Why is 35x per inch of aperture the magic number for planetary observing?

[Saturninus]

I often hear the optimal power for viewing certain objects as expressed in magnification per inch of aperture. For instance, I hear that Jupiter is best observed at 35x per inch of aperture. I guess this equates to an exit pupil of 0.7

Is this advice true no matter what size scope you use? For instance, in a 6 inch refractor, that is 210x. In a 4 inch refractor, that is 140x. I would have thought that as long as seeing conditions permit, something like Jupiter would look best at 200x no matter if it is a 6 inch or 4 inch scope.

So if I have a 4 inch scope and I want to do planetary viewing, and seeing conditions generally permit 200x on  a regular basis, should I be clustering my EP collection around 200x magnification, or would I get better views at 35x per inch of aperture (140x total magnification) even though the image would be smaller?

[oldpink]

max mag is a bit of a grey area
Jupiter doesn't take well to high mag while mars does

also your viewing and transparency greatly effect the max mag you can use
my 200 can in theory go to x400 but in practice 200 - 300 is more in line with what can be achieved in the Uk

[Saturninus]

Yes I experienced this last night. I was looking at jupiter, and every time I tried to go past 250x it got sloppy. Then after a short nap i woke up again and Saturn...maybe the atmosphere was much better at 2am, maybe it was much higher in the sky, or maybe I was just seeing things, but I was getting up beyond 400x and it seemed like Saturn was getting better and better!

[Kirscovitch]

I haven't been doing as much visual work since I started dabbing in imaging but most nights with my 130SLT, 144x was as high as I wanted to go. On a good night of seeing I could push it to 216 but that was a rare occasion. Jupiter can be very finicky. Mars and Saturn tend to take higher mag better.

[ChrisLX200]

Increasing magnification (or rather, image scale) can have a negative effect because it spreads the same amount of light over a larger (apparent) area. So colours become weaker, contrast is reduced, and by the same token it appears dimmer. Backing off the magnification reverses these effects and often provides a more pleasing result on planetary disks (for example). There is no fixed limit to this because it depends on the brightness of the target - this ignores the limits of resolution which is a matter of physics. However, sometimes you can use high magnification to good effect even on faint subjects - galaxies and nebulae - by isolating the target and darkening the background. It's best to try a variety of eyepieces - which is why we have so many, right?

ChrisH

[ronin]

So far I have read 25x, 50x and now 35x.

There is no real magic number.

It all depends on the scope, the eyepiece, the atmosphere and the target, and very likely a few other bits, one of which is luck as in does that eyepiece and objective just happen to work together.

[orley]

It all depends on the scope, the eyepiece, the atmosphere and the target, and very likely a few other bits, one of which is luck as in does that eyepiece and objective just happen to work together.

Oh so true.

We're all built differently, and all have subjective preferences.  Experiment over several nights and see what works best. That's part of the fun

[John]

I think the seeing conditions are the key thing for planetary observing. Everything else is subservient to this in my opinion. Under poor conditions even 100x or 120x might be pushing things too far in, say, a 5 inch scope. Under, rare in the UK, perfect conditions, the same scope could be delivering wonderfully sharp and contrasty images at 250x or more.

We do spend a lot of time discussing optimising the scope, collimation, eyepieces and even observer technique because these factors are within our control whereas the seeing conditions are not !

[Stu]

I think x35/" sounds a little low. With my 4" apo in good conditions, x180 is often the optimum which is x45. x200 is normally too high in this scope.

I've had less time with my 6 and 12" scopes but reckon they could be pushed higher on Jupiter.

Other targets take mag better such as Saturn and Mars, I would use x230 to x250 in the 4" on these.

I would tend to avoid rules and go with your own experience as we all prefer different things, and observe under different conditions. Higher magnification depends upon seeing conditions, collimation (less relevant with a frac, though still important), cooling and quality of optics.

Buying used eyepieces should allow you to experiment without loosing much value.

Cheers,

Stu

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