what lens should i buy

[dwooll]

hi guys,

I am just starting out and could do with some advise.

I have just bought a SKY WATCHER SKYLINER 200P 200MM (8") DOBSONIAN TELESCOPE it came with the standard 2 lenses 10mm and 25mm, I have just ordered a x2 barlow lens and I would like to know what would be the best size lens to buy to accompany this,to give me the best possible views of space.

the two that came with the scope are great for looking at the moon but everything else too far away to make out..

any help appreciated..

[ronin]

Little lost by the comment "but everything else too far away to make out".

Optically the moon and the rest are all at infinity.

If you mean too small then what are you looking at?

Stars will always be points.

Now the choice f/5 or f/5 200P. Will assume that the budget is not for TV Ethos and Delos.

With a 2x barlow then try a 12mm BST Skyguider, that gives 12mm and 6mm.

Alternatively the 15mm to give 15mm and 7.5mm.

Part of the choice is the f number of the scope, the main bit being what do you want from the eyepiece and what do you observe.

Bet you never thought it could be this complicated did you? :grin:

Slightly more costly are the Celestron X-Cels at £65, lower cost are Vixen plossls at £35 (this may well depend on the f number and if you wear glasses), finally the BST Wide Angles (actually narrower then the Skyguiders ) at £43.

P.S. Which barlow are you getting?

[Stargazer 2013]

P.S. Which barlow are you getting?

Peter Barlow

[dwooll]

it does all seem a little confusing but I'm sure I will get the hang of it.

what I want to look at are planets like mars and Jupiter.

the barlow lens I have ordered is a

31,7mm (1.25") Achromatic Super Barlow Lens Eyepiece 2x.

[Qualia]

what I want to look at are planets like mars and Jupiter.

When it comes to planetary viewing, Dwool I've noticed that slight changes in the focal length of eyepieces makes a difference to what can be tweaked from the given object. In terms of general useful magnification I have a small run of orthos and plossls which offer:

f/10: 200x, 166x, 142x, 125x, 111x, 100x, 90x, 66x.

f/5: 250x, 208x, 178x, 156x, 138x, 125x, 113x, 83x

For Lunar work, I feel all magnifications offer up something to be amazed by. Low powers help frame the Moon, the mid-range frames craters and seas and mountain ranges and the high-powers helps tweak out subtle features like terraces, tiny craters, ray grazes etc.

For Jupiter, I typically use mags between 140x to just over 200x. I have gone to 250x on a number of occassions but generally find I'm working between 180x to 200x in the 10" and between 140x to 160x in the 4".

For Saturn - Again I have found 140x to over 200x useful mags to play around with. You can go higher here, but a lot will depend on the night's atmospheric conditions.

For Mars, I figure Jupiter is about 3x the size of Mars and to see Jupiter nicely is, say, at around 200x which means that for me to get a 'similar' view of Mars I need 600x. Well, I'm just not going to get that kind of viewing quality. So, I figure a compromise is needed, say, around 250x which still means seeing conditions will have to be very good. As such, I haven't found Mars a particularly giving planet..

For Solar work in white light I find either 40x or 50x are the nice working magnifications which can be upped to around 100x, or a little more for more detailed work when skies allow.

Other than that, in general, try to view the given object as close to the zenith as possible and bear in mind that as a general rule of thumb the brightness of an object will decline as you up the magnification. If I up the mag twofold, say, I'm reducing the image brightness by a factor of four. If I keep on doing this eventually details just disappear.

On the other hand, increasing the mag does make detail more apparent, so, as you can appreciate, we're now at a trade-off: will increasing magnification gain more detail even though I'm making the object fainter?

I've found that playing around with this trade-off - dependent on the evening's seeing (LP doesn't really affect planets, Moon or the Sun) - does make a difference. Even as little as 1mm increase or decrease in the mag - about 10% to 15% difference of magnification - can be quite surprising which is the main reason why most planetary observers will have quite a run of high-mag EPs.

For planetary work, I either use Orthos and in particular those that were made by Baader - the Baader G.Os which are no longer in production, or Tele Vue plossls which are also amazing planetary eyepieces. The former do crop up from time to time on the secondhand market (often overpriced). Nevertheless, John (a senior member here at SGL) has made some excellent reviews of the Hutech and Baader Classic range which appear to be of very similar quality and sharpness.

