the telescope. I continued screwing the knob just another 1/4" beyond the entrance of the second O-ring since additional tightening would not add any value and would make removal of the replacement plunger more difficult. Finally, I connected the other end of the vinyl tube to my controlled pressure source - either a hand pump or regulated compressor air. In each case a pressure gage indicated the pressure supplied to the etalon.
To remove the replacement plunger after use, I completely unscrewed the knob and then gently applied air pressure to slowly push the plunger out of the cylinder on the telescope, being careful not to have the plunger exit too quickly.
To tune my etalons, I gradually applied external pressure while watching the pressure gage and the image of the Sun on the computer monitor, never exceeding 13 PSI. I found that my telescope is optimally tuned at between 10 and 11 PSI. This value could vary with telescope, elevation, and temperature, etc. After applying this pressure, some time (~5 minutes) must be allowed for the pressure to stabilize. By using a tee, I could apply the same pressure to both etalons in my double stack telescope having a replacement plunger in each. This yielded a good tuning. In the future I will try tuning the etalons individually using an independent external pressure for each.
This YouTube video demonstrates the use of two pass-through replacement plungers to tune an 80mm Lunt double stack solar telescope with a common pressure:
And this YouTube video demonstrates remote pressure tuning of a Lunt double stack solar telescope using independent pressure regulators for each of the two etalons:
I look forward to any comments or questions on the pass-through replacement plunger. If you decide to make a replacement plunger for your own use, you do so at your own risk.
I have added three additional STL files for a box for holding the two pressure regulators and two pressure gauges for independent control of a double stack solar telescope. The new STL files include one for the box, one for the cover, and one for the panel mounting nuts to hold the pressure regulators to the box (two panel mounting nuts needed). The two pressure regulators connected with a brass tee must be just under 123mm wide to fit properly in the holes provided in the box as shown in a photo. The cover is held on with with 7 #8-32 x 3/8" long flathead machine screws that go into heat-set threaded inserts as shown in a photo.
A complete parts list and assembly steps for a double stack remote pressure tuner is now available as a PDF file here:
https://drive.google.com/file/d/1mJRsDiwYfiztM7wZUNN9-HCqKzLmn5uq/view?usp=sharing