*Smallest* possible magnification

[virtualpilot45]

I've heard about people trying to achieve too much magnification and getting a fuzzy dim blob, but is there a limit to how little magnification can be used? Surely a really long eyepiece would give small, but very crisp and bright images. Is there a limit to how long the eyepiece can be?

[Ags]

Yes - low magnification can make the exit pupil too large causing the image to get fainter. In scopes with central obstructions low mags start to show the shadow of the CO.

[brantuk]

Most scopes come with a stated maximum magnification. In reality though the maximum is governed more by the limits of the local atmosphere, which in the UK is around 250x for most scopes. Even though the scope may be able to go up to 600x. You'd need an extremely clear sky like that found in a cold desert to get to 500x or 600x.

Edit : think I read this question the wrong way round - ignore me if I did lol

[virtualpilot45]

Most scopes come with a stated maximum magnification. In reality though the maximum is governed more by the limits of the local atmosphere, which in the UK is around 250x for most scopes. Even though the scope may be able to go up to 600x. You'd need an extremely clear sky like that found in a cold desert to get to 500x or 600x.

Edit : think I read this question the wrong way round - ignore me if I did lol

You did, but I still learnt something I didn't know before!

[virtualpilot45]

Yes - low magnification can make the exit pupil too large causing the image to get fainter. In scopes with central obstructions low mags start to show the shadow of the CO.

After looking at your answer I checked my scope's minimum mag. EP (25mm) and found it had an exit pupil of 5mm, which means it is probably close to my eyes' pupil sizes, therefore near the best my eyes could get to. Thanks!

[CGolder]

it will depend on the telescope you have, the smaller the focal length and objective the lower min magnifcation you can have without losing light due to exit pupil.

min useful magnication is another thing, i mean my finders 8x mag is very useful

[barkis]

If I remember, the full moon would be 20mm diameter at the prime focus of a 2000mm focal length mirror. But that is on film.

It's a different ball game using CCD sensors.

Ron.

[Merlin66]

The minimum useful magnification is one which gives you an exit pupil of 7mm ( for young eyes)

Exit pupil is the objective diameter/ magnification so, for 7mm with say a 200mm scope this would be 200/7 = x28 which for an f5 (200 x5 = 1000mm fl) would need a 1000/28 = 35mm eyepiece.

[PortableAstronomer]

I guess if your scope objective or mirror has 1000mm focal length, and you could make 1000mm 'eyepiece' in the hypothetical attempt to get a x1 magnification, then the exit pupil would be as wide as the scope itself. Even if the 1000mm 'eyepiece' were only 50mm/2" wide, then most of the light would just bypass the 'eyepiece' and be wasted.

I don't think you can use a telescope for brightening dim objects without having magnification. I read somewhere that it would break a rule of thermodynamics.

If you look down the reverse end of a refracting scope, with an eyepiece at the other end, then you have <1x magnification. You will also notice that it won't make the image brighter either, because the light has been spread out across the width of the objective lens and a tiny fraction enters your pupil.

For photography, I would think the minimum magnification limit would be caused by the limitation of 2" or 3" eyepiece barrels (which the camera would be mounted on) would not be wide enough to allow the full light path to flow through as such low magnifications; you would also see the internal view of the sides of the telescope around the edges of the frame, and of course with a central obstruction you would start to see that in the image at extreme cases.

[Merlin]

As a general rule the minimum magnification shouldn't be more than about four times the diameter (4D) of the objective or mirror in inches.

For an 8-inch mirror this would be 8X4= X32.

[Telrad]

What an informative thread.

I too have learned a lot from this. Thanks Kim and Ron.

[Staypuft]

Hear Hear!