To Barlow or not to Barlow : that is the question

[PaulG]

Right then my little head is running around in circles (doesnt take too much). I enjoy squizing at the Moon as this floats my boat. A mirror image Moon map is also on its way as a normal map is too confusing for me at the minute. While I’m pondering which way to go with my next telescope purchase (subject to authorisation by HID) I thought I would buy a cheap 6.3mm Plossl (I can see you cringing :nono:) just to get a feel for magnification and view.

The 20mm I have allows a full view of the Moon which fills most of the FOV and is clear and easy on the eye.

The 10mm I have is again clear and easy on the eye.

Now I know the 6.3mm is pushing my little scope somewhat but it’s not as clear as the other two and creates a degree of eye strain for me (almost like looking through a hollow BIC pen). I noticed that when testing the 6.3mm in the daytime, the view was not so bright which I fully expected but the clarity of the view looked like looking at an old photo with a little grain. It’s hard to describe being a newbie but it exhibited like a matt type view which lacked contrast.

Do 6.3mm Plossl eyepieces act in this way or should the view be not so bright but with clarity without the matt effect?

Would I be better off scrapping the 6.3mm and buying a Tal x2 Barlow (my kinda budget) with a 12.5 -15mm Plossl.

It’s difficult for me to visualise what is best without having access to different lenses.

Paul

[blinky]

I would suggest the Barlow route, a cheap 6mm EP is not going to be very good (as you have found out) the TAL Barlow on the other hand gets a great review

[themos]

Why not go for the 3x?

[PaulG]

I had thought about the x3 but I'm not sure by how much this will degrade the view ?.

It's tricky when I havnt looked through a Barlow before.

Paul

[themos]

If you are in need of magnification, it's more likely that you will need x3 than x2. That's what I have found over the last year.

[PaulG]

I do need magnification.

What I’m trying to figure out is what would give me clarity without too much eye strain and without degrading the view too much.

So would it be

1. A quality 6.3mm Plossl

2. x2 Barlow with 12.5mm – 15mm Plossl

3. x3 Barlow with a 20mm Plossl

Paul

[Darren B]

If the 6.3mm plossl is giving you eye strain, go for the 3x barlow and 20mm plossl. The 20mm plossl will have better eye relief and therefore more comfortable to use. Using the three times barlow will increase the magnification to what you want, but you will have the same eye relief with the 20mm plossl as before.

[Ajohn]

I would be inclined to forget the barlow unless you go for a decent one but that isn't what you need in anycase.

I can explain you fuzzy image of the moon and give you some idea of what to expect with a given eyepiece but you will have to do some simple sums.

Using the 6.3mms eyepiece the magnification is about 143. It's a 70mms scope so the exit pupil - the bit that goes into your eye is 70/143=0.49mms. Exit pupils are the best way to calculate which eyepieces to use. They account for the resolution capabilities of the scope. At 0.5 they are most suited to viewing stars or testing the telescope against a star. The optimum planetary views will be obtained with exit pupils in the range of 0.8 to 1mm or so. Magnification wise thats 70/0.8 to 70/1 = 87.5 to 70X = 10 to 12.8mms eyepieces. These will show the most detail with the most clarity, longer focal length eyepieces being needed in poorer conditions especially on scopes over 6ins diameter. Go shorter and the image won't be so good. Some high quality scopes can be pushed a bit further but the image will be dimmer and no more detail will be shown.

The magnification for an exit pupil of 0.8 mms on a 6ins scope is 190 giving a bigger image than you can realistically get with your scope. A bigger scope is the only answer. There is a catch though. As the size goes up the magnification can go up too but atmospheric conditions start having an effect so lower than optimum powers have to be used. At some point the observer has to put up with an unsteady image or in the extreme fit adaptive optics and generate a 'star' high in the atmosphere with lasers. The truth is that it can be hard to get maximum performance out of even an 8 ins scope. It all depends on the observing conditions.

John

[The Warthog]

This is an entry-level scope, and I would not suggest pushing it too hard. A magnification of 140 is the theoretical maximum, but somewhere around 100 would probably be the practical maximum unless you have amazingly clear and still skies. I would suggest a 30 - 32mm plossl, and a decent 2x barlow. Plossls usually take well to being Barlowed, but some Wide Angle eps kidney-bean like crazy witn a barlow.

You will get a more comfortable view with the 20mm and a Barlow than you will with the 10mm, and the 10mm barlowed will have limited use. Still, you will have ep lengths of 30, 20, 15 and 10mm, which is a nice selection, without breaking the bank.

The odd performance with the 6.3mm Plossl may be due to overpowering the scope.

