Tunnel vision

[Highburymark]

Ever tried (artificially) star testing an 8" SCT in a small flat because you've got the day off work and there's absolutely no prospect of the clouds and rain dispersing? 

Think I need a longer flat. 

[michael.h.f.wilkinson]

No I haven't and yes, I think you do

[Cornelius Varley]

Remember, that light at the end of the tunnel may be an express train coming towards you. 

[YKSE]

I've done the collimation in my house, sort of larger than a small flat, almost the other end of focus travel with a normal EP.  Your long tunnel might need a flat of Football field size, and you might need to drill a hole in upstair neighbor's floor to see the light in tunnel

[DirkSteele]

To artificially star test you need a really long base line otherwise you can introduce spherical aberration into the test as the wavefronts entering the telescope will still be spherical rather than plane wave.  This can either make you think the scope is no good, or potentially cancel out an inherent problem hiding an issue that is present.  Once you the artificial star is sufficient distance away that problem will fall below your detection limit and the waves will essentially be plane.

[Highburymark]

The good news is that there's a slightly larger flat for sale down the road. The bad news - it's £400,000 more expensive than ours. Imagine, I could get a whole Ethos set for that.

[Peter Drew]

As Matthew has said, don't confuse collimating with testing for optical quality, the latter requires a very distant light source or an optical test bench generated infinity for indoor use.

[Highburymark]

21 minutes ago, DirkSteele said:

To artificially star test you need a really long base line otherwise you can introduce spherical aberration into the test as the wavefronts entering the telescope will still be spherical rather than plane wave.  This can either make you think the scope is no good, or potentially cancel out an inherent problem hiding an issue that is present.  Once you the artificial star is sufficient distance away that problem will fall below your detection limit and the waves will essentially be plane.

Thanks - I won't be trying this indoor again....