Highest Practical Power (potential)

[Bried]

Hello There,

I bought a skywatcher 150p around a month ago. I've been having a great time with it but decided it was time to buy some better eyepieces than the standard ones that came with it.

The supplied eyepieces were 10mm, 25mm and a x2 Barlow. This meant that given the focal length of the scope (750mm) I would be able to get x150 as my highest magnification given that 750/10=75 then 2*75=150.

On the spec for the skywatcher it states that the "highest practical power (potential)" is x300. I did a little maths and worked out that if I bought a 5mm eyepiece and used it in conjunction with a barlow I would be able to obtain x300. I've been observing Jupiter on a regular basis at x150 and although I've been really happy with the results the thought of looking at through better optics and at a higher magnification was too much to resist.

I popped down to my local astronomy shop and after having a chat with the guy was led to believe that the magnification I was hoping for would be too much for the scope, despite the manufacturers specification stating otherwise. I ended up buying a 5.2mm Ascension ED eyepiece for £55 anyway, as just the improved optics without higher magnification will be something to look forward to anyway.

I was wondering if anyone had any views on what I was advised and whether this means that the manufacturers specifications are a little on the optimistic side. After all with a 5.2mm eyepiece I should be able to get x288 with a barlow which is within the manufacturers specs. I've had my new eyepiece for around a week now and this cloud just isn't shifting so I've not been able to try it out.

Thanks for any advice in advance!

Brian

[tooth_dr]

I believe the atmosphere and 'seeing' has a lot to do with the maximum magnification. You're scope may well be designed to cope with up to 300x, but skies have to play ball too.

[barkis]

High quality eyepieces are without doubt, a prime requirement for any serious observer in my opinion, for what it's worth. Many a good instrument can fail to deliver using low grade EP's. However, however good a telescopes optics are, there is a limit to what is possible. A guide to power is usually a power of 40 times per inch of aperture. I've also heard of 50 times, but before I spent money on an expensive eyepiece to give that sort of magnification, I would beg to borrow such an eyepiece to test the water first.

UK skies are not the best on the planet, and the nights of pristine seeing are not going to be many in any given year.

High magnification, if too high, will degrade rather than show better detail, so beware before committing your money. You will no doubt get some conflicting opinions to mine though.

Ron.

[brantuk]

Rons spot on with his advice - and it's allways a good idea to try before you buy where ep's are concerned.

[John]

The manufacturers specs mean nothing in practice while local observing conditions, the object being viewed, the quality of the eyepiece, the state of collimation of the scope and the experience of the observer are the factors that actually do come into play.

What's important is using enough magnfication to get the best definition on a given object and this is usually way below the theoretical max.

[Bried]

Thanks for the advice guys. I've just been on metcheck and it looks like tonight will be a chance for me to try it out anyway. Even if I can't step up the magnification with a barlow I suppose I've still got my first quality (to my standard anyway) eyepiece to try out.

Cheers

Brian

[michael.h.f.wilkinson]

It also all depends on your own visual acuity. Abbe apparently had sharp eyes, so would tolerate LESS magnification, people with lower visual acuity could take more. On bright objects such as Mars, people often "transgress" the rule of twice the aperture in mm. I never do, because I have high visual acuity.

[Earl]

If your going to spend on eyepieces take a serious look at the Tele Vue plossl range.

Excellent quality and also hold there value.

[ollypenrice]

I second anything TeleVue. I just bought another one second hand. Perfect!

300x is a rule of thumb maximum for any telescope looking through the atmosphere, however large it is. Smaller scopes can't reach that because they lack the resolving power.

Our best instrument for taking high 'planetary' powers is probably a 5.5 inch apochromatic refractor. From space it would laugh at 300x but down here on earth (albeit at 900 metres) I have no eyepiece capable of exceeding 245x. I don't miss it. It needs to be a decent night, thermally stable, even for that. In the big Dob I never exceed 200x, partly because tracking becomes too bothersome.

