PST focuser

[moriniboy]

Out with my son solar watching with the PST yesterday when I noticed that he had turned the focuser knob all the way out and the knurled bit had started to move away from the PST body.

Luckily I spotted this in time, but if I had not would the knob carry on winding until it fell out or is there some sort of safety mechanism to stop this? Would appreciate some comments back from people who have taken these apart.

Nigel

[Merlin66]

Not an unusual occurance!

There's no safety stop per se in the unit......

I'd also recommend the "Vegemite precision focus controller" - a Vegemite lid with a central hole fitted ove the existing knob..makes focusing 100% easier....

[moriniboy]

Thanks Ken

Vegemite lid, got me thinking about what I had hanging around in my astro odds and sods box.

Found a black anodised 1.25" eyepiece plug which I got on Astroboot for £2.50

The knob on the PST is 12mm dia. One 12mm dia drill later, and it fits perfectly

Not sure if i want to araldite it in place or use a grub screw, also should I re-use the Astro Engineering plug cover, decisions decisions?

With regard to the original post if the PST focuser did drop out is it a simple re-attach or a PST strip down?

[iwatkins]

It's very easy actually. I'm not sure you could get the focus knob/shaft to fall out though, but may be possible to get the thread to pull out of the pentaprism block.

The focus knob goes into the body. On the inside is a spring around the shaft with a retaining ring. Further up the shaft is a thread. This thread passes through a block that is on the side of the pentaprism. So basically turning the focus knob, turns the thread and the pentaprism moves up and down, it simply slides up a small rail on one side and a cloth pad on the other.

If you remove the side cover, you can see all this.

To remove the pentaprism you need to wind it all the way up and you then need to pull the focus knob outwards, compressing the spring, to wind the knob further, to get the pentaprism off the top of the shaft. Reassembly is the same but in reverse.

Worth doing though, giving the inside a clean and regreasing the thread, focus knob, pentaprism block etc. Makes focusing much smoother.

My next project is to see if I can get two small DC motors in the case so that focus and etalon tuning can be done remotely.

Cheers

Ian

[Adz]

Its a silly feature isnt it to not have a safety stopper...! I am going to make a fine tuning knob thanks for the pics

[moriniboy]

Final version.

Got original Astro Engineering cap, immersed in water to remove the label and then once dry I painted inside face black.

Later on after the paint had fully dried I Araldited the painted cap to focuser assembly then Araldited whole assembly to the PST knob.

Well chuffed with the end result