LX200R backfocus distance ?

[Bassnut (Fred)]

I fitted 350mm of image train gear to the back of my LX200R 12" OTA, and to my supprise it still reaches focus, with adjustment to spare .

And it all works (rotator, focuser,AO8,OAGer,filterwheel,cam)

Anywayyyy, test stars look a bit bloated tho, got me thinking as to the max BFD on an SCT. Searched the net, nothin, except one comment that, unlike an RC, BFD was essentially infinite on an SCT (I guess the LXR qualifies as a modified SCT, having a corrector plate)?.

Based on test images, im a bit concerned now that a 350mm image train maybe somewhat longer than optimum, despite the ability to still reach focus. Anyone know if that might be the case?.

[Doomsayer]

Hi Fred

There is some theoretical explanation here I think.

http://www.telescope-optics.net/SCT2.htm


cheers
Guy

[multiweb (Marc)]

just emailed you some stuff. There's some info here too.

[Bassnut (Fred)]

Thanks Guy and Marc. Both links are fairly heavy going, but I suspect Ill have to wade through them. The 1st seems to relate to close focus, and the 2nd mentions "Critical distance" between primary and secondary, which doesnt sound good at all. Its a shame Meade just doesnt specify optimum BDF without needing to do all the math to work it out for yourself.

[AndrewJ]

Gday Freddy

Most people believed that the critical backfocus distance was initially set at a std 35mm Film camera on the back.
Later comments were it now includes the length of a microfocusser as well.

However, the big problem is associated with SA
Whilst an SCT ( or ACF ) can theoretically project the image plane way out behind the rear, the SA also grows worse as you go from the critical design position.

Several users on the Yahoo groups have done empirical tests, but i cant ever remember seeing anything definitive.

Andrew

[Bassnut (Fred)]

Thanks Andrew. Ummmm, what does SA mean?.

[casstony]

I read an interferometer test done by Wolfgang Rohr on this subject but I can't find it now - the result was negligible change in SA(spherical aberration) with different amounts of backfocus, but I don't think he checked quite as far back as 350mm.

[casstony]

Found it: only tested back to 157mm on a C11, a bit better up close, no strehl change between 127mm and 157mm:

http://rohr.aiax.de/@C11Jul07-05.jpg

[multiweb (Marc)]

Originally Posted by casstony

Found it: only tested back to 157mm on a C11, a bit better up close, no strehl change between 127mm and 157mm

Just wanna make sure I'm reading this right. Is 97, 127 & 157mm the distance measured from the back of the C11 mirror cell to the point of focus?

[Bassnut (Fred)]

OK, thanks Tony, that looks promising, although Im out more than twice as far. Anyway, its all connected, so nothing to loose trying it again.

[casstony]

Originally Posted by multiweb

Just wanna make sure I'm reading this right. Is 97, 127 & 157mm the distance measured from the back of the C11 mirror cell to the point of focus?

Correct. There's a wealth of information at the optics testing section of this forum: http://translate.googleusercontent.c...V8V-hceZUrPIIg

[multiweb (Marc)]

Originally Posted by casstony

Correct. There's a wealth of information at the optics testing section of this forum

Thanks Tony. I'm in the process of aligning my C11 optics and I want to determine the optimum spacing between my primary and secondary/corrector assembly. Building a rig this week-end to spin the OTA on its baffle tube and check corrector cell run out. This link you've sent is a gold mine.