What is optimum magnification for doubles with small scope?

[RobertI]

Hi all,

I've been doing some visual with my little Megrez72 refractor recently. I have two Baader Hyperions of 21mm (giving 20x) and 10mm (giving 43x).

The 10mm is best of the two giving nice views for open clusters, brighter nebulae and asterisms. 

However I feel more magnification is required for planets and doubles, although I realise there is a resolutio limit on such a small aperture.

I have read that around 30x per inch of aperture (85x in my case) is optimum for planetary and doubles. This means an eyepiece of 5mm or a 2x barlow.

Can someone advise whether this actually works in the real world?

Also any advice on an good eyepiece to match the scope or opinions on barlow vs eyepiece would be much appreciated.

Thanks

Rob

[ronin]

My opinion is to drop an eyepiece in that is the same as the f number of the scope, that gives a magnification equal to the diameter and an exit pupil of 1mm.

The M-72 is about f/6 (bit less I think) so a 6mm or 5mm should be good without it pushing the capabilities too far.

Thing is I have read so many thoughts that I think you have to make your own decision, and the above is nice and easy.

[Peter Drew]

I find that using the highest magnification the telescope can stand on a given night helps with close double stars, it won't separate stars below the optical threshold of the aperture but it will make those that are possible easier to see. 

[NGC 1502]

Hi Rob. I have two small refractors, a TV Pronto (70mm) and a SW ED80.

They are both good up to 100-120x on planets and 150x on double stars.

A short focal length refractor is very versatile, gives wonderful low power/wide field and medium to high power all in one neat and

portable package.

A top quality barlow used with a good eyepiece works fine for me, but I do prefer single high power eyepieces as they are more compact

and just seem nicer than a long barlow + eyepiece combo.

Regards, Ed.

[John]

I really push the magnfication when trying to split close doubles. As high as 220x for my 102mm refractor and 300x for my 120mm.

[Stu]

I've used a variety of small refractors extensively over the past few years, including a fair amount of double observing. They range from 60mm to 85mm and generally are around f6 ish, so quite similar.

I find that decent quality optics are capable of being pushed quite hard, and regularly use exit pupils down to x0.5mm in order to get decent image scale on planets and for separating doubles.

My 60mm will take x100 or sometimes x120 and the 76mm will take x140 odd, more on good nights. This generally equates to around a 4mm eyepiece or perhaps 3.5mm which I have found to be a good maximum in a number of these scopes. Even the 60mm will quite easily split the double double and Izar for example.

I have a 3 to 6mm Nagler zoom which does a very nice job, and obviously gives the flexibility to tune the magnification. Orthos are nice too but the eye relief gets very tight at these ranges so a 7mm with good x2 barlow would be great.

At these small exit pupils, floaters in your eye do become an issue. You may find this a problem or you may get used to it. I find it more annoying on planets as often the floater in my eye appears over the disk. For doubles it is rarely a problem for me but you will find out through experience.

Finally, sometimes magnification is not the only trick, and backing off it a little can show some doubles which don't show at max mag.

Stu

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[Mark at Beaufort]

When I had a William Optics SD66 the highest mag that I ever used was with a 2.5mm EP. It was only used to split doubles and it was not an expensive EP. - TS Planetary HR. I think it gave a mag of about 155x.

I never used it on Planets but I could split the double double quite easily.

[philj]

I could sometimes get away with 150x plus on my old 60mm Swift with doubles on good steady and clear nights. Its all down to the limit of your objective and conditions in the end as to how much good high mag will be on splitting close doubles.

+1 with Stus comment on backing off a bit though. 

Higher mag isn't always the answer, sneaky little things doubles are sometimes. Occassionally Ive lobbed in a couple of hundred mag in my larger scopes thinking I would spilt sub arcsec doubles only to find nowt there, then backing off a bit reveals a gap. Same thing with Saturnian moons sometimes 

[orion61]

I go as high as the seeing and the quality of my scope will go. I prefer the appearance of the Airy Disk when you push the Magnification up to 50-60X per Inch..

OOPS forgot you lads use Metric across the pond..

I want to wish you all a Merry Christmas from the heartland Of America,

Iowa, where the Best Beef in the world comes from.

[John]

Backing off the power a little is definitely worth trying. I find E & F Trapezium are best seen with a "goldilocks" magnification but this can vary session to session. Often though I find 8mm "just right" for these stars, ether in the shape of the Ethos or the TV plossl. Thats 113x with my ED120 and 199x with my 12" dob.

[RobertI]

Many thanks for the helpful replies everyone, really appreciated.

From what i hear It sounds like a 5mm (x86) would be a safe bet to start with (giving 86x) and I could then investiagte look at the results and then try a 3mm to 4mm if I feel the need and feel the scope is capable.

The other consideration is the mount - I may be able to get away with x86 on the Horizon tripod I use but anything more will require the Skytee 2 which is less grab and go. So again starting with the 5mm seems like a good idea.

When I had a William Optics SD66 the highest mag that I ever used was with a 2.5mm EP. It was only used to split doubles and it was not an expensive EP. - TS Planetary HR. I think it gave a mag of about 155x.

I never used it on Planets but I could split the double double quite easily.

