Magnification

[Naemeth]

Okay, so I was wondering, what is the ideal magnification (or range) to have a look at:

1) Clusters

2) Nebulae

3) Double Stars

I've heard that Double Stars usually need a high magnification and usually can support a high magnification, but that Clusters and Nebulae can't.. can someone clarify and offer suggestions?

[ollypenrice]

All you want to do in double star splitting is get the separation. Getting attractive, resolved stars is not the point, so on some scopes you might push to 500x or more.

For clusters you first need to fit them into the view, which might mean using the lowest power you can. In my case this can come down to about 15x for the Pleiades, the Coathanger, etc. A wide 2 inch EP is a boon here. Globulars can look great at 200x or more, however, if the aperture and resolution of your scope can handle that.

High magnification is at the mercy of the seeing (the stability of the air)so sometimes you can be limited to something as low as 100x.

There are calculators on planetarium software programmes which allow you to model a particular EP/scope combination on a map of the sky. I use SkyMap Pro for this but there are free ones. Someone may come in with others to recommend.

Olly

[michael.h.f.wilkinson]

Nebulae are very varied beasts. Some are huge, like M42, and require little magnification to get the picture. The same holds for some very extended but faint nebulae, like the Veil, the Crescent, California, Flaming Star and the Cave Nebula. You need low magnification and something like a UHC filter to pick them out. Low surface brightness means low magnification. The Veil was just stunning in my 80mm F/6 at 12x with my 40mm EP and UHC filter.

Planetary nebulae are often bright and small, so I use my 17 and 12 mm Naglers most often, giving 119 and 169x magnification. These too are often better with a UHC filter. Galaxies are different again. They require middle magnification as a rule. I tend to use my 22 and 17 mm Naglers most, at 93 and 119x, but for larger ones and groups I go to either the 40mm (50.8x) or the 31mm (65.5x). UHC filters are not useful at all, though broad-band LPR filters can help. M31 and M33 are best either in my 15x70 bins and in my 80mm APO at 15.5 (31mm Nagler, 5.3 deg FOV) or 21.8x (22mm Nagler, 3.76 deg FOV).

[NGC 1502]

Olly has good advice.

Some double stars are well seen at low power, 61 Cygnus is one example, binoculars will give a nice view.

For instance, in Cygnus are two Messier open clusters. In most scopes use your lowest power/widest field for M39 as it is quite large, M29 is quite tiny and is well seen at medium power, around 100x. So match the magnification to the apparent size of the object. As Olly says, Globulars take higher powers well. M13 is easily seen in binoculars, even from a light polluted town,

but at 150-200x in a scope, resolves into a ball of stars. I've looked at M13 under a good sky with a clubmates 20" Dob - wow !! is a good description.

Same with nebuae, some are quite tiny - NGC 7662 the 'blue snowball" is one example that takes magnification, at the other end of the scale the Veil can be seen well at very low power/wide field.

Regards, Ed.

[Naemeth]

So a lot of DSO's will be able to take high magnification then? The current EPs I have are at 26x and 65x respectively, and am getting one for 162.5x which would be good for Planetary Nebulae, I may get one for 100x (6.5mm or 7mm for my scope), and probably eventually a middle one (15mm) for 43x.

The most I should ever be able to squeeze out of my scope would be 260x, but am I right in thinking that would split a lot of doubles? Does seeing matter for splitting doubles?

Also, I now know a UHC filter would be a useful addition (thinking the one by Baader on FLO ), but would a OIII filter be good for Nebulae too? In addition, would a Light Pollution filter be a good idea?

_{This shopping list is becoming huge...}

[ronin]

Many DSO's will not take high magnification.

Many are pretty faint to start with and all magnification does is spread the collected light out over a greater area, so what you see is even dimmer.

Also magnifing a cluster too much will lose the cluster effect, looking at one star in a cluster is not looking at the DSO.

Your question is wrong, magnification is NOT the answer and to many aspects of astronomy it is meaningless, at least without other factors coming into play. Unless you have collected sufficent light then magnifying the result is detrimental. In general in astronomy you can collect very little light from the object.

[Double Kick Drum]

The advice above should help but i'd also suggest a little trial and error. There is nothing wrong with starting on a lower power and increasing it once you have found your target.

How much magnification an object can take depends on many things. If you suffer from light pollution. Many objects may already be washed out and may not take enlargement very well.

