Hi and Resolving power of an objective

[Debasisswan]

Hi,

I am a new comer. Thanks for taking me in.

I need your help to clarify my doubt on one issue. Please forgive me in case this is not the right place for this question or in case the topic has already bee discussed here.

As a rule of thumb, we know that resolving strength of an objective is approximately 50x per inch of Objective Diameter.

So, for a 8" Objective- resolving strength should be 8 x 50 = 400x

Now, if I use a 3" musked filter like Solar Filter (The off axis kind ones normally used in Cataoptrics or Reflectors), the effective dia of the objective becomes 3" though the objective material remains the same.

Will it alter the resolving strength of the system? I mean, whether the resolving strength will remain 400X or it will come down to 3 x 50 = 150x?

I am not considering the change in focal ratio aught to come due to masking of the objective.

Thanks in advance-

Debasisswan

[FraserClarke]

Yes, it would be 150x because it scales with the effective aperture.

The 50x per inch rule of thumb comes from the magnification it takes to make the theoretical best resolution of the telescope (which is comes from the effective diameter of the object) just resolvable by a normal human eye. Any more magnification than that will not show you anymore detail, because you're just seeing the intrinsic 'blur' of the telescope.

So in your case, because the telescope diameter is now 3/8x smaller, the magnification at which you resolve the telescope's intrinsic blur is also 3/8x smaller (i.e. 150x instead of 400x). So there wouldn't be any point using more than 150x in this case.

[NGC 1502]

Hi Debasisswan, you are very welcome here.

The 50x per inch rule ( to give 400x for an 8" telescope ) is theoretical, an excellent

telescope used under ideal conditions. There are times when that occurs, but it's not very often.

Much of the time the atmosphere is too turbulent (called poor seeing) and most

optical systems, even if very good, are not perfect.

When I had an old but very good 8.5" reflector, I rarely used more than 200x.

And of course lower powers are often needed to give a larger field of view to fit all

of the object in.

With my 10" telescope I use from 44x to 150x, sometimes 200x for planets and double

stars on a good night.

Hope that helps, Ed.

[Capricorn]

The 50x per inch rule is as said theoretical and as much to do with marketing as anything.

If a 4 inch achro could give the same as a 4 inch triplet apo why do we buy 4 inch triplet apo's at a much greater cost?

Some years back the maximum was reckoned to be around the 37x per inch ( 1.5x dia in mm).

I also recall reading on a US site and it had the reason why that the best is achieved at a magnification equal to the dia of the objective in mm (or 25x per inch).

I would suggest that you do not get drawn in by tales of huge possible magnification. You will get disappointed ultimately.

P.S. The resolving power is not the magnification. They are different. Magnification is a multiplier, resolving power is measured in arc seconds.

[Debasisswan]

Thanks to you all. Now it is clear to me that the resolving strength of any objective is directly proportionate to its effective aperture, not the actual aperture.

If I have understood you correctly-

So, suppose a good 8" scope (Actual Dia is 8") gives actual resolving strength of 200X against its theoretically possible strength of 400X.

But if fitted with a 3" Solar filter, its effective Dia becomes 3" only. So, now it will give 200/8 x 3 = 75x and it will have its theoretically possible resolving strength of 400/8 x 3 = 150x.

Thanks a lot-

Regards-

Debasisswan