[ausastronomer (John Bambury)]

Originally Posted by technofetishism

John, the telescope doesnt matter in this case at all.

It matters a whole lot.

For instance in a 14"/F10 SCT (FL 3.56 metres) you would choose either a 40mm ~ 70 deg Apparent Field of View (AFOV) eyepiece or a 55mm/56mm plossl with a ~50 deg AFOV eyepiece as your lowest power widefield eyepiece. These eyepieces give the maximum True Field of View in that scope. Similarly, those eyepieces would also be the best choice for low power widefield views in say a 100mm F8 refractor (FL 800mm) In those respective scopes, an eyepiece say for example a 40mm Pentax XW (70 deg AFOV) gives 90X magnification and a TFOV of 47'. The same eyepiece in the 100mm/F8 refractor will give 20X with a TFOV of 3.5 degrees. Worlds apart.

Then insert that same eyepiece in a 32"/F4 reflector (FL 3.25 metres) and its almost unusable because of the 10mm exit pupil it creates. With that 32"/F4 reflector your longest "practical" eyepiece focal length would be ~30mm. Lets take a 30mm Pentax XW (70 deg AFOV). In the 32"/F4 reflector that eyepiece gives 108X with a TFOV of 39'.

Let's go a step further and consider Jimmy Lowrey's 48"/F4 reflector (FL 4.9 metres). The longest focal length eyepiece he uses in that scope is a 31mm Nagler T5. In his scope that eyepiece gives 157X with a TFOV of a fraction over .5 degrees. Not your classical low power widefield view.

A combination that gives 20x will still give 20x in a 25" SDM or a 80mm ED.

They might both be giving 20X but there's a whole world of difference due to the significantly greater resolution of the larger aperture telescope. A 25" telescope at 60X is a whole world of difference to a 6" telescope at 60X.

Further there is no eyepiece available which will allow you to get anywhere near as low as 20X in a 25" telescope. Assuming a 25"/F5 you would need an eyepiece with a focal length of 160mm which would need a 4" plus diameter barrel, so that it did not vignette; and it would create a 32mm exit pupil which an owl would find difficult to use.

I suppose if the question is phrased in the manner of something like, what would be the ideal magnifications for OC, GC, Nebula, Planetary etc targets it might be more clear.

This depends enormously on the aperture of the telescope.

In a large telescope most of the bigger brighter globular clusters are easily resolved at low power. For instance in my 18" Obsession most of the Messier and Caldwell globulars are easily resolved at under 100X. However, you need a lot more magnification to resolve some of those same globulars in a 6" or 8" telescope and in many cases a smaller refractor will not resolve them, regardless of magnification. I like to observe globulars at about 120X in my 18" Obsession, using a 20mm Pentax XW with a paracorr, or my 22mm Nagler with a paracorr for 110X. In my 14" SDM I usually observe globulars using my 14mm Pentax XW with a paracorr for 130X or my 12mm Nagler for 153X. The effect can sometimes be similar with open clusters. The bigger the telescope the less magnification you need to make them look good because of the brighter image and greater resolution at any given magnification.

Cheers
John B