The B.G.Os and TV Plossls - and I imagine it to be a similar story for the Hutechs - will give you a quality of image just about as good as it gets. If you want a similar quality image EP but with a wider field of view and more comforting eye-relief, you're going to have to spend more, a fair bit more.

Final thought, whenever possible try to sit with your given planetary object for a peaceful twenty to sixty minutes or so (at least) and you'll find that they'll be moments of great clarity and seeing. By practicing attentive sitting you come to notice more and more detail from the given object.

Needless to say, apart from the Moon and the Sun, the actual size of the objects in question are not going to be much bigger than a large pea in the palm of my hand, the Jovian moons no bigger than a . but with sufficient time, questioning and observing you will be able to see more detail than you can ever hope to record in memory or sketch.

If you haven't already, you might also find this post of some help.

Hope this has cleared the noted and understandable confusion a little and please don't hesitate in asking more questions if the need or desire should arise

[dwooll]

thank you very much for that.

[Reeny]

This is tricky.

Ask 10 people, and you will get 10 different answers.

I suggest you take a deep breath and wait for the Barlow to arrive.

If it is a good-un, it will be OK with the 10mm eyepiece.

The 10mm eyepiece is OK, but it isn't the best - so if the barlow works well on this one, it will work on anything you throw at it.

The 8" dob has a focal length of 1200mm

Divide 1200 by the eyepice size - eg 1200mm/10mm = 120x magnification

A 6mm or 5mm eyepiece is normally the maximum you can use due to atmospheric conditions.

If the Barlowed 10mm is good at that magnification (240x) - try to get a decent 5mm eyepiece to get the best out of the scope.

BST's are £49 each, and have 60-degrees wide angle view.

http://stores.ebay.co.uk/Skys-the-Limit-Astro-and-Optical/BST-Starguider-ED-/_i.html?_fsub=2568750014&_sid=53377064&_trksid=p4634.c0.m322

If you like the BST range - you can decide if you want the full set of 6 BST's / or just need to buy another 2 or 3 EP's to use with the Barlow (eg 25 / 18 / 12)

25mm = 48x / 12mm = 100x / 18 barlowed = 133x /  12 barlowed = 200x / plus the 5mm = 240x

You will get better views with out the Barlow, but you will need more eyepieces.

If you want more than the BST's can offer - go for Maxvision 68 degrees 24mm / 16mm +barlow / and an 11mm Explore Scientific 82 degree with barlow.

http://www.explorescientific.de/eyepieces-c-8964.html?page=2&sort=1a

That is the route I took - so I have never tried the chance to try the BST's (=£150 half set / £300 full set).

I have the Revelation 3-element Barlow (£32)

And MV 24mm=50x / ES 11mm=109x / 2.1x 16mm=158x / 2.1x 11mm=230x

So your budget will play a very big part in your final decision.

BST half set = £150 - £200

BST full set = £300

Maxvision 24 and 16mm = £130 (plus the 11mm ES at £120 EU / or £90 USA) = £220-£250

[Reeny]

I think the 12mm BST might be a better starter than the 5mm BST

You already have the 10mm to barlow down to 5mm.

The 12mm will give you 100x and 200x magnifications.

And introduce you to some of the best views the scope can offer.

Seek out the Ring Nebula and Dumbell Nebula at 100x with a quality eyepiece - amazing.

[Tonys]

I'd just like to echo what Qualia says about having patience when observing the planets. Those brief moments when the seeing improves dramatically can be unforgettable.

[acey]

Take some time to get used to the equipment you've just bought before rushing to buy more. The 25mm and 10mm EPs are fine, and the Barlow will give you an added luxury of 2 more magnifications if you want them. You'll probably find that the 10mm plus barlow is too high to be much use. The 25mm plus barlow should give you a nice medium power view.

Planets will always look small, because of the way our visual system works. We tend to see a planet in a telescope as a little thing that's close up, rather than a big thing that's far away. Hence people talk about Jupiter being "pea size" etc. Deal with the psychology before spending money.

