Best way to achieve 250x with 120mm f8.3 refractor??

[Wurzil]

:icon_salut:Hi there everyone!

Just after a piece of advice about eyepieces/barlows etc.

Ive got a Celestron OMNI XLT 120 refractor (1000mm focal length 120mm apeture). I want to be able to try up to 250x with it at some point (maximum useful mag is stated as 283x).

I currently have the 25mm eyepiece that came with the scope and a 10mm Skywatcher super plossl. I WAS planning on buying a 6mm EP (which will give me 167x) and a 2xBarlow lens - but Ive just realised that the barlow+6mm will not be a usable combination is it goes over the 283x useful mag for the scope - so the best mag I will be able to achieve with that set up will be 200x (10mm+barlow).

So, my question is - should I forget about the 6mm EP and get the 2xbarlow and an 8mm eyepiece instead (so I will be able to get 40x,80x,100x,200x,125xand250x) - or should I get the 6mm and barlow like I was going to in the first place and just buy an additional 4mm EP later on when I can afford it?

Ill be looking at reasonably priced EPs by the way (£30-40max).

Thanks guys

Warren

[pvaz]

You need to buy a 4mm EP or a 8mm and barlow it. I'd go for the 4mm, my experience with a barlow wasn't that good.

[Wurzil]

Maybe I should get the 6mm (167x) and the 4mm(250x) and just forget about the barlow altogether - Id not get a 200x mag that way though.

hmmmm...Methinks then, that I will have to buy 3 more EPs instead of 2 to get the range of mags I want - and a moon filter - and then later buy a medium-low power EP - oh and an LP filter.....stupid blumming stupid expensive blumming hobby.

The high powers are for mars and saturn by the way.

W

[great_bear]

Your scope will start to lose detail if you go much over 140x - the more you magnify past this figure, the less detail you'll see in the view.

An eyepiece of 7mm will resolve all that your scope is capable of resolving - so you'll want something of that length as your "maximum clean magnification" eyepiece, and a 9mm backs off the magnification a bit for when seeing is poor.

"Circle-T Orthos" eyepieces (£25 second hand and known in the USA as the famous "University Orthos") produce fantastic planetary views beaten only (marginally) by Baader Genuine Orthos, and rare, expensive collector's eyepieces.

So in your scope you'd be hard-pushed to get better planetary views than with 7mm and 9mm Circle-T Ortho eyepieces.

[pvaz]

Everyone says you shouldn't push the max mag of your scope, and I mostly agree with it. But the truth is, on those nights with good seeing conditions, nothing beats a high mag and a sharp and large Jupiter in your EP.

I just aquired a 5mm and in less then 2 weeks I had 2 chances to use it. The 1st time I sow 3 bands on Jupiter, a 4th faint one that seamed to come and go, and my 1st festoon. A couple of days later, at 6am, I soo the polar cap in Mars (clear and sharp) and Saturn rings with 2 faint dark equatorial bands on the surface of the planet. The moon is incredible at 240x. On less good seeing I can use the 9mm.

My next buy will be a 4mm to get my max recommended mag on my scope. True I may use it just a couple of times, but those times are well worth it IMHO.

[Doc]

I agree it's worth having a 4mm just in case conditions allows it's use. A little while ago I was looking at the Ring Nebula M57 and thought for fun and giggles I'll chuck in the 4mm Uwan, this gives me x 457 and yes the view was soft but I was inside the nebula I was looking at this nebula and it nearly filled the eyepiece up.

[Mark at Beaufort]

Warren

What do you want this higher mag for? Is it for lunar and planetary viewing or do you want to split double stars? Do you belong to an astronomical society or is there one nearby?

Eyepieces are a very personal thing and different aspects of a product i.e small eye relief can make you like or dislike it.

I have a 4" APO frac and the best EPs for lunar/planetary is a 5mm and 4mm giving me 142x and 177x. Because the 5mm is a Hyperion I can increase its focal length to 3.2 and 2.6 but I only use this mag of 221x and 273x on splitting double stars.

Mark

[great_bear]

I agree it's worth having a 4mm just in case conditions allows it's use.

