Object Size in FOV

[Globular3 (Jeff)]

Hello,

I'm thinking about getting some new astro equipment and was wondering if anyone can help with these questions.

Q1) Does the angular size of an object in the field of view (FOV) of the eyepiece appear the same at equal magnifications BUT with different objective lens sizes of telescopes? For example, would Saturn (or any astronomical object) appear the same size at say 140x in a 70mm telescope and 140x in a 100mm telescope?

Q2) Does the angular size of an object in the FOV depend on what eyepiece is used? For example, if I used say a Televue Delos 8mm eyepiece to get the 140x with the 70mm telescope, would Saturn look bigger than it would in a Plossl 8mm eyepiece at 140x in the same 70mm telescope?

Thanks for any enlightenment anyone could give me here. As astro equipment can be potentially very expensive, I'd like to know what difference (if any) there is so I can make an informed choice.

[casstony]

Hi Jeff, the magnification or size of an object only depends on the focal length of the telescope and the focal length of the eyepiece.
A bigger objective diameter can give greater resolution, brightness and contrast, even though the planet is the same size; a wider afov eyepiece will show more black space around the same sized planet.

[Globular3 (Jeff)]

Thanks, this does make sense.

All the best.

[pluck]

Originally Posted by casstony

Hi Jeff, the magnification or size of an object only depends on the focal length of the telescope and the focal length of the eyepiece.
A bigger objective diameter can give greater resolution, brightness and contrast, even though the planet is the same size; a wider afov eyepiece will show more black space around the same sized planet.

With respect to magnification versus resolution, an 8" f6 telescope has exactly the same native magnification as a 6" f8 telescope (ie, they have the same focal length of 48 inches). To borrow from your example, Saturn will be the 'same size' using the same eyepiece in each of these telescopes.

However, the 8" will be able to resolve more detail due to its greater 'light gathering' capability, which in turn usually means that you can use higher magnification eyepieces.

It's the reason why there is so much consternation when it comes to cheap and small telescopes that advertise ridiculous "magnifying power". It's the 'resolution' that actually counts, and this is mostly a function of aperture for a given telescope design.

Paul

[Merlin66 (Ken)]

You can use the freeware CCDCalc to get a good representation of what the FOV will show....