Is there a 'minimum practical' magnification?

[Steve922]

I'm using a TAL100RS F10 refractor and have a nice range of eypieces - 25, 15, 10 and 6 mm plus a 2x Barlow which takes me nicely to a bit above its 'maximum practical' magnification.

I was thinking of extending the range on the lower magnification side, perhaps with a 40mm plossl. The idea is for viewing the larger objects, e.g. M31 and trawling the Milky Way for open clusters and maybe even nebulas?

Is this idea sound? Buying a 40mm eyepiece to gain a wider (and brighter) FOV ? All comments most welcome.

Steve

[Demonperformer]

The limiting thing here is the exit pupil, which is the diameter of the objective divided by the magnification. In the case of a 1000mm focal length telescope and a 40mm eyepiece this will be 100 / (1000/40) = 4mm. If this figure is bigger than the diameter of the pupil of the eye, you cannot see all the light that is coming out of the eyepiece. In young people, this is generally reckoned as 7mm, but reduces to about 5mm as you get older. So 4mm is ok. If you tried to use a 0.5x reducer to give you only 12.5x magnification, this would give a value of 8mm which is more than the eye can see and so the light would be wasted

[brianb]

The limiting thing here is the exit pupil

True but a 'frac has no "secondary shadow" which can get to be an issue at very low power. An oversized exit pupil does "waste" light grasp but you need to get the exit pupil as large as your eye's to maximize the brightness of extended areas (faint fuzzies, especially emission nebulae, are especially sensitive). A lower magnification even than this can help make low contrast detail in faint objects more visible by increasing the contrast gradient. If your skies are dark enough or you're using an effective light reduction filter with a small short focus 'frac, try loading an eyepiece that gives an 8mm or even a 10mm exit pupil ... for some objects (e.g. California Nebula) it can make the difference between a reasonable view and not seeing the object at all.

[Macavity]

The main problem with 40mm Plossls is that, at least in 1.25" format, they will have an apparent field limited to more like 40 deg and not the typical 50 deg. This by virtue of the limiting internal diameter of the nosepiece. In simple terms, you would gain the maximum True field in 1.25" format with a (common) 32mm Plossl.

Assuming the focal length of the objective is 1000mm, this gives 31x mag, 1.6 deg True field, 3.2mm exit pupil etc. All well within usable limits. Indeed, unless you're going to do something VERY exotic (expensive!) with a 1000mm FOCAL length and available eyepeices, you need never worry about exit pupil size...

It's generally worth while getting a decent 32mm PLossl (24mm 70 deg etc.). The, albeit small, additional, TRUE field might be useful in "framing" objects, such as M45 etc. You're always going to want to try?

[John]

One issue with very low powers (which depends on your local conditions) is that the light pollution shows more at low powers so your background sky is more dark grey than black. This makes picking out dim, low contrast deep sky objects rather difficult. It's one of the reasons that wide angle and ultra wide angle eyepieces are popular - eg: a 24mm wide angle 1.25" eyepiece can show the same amount of sky as a 32mm "normal" eyepiece but at a higher magnification and therefore darker background sky.

If you observe under really dark skies with little or no light pollution this not such an issue but most of us don't have that luxury !.

John

[Steve922]

Wow, this sounds really complicated. I';m now thinking of putting off the idea until I can borrow one to try out so I can see if the idea is sound or not! :-)

Steve

[brianb]

this sounds really complicated.

No it's not; you have a 25mm, a 32mm is so close you don't need both, a 40mm gives a useful reduction in magnification / increase in contrast and iy doesn't matter that the actual field of view will be no bigger than the 32mm. And with your scope 40mm gives a 4mm exit pupil which is nowhere near the "too large" category, when used for observing astronomical objects after dark.