You can't generalise about "4mm" - it's down the the f-ratio of the scope.

No problem for your Lightbridge - conditions permitting - but in my f15 Mak however, 4mm is awful under any circumstances.

[great_bear]

Everyone says you shouldn't push the max mag of your scope, and I mostly agree with it.

The trouble is "max mag" means different things to different people.

To get the maximum detail a scope can resolve, multiplying the f-ratio by 0.85 is about the best you can hope for in terms of eyepiece millimeter length. Then of course, you'll need to back off depending on what conditions you'll be observing under.

You can go bigger on the mag - but there's no point.

It's just "empty magnification"

[pvaz]

When I said "max mag" I meant the max theoretical magnification in the scope's specs written by the manufacturer.

What you say makes some sense, if the amount of light gathered haves a limit, then it makes sense to have a limit on the mag you can use. But, having tried on the same occasion a 9mm (133x) and a 5mm(240x), both Baader Orthos. on Jupiter this is what I sow: The 5mm showed a very noticeable dark spot moving along one of the equatorial bands. With the 9mm it was very small, I only sow it after having a look with the 5mm when I knew it was there.

Besides that, I know from studying image compression technics, such as jpg compression, the human eye haves a good threshold to look at grainy images and smooth then. I'm not sure if this characteristic of the human eye helps at the EP or not, but after experimenting I would pick 240x over 133x on planets anytime I have the chance.

And yes, I'm well aware the vast majority will disagree with this personal opinion/experience.

[great_bear]

When I said "max mag" I meant the max theoretical magnification in the scope's specs written by the manufacturer.

Well - that still doesn't help - how is "max mag" defined? Limit of light? Limit of detail? etc. etc.

The 5mm showed a very noticeable dark spot moving along one of the equatorial bands. With the 9mm it was very small, I only sow it after having a look with the 5mm when I knew it was there.

And yes, I'm well aware the vast majority will disagree with this personal opinion/experience.

Not really. As I've said before - these things are not about "opinion", they're about science. Telescopes don't have "magic powers" misunderstood by mankind; they're designed by engineers within known physical design constraints.

Your experience is completely in line with that. i.e. multiply 5.9 (the f-ratio of your particular scope) with 0.85 and you get 5 - the eyepiece which you say gave you the most detail. No surprise there then. If you went past 5mm on your scope however (say, a 4mm eyepiece) - yes, you'd get a bigger image - but you'd see less than what you get with your trusty 5mm eyepiece.

In my case - in my F15 scope - the 12.5mm eyepiece is where detail "peaks" (conditions permitting of course).

As I say - these things aren't about "opinion" - they're about optics. Some people like to go really high and see large airy discs when splitting stars, and if they enjoy that, then there's nothing wrong with it - but they're not seeing any more detail (as such) than you'd get at 0.85 x f-ratio.

[pvaz]

Ok. Althought that formula is new to me I'll accept your point of view.

But, as you said telescopes are made by engineers and if the ones that made mine say it can reach 300x I'm gonna try it!

Besides, in all literature I read so far the rule of thumb is 50x per inch of aperture on Refractors and 40x per inch on Newtonians (due to the waste of aperture caused by the secondary + spiders obstruction in comparison to refractors). That doesn't mean I don't believe you, and you may well be 100% right, but I'm still going to give it a try.

[Wurzil]

Ok thanks for the info guys. I wont bother with a 4mm for my scope.

I cant find any of those circle-t orthos in my price range really - Ive seen 6.4mm meade 4000s going quite cheap - maybe I should get one of those???

W

[AWR]

If my maths is correct:

A limit of 50x per inch of aperture (or 2x per mm of aperture) equates to an EP limit (minimum) of 0.5x the f ratio.

A limit of 0.85x f ratio equates to a limit (maximum) of 1.18x per mm of aperture or about 30x per inch.

Andrew

[pvaz]

If my maths is correct:

A limit of 50x per inch of aperture (or 2x per mm of aperture) equates to an EP limit (minimum) of 0.5x the f ratio.

A limit of 0.85x f ratio equates to a limit (maximum) of 1.18x per mm of aperture or about 30x per inch.

