What can I expect to resolve?

[Starfazed]

Sort of extension to the "what can I expect to see" question, based on available equipment. I appreciate my new telescope (Star Discovery 150) is going to be no match for the Hubble telescope in aperture, mirror quality or lack of atmosphere. But just how well can it resolve before detail is lost to mushy blur I wondered?

To test this I pointed my scope at a distant terrestrial object, in this case a sign on a building which according to Google Earth was 325m away, and tried a variety of eyepieces. I could clearly make out the layers of bricks and mortar of the building, the mortar being around 1cm thick. I could just about make out capital letters 2cm high - the bars and spaces between the bars of the letter "E" for instance would have been about 4mm high and I could separate these using my sharpest and highest power lens (6mm redline) though they weren't very sharp. I can't say for sure the telescope couldn't resolve to under 2.5 arcseconds as the only other text on the notice was much too small to read but I could see there was text on it.

Adding the lens onto a Baadar didn't make the letters any clearer; indeed, if anything the reverse. And my 7-21 zoom was more blurry at the 7mm end than the 21mm end meaning I had to really squint to resolve the "E", as it was sharpest when zoomed out midway.

According to trigonometry a 4mm space at 325m is 2.5 arcseconds so more than twice what I think I could clearly resolve, and according to formulae for Rayleigh resolution a 150mm telescope's theoretical resolving limit is 0.9 arcseconds.

On the basis of my observations with a beginner scope (albeit one with a fairly decent mirror) and a £50 eyepiece, does that sound about right, or should I have been able to read closer to the small print of a newspaper from that distance? (Returning to the Hubble analogy, had I been looking through that in my room I reckon should have been able to see individual bacteria on the wall from 325m away!)

For comparison with celestial objects, I gather Saturn (including rings) and Jupiter are both in the region of 30-45 arcseconds wide depending on position relative to Earth. A brick at 325 m is 41 arcseconds across so approximates to the size of either planet at its closest. I couldn't see much detail on the bricks but could clearly make out the layers of mortar between, which implies I'd be able to at least see major cloud bands on either planet - again, does that sound reasonable?

[bosun21]

Theoretically the Dawes limit for your scope is 0.77 arc seconds. This is however influenced by many variables like mirror quality, collimation and atmospheric seeing.

[Starfazed]

Indeed - so what I'm angling (pun intended) towards is, how close to the theoretical limit can a consumer grade telescope get and how would you go about testing? 0.77 arcsecs at 325m is about 1mm. So in theory with a perfect mirror and no atmosphere I presume a 6 inch telescope could not resolve two lines as separate if they were 1mm or less apart. But in practice if you could only resolve lines 2mm apart, or 5mm apart? Where do you, er, draw the line?

[Zermelo]

I have the same scope, but I've never tried to test the resolution in daylight, just at night with tight double stars.
In ideal conditions, on pairs with equal mags, I've split down to 0.9" more than once.

[great_bear]

Also worth noting that you can’t compare daytime viewing with night. 
 

The best bet is to find some double stars to test with on a clear, still night.