Had a look at these eyepieces and they seem good value with a nice range of FLs. Will definitely look at these.

Any further thoughts on recommended eypieces would be appreciated (perhaps I should post in the e/p forum?)

Thanks again

Rob

[Stu]

Simple designs like Orthos often perform well for doubles, and planetary. The problem with short focal length ones though is that the eye relief is very short. If you wear glasses they are a definite no, and even if you don't they are not for everyone. They are very light though, so it makes scope balance easy.

Something like a Baader Classic Ortho perhaps, around £50 or £60 I think.

It also depends on your budget. A used Televue 5mm Radian would be nice at around £100 to £120.

I'm sure others will suggest plenty of options to consider

Stu

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[John]

Personally I use Pentax XW's and Tele Vue Radian's for high power viewing but I've read some great reports on the William Optics SPL's too which cost quite a bit less.

[Stu]

I guess the Vixen SLVs would probably be pretty good John?

[John]

Yes, they are great eyepieces for high power viewing

[Lowjiber]

Resolution is purely a function of the scope's objective diameter.  Google "Dawes Limit" and you'll find a lot of factual information.

Clear, Dark Skies

[Stu]

Resolution is purely a function of the scope's objective diameter. Google "Dawes Limit" and you'll find a lot of factual information.

Clear, Dark Skies

True enough, but ability to resolve tight doubles also depends on many other factors including optical quality, collimation, cooling, seeing conditions and observer eyesight and experience.

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[Merlin]

The point about resolution and the 50X per inch limit is that it only applies to extended objects.

It's different with point sources,i.e. stars. Here we can go much higher. William Herschel used "astronomical" magnifications on close doubles; up to 7000X, I read somewhere.

[philj]

I use my longer fl circle t orthos and vixen lvw shorter fl eyepieces on doubkes usually, depends on the scope but the shorter orthos are a bit tricky

[jetstream]

The point about resolution and the 50X per inch limit is that it only applies to extended objects.

It's different with point sources,i.e. stars. Here we can go much higher. William Herschel used "astronomical" magnifications on close doubles; up to 7000X, I read somewhere.

I'm not sure if I agree, it would depend on what we call extended objects. I consider extended objects galaxies etc,that when magnification is increased both the sky back ground and the object get dimmed equally. If this is what we consider extended objects then there is a magnification limit imposed by our eye and brain around 2mm exit pupil. This is much lower magnification than 50x/inch.

I have an article here somewhere stating Herschel went up to 3000x magnification, but this is misleading to the OP as he will never use this.

Personally I don't like to go above the mag where the little Airy disk stars to show on doubles when splitting stars, I like them to look like pinpoints, just my thing.

Lots of good advice from John,Stu,Ed, Ronin etc Its hard to beat a half decent zoom for dialing in mag to suit the conditions, but there are better eyepieces optically than some zooms.

[Merlin]

W

As you said, we all like to see doubles as nice sharp points, but some doubles are so tight that they can only be split at mags that are so high that the Airy discs show up.

[F15Rules]

I grew up with Sir Patricks mantra that x50 per inch of aperture on good optics in good conditions were a good guide, and I've never found any reason to doubt that in practice. I do find that our seeing here in the UK is lousy for much of the time (note I said seeing, not transparency - although lately they are both rubbish! LOL), and that does limit the power you can usefully employ.

I'd also second Phil's feeling that traditional orthoscopics are hard to beat on double stars. The field of view is narrower but that doesn't matter for doubles, and it's very, very flat, with almost no induced aberrations, so "what you see is what you get"..on the OP's scope focal length of (I think 430mm?), a 6mm Circle T ortho would give x72 approx, or x144 if used with a decent quality barlow lens...the latter being right in line with Sir Pat's x50 per inch guideline.

I quite like Hyperions (and really like the zoom), but do find them a bit soft compared to orthos when pushed to their limits...

HTH

Dave

[ollypenrice]

Resolution is purely a function of the scope's objective diameter.  Google "Dawes Limit" and you'll find a lot of factual information.

Clear, Dark Skies

An interesting theory but, I think, incorrect. According to this theory there would be no difference in resolving power between a cheap F5 4 inch achromat and a not at all cheap F5 Takahashi apochromat. In reality the 4 inch Tak brings all visible spectrum colours to almost the same focal plane whereas the cheap apochromat does not, so much of the visible spectrum light in the cheap scope is out of focus and cannot possibly resolve the detail attributable to its aperture.

The reality is that what can really be resolved is the product of complex equations and balancing acts. This would not be the case if we had perfect optics, but we don't. Far from it.

Olly

[RobertI]

Thanks for the helpful replies everyone. It seems there are just too many parameters to hit on the right solution straight off, so I decided to go for the 'Try and Buy' offer from Skys the Limit.

I shall be ordering a 5mm BST ED Starguider and TMB 4.5mm PL2 (no 5mm in stock) and keeping one of the two.

The BSTs seem to have had very good reviews on SGL and both are reasonbably priced at under £50.

I will share the results when I have tested them.

Rob

[Mr Spock]

I find for planets x1 per mm gives the right balance between brightness, contrast and resolution.

For close doubles you can go to x1.7 - x2 per mm.