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"Fun" with back focus


Endolf

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So, this is part ramblings and part warning :)

I got myself a new scope (SharpStar 61 EDPH Mk II), with the reducer/flattener in place the back focus is different to what I had adapters for before when using my newtonian with a coma corrector. So had a look at some options and ordered a Omegon M42 to M48 adapter that said it only added 2mm to the optical path, and then an SVBony set of extensions to build it up.

The SVBony M48 extension tubes arrived first, I quickly discovered that the threads on the extension don't quite work with the M48 threads on my baader MPCC. Weirdly, they work fine with the ZWO M48 adapters, and the male M48 thread did screw in to the MPCC, I'm guessing the SVBony adapter is right on one edge of tolerance, and the MPCC the other extreme.

Then when looking at one of the other SVBony extension tubes I realised that the threads are not cut very deep, so it wouldn't screw on to the new scope's flattener/reducer far enough.

I ordered some Altair extensions.

The new extensions arrived and all was looking good.

This morning the M42 to M48 adapter arrived. Set up for one scope and it all looked good. The extensions needed for the other scope are different, but the M42 to M48 is the same, when I tried to swap it over I couldn't get the adapter off of the extensions. I eventually managed it, but the outside had got scuffed up and the extension was bent out of shape. I'd had to apply heat too. So one of the extensions trashed, but I did manage to get it off eventually.

So then I tried the other setup. All was looking good till I measured the back distance an discovered it was 5.5mm too long. The sums all worked out, but measuring it didn't. I found out the M42 to M48 adapter added 7.5mm to the optical train, not 2mm.

I hunted around online to see what adapter and extension tubes were around, it took a while, but I eventually realised that if I machined 1mm off the adapter, I could use extension tubes from off the shelf. Luckily, one of my other hobbies is miniature steam locomotives, so I have a little workshop.

I took some images, from problem, to solution, to result.

IMG_20200902_130226.thumb.jpg.ff1935285aacf3df9fbe32a5e71997c1.jpgIMG_20200902_161550.thumb.jpg.b89f7a47330e1c2563b4ac2f691769c5.jpgIMG_20200902_161552.thumb.jpg.57a594fedfff7c9c127221dc178a84e6.jpgIMG_20200902_162409.thumb.jpg.3f1a36f2234ded032bac59831e1ef846.jpgIMG_20200902_162626.thumb.jpg.7c95d716fb5c19ce5836cf86def975c9.jpg

IMG_20200902_162347.jpg

Edited by Endolf
Remove extra picture
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