Which magnifications?

[Dom1961]

What is the best useable magnification for an 8" DOB on Planets, Moon, DSOs?

[ngwillym]

The highest mag should be about x50 per inch and maybe a bit more on nights of exceptional seeing. the best usable will very much depend on the seeing conditions.

[John]

With respect to the previous poster, I think 400x is going to be very, very rarely useful on any targets.

When I had a 200P dobsonian I found around 40x - 70x useful for deep sky objects, sometimes more on smaller ones such as planetary nebulae and small galaxies. Lunar and planetary viewing seemed to usefully start at around 130x - 200x for Jupiter, perhaps 150x - 220x for Saturn and Mars, maybe a little more if conditions were great. Lunar viewing and double stars sometimes benefited from 250x - 280x but that was about the max I found usefull.

It's not a precise art though and you must be guided by the conditions when viewing as what gives the most satisfying views so a range of options is good to have. Say 4-5 eyepieces that span 40x to 250x and that should keep you going for some time

[Peter Drew]

Assuming reasonable seeing conditions, magnification depends a lot on the object being observed. In general planets require higher magnifications than DSO's but this varies according to which planet. Mercury and Venus require the lower end, Jupiter middling, Saturn more and Mars the higher end. Large bright DSO's benefit from low powers and wide fields of view, small DSO's and globulars need higher powers and close double stars and often the Moon will stand maximum magnification. Throw into this mix seeing conditions, size, type and quality of the telescope used and you find that there is no straightforward answer. 

[ronin]

The maximum you are likely to get to use on occasions is 200x-250x, but that will not be every time you go outside and point at something.

Conveniently for a 200P if you get an eyepiece equal to the f number you get 200x. So likely to be a 5mm eyepiece or a 6mm eyepiece.

I suggest you limit yourself to that, ie 200x.

Now if the scope is f/6 you can easily try a 5mm eyepiece at no great cost, and that will deliver 240x.

Just do not expect it to bew useable often.

The other factor is there seems to be a sort of cut off at 5mm for eyepieces. At shorter you tend to need better (=more expensive).

Combine the 2 and really 200x to 240x is your maximum if and only if conditions allow.

The most useable is likely to be 150x to 180x. And I bet that 120x will actually be the most common magnification used.

In all honesty I would forget the 2x diameter "rule" just about everyone that tries for it gets disappointed as it doesn't work.

Have a look round and you will find that magnifications greater the 2x are now becoming quoted, but it is the same scope but 300x sounds better then 250x and that sounds better then 200x.

[Dom1961]

I'll probably get 150x as there is a 8mm BST, other than that it is a 5mm 240x

[Roysten]

Just to follow up on some of these responses - why is it that Venus/Mercury can take less magnification than Jupiter, which then takes less than Mars/Saturn?

[John]

Just to follow up on some of these responses - why is it that Venus/Mercury can take less magnification than Jupiter, which then takes less than Mars/Saturn?

Good question and I don't know the exact reasons but it's how I've found things work out over the years that I've been observing these objects with a range different equipment.

I found this posted by someone else though that seems to make some sense:

"Jupiter is seen best under mid-high magnification. It's rare that more than 200x is beneficial. This is because it's a very low contrast object, and additional magnification comes at the cost of less contrast, which makes things worse.

Saturn works best at high-ish magnification, bit more than Jupiter but maybe not much more. Around 200 ... 250x usually works. It depends on what you do - if you're trying to see the ring divisions, push it a bit higher.

Mars can use the highest magnification that you could generate, given the instrument and the conditions. It's a very small object, contrast is not bad, so crank it all the way up. Most instruments are limited by seeing when observing Mars."

My interpretation is that what we are trying to resolve on Jupiter are generally low contrast surface features wheras on Saturn it's a mix - the ring features are high contrast but the surface features are lower contrast than Jupiters. Venus is very high contrast as an overall target but any surface features comprise of very subtle contrast variations. Mars disk is often small and the surface features are quite high contrast, at least when there are not dust storms obscuring them !

[DRT]

When I had my 8" Dob I had a set of Celestron X-Cel eyepieces (5mm, 7mm, 9mm, 12mm, 18mm & 25mm) - the 5mm (x240) rarely came out the box, the 7mm (x171) came out a few times and the 9mm (x133) was my favourite for nice crisp views.

[damnut]

There is a common thread of around 150x, even when I had a 10" sct rarely dd I exceed that value.

[DRT]

There is a common thread of around 150x, even when I had a 10" sct rarely dd I exceed that value.

Indeed. The sweet spot of my 11" SCT on a clear night is with a 17mm, which is 164x. I can use the 13mm Ethos on some targets at 215x but those occasions are rare.

[Ags]

In my 102mm scope I too use 150x on Jupiter and 220x on Mars and Saturn.

[jonbarr87]

200 will most likely be a comfortable upper limit. The real problem with a dob is actually aiming/"tracking" at high magnifications (I find anyways). But I have to agree that the 70 - 150 range is the most common for me. 

