Resolution calculations (1 Viewer)

[BoldenEagle]

Hi,

I (finally) ordered Edmund Optics USAF 1951 pocket size resolution target and would like to know the formula how to convert the results to arcseconds?

Regards,

Juhani

 

[BoldenEagle]

The forum search did the job and I found this formula:

R= 104/(D×LPM)

Multiplied by 2 as using line pairs per mm.

Now, the print quality with the smallest patterns is not so great so would it be wise to operate with the largest ones (group 0 or 1) by increasing the distance so that resolution limit could be seen?

Also, is 96x magnification enough (using Kowa 883 + TE-11WZ- zoom + 1.6x extender) and how accurate is this method when using just this pocket chart instead of more high quality glass slide?

Regards,

Juhani

 

[henry link]

I have the Edmund plastic card in a drawer somewhere beyond hope of locating. My recollection is that its printing quality is pretty poor, so my guess is that group 1 is the smallest group that can be trusted, which means you would need to place the card about 65 meters from a diffraction limited 88mm scope in order to resolve group 1/ element 5, leaving element 6 unresolved. You could try group 2 at half that distance to see if you get the same result. The pocket chart can be perfectly accurate if you stick to the large groups, but then the long distance to the target can make air turbulence a significant factor.

If your scope is diffraction limited its line pair resolution on the USAF 1951 should be around 1.31 arc seconds. The magnification needed to see that depends partly on your eyesight acuity. A person with 20/20 vision can resolve 120 arc second line pairs, so theoretically could resolve 1.31 arc second line pairs at around 92x. In my experience it's much easier to resolve line pairs if they are magnified more than that. For a 1.31 arc second scope (with the USAF 1952 placed in sunlight) somewhere between 120x-160x would probably be the sweet spot for my 20/15 acuity. The standard for resolution I apply is the smallest element that definitely shows the direction of the lines, not a completely clean split of the lines.

Henry

 

[Tringa45]

Hi Juhani,

I have the intermediate Edmund optics glass slide, which goes from Group 0 to Group 3, Element 6 (14,3 lp/mm) and did some resolution measurements on my ATM65 HD and 883 using a backlit matte light source. I used a 3,5 mm Televue Nagler for 130x and 140x magnification respectively but think that would pose a challenge for me today.
With the little Swarovski at 23 m I resolved Group 2,3 (1,78" and the diffraction limit) and with the Kowa, Group 2,5 (1,42"). A little closer and I might have managed 2,6 for a diffraction limited 1,32".
Very bright illumination of the target is important at such small exit pupils, so some additional lighting would be advisable as sunlight might introduce mirage. You will also need an accurate measurement of objective to target distance. I used a folding 2 m ruler but there are cheap laser rangefinders available, which are intended for indoor measurements and are very accurate up to 40 m.
Good luck!

John

 

[BoldenEagle]

Henry and John, thank you very much for your informative (as usual) responses.

Earlier I did do some comparisons between my Kowa 883 and Zeiss Victory 85 using € bills and figured out that choosing a tiny square details that had many parallel lines in them, I was able to get difference between the scopes exactly by which one could let me see the orientation of the lines; not all the lines themselves. It was quite easy to tell that with Zeiss I could see it from more distance (which was no surprise because by comparing them with any target, Zeiss allmost allways seemed to be just by a little the sharper one). That comparison was done @75x which was the max. I could achieve with Zeiss zoom.

Target illumination also came very apparent when I was comparing my 883 to Swarovski Atx95 at indoors without any: Atx's combined bigger aperture and 86% light transmission showed dramatically brighter image and by that much more detail (but it most probably was better sample anyway, judging also by the star test). I had no extender with Atx so I had to use 70x mag. for this not so accurate comparison.

I do have 25m meter measure tape so I think I can get distances accurate quite easily.

At the moment I don't have available more than 96x magnification and my eyesight is 20/20 but I will still give a try within couple of days when weather will be fine and I have time.

Regards,

Juhani