Planetary nebulae usually take quite a bit as many are smaller and have a high surface brightness. M57 the Ring nebula is a good example. It usually requires more than 60x magnification to see the hole in the middle.

Galaxies are comparatively large and usually only need low or medium power. Detail is difficult without very good skies and a 5" scope is likely to show shape and density but little more.

Large open clusters are usually best on lowest power. Smaller clusters (including globulars) are worth magnifying a little more. Dimmer globular clusters will not take as much and will not resolve stars so easily as say M13 or M5.

Double stars magnification will depend on the level of separation.

Other nebulae vary greatly and so magnification necessary will follow. With a few exceptions (Orion nebula, Omega nebula etc.) many nebulae have a low surface brightness and so enlargement will hinder viewing.

Happy hunting!

[Moonshane]

the answer to the question 'what magnification is best' is 'that which is provided by the eyepiece in your collection that resolves or best frames the object you are looking at / want to see'. as suggested this means different things for different objects and different scopes.

I like to see things in context wit their surroundings so although I can achieve reasonably high magnifications with my kit, I often choose to use lower powers to give a nicer overall 'picture'.

[michael.h.f.wilkinson]

Actually, perhaps the question itself should have been put: "what is the best exit pupil?" It is the exit pupil (or magnification relative to the size of the objective) which determines the optimal viewing conditions, as it dteremines the brightness of the image through the scope.

Very large, diffuse objects are best seen at between 3 and 6mm exit pupil, with a slight preference for 4-5, depending on how dark your sky is, and how far your own pupil will dilate, and the brightness and size of the object. For an F/5 scope this means an EP between 5x3 = 15mm to 5x6=30mm. In my F/10 scope the figures become 30-60mm. For galaxies an exit pupil between 1.5 and 2.5 mm is usually best, and an in my scope that becomes 15-25 mm (my 17 and 22mm EPs are the galaxy-hunting workhorses, with occasional help from the 12mm). In an F/5 scope 7-13mm EPs are usually the best. For planetary nebulae, I do use the 10, 12 and 17mm EPs most, indicating an ext pupil of 1-1.7mm, but then the Helix is different, being large and diffuse, as it showed up best in the 31 and 22mm EPs in my F/10 scope. Higher magnification only get you better views on many DSOs if there is enough aperture to give you sufficient photons.

[Naemeth]

I think I'll have to build my collection of EPs up to certainly include a 15mm (and probably 12 and 18) and the BST's will probably be the best for that.

[acey]

For viewing DSOs, start at low power and work up gradually until you find there is no more to be gained. When building up an eyepiece collection do the same thing.

You don't mention galaxies (unless you count them along with nebulae) but in that case, taking visually observable galaxies as a whole (rather than just the very brightest, such as Messiers) the appropriate magnification depends principally on the angular size of the target. With my 12" scope I do most of my galaxy observing at x188 or x250, occasionally going up to x375. This is mostly on small objects (angular diameter of the order of an arcminute). Herschel used a sweeping power of x157 on his 18.7" scope and this was sufficient for him to detect over 2000 DSOs.

That is for viewing under a dark sky (dark enough for the Milky Way to be seen with the naked eye). In a brighter sky it is much harder to go to high magnification without losing the object from view, hence people who observe at light-polluted locations tend to recommend viewing galaxies at lowest power.

[umadog]

Thinking by exit pupil rather than simply by magnification is good advice. 200x on a 16" Newt is a different thing to 200x on a 6" Newt. Remember that poor seeing will impede the views of DSOs in the same way that it impedes the views of planets. A turbulent atmosphere is less obvious in DSO observing but, just like planets, there are nights when you can use over 200x and there are nights when you can't. Pay attention to that and you'll see more. The magnification that is best for a given object will vary by scope and conditions.

[michael.h.f.wilkinson]

Thinking by exit pupil rather than simply by magnification is good advice. 200x on a 16" Newt is a different thing to 200x on a 6" Newt. Remember that poor seeing will impede the views of DSOs in the same way that it impedes the views of planets. A turbulent atmosphere is less obvious in DSO observing but, just like planets, there are nights when you can use over 200x and there are nights when you can't. Pay attention to that and you'll see more. The magnification that is best for a given object will vary by scope and conditions.

Good point. There are some planetaries which are near stellar. I picked one in Delphinus up with Olly Penrice's 20" Dob on the second night of observing, but not the first. The seeing was way better the second night, and that made it possible to resolve the tiny disc.