[Macavity]

With apologies, for I don't want to get into this overmuch: But isn't the general idea that contrast INCREASES with magnification. Thus e.g. a 32mm eyepiece giving about same TRUE FoV as the 40mm is going to be more contrasty? I happily concede that a 32mm eyepiece is CLOSE in True field as a 25mm, but that LITTLE bit extra might be somewhat useful? I'm also incorporating the notion that the 10mm and 25mm eyepieces may be the "default" eyepieces that come with the scope, and whereas TAL probably make fine eyepeices, they may not be the very BEST available?

I suspect I'll leave it now. It's my hope not to muddy the waters in such things.

P.S. I do sense some of this begs another question as to WHICH 40mm is being considered (I can't think of many!) and particularly the format... 1.25", 2" etc.

[John]

I had a TAL100 a few years ago and bought a 40mm 1.25" plossl to use with it and quickly came to the conclusion that it was like looking down a drinking straw with just a 43 degree aparrant field of view and was not providing the sort of wide field views I was hoping for. I eventually got a 25mm wide angle eyepiece (also 1.25") with a 70 degree field and that gave me nice wide, immersive views and proved ideal for deep sky sweeping.

The 25mm eyepiece for sale here (not by me) is very similar to the wide angle that I had:

http://stargazerslounge.com/sale/86949-pair-eyepieces.html

OK you already have a 25mm but this would make an interesting replacement for the one you have IMHO - and you could always sell yours to help fund it - if it's not already been sold of course.

That's just my experience - "your mileage may vary" as they say !.

John

[brianb]

But isn't the general idea that contrast INCREASES with magnification.

No.

The geometrical contrast (the ratio of the light intensity in the lightest & darkest parts of the field) is the same whatever the magnification. But increasing the magnification spreads the light around, making everything fainter, and the eye is not as capable of distinguishing tones when the brightness is reduced, so apparent contrast is reduced when magnification is increased ... providing what is being magnified already has an apparent size, i.e. not a point like a well-focused star is at low magnification.

Also, placing blocks of slightly contrasting tone closer together (by reducing the magnification) makes the contrast between them more apparent.

You can see fainter stars by increasing the magnification (up to a point) because the sky background is dliuted whilst the stars remain a point. But once you get up to about x15 per inch, even with perfect seeing, stars are not point like enough for this to carry on working; above that, you will start to lose the faintest stars if you increase the magnification. When the seeing is poor, or the scope isn't properly cooled, or is badly collimated, the stars will be blobs even at low power & increasing the magnification will do no good at all.

As for 43 degree apparent field being "drinking straw" - sorry, but that's what we used to consider wide; only 30 or 40 years ago, 35 to 40 degrees was the norm, & the best eyepieces for planetary work (monocentrics) were usually more like 20 degrees.

[FLO]

The 'exit-pupils over 7mm waste light' rule is less of an issue with refracters as there isn't a secondary obstruction to worry about and there is plenty of brightness available at lower magnifications. Just use whatever best frames your target. Keep in mind though that eyepieces with focal lenghs longer than 25mm will need a 2" barrel, otherwise the field of view will be restricted.

HTH

[Macavity]

No.

... so apparent contrast is reduced when magnification is increased ... providing what is being magnified already has an apparent size, i.e. not a point like a well-focused star is at low magnification.

I sense when MOST people refer to contrast they are assuming precisely a (collection of) point sources, against a dark sky... a.k.a. Stars. The argument re. extended sources, and "smeared" point sources is indeed interesting / obvious / compelling though...

As for 43 degree apparent field being "drinking straw" - sorry, but that's what we used to consider wide; only 30 or 40 years ago, 35 to 40 degrees was the norm, & the best eyepieces for planetary work (monocentrics) were usually more like 20 degrees.

Again, hard to argue with such truisms. But, on such a basis, would we not trade in all our cars for bicycles...

Albeit very HIGH CLASS, vintage bicycles.

[John]

...As for 43 degree apparent field being "drinking straw" - sorry, but that's what we used to consider wide; only 30 or 40 years ago, 35 to 40 degrees was the norm, & the best eyepieces for planetary work (monocentrics) were usually more like 20 degrees.

Gosh !

Meanwhile .......... in 2009

John