breaking the rule of maximum usable magnification?

[deejayry]

I was wondering how much you need to stick to the rule of maximum usable magnification, the recomended maximum power for my 6" f/8 newtonian is 354x.

The more I look at the planets the more i am amazed by them, after trying out my new william optics Swan series eyepieces and being amzed by the quality, i thought of getting some of their SPL series eyepieces for planetary use, they make 3mm, 6mm & 12.5mm versions. The 3mm should give me a magnification of 400x. Will this be usable or would i just be wasting my money?

[JustAFriend]

Lots of people confuse the term 'rule' with 'guidelines'.

Use.Whatever.Works.

No one is going to throw you in jail for using any particular eyepiece.... and if it don't work you'll just grab something else that will.

[winensky]

You will get a larger image but no 'clearer'. The rule of maximum magnification should perhaps be interpereted as 'limit of resolution'. If you are getting a clear image with your smallest lens at present you may get a clear one by dropping to a slightly smaller diameter however:

1. You probably won't be able to push it much further and still get a satisfying image so is it worth the expense?

2. Anything below a 9 mm eyepiece in my experience is nearly impossible to comfortably view with though I have not tried a super wide angle 5mm.

3. I have yet to use a newtonian that I was able to push to maximum magnification as abberation is compounded with higher powers.

4. Atmospheric effects such as convection and turbulence become very obvious at high magnifications so again, there is no gain in image quality.

I now feel like a complete wet blanket but it would be a pity to spend big $ on a lens with which you would be dissapointed. If you belong to a club or get a chance to take your scope to a star party see if you can find someone with a small diametre lens you could try before you buy. Untill then clear skys.

Kind regards
Matt

[tdvance]

I recall reading an article saying that the "rules for max. magnificaiton" are too strict in some cases (clearly not for resolution, or else quantum mechanics is badly wrong, but for making an image your eyes can see things you can't at lower mags because of image size, contrast with sky in the case of stars, etc.).

On the other hand, it could be my eyes, but I find the max useful magnification is too high for me and my scope for nearly everything I look at. It helps to have a really good scope and really good mount (vibration is a real problem with high magnification if the mount isn't rock steady) and a Crayford 2-speed focuser (my el-cheapo rack-and-pinion just won't focus at too high a mag, though I made a minor modification to it to correct for unsteady hands--drilled hole in knob and inserted a metal rod to use "reverse leverage" and get small adjustments for big hand motions on the rod).

[Dave Mitsky]

Matt,

Eyepieces that are designed to have long eye relief (typically ~20mm) such as the Tele Vue Radians, Burgess Optical/TMB Planetary Eyepieces, and the Pentax XW series are certainly comfortable to observe with in very short focal lengths.

Dave Mitsky

[aurora]

Where I live, the maximum maginifaction is only useful on about 2 nights a year (when the seeing is exceptional).

Only you could decide if that was worth spending money on.

If you attend a star party, you may be able to find someone who would loan you the eyepiece in question and you could try it in your scope and compare the view to your other eyepieces.

[deejayry]

Right, well i guess there is no reall definitive answer, but all of your comments are welcome.
These WO SPL have an eye relief of 20mm which is reasonable, and i've got fairly young eyes, so I dont see that will be an issue.

My mount is fairly sturdy although there is quite a lot of leverage from my scope. the current R&P focuser is lousy, but I have a Moonlite crayford with the hi res stepper motor which is due to be arriving in the next couple of days, so my focus issues will soon be a thing of the past.

I do understand that the seeing conditions will rule when I can and cant use such high power, but it would be nice to take full advantage of the really still nights.

These eyepieces are near the bottom of my shopping list, there are plenty of more important things to buy, But if i find someone with the same ones i will ask to give them a try.

[RickJ]

It depends on a lot of factors:

When I was young I used really high power, up to 80x per inch, to cut through light pollution when looking at small but bright planetaries and galaxies of less than a few seconds of arc in size. While fuzzy the sky glow dropped far faster than the object making the invisible visible. Only a few objects qualified for this treatment.

As I've gotten older and my averted vision gotten less sharp higher powers do help me see detail in faint fuzzies with averted vision. Not like I did with young eyes but a lot better than I would today without it.

Still for planets and bright objects in dark skies I fine about 25x per inch shows me the most detail with the highest contrast. But then I can still separate about 1 minute of arc detail which is well above average. My wife needs close to 50x per inch to just barely see what I see at 25x per inch. Going higher actually lowers the contrast to my eye. It doesn't matter if I'm using a top apo refractor or a basic Newtonian other than the highest power is about 20% higher in the apo, likely due to the lack of the secondary as my 6" f/12 with 0.5" secondary uses top power like the apo.

So it depends on what you are looking at as far as I'm concerned and a bit on whether it is a scope with a central obstruction.

Another consideration is the atmosphere. A 6" scope can push the 50x limit far easier than a 10" due to the 10" seeing finer detail only when the atmosphere allows. So a 6" can work at 300x far more nights than the 10" can work at 500X. Thus I'd push a good small scope farther as to power per inch (or mm) than a good large one.

That supposes the mount is steady enough to take the power and still allow you to attain spot on focus.

Give me a few more minutes and I'll add some more conditions I haven't thought of until I hit the post button.

Rick

[redshifter]

I was wondering how much you need to stick to the rule of maximum usable magnification, the recomended maximum power for my 6" f/8 newtonian is 354x.

The more I look at the planets the more i am amazed by them, after trying out my new william optics Swan series eyepieces and being amzed by the quality, i thought of getting some of their SPL series eyepieces for planetary use, they make 3mm, 6mm & 12.5mm versions. The 3mm should give me a magnification of 400x. Will this be usable or would i just be wasting my money?


IMO you're wasting your $$. It's highly unlikely that you'll ever get seeing conditions that will allow you to use 400X with a 6" scope. In fact, I'd venture that the overwhelming majority of nights, you'll be lucky if the seeing permits anything more than 150-200X.

[winensky]

I stand corrected re the eyepieces Dave though I still believe the other bang for buck issues are valid.

Kind regards
Matt