So don't worry about 300x. To find out how manufacturers arrive at these numbers just Google 'The Dawes Limit.' A practical high power for UK seeing would probably be around 200x and this you would use regularly, I think.

Olly

[michael.h.f.wilkinson]

I get 250x out of my C8 only on the very best nights (but then the view is stunning), with my old 7mm LV I very rarely got descent views at 288x (250x with the Radian 8 is much better). 200x is more useful (so I must get that Radian 10mm).

[perrin6]

I use 300X (William Optics SPL 6mm) on Jupiter as I find it easier than the 9mm Nagler at 200X to see all the detail plus I don't get coma with the higher power view. I have to 'stop' the aperture down to 297mm (11 3/4 inch) to get sufficient contrast as my main mirror isnt of great quality. Perhaps my middle-aged eyes means I have to use higher magnification than a younger person.

Alan

[Moonshane]

all good advice above and from people with a lot more experience than me! as suggested if you were in space and looking with your scope then the theoretical maximum might be achievable but of course, we're not. I have found Jupiter to be best at give or take 150x, other targets will vary in their ability to take or even require higher magnifications and conditions definitely affect it. good seeing is not always when it's completely clear by the way. sometimes the best seeing conditions for higher magnifications exist when there's a fine haze in the sky. this makes galaxies and such hard to find but planets, double stars and the moon then take really good magnifications - in my 12" dob I have used my 6-3 Nagler zoom at the 3mm end giving 500x+ on the moon and the image was still ok one good night - this was rare but the moon will normally take a little more magnification than some other targets. galaxies are best at low magnifications and then slightly higher to see a little more detail, globular clusters and open clusters need medium magnifications.

also don't forget that you need a cool, well collimated scope for anything like high magnification and the lower the target the more atmosphere you are looking through. targets high in the sky (the Zenith) are more likely to show a good image and take better magnification.

hope this helps.

Shane

[Astro_Baby]

I'd suggest the dealer was being honest with you. The math for maximum theoretical power is often stated as x2 per mm of aperture giving a 150 the max of x300.

But I have also seen it stated in various places that you should only generally reckon on 60% of the theoretical maximum being available which would give you about x180.

Generally UK seing conditions dont permit much above x200 and quite often less than that.

With all that said I have pushed a Nexstar 4SE (by no means a mega scope) up to x260 (its theoretical max is x180) and its been ok for bright enough objects.

Reflectors also seem to suffer more with limits than refractors. Theres all sorts of theories why that is

In a nutshell the rules are what you find works well but I'd not have argued with the dealers advice as a general rule there. I dont think you were badly advised there.

[Ogri]

" I ended up buying a 5.2mm Ascension ED eyepiece "

I've got one of these, and it's nice, managed 230 x mag in my 200p dob ok (115 x mag in my ST120) on Jupiter, with comfortable eye relief.

I have had a couple of nights of 'minor struggles' with it, but that's been down to atrocious viewing conditions.

I hope yours is at least as good as mine.

You will get the occasional night where you can push the magnification a bit, but perhaps the best approach is indeed to have an EP such as that 5.2mm for more general use, and when viewing conditions allow the attempt, Barlow it and see what happens?

I did that with my ST120 one night on the Orion Nebula, in stunningly good conditions, and ran out of EP and Barlow to go high enough. So there is hope.

PS as a self-confessed DSO nut, I have to admit I am more concerned with the other end of the scale, and getting really nice views around 30 x magnification is my main priority.

[LukeSkywatcher]

Rons spot on with his advice - and it's allways a good idea to try before you buy where ep's are concerned.

But do you not also agree that even the best optics and EP's are also let down by atmospheric conditione etc.............as Tooth_dr said.

Yes the views will be far better with top quality optics under poor conditions ,but they still are let down by poor seeing etc.

There is no escaping it.

[Bried]

Thanks again for the advice everyone. I managed to get a bit of time out with my new eyepiece tonight and am pretty happy with the results. I tried using a x2 barlow with it just out of curiosity whilst looking at Jupiter. It made it a lot bigger but it was so fuzzy I quickly reverted back. After reading all the posts I was kinda ready for that anyway.

