To Barlow or not to Barlow : that is the question

[KaStern_Former_Member]

Hello Paul,

I would not barlow an f/13 scope. You can acheive the maximum magnification with a simple 6,5mm eyepiece.

I agree to what Ajohn wrote, the exit pupil ist the most suitable way to calculate the eyepiece focal length.

Devide the eyepiece focal length ba the F/number of your scope to find out the exit pupil.

I would go for a good 7mm Ortho (University Optics HD Ortho, Baader Ortho, ect) for moon, star testing and

double star splitting. If that is too expensive than try a fully multicoated taiwanese plössl.

And a 9mm Ortho for Planetary viewing, that is about 0,7mm exit pupil.

Regards, Karsten

[The Warthog]

You can also achieve the maximum, or nearly, with a 13 - 15mm Plossl, Barlowed. This gives a much more relaxed view, and a bigger piece of glass to look through, and saves the expense of another ep that isn't going to be used much. In practical terms, this scope will probably perform best at 90 - 100x, and that is what Paul should aim for.

[Ajohn]

I think a decent barlow will cost a lot more than one or two decent eyepieces.

On exit pupils I do it the long way. Say the scope is 150mms dia and I want a 0.8mms exit pupil. The magnification needed will be scope diameter/exit pupil that's 150/0.8 = 187.5. So if the focal length of the scope was 750mms the eyepiece needed would be focal length/magnification and that's 750/187.5 = 4mms.

Larger exit pupils mean less magnification. Just change the figures to suit but note that 0.8mms on a high quality 8ins (200mms) dia scope may suffer from atmospheric seeing problems. Less likely with smaller scopes much more likely with bigger scopes. The other point is optical quality of the scope and eyepiece. At this sort of exit pupil defects are more likely to be apparent. Smaller scopes / longer focal lengths tend to help in this respect but there isn't really any way of finding out without trying it. A vixen lanthium zoom is an excellent buy for sorting this sort of thing out if the focal range is suitable. Also finish up with an eyepiece that will always be useful - even if it's doesn't fully exploit the scope.

John

[The Warthog]

Talking about exit pupils is a waste of time. The ep that will maximise your scope, at an exit pupil of 0.5mm, is the one with the focal length half of your scopes f/number. The ep that will be the most comfortable and useful is about 1.5 times the length of the shortest one you will use. At the other end, you are shooting for about 7mm exit pupil if you are young, 5mm if you are older. However, if getting a nice wide field of view wastes a little of that exit pupil, I'm not going to sit up all night worrying about it. You can figure out what is the most useful set of eps even if you've never heard of exit pupil.

How much a decent Barlow costs depends on your definition of 'decent.' I worked with a pair of Celestron SMA (Kellner) eps, and a standard Barlow that cost me about $40. I enjoyed wonderful views for years, and later moved on. I eventually got a Celestron ep kit, including another bog-standard Barlow, and used it with pleasure, and over the ensuing three or four years arrived at the kit I have now, with scopes that make the investment reasonable.

The point is, most of your brand-name plossls give good results with any scope, and the ordinary humble Barlows give good results with them. You don't need a Powermate to look at planets, DSOs, or the Moon. You can buy a Celestron 130 for not much more than the price of a Powermate, and that may be a better investment.

At present, the gentleman who started this thread has eps that provide 45x, 90x, and 142x. As any ep will give decent results in a f/13 scope, what more does he really need? A new scope, perhaps.

[rusirius]

I can vouch for the quality of the UVO HD's and the Baader Orthos as I have a set of 6 UVOs and 12.5 Baader. The 18, 12 and 9mm UVOs work well with my 8in f10 SCT, whereas the 7,6 and 5mm ones work well with my wifes Skywatcher 120/1000, f8 refractor. The Baader is practically identical to the UVO and apparently comes from the same Japanese workshop. Both UVOs and Baader seem to outperform Meade Plössls of the same or similar focal length by some margin. The Kellners that came with the Skywatcher also give very good results with the refractor.

Having said that, I bought the UVOs for a reasonable price and was prepared to put up with some discomfort due to the short eye relief. As it happens, I quickly got used to them and do not find them too much of a problem to use.

I do appreciate though that Orthos may not suite everyone and a good quality Plössl + barlow may still offer a clear, sharp and yet more confortable view. I have a Meade barlow (Telenegative Amplifier), but I find it a poor performer. One day I may get a decent barlow but until then to be fair, I don't really have a basis for comparison of the Orthos with a decent barlow + lower magnification Plössl or similar eyepiece.

[Ajohn]

You will do much for eyepiece sales warthog. Many people have no idea what to use and exit pupils are an excellent way of getting it across to them. As usual with anything to do with scopes it's a bit fuzzy but the only complication is size. The bigger the scope the higher the magnification needed and the atmosphere etc starts having an effect. Many many people can fully utilize the capabilities of an 8ins scope at times but it's a sad fact that the going get tougher and tougher above that. Some live in places with excellent seeing conditions many don't.

John

On the calc fine but the other way does illustrate exactly what's going on.