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Removing focus screen



I was sitting here thinking about trying to get a clear view in the viewfinder using a 500mm mirror and a 2x tele-extender (I know, I know!) for moon shots. I wondered if removing the focus screen would work?

Has anyone ever tried this? What kind of results did you get? It seems like it should work, since the lens will be set to infinity anyway, so there really isn't a need to actually focus it.

Thanks in advance for any insight anyone can provide.


Well-Known Member
Hi Jim,

I have taken the screen out of my RX and all I get with a 200 is a circular image in the middle of the viewfinder. It shows more of the image than it does with the screen in as it is not focussed at the right point.



Well-Known Member
Hi Jim .... you might be surprised that a lens set to infinity mark might not be focussing at infinity! Longer lenses are designed(?) to focus past infinity to allow for temperature variations (amongst other reasons). Adding a 2x will throw in other variables.

If you want really sharp results, you will need a screen in. Use the matt area of the field, or obtain an FS-5 screen.

One method with telescopes and very long lenses is to use a Hartmann mask on a bright star (or the Moon), this will give you precise focussing. Do a search on Hartmann mask, astrophotography etc. Its easy and cheap to make one yourself (but don't forget to take it off for the I wonder why I might say this

Keep checking your focussing during the will alter slightly as things cool down.

Cheers, Bob.


Ah, well, it did seem a bit too easy.

Thanks for the feedback and I'll look up the Hartmann mask.