Get those two, the 32mm and the Barlow, and consider a larger scope as your next purchase, before you spend a lot on accessories.

[PaulG]

Hello John & Warthog - Thanks for taking the time to explain things in more detail. I have perhaps foolishly only been looking at the maximum magnification of my scope at x140 but now you have thrown exit pupil size in the mix it’s now deflated my balloon regarding choosing my next telescope. Even with a 150/1200 refractor, the view with optimum clarity will have a magnification of only x190. My goal was to buy a scope to achieve a magnification of say x200 to x250 as most people say x250 will be all you can realistically use. I had short listed two telescopes for my next purchase, Phenix 127/1200 refractor and a Sky Watcher Skymax 127/1500 Mak. So I guess I’m going to have to lower my sights a little regarding ultimate magnification and work to achieve the best compromise regarding magnification and exit pupil size. Thanks again Paul

[John]

Hello John & Warthog - Thanks for taking the time to explain things in more detail. I have perhaps foolishly only been looking at the maximum magnification of my scope at x140 but now you have thrown exit pupil size in the mix it’s now deflated my balloon regarding choosing my next telescope. Even with a 150/1200 refractor, the view with optimum clarity will have a magnification of only x190. My goal was to buy a scope to achieve a magnification of say x200 to x250 as most people say x250 will be all you can realistically use. I had short listed two telescopes for my next purchase, Phenix 127/1200 refractor and a Sky Watcher Skymax 127/1500 Mak. So I guess I’m going to have to lower my sights a little regarding ultimate magnification and work to achieve the best compromise regarding magnification and exit pupil size. Thanks again Paul

Hi Paul,

I think the pursuit of high magnfication is something that many people new to this hobby get hooked on, because of the, perhaps understandable, belief that more magnification = more detail.

A telescope is primariliy a light gathering instrument - magnification of the image is really a secondary function.

The real key to more detail is more aperture, hence the oft quoted saying, "aperture is king".

John

[Ajohn]

At least exit pupil calculations can give you some idea of what to expect. They aren't hard and fast but a sensible guide especially when buying eyepieces. Odd thing is that I came across them in a telescope making book by a french optician called Texereau. He was the one that made plossls famous (once the US book was published). I bought the book a long long time ago and didn't take that much notice. Over time I've come to realize that there is much in what he says on this and other subjects. One of the things he points out is that it is difficult for the average amateur to fully exploit any scope over 8ins dia in terms of using it's maximum resolution. I think he might be a bit over the top on that point but it's worth thinking about especially on planets.

John

[The Warthog]

One little hint. An eyepiece with a lenght equal to half the focal ratio of your scope, e.g. a 4mm ep in an f/8 scope, will maximise the power of the scope, and produce an exit pupil of 0.5mm. I suggest not buying eps of less than the focal ratio of your scope, (8mm for an f/8 scope) and using Barlows on the rare nights which reward such high magnifications.

Like all rules of thumb, this doesn't apply all the time, but with Plossls and smaller scopes, it's infallible.

[philsail1]

Hello "PaulG"

Have you considered a Televue "Powermate" instead of a Barlow?

The Powermate is supposed to have many advantages over and above those of even the best Barlows.

Apparently, the Powermate allows you to increase the magnification of your eyepiece by approx 2.5, but does not reduce the field of view. It also increases contrast.

You can read about the powermate in this review on "Cloudy Nights"

http://www.cloudynights.com/item.php?item_id=473

Hope this helps,

Regards,

philsail1

[The Warthog]

Phil, I agree on the quality of the powermate, although I'm not sure what you mean by not reducing the FOV, as the apparent FOV of an ep should remain unchanged. I question the wisdom of buying an appliance which costs as much as the scope, especially when the scope is a very small entry-level scope.

Paul can achieve very satisfactory results with plossls and a kit-standard Barlow, and save money for his next scope purchase, which will happen sooner than he expects , and still be able to take the plossls to the next scope.

Despite all the hype about fancy wide angle designs, Plossls are stil the chevy sedan (ford prefect saloon?) of eps, and perform well in most scopes with a usable 52 degree AFOV, and Barlow well.

[PaulG]

Hi Philsail1

Thanks for the recommendation of the powermate, unfortunately my pockets aren’t quite that deep at the moment . I’m now in deep thought thinking about a scope upgrade (Warthog knows me better than I do at the moment). I know that you have a 90mm Mak Phil and have had a lot of positive things to say about it, so a Mak is on the short short short list. I always fancied a big refractor but the thought of pulling my back or groin out of place doesn’t have me running to get the credit card out. So I’m looking at the 127mm OTA for around £215.00.

Paul

PS – Warthog thanks for the little hint :thumbup: it is appreciated.