[SpaceWalker]

I'll probably get 150x as there is a 8mm BST, other than that it is a 5mm 240x

I guess, it's a good idea. And if you also buy a 1.5x/2x Barlow with a removable nosepiece like this 2x GSO Barlow you'll be able to screw the nosepiece into the 8mm BST to get ~5.3mm (226x) when the atmosphere supports this magnification. That removable element at the bottom of the GSO Barlow works as ~1.5x.

[SpaceWalker]

What is the best useable magnification for an 8" DOB on Planets, Moon, DSOs?

For planets ~180-200x, for the Moon ~250-300x, for DSOs ~100x.

[Dom1961]

Thanks for your help everybody! I've been flicking through threads looking for the magnification but to no luck, but now I'll be able to use your advice to 'cherry pick' my eyepiece, thanks!

[ronin]

Half the truth is you put a wide eyepiece in to find the object, get it in the centre then go for a bit more magnification.

This allows you to keep it in the centre (hopefully) and have a peek to determine what it is like, you then go down again and repeat the get in centre, what's it like bit.

Eventually you go from say 8mm to 5mm or 6mm and either it works and "Yeah, thats good!" or you think "Yuk, looks awful."

There is also the aspect that you could magnify say a cluster and see the core bit only, but that loses the "cluster" effect. Andromeda is a prime (extreme) example, look at Andromeda with more then 20x and you do not see all of it and the "experience" is lost.

Whatever you do do not get hung up on magnification, it has it's place but it is not really that high up the list of importance.

It just takes time and a few eyepieces to realise that. :grin:

I would half guess that a manufacturer can get say "400x" from a 200P scope, but that is in an optical test laboratory, in a temperature controlled environment using some strange resolution chart and I suspect no eyepiece just a measure of the reproduced image by the primary. Not stood outside in N Lincs, in the wind, moisture in the air and looking through 5 layers of atmospheric turbulance.

[YKSE]

 look at Andromeda with more then 20x and you do not see all of it and the "experience" is lost.

If you try to convey your small scope experiences, it'll be much better thinking in term of exit pupils. 20x in your 70-90 scopes gives exit pupil of 3.5-4.5mm, but in a 8" (200mm), it's a huge 10mm exit pupil, i.e. loads of wasted light. 40x for 8" mentioned by John earlier is a much practical choice.

[ollypenrice]

I'd only use very high powers on a Dob if the EP had a very wide field. I like a long enough 'dwell' on a planet to be able to relax into seeing all that's there before a nudge.

Olly

[Dom1961]

One more thing, sorry, if I get an eyepiece next month, is it worth getting a planetary or a dso eyepiece? Because just looked on stellarium and it says the planets will be really close to the horizon. Therefore out of view?

[ronin]

If you try to convey your small scope experiences, it'll be much better thinking in term of exit pupils. 20x in your 70-90 scopes gives exit pupil of 3.5-4.5mm, but in a 8" (200mm), it's a huge 10mm exit pupil, i.e. loads of wasted light. 40x for 8" mentioned by John earlier is a much practical choice.

Maybe but to get it all in you need the field of view and that means low magnification and therefore the large exit pupil and some loss.

A classic case of you cannot have everything, and the 200mm should be collecting sufficent to make the loss of light through a large exit pupil and eye ball combination acceptable.

I would suggest the 8mm BST STarguider as it seems to be a good all round eyepiece, planets may be a bit rare but somethng like M57 will be appearing and the 8mm will suit that. There are other planetary nebulas - Dumbell, Owl and Blinking Nebula. Also I guess that quite a number of cluster will also fit in the 0.4 degree view.

Other suggestions will no doubt appear. :eek:  They always do. :grin:

[YKSE]

Maybe but to get it all in you need the field of view and that means low magnification and therefore the large exit pupil and some loss.

A classic case of you cannot have everything, and the 200mm should be collecting sufficent to make the loss of light through a large exit pupil and eye ball combination acceptable.

I don't think there's anyone who bought an 8" scope just want it to work as 5", by using longer focal length eyepiece(too low exit pupil) for stopping it down. The result will be that they got an unusable eyepiece because of your advice. How you want to  spend your own money is up to you, but giving beginners wrong information, thus spending their money freely,  is NOT appropriate, IMHO.

[Mr Spock]

With the 200 Dob I'd be looking at two eyepieces.

For looking at fuzzies I'd want a 13mm. x92 with a wide field would provide a nice dark background and a good image scale for galaxies. Lower magnifications take in a wider field for some clusters, but it also brightens the background. Unless you have really dark skies there isn't much to gain on nebulae and galaxies.

For planets I'd want a 6mm. x200 is an ideal x1 per mm for best image quality and is enough to reveal the finest detail without compromising contrast. Lower magnification will lose some detail and higher will lessen contrast. The moon and Mars could take x300 on a night of excellent seeing.

[michael.h.f.wilkinson]

When your exit pupil exceeds the dilated pupil of the eye, you do not only lose light, but the central obstruction becomes effectively larger. This does degrade the image, and can make viewing uncomfortable. Eye placement becomes critical, sometimes the secondary causes a clearly visible darker patch in the field of view. Wide-angle EPs are a much better solution (not cheap, but much better)

[Ags]

To be fair, to get 20x in a 200mm dob would require a 60mm EP - you don't see many of those around.