2/3rds rule!!!

[Andy78]

Hi all... Does this max mag rule apply to refracters aswell as reflectors? And if so... what refractor would equal an 8" dob?

[ronin]

Forget rules.

Just find out what the scope can deliver that you find acceptable.

That then is the maximum magnification for the scope and you.

[luminova11]

I don't know how much truth there is in it, but an experienced astronomer friend, who also happened to be a telescope designer, once told me that a refractor was roughly equivalent to a reflector 2" greater in diameter, so in this case 6". So you could probably push the magnification to the same levels as on the reflector.

[Scott]

There are so many more things that determine magnification than aperture. seeing, eyepiece quality, and an important one is experience. an experienced astronomer is likely to see more detail than an inexperienced one. yes it's important, but so are the other factors

[acey]

What's the two-thirds rule? I know of two "maximum magnification" rules: one is 50 times aperture in inches, the other is 2 times aperture in mm. Both of these are saying the same thing, which is that the exit pupil should be no smaller than 1/50th of an inch or 0.5mm. Some people's eyes can cope with smaller exit pupils, many can't.

Instead of aperture one should really speak about "entrance pupil", which is the actual light-gathering area. A reflector has a central obstruction (the secondary mirror) so its entrance pupil is slightly less than the full diameter of the primary. If the primary has diameter P and the central obstruction has diameter S (and we ignore the area obstructed by the spider, mirror clips, intrusion of focuser into light path....) then the actual light-gathering area is pi(P^2-S^2)/4 so the entrance pupil diameter is sqrt(P^2-S^2). A 200mm scope with a 50mm secondary would have entrance pupil 194mm.

In comparing reflectors and refractors there are lots of other considerations apart from light-gathering area. Until perhaps about the mid twentieth century it was common to say that a 6" reflector was equivalent to a 4" refractor. Mirror coatings have improved since then, but it remains true that Newtonians always have diffraction issues because of the central obstruction, and this affects contrast.

However, as far as magnification goes, it's the eye and the atmosphere that are the determining factors, assuming the optics are good. Hence the rule of thumb about minimum exit-pupil size, whichever type of telescope you use.

[cotterless45]

I think the two thirds rule is the maximum that you should pay for pre loved gear. I usually go 50% then add 10-15%.

As far as magnification goes, it's very overrated. You should aim for the minimum that you can get away with. Most high mag gear is deigned to work in dry still deserts. Here in the UK we get the opposite,

Nick.

[acey]

Maybe the 2/3 rule is how much time people spend collimating, cleaning or modding their scope compared with actually using it.

[AstroJOE]

I thought the 2/3 rule was the stage you reach when setting up your gear before the clouds roll in!?!

[michael.h.f.wilkinson]

Apart from many factors mentioned, the maximum useful magnification (that magnification beyond which no more detail is spotted) depends on the target. Mars and the Moon can take a lot under good conditions, Jupiter less (although I have pushed my scope to 290x even on Jupiter). On planets I (and many others) like to have fairly closely spaced magnifications available to tune the magnification to the object and conditions. Even on DSOs I find this helps, hence my fairly closely spaced set of wider-field/mid-magnification EPs. I have never profitably gone much beyond 400x on my 8" scope. Mars, on one or two magically stable nights did benefit.

[timwetherell]

Back when I was a kid it was the 1/2 rule (refractor equal to reflector of double aperture) I guess that's inflation for you

Refractors do have a few things in their favour but at the end of the day aperture is aperture. It might be true to say that often a refractor will thermalise to optimal performance faster than a reflector and be less affected by things like the observer's body heat so may give the impression of better performance particularly in short observing sessions. But you have to have pretty deep pockets to get any sort of serious aperture in one!

[michael.h.f.wilkinson]

Very true. I looked through Olly's 20" Dob (AWESOME views of M101 and M51, amongst many others), and I could aspire to such a beast one day. The only 20" frac I have seen was one by APM going for 800,000+ euro (OTA only, mount adds another 500,000). Not a realistic option, unless you already own a small, oil-rich country, or similar assets. Even a "mere" 12" frac is beyod what most of us could afford (and if any SGL member can afford it: can we be friends? ).

[Andy78]

Thanks for all yr comments. I read warthogs guide to eyepieces on here and thought id ask the question to see what others thought. I guess its still trail and error to see what works for the individual.