Andrew

You can't make a direct relation because the focal ratio depends on both the aperture and the focal length. So 2 scopes, bout with the same aperture, will have different focal ratios as long as their focal length is different.

[Wurzil]

Warren

What do you want this higher mag for? Is it for lunar and planetary viewing or do you want to split double stars? Do you belong to an astronomical society or is there one nearby?

Eyepieces are a very personal thing and different aspects of a product i.e small eye relief can make you like or dislike it.

I have a 4" APO frac and the best EPs for lunar/planetary is a 5mm and 4mm giving me 142x and 177x. Because the 5mm is a Hyperion I can increase its focal length to 3.2 and 2.6 but I only use this mag of 221x and 273x on splitting double stars.

Mark

The high mag is for lunar and planetary viewing (mars and saturn). I havent got into double stars just yet.

I havent managed to get to a public evening at my local society yet because the days its on ARE ALWAYS CLOUDY!

I guess that when I do go, I can try out other peoples EPs to get an idea of what ones I want/need.

I think Im gonna grab the meade 4000 6.4mm for now and not even bother with a Barlow lens at all.

I have to admit I didnt realise that my scope would only push up to 140x and then start losing detail - maybe i should have got the 8" dob after all?

thanks again

w

[Whippy]

But, as you said telescopes are made by engineers and if the ones that made mine say it can reach 300x I'm gonna try it!

Besides, in all literature I read so far the rule of thumb is 50x per inch of aperture on Refractors and 40x per inch on Newtonians (due to the waste of aperture caused by the secondary + spiders obstruction in comparison to refractors). That doesn't mean I don't believe you, and you may well be 100% right, but I'm still going to give it a try.

Not quite sure where you got differing theoretical maximum magnifiactions for different designs of scope (central obstructions, spiders etc etc aren't important) but the general rule of thumb for maximum magnification is 2x the aperture in millimetres ie: a 200mm scope theoretically is capable of x400 magnification. This is based on optimal atmospheric conditions and the scope being perfectly collimated and of good quality. Normally, this isn't possible mainly due to the conditions, you may find that you're only able to go to maybe 2/3rds of the limit on a normal observing session.

Another thing to bear in mind is that smaller scopes are less affected by atmospherics and are more likely to be able to give a better view at higher mags. For example on one particular night I was 'just' able to extract a decent view at x500 on the Moon out of my old 6" f12 Mak-Cass, partly because it was an excellent night conditions-wise, partly it was an excellent scope optically. Plus not being very large, it was less affected by the seeing. Mind you, it only happened once while I owned the scope, nights like that don't happen very often!

Tony..

[AWR]

You can't make a direct relation because the focal ratio depends on both the aperture and the focal length. So 2 scopes, bout with the same aperture, will have different focal ratios as long as their focal length is different.

But magnification also depends on focal length.

FR = focal ratio

FL = focal length

EL = Eye piece focal length

AP = Aperture

MAG = Magnification

now MAG = FL / EL

and FR = FL / AP

ie. FL = MAG x EL = FR x AP

re-arranging: MAG / AP = FR / EL

ie. given a MAG / AP limit you can work out an EL / FR limit and vice versa

HTH

Andrew

[pvaz]

Indeed. I should actually have thought about it before I opened my mouth!

[great_bear]

As mentioned previously, discussions of "maximum magnification" are meaningless unless you specify by what criteria you define "maximum". What happens if you go past "maximum"? - does the telescope explode? If not, then what's so special about that point that defines it as being the so-called "maximum"?

By the time a telescope reaches 30x per inch, detail has already maxed out. Yes, you can get a "bigger" image by magnifying further - (and why stop at 50x? SkyWatcher quote my Mak at 75x per inch!) but there are good reasons not to do so:

- Past 30x per inch, you won't see anything you couldn't already see at 30xpi (i.e. aperture(mm)/0.85)

- The image is no longer "crisp" - even if it is "acceptable"

- When the exit pupil gets less than 0.85, the debris (floaters) inside your eyeball get very distracting

Of course if an observer likes going past 30x per inch, then of course they must do what they want.