Planets often look fuzzy because of atmospheric turbulence. A high-quality high-power eyepiece is only as good as the air you're looking through. Start saving up for one (e.g. TeleVue 8mm plossl) but in the meantime enjoy what you've got. When you've mastered the knack of seeing a fair amount of planetary detail with the 10mm EP, that's the time to think of upgrading in order to see even more.

For DSO viewing it's not eyepieces that count, it's the darkness of the sky. Check out any possibilities for getting your self and your scope to a place where you'll be able to see the Milky Way naked eye. Then your 25mm and 10mm EPs will show hundreds of DSOs.

Buying gear is a hobby in itself. But try astronomy first, which to my mind is a far more interesting hobby.

[Charic]

dwooll.........Hi,       My first upgrade was the 8mm BST from skies_unlimited, a brilliant lens in-place of the SW 10mm. Then I bought the 18mm and then a 25mm second hand. My Barlow arrived yesterday,  a Revelation Astro 2.5x,  so as soon as I get the cloudless night, Ill be checking to see if I need the 5mm or just the 12mm to complete my collection, Then possibly one more lens, either 32/42mm for wide views at my special dark sites, maybe in 2" format, although I wont be Barlowing those (yet ?) I would suggest if you stick with the BST's, you can buy both the 8mm & 12mm from Alan at skies, and try them side by side. You`ll like both of them for sure. If you decide you dont want either one of them, then Alan refunds. Its a great service. I want a 5mm. But if my 8mm Barlowed looks good enough, then I`ll probably just get the 12mm in my collection. The 5mm at 2.5xBarlow would give me 600X magnification, how excessive is that! so I could  only use it at 5mm to be practical? The 8mm and/or 12mm will give  better practical results for you. 150x-300x from the 8mm and 100x-200x from the 12mm. Much more practical. These scopes are good, but your own health & eyes, Urban polluted skies and UK visibility, brings theoretical  use down to practical  use only. Your SW 25mm is good to go, Barlow that,  like using a 12.5mm  only with better eye relief, but wont be as bright as the BST.  So a BST 8mm & 12mm and your SW 10/20mm lenses would give you, theoretically , a 4mm,5mm, 6mm and 12.5mm from the 8,10,12 & 25mm lenses, a total of 8 lenses. Please remember though, a Barlow does not turn an 8mm lens into a 4mm lens. The barlow  changes the focal length of the telescope. Its the result of the new higher focal length, divided by the eyepiece focal length, that provides the magnification. The 8mm is still an 8mm lens( I do like to keep things simple????)

EDIT* I can clearly see the cloud belts on Jupiter from my light polluted garden using a straight 8mm BST. Hopefully  Barlowed will provide a slightly closer / larger image in the viewfinder.

[Reeny]

EDIT* I can clearly see the cloud belts on Jupiter from my light polluted garden using a straight 8mm BST. Hopefully  Barlowed will provide a slightly closer / larger image in the viewfinder.

I have tried repeatedly to get the 9mm Revelation Plossl to Barlow down to a calculated 280x magnification - and failed 99% of the time.

It kinda worked on the moon - and when it did the craters were massive, and a little blurry.

We are still waiting for our 11mm Explore Scientific to arrive from the USA = 109x (or barlowed to 230x)

At the moment the weather is making hard work of the 12mm (at 210x)

So I was wondering if 230x was being a touch too ambitious.

The limit for my scope, at our location, with the turbulent weather, appears to be around 200x at the moment.

Charic - the Barlowed 8mm at 2.1x will be a massive 315x magnification.

Hopefully you will get a chance to try it out soon.

We should be due some high pressure to take all of this moisture and cloud out of the air.

[Charic]

Reeny.........Hi, we spoke  via  PM on this before! How are you measuring  2.1X  from the Astro, how do I check?

[Reeny]

It was mentioned on here by an American member who had taken the trouble to test the Barlow.

I don't know the full details.

But when he said it was 2.1x, it made sense of the all the trouble I was having to match up different combinations.

If your new Barlow is a true 2.5x, the 8mm will be magnified up to 375x

[acey]

The factor by which a Barlow multiplies magnification just depends on the distance between the Barlow lens at the bottom and the eyepiece inserted at the top. Try sliding the eyepiece in the Barlow and you'll see that the magnification changes (you need to refocus with each new position). The further out you slide the eyepiece, the higher the magnification. If you double the length of a x2 Barlow it becomes x3.