[cotterless45]

Most planetaries are pretty and small , as they're fairly bright, they'll take considerable magnification with the bonus of searching for the central star.

It amazes me the difference in colour and appearance of clusters and multiple stars in Newt compared with a refractor. There are no diffraction spikes and the clours are saturated.

With public viewing, it always produces the wow factor to see the Perseus double clusters and the Auriga clusters at x30, then fill the fov with the Pleiades at x60.

If you get stuck finding the right mag, unscrew the bottom of a x2 barlow and screw it onto the bottom of your ep, that'll give you x1.6.

The very basics I'd go for are x30,x60,x100.

Nick.

[acey]

Totally agree that exit pupil is the relevant parameter (along with observer's pupil size, target size, sky quality, optical quality). As to the "best" exit pupil, there can be no general rule: just work down in exit pupil (up in magnification) until nothing more is gained.

[8kids]

I have found I use 26mm and the zoom at around 14mm for most of my viewing, which is around 57x and 107x. Although if needs be I can zooooom in!

[Moonshane]

I was looking at Saturn last night and the image at 128x was pretty much solid, sharp, almost etched, and showed all of the detail visible at 160x and higher, including planetary banding and Cassini. in fact the moons seemed to be more obvious with less power - I saw at least four in my 6" dob. the higher magnifications though were sharp, sharp, bleary, sharp etc. higher mags are great when possible/required but not essential all the time. I did need some power to split Delta Cygni (about 250x) and Epsilon Bootis (about 200x).

[knobby]

If you get stuck finding the right mag, unscrew the bottom of a x2 barlow and screw it onto the bottom of your ep, that'll give you x1.6.

The very basics I'd go for are x30,x60,x100.

Nick.

I like that ... does that work with any Barlow?

---Co. founder of the Essex cloud dodgers social group (possibly? )---

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[michael.h.f.wilkinson]

I like that ... does that work with any Barlow?

---Co. founder of the Essex cloud dodgers social group (possibly? )---

http://stargazerslou...ection-Please-?

No. In fact, this is a solution which can increase chromatic errors. I am no great fan of barlows, though TV PowerMates and Meade TeleXtenders do work very well indeed. Removing one element of a simple barlow compromises a compromise solution.

[Stu]

Hi Michael

My understanding of this is that all you are doing is removing the entire lens element from the barlow (by unscrewing it from the tube) and then re fitting it to the base of the eyepiece. By reducing the distance you reduce the magnification. Does this still increase errors of other sorts, CA etc?

I totally agree in terms of barlows being a compromise, I use a Powermate and don't notice any reduction in image quality but previously had a relatively cheap one and hardly used it because it degraded things too much.

Stu

[acey]

My understanding of this is that all you are doing is removing the entire lens element from the barlow (by unscrewing it from the tube) and then re fitting it to the base of the eyepiece.

That would be my understanding too. I don't see that it would do anything to change whatever aberration the barlow introduced at its normal working distance from the eyepiece (if anything I would expect the aberration to be less, because the magnification increase is less). But I, too, am no fan of barlows: I bought a Meade apochromatic one years ago and quickly decided I had no real use for it.

More useful for deep-sky viewing, I find, is a zoom eyepiece: I use the Baader Hyperion 8-24mm. Small, faint nebulae or galaxies can be indistinguishable from stars at low magnification (or simply invisible). The zoom at low power can be used for finding the field, then magnification is increased until the target pops into view. The Baader Hyperion zoom is not cheap, and I wouldn't consider it a replacement for a whole eyepiece collection, but I get a lot of good deep-sky use out of mine.

[michael.h.f.wilkinson]

I may have misunderstood. If you remove the entire lens, and the screw that directly into the bottom of the EP, you have indeed just changed the distance to the focal image plane of the EP, which changes magnification (like the fine-tuning rings of Hyperion EPs). However, even that may have a (small) impact on lens quality. An achromatic negative lens is corrected for a particular distance to the image plane, and though small deviations from that distance are inevitable (unless parfocal EPs are used), a large deviation may mean chromatic errors increase. The fact that you are reducing magnification may mitigate that effect.

[Stu]

All clear, thanks Michael

Stu

[cotterless45]

If you're able to get a decent Barlow, such as a Tal,you'll soon make room in the ep case.

Nick.

[michael.h.f.wilkinson]

If you're able to get a decent Barlow, such as a Tal,you'll soon make room in the ep case.

Nick.

I am afraid in my case I end up finding yet another useful focal length, and end up buying another EP