[AWR]

Indeed. I should actually have thought about it before I opened my mouth!

Sorry - hope I didn't come across too much like a teacher!

I'd never come across a magnification limit given in that format (in my very limited experience) and was just interested in how it converted.

Andrew

Now write it out 3 times for your homework

[Moonshane]

hi Wurzil

I am a total novice but have the same refractor as you. I currently have the 25mm that came with the scope (in my view this is my best lens and the most comfortable to use - see below), a 40mm plossl and a 5.2mm ED. I also managed to negotiate a free 'ascension' barlow from the seller of the scope.

I find the barlow useful and have recently also considered a zoom but now reckon that there's nothing better than building a small supply of decent quality eyepieces. First of all, don't rush in and buy something for now. I think this is a waste of money. let's face it, astro targets are going to be around long after we fold our tripod for the last time! Better to save for a bit longer and get a good/better quality eyepiece. Personally, I am from the sounds of things like you - I don't want to and don't have the funds to, buy £100+ eyepieces but from looking online there are lots of good eyepieces in the £50 approx. range. I am about to order some Paradigm ED lenses which seem good quality from reviews in the states, and meet my eye relief requirements (I wear specs) and also my viewing preference (I take off my specs once the target is in view) as they have twist up eyecups which helps block extraneous light. These come in well below £50 each. Others might feel that other eyepieces are better but these are not so expensive you hate them every time you look at them when not used too much!

Back to performance of the lenses I have. The 40mm plossl and the 25mm are great for general use and with the barlow there's not too much loss of detail. Don't forget with a barlow you can also insert before the diagonal to get 3x but I am not sure if there's a major loss of light this way.

The 5.2mm is useful on my scope only for splitting double stars as the detail is not great and the light somewhat dim. On the moon it's OK but again the above all applies. I now know why as using the 0.85 rule referred to above, my scope's max is 7mm. Thankfully I have ordered an 8mm Paradigm so this should work ok on the planets and maybe I will also occasionally use the 5.2 on planets when seeing allows.

The 5.2mm is actually slightly better (due to the 0.85 rule again) on my f5 150mm XLT reflector as the max on that scope would be 4.25 when seeing allows.

I have to say I am absolutely delighted with this scope. I have managed to see the double double, ring nebula, split castor and of course the orion nebula. I also managed to see the triangulum and resolved at least 4 stars, and saw four stars on Sigma Orionis (just) with the 5.2mm (albeit dim and blurry!). andromeda is visible but from my garden very faint and wispy, better in the 6 inch newtonian. pleiades is great with the 40mm.

Hope you find these direct comparisons with your scope useful. If I were you, I'd probably get a lens at about 7-8mm and one to bridge the gap between this and your 25mm, maybe a 12-12.5mm. Possibly a cheap barlow would be useful as this would be useful just with the 12.5mm and the 25mm. With these you will be able to see most things viewable with your scope. Bear in mind, all this is said with the mind of an astronomical baby/numptie compared with others on this forum!

cheers

Shane

[RobH]

You'll tend to find that the maximum useful magnification you can get, most of the time, is about x30 per inch of aperture.

As said previously, in really good seeing more is sometimes possible.

You can push a good apo more than an achromat as you won't run into colour fringing problems.

I have a top end 6 inch TMB apo, and (on the few occasions it gets used visually), I find the best views are when I don't push to the limit.

Regarding barlows, I find using a good quality x2 barlow with a 10mm EP is much better than a straight 5mm EP as the eye relief is much better and it's just generally easier to view through...4 or 5mm EP's give a tiny light cone.

Cheers

Rob

[great_bear]

telescopes are made by engineers and if the ones that made mine say it can reach 300x ...

They don't - that'll be the Product Marketing department. They'll say anything that they can legally get away with - wouldn't you rather believe someone from NASA?

the waste of aperture caused by the secondary + spiders obstruction in comparison to refractors

In an inch-per-inch aperture comparison, light lost through a glass lens is more - so the only concern is diffraction spikes rather than light loss.

[Brunty12345]

im gonna keep looking at this thread im learning more things 8-)