Thanks again

Brian

[Ags]

As a relatively inexperienced stargazer, I'm finding that 150x works on Jupiter quite well, but some night I can go higher - rarely on Jupiter and more often on the Moon. On those good nights I get up to around the manufacturer's suggested maximum (i go to 220x, with 240 max recommended by Celestron for my NexStar 4SE). Doubles can take the highest mags I find - it seems you can go as high as you like if you are splitting a double. I think my ideal magnification would be 180x-200x, but I don't have that EP yet.

[LukeSkywatcher]

As a relatively inexperienced stargazer, I'm finding that 150x works on Jupiter quite well, but some night I can go higher - rarely on Jupiter and more often on the Moon.

Same here. On a really good night i can push my scope to the max only on the moon (even with a 4mm EP) and get amazing views.

Even on the same night i find that Jupiter (or any planet) is better if i hold back on the magnification.

Understandable really....................the moon is HUGE compared to planets.

[brantuk]

"But do you not also agree that even the best optics and EP's are also let down by atmospheric conditions"

Absolutely Paul - I was just backing up the idea of trying before buying - there are many factors but generally better quality ep's are a good way forward I've found.

[ollypenrice]

Worth noting that where I live the best seeing comes in the wee hours when the temperature has stabilized. I imagine it is the same in most places. Wind also really screws up the seeing, presumably because of chaotic temperature variations at different altitudes. A little morning mist is no bad thing either for planetary high power, where seeing matters more than transparency. This s a well established thing as witness the successful Mr Peach in steamy Barbados!

On the value of good EPs, it does depend rather on your scope. A fast Newt will see a bigger benefit than a premium refractor. But I feel that good EPs make a big difference.

Olly

[Mr Spock]

My best seeing is early evening. Close to midnight seeing tends to fall apart. Because I'm on the side of a hill this is due to katabatic winds. I can follow it in the temperature curve. The evening cools down, but around 11pm starts to rise again, only starting to drop around 3am.

I get my best seeing on the nights where the hill top/valley bottom temperatures are similar.

In good seeing my 250mm newt is crisp at x300 and my OMC-140 can hold x333 due to its better optics. The moon, double stars and saturn are good at this, jupiter is better at x200.

I suspect a bog standard 150mm shouldn't really be pushed much above x200.

[brantuk]

My best seeing is on a Sat night - largely cos I'm too knackered from work and travel in the week (and Fri). Sun is early bed time cos of work the next day. So the weather does limit me quite severely. That's why I really look forward to star camps

[Capricorn]

I tend to assume that a magnification equal to the mirror diameter in mm is where the scope works best.

So a 5mm eyepiece would be that for your scope. You may occasionally get better but not 300x which means a 2.5mm eyepiece. A 4mm may occasionally be useful, I would say a 3mm is too small.

Whoever gave you the advice was being sensible and honest in my view.

Another "problem" is that the scope is f/5 and that is really seems to be the limit of getting decent optics, at least on the amateur side, I think the 150PL would often be a better option. Wider range of eyepieces for the higher magnifications and you wouldn't need to consider the higher end eyepieces, although they would work well on any scope.

[michael.h.f.wilkinson]

My best seeing is early evening. Close to midnight seeing tends to fall apart. Because I'm on the side of a hill this is due to katabatic winds. I can follow it in the temperature curve. The evening cools down, but around 11pm starts to rise again, only starting to drop around 3am.

I get my best seeing on the nights where the hill top/valley bottom temperatures are similar.

In good seeing my 250mm newt is crisp at x300 and my OMC-140 can hold x333 due to its better optics. The moon, double stars and saturn are good at this, jupiter is better at x200.

I suspect a bog standard 150mm shouldn't really be pushed much above x200.

Seeing here is often terrible early in the evening (bigger temperature differences), and calm down around 10-11, when most of the heat from roads etc., has dissipated. Depending on the weather, it may stay stable throughout the night.