Simple but effective adjustment for some mirror lenses

ForenSeil · Joined: 15 Apr 2011 Posts: 2726 Location: Kiel, Germany.

I've just found that my Celestron C8 2000mm F10 is very easily adjustable, if mirror(s) are slightly tiltet.
That works with most Schmidt-Cassegrain and Maksutov-Cassegrain lenses but sadly not well with mirror lenses with rotating front elements.

All you need is a small light in a far distance* (in darkness), a screwdriver and 15min of time.
*I used a street light which was ~5km away, of course other artifical stars or even real stars could be used. You can also do it indoors if you have a VERY tiny light (torch light behind pinhole lens for example) and at least lets say 10m distance between lens and light. Close distances don't work for some reason.

The trick is that you can easily use the "bokeh" of the lens to see if it's mirrors are misaligned and when you have a perfect donut/onion rings everything is fine.
If primary mirror is heavily tiltet it doesn't work nice of course - than you would have to disassamble the lens and use this step as fine adjustment.

Remove red protection cap

Simply watch live view of your camera while tightening/loosing screws until donut becomes perfectly round

Before

After

Results: IQ is somewhat better than before visually! Now it's really breathtaking, you can read newspaper in 100m distance, the effective resolution is really amazing. I'm waiting for good weather to take some moon shots Smile

I whish I had already known that trick when I had it's "little" brother Celestron C130 2000mm F16, it was also misaligned when it arrived but I didn't know that there were screws for adjustment. Anyway it already delivered good moons shots http://forum.mflenses.com/celestron-c130-2000mm-f15-4-t54777,highlight,%2Bcelestron.html . Visually the my Celestron C8 is somewhat better after alignment than the C130, I'm very curious about its IQ when used on the moon and on planets.

Do you know if there are there any more easily adjustable (photographic) mirror lenses?

RSalles · Posted: Thu Nov 07, 2013 4:22 am Post subject:

Thanks for sharing, I have the idea to get one for us the next year, very useful post,

Renato

SonicScot · Posted: Thu Nov 07, 2013 11:36 am Post subject:

Great tip, I'll have a look at my doughnuts later. Very Happy