Going H-alpha: Delving in the PST "Innards."

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WARNING: SOLAR OBSERVATION REQUIRES GREAT CARE AND SAFE FILTRATION.
INSTANT PERMANENT BLINDNESS CAN EASILY RESULT FROM SIMPLE MISTAKES.
NEVER LOOK AT THE SUN THROUGH ANY LENS, MIRROR OR INSTRUMENT UNLESS IT HAS BEEN FULLY TESTED AND APPROVED FOR SUCH USE. YOU FOLLOW MY EXAMPLE ENTIRELY AT YOUR OWN PERIL!

As the solar image seemed so dark I thought I'd better check the ITF [UV/IR blocking filter] for rust. Then I noticed two chewed up screws on the PST housing. The hex key which fitted the other, undamaged screws quite nicely, just rotated in the worn ex-hex holes.

I dripped a little penetrating oil around each worn screw head, wiped the surface clean and then left it to soak overnight. Hopefully they will shift with a little persuasion from a small screw extractor tomorrow. Failing that I shall have to drill through the screws in half millimeter steps.

The eyepiece pinch screw was also absent. A bit disappointing as no other thumb screw or plain metric screw in my collection fitted the thread in the PST's eyepiece holder. I spread all the packaging foam out on a large piece of polythene in a wheelbarrow but there was still absolutely no sign of the missing thumb screw.

Today I found the housing screws would still not move. I even tried a T10 Torx bit which fitted tightly in my 2mm pre-drilled holes but that just turned when torque was applied. A 2.5mm drill quickly spun the screw heads free but leaves me with a much more difficult cleaning job in the threaded holes. BTW: The screws are 3mm, domed, hex socket head in case you were wondering. I think I shall order some stainless steel examples to replace the originals. A 2.5mm, left hand drill would be very useful to clear the remaining debris if I knew where to find one.


Most of the weight of the instrument is in the sturdy, cast aluminium, prism housing. Which is fitted with standard tripod screw holes in the base. A simple finder throws a tiny image of the sun in the top, round window when aligned. A handy feature when the sun is too bright to point the telescope.

As I was unable to unscrew the eyepiece holder I decided to remove the prism to allow easy access to the UV/IR blocking filter. The images show the prism guide groove and Sun finder projection prism. The main prism is best handled by the attached blocks to avoid smearing grease on the active prism faces. Two projecting pins slide in the body's guide groove. The block is [allegedly] fixed to the prism by a high viscosity glue so the prism can be gently 'righted' perpendicular to the housing. In this PST the prism is rotated slightly anticlockwise. Which seems to be quite typical judging from PST owner's images posted online. It did feel as if it was giving slightly to finger pressure but I'll wait for re-assembly to check prism alignment.

Removing the prism allowed me to slip a small mirror in at 45° to photograph the ITF, UV/IR filter 'head-on,' with flash. It looks awful in the image though the reflective outer surface is actually flawless and nicely blue coated. So I'm not quite sure where the 'orange peel' is actually situated. Presumably on the back side of the ITF, UV/IR blocking filter? Another filter, in the form of a square block lies under the UV/IR filter.

Further online research for PST and rust resulted in more information. The rust appears between the filters as a result of moisture and/or heat stress. Meade offered a replacement for the rusty objectives and repaired other filter problems under guarantee but the long term reliability of the PST remains in doubt. Repairs outside the guarantee period are reported to be very slow and costly.

I still need to remove the eyepiece holder to unscrew the ITF, UV/IR filter's locking ring. A large DIY outlet in the city has the 'Boa" type strap wrenches I need. I'd order a pair online but need to visit another outlet nearby which has 2mm screw extractors in their cased, multiple sets. Which I am very unlikely ever to use again. I already have a set but of five but the smallest is much too big.  Unfortunately this second chain doesn't offer online sales. So a 50 mile journey seems likely. Though I could postpone the screw extractors until later. Dismantling the PST takes greater precedence if I am to progress.

Click on any image for an enlargement.

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