[Macavity]

I think, on a budget, the Maksutov is capable of exploring "high powers" - And you may never be able to resist THAT temptation... until you have? Then you'll want a budget achromatic FAST refractor, to explore WIDE fields etc. Most folk would (wisely) suggest a decent reflector, Dobsonian etc. for general purpose. But I think (especially for the "weight-challenged") the 'Frac + Mak is an interesting alternative... They can e.g. SHARE eyepieces/mount too.

[philsail1]

Sorry Warthog (and PaulG), I didn't realise that budget was a deciding factor.

When I mentioned about the "Powermate" not reducing the field of view of an eyepiece, I meant that it doesn't reduce the "existing" field of view of that eyepiece - as the review on "Cloudy Nights" says.

Paul. If you haven't yet got a Barlow, and you would like to try one, I have a standard Skywatcher 2x Barlow that you could try if you wish? (I could post it to you, and if you liked it just send me a cheque for £6.00 (inc P+P) - If you decide you don't want it, just post it back to me!). Just a thought - ignore, if you already have one, or at not yet ready to try one.

P.S. I bought a Celestron "Ultima" 2x Barlow for £79 last year. And although it is better than the Skywatcher in terms of reducing slightly the "colour fringeing" around the edges of stars and planets, and giving a slightly sharper view, the differences are hardly that noticeable. In fact, on my "binoviewers" I find the Skywatcher "delux" 2x barlow better that the Celestron.

As in all hobbies, accessories tend to be expensive - this is the main reason I am doing this "eyepiece survey" that you may have seen in other sections on SGL. It's all about trying to save people money in buying an eyepiece that may not perform as good as advertised - on their particular scope. (My 90mm Macsutov came supplied with two "Kelner" eyepieces. A 10x and 25x, and after trying Plossls and Orthoscopics, I find that the Kelner's give the best results. (I'm not saying that there may be other eyepieces out there that may give far better results than the Kelner's, but they are the best out of they limited range of eyepieces I have).

Also, if you are thinking of buying another scope, think carefully about what you really want to view. I say this (and I'm sure others will agree), as some scopes (Maksutovs) tend to have quite narrow fields of view which may reduce your enjoyment of looking at wide field views of stars (unless you are happy with a pair of binoculars to view wide field), and other scopes (short f4/f5/f6)refractors) will give you wide field - as well as good views of the planets.

Hope this all helps in some way!

Regards,

philsail1

Regards,

philsail1

[Ajohn]

I think that people must realise that it doesn't matter how you achieve it the exit pupil is the bit that counts also that a 0.5mms exit pupil is unlikely to give the best planetary detail. I have read comments that a very high end well corrected apo will give what might be called decent images of planets at that sort of magnification and my own 5ins apo backs that up to a certain extent but the basic problem is that the enlarged image doesn't add any more detail and being a bigger image it will be darker effectively reducing contrast. On top of that optical defects will be more apparent. Better more detailed views will be obtained with exit pupils in the range of 0.8 to 1 mm or so. Even those may be difficult to get to on larger scopes as atmospheric effects start getting more apparent. Optical quality comes into it too. One fairly easy way of judging max mag for any scope accounting for optical quality as well is just how easy it is to focus it. Images should more or less tend to snap in and out of focus.

The other point is that these comments relate to all scopes of all types on small on axis objects. One of the advantages of a refractor apo or achro is that they can be well corrected for off axis images too as can maks and some other forms of scopes but commercial products only go so far in that direction unless one spends many £K.

On bigger refractors. A skywatcher 120 with a 1000mm focal length is easy to move around and sits perfectly on an Eq4 mount even at high magnification with just one counter weight. Only problem with that is that the eq4 now seems to have become the eq5. As to quality the one I have for sale will give slightly fuzzy but focusable terrestrial views at 300+ X. This might be a fluke but the type of lens used in it is famous for being very easy to make using relatively in expensive glasses but still offering more than adequate performance. The 6ins scope is much more of a handful. I have come across people who have gone down one size for that very reason.

John

[philsail1]

Yes, I take your point about the "exit pupil" being the critical factor John. Is it true that the human lets in 7.0mm light at maximum opening? So a pair of 10x50 binoculars giving an exit pupil of 5.0 would represent a reasonable light gathering ability? (As we get older our eyes don't let in quite as much light?).

I don't care much for the "tiny" exit pupils of the very small eyepieces. I find it quite uncomfortable looking (or squinting!) through my 6.0mm orthoscopic eyepiece, even when it's fitted to my 200mm Newtonian (giving an exit pupil of approx 1.2mm). It's difficult to focus, and the view isn't all that sharp either. I find one of the nicest eyepieces being my 10mm Vixen NPL (and when using it with the Celestron 2x Barlow, (giving an exit pupil of 1.0).

But I do obtain the best views of the planets with my Binoviewers (using 2x25mm GSO Plossl eyepieces and a Skywatcher de-lux 2x Barlow (giving an exit pupil of 2.5mm). Once collimated (which is easy on these binoviewers) I find the views are very easy on the eyes, relaxing and yet so superb (in binocular vision) that I'm quite mesmerised!

I digress!

So is it true that to obtain a decent "exit pupil," and "magnification," one simply has to go for a scope with a bigger main mirror, or objective lens? Or would the quality of the optics on a top class instrument make up for lack of objective mirror/lens size?

Regards,

philsail1

[Ajohn]

Given that you are having difficulties at 1.2mms on an 8ins I would be a bit suspicious of the optics. Might be collimation or could be the optics themselves. The seeing conditions could also be to blame but that usually results in an image that comes and goes. Best seeing conditions are often in the winter especially after rain which from your blanket you have already experienced. Lot's of water vapour and pollution - sort of grey mist look to distant objects also has a dramatic effect. It's every where most of the times these days. Stars or terrestrial objects are the best things to try that focus test on. Look for a round star image in and out of focus too which can give a clue about collimation. Ideally it's best to see diffraction rings - exit pupils of about 0.4 to 0.5 mms - as high as you can go. A dark clear sky helps a lot with that also some sort of hood over your head to keep away any light that's around.

There is also the question of eyepieces. I've made a lot of use of seben ed eyepieces of ebay on an f9 5ins apo and had extremely good results. Bought a whole set from 2.5 to 25mms. He's restricted his range now (and they have gone up a lot) but others do the same eyepiece. They show as good images as a nagler zoom. That isn't the case on an F6 110 apo I've just bought. The nagler is streets ahead. The same will be true for any short focus scope. From samples I've had of skywatcher super plossls I just couldn't recommend them for higher power use either. My advice after checking collimation would be to buy better eyepieces. Having bought most of mine years ago though it's hard to say which ones would be the best to go for. I have some early vixen plossls that work very well on all scopes I've tried them on. These are of the tiny bits of glass variety with hardly any eye clearance - wouldn't surprise me if they were a direct copy of clave's. I bought a vixen lanthium zoom more recently. That works well too but may not give you the powers you are after. I haven't used it at very high powers either. When choosing eyepieces I would suggest going for a manufacturer that makes high end short focus scopes. Price wise WO may well be the best. Vixen too but I can't give any guarantees. May seem silly spending a lot on eyepieces on cheap scopes but it is worth while. Skywatcher claim diffraction limited scopes on there reflectors. There is also the 2nd hand market expect to save about 50%. Even more in some cases. If not make a sensible offer. The most active uk sales site is

http://www.astrobuysell.com/uk/index.php - 8) hope the mods don't mind but it is. Most people place adverts in several places anyway. (including me) There is also cloudy nights. Even after paying a total 25% duty and shipping it can still be a good place to buy. Ebay used to be regarded as the place to sell duff kit but I'm not so sure that's the case these days. 2nd hand sales seem to be stagnant now and all sorts of amazing things crop up on there from time to time. To easy to bid over the top is the main problem.

On barlows I think the same comments hold. If it isn't a power mate or WO forget it. I've had a couple of cheap ones over the years and they have been hopeless. Fortunately my power mate came in part exchange for a scope I sold a while ago. Also a tele vue 2x barlow. Both work well. Meade and Celestron's offerings are most likely to work best on F10 or so scopes eg C8 sct's etc. Personally I wouldn't risk the outlay.

John

[Ajohn]

Whoops missed the max exit pupil one. Opinions vary and of course some one just has to have bigger pupils than other people. I believe it to be 6mms not 7. I reckon on 5mms as do some companies that supply a scope plus eyepiece. I do use much larger exit pupils though - some of it just doesn't go into my eye. One can move ones head around. That seems to work out better on some eyepieces. I have no idea why.

John

[PaulG]

Phil

Thank you for the kind offer of the Barlow, having never experienced one I would like to take you up on the offer. It’s interesting what you said about the Skywatcher and Celestron comparison, I suppose it’s all down to your eye sight being able to detect the sometimes subtle differences. I will email you with my details. Thanks again Paul

[philsail1]

It's on its way to you Paul - Details as in "pm" I sent!

Good advice from John there Paul.

regards,

philsail1

[PaulG]

Thanks Phil

I wasn't expecting you to post that quick, not complaining though .

Cant wait to try it out and experiment a little.

Thanks again for the offer and for posting so quickly.

Paul