Eyepiece Choices

[technofetishism (Nick)]

Now, this isn't looking at eyepiece types, focal lengths, manufactures or telescope type, but more in regards to pure magnification bands.

I'm currently planning some eyepiece replacements and growing the collection a small amount and was trying to figure out what magnification bands are desirable in order of usefulness. Just like a super high maginification isnt that much use 95% of the time, and that a very low magnification wont allow you to look at that many objects.

Any thoughts on the matter from those of you who have grown a reasonable collection size?

[Vegeta (Ibrahim)]

4 - 5 eyepieces are all that you really need. The first eyepiece I would go for would be a big widefield 20 - 30mm depending on the scope. these would normally be used to view extended DSO's that span larger than the moon like the andromeda galaxy which cover 6 full moons of the sky! their also used for Hunting down objects.
As for the second eyepiece, probably 15 - 18mm for a closer veiw of faint DSO's. The FOV of the eyepiece would vary depending on the scope and what you intend to look at.
3rd choice for me would be a 9 - 12mm for globular clusters, PN's and planets. As for a 4th choice, a 5 - 6mm eyepiece for really high magnification on planets and resolving the tight globulars and PN's. Keep in mind, these will only work when seeing is steady. If you get a Barlow lens, it will double your eyepiece collection by doubling the magnification on a given eyepiece. Remember it all depends on the scope and what you intend to view.
The formula for working out magnification is:
Telescope focal length/eyepiece focal length
Hope this helps

[jjjnettie (Jeanette)]

I have a fair collection of ep's. None of them premium. But I still enjoy the views nonetheless.
I've listed the top 5 in the order of which gets the most use.
15mm
25mm
32mm
9mm
4mm

[technofetishism (Nick)]

Without knowing what the focal length of the scope your using, giving an eyepiece in mm doesnt answer the question.

Things like 10-20x, 40x, 100x are useful.

[omegacrux (David)]

In order of my most used ep's
17mm 44x used most
30mm 25x gso it gets a lot of use nice widefield
21mm 35x Saxon was my fav till got the 17 ultima
9mm 83x Orion LER good on Saturn and moon
6mm 125xTMB rarely used only on good seeing
Most of mine bought on budget sec-hand
Cheers

David

[AstralTraveller (David)]

My approach was to start with the widest fov I could get. I wanted a 31mm Nagler but wound up with the 28mm WO UWAN. I then increased magnification in steps of about 1.5. So I have Naglers of 20mm, 13mm and 9mm. That takes me up to 133x on the 6" and 10" and 200x on the 16". The 16 is pretty new so I don't have a good feeling for what will be well-used but I find uses for all my eps on the others, with the 20 and 13 getting most use. Remember though that with bigger scopes you will tend to look at fainter, and so normally smaller, objects.

I then have a my planetary/lunar eps and for these I chose orthoscopics. I have 10mm, 7mm and 5mm. The 7 has generally had the most use but with the 16" it might be the 10 that I use most. Generally the 5 is too much mag for the conditions.

[wavelandscott (Scott)]

There was a very good article on Televue's website that outlined Al Nagler's advice og good magnifications to have...

While he advocates for his own gear, the information contained in the article is stiil useful even if you don't choose the Televue brand

[ausastronomer (John Bambury)]

Originally Posted by technofetishism

Without knowing what the focal length of the scopeyour using, giving an eyepiece in mm doesnt answer the question.

Hi,

I agree with this comment. Without knowing what telescope(s) you have it's impossible to recommend anything. You run a completely different set of eyepieces in a 25" Obsession to what you run in a 80mm refractor.

Tell us your Scopes and we can help.

Cheers,
John B

[technofetishism (Nick)]

John, the telescope doesnt matter in this case at all.

magnification is magnification, its a result of eyepiece and telescope. A combination that gives 20x will still give 20x in a 25" SDM or a 80mm ED. In either case its not going to be very useful for planetary.

I suppose if the question is phrased in the manner of something like, what would be the ideal magnifications for OC, GC, Nebula, Planetary etc targets it might be more clear.

[Vegeta (Ibrahim)]

we can't give ideal magnifications if you don't know the apeture of the scope. e.g. If I told you to use 400x on the planets under excellent seeing, but your scope is only 4", the magnification would be too high, and the image distorted. I can give you generalized bands, but nothing specific.
OC's - Low magnification (60x and below)
GC's - Medium to high (100x - 250x)
Planets - low to high (depending on seeing)
PN's - Medium to high (100x - 200x)
Nebulae - low (60x and below)
Galaxies - low to high (depending on the galaxy e.g. M104 handles 250x well in my 12", but i can only view M83 at 60x - 100x due to surface brightness)

Key: Low = 60x and below Medium = 60x - 150x High = 150x +

Remeber that this is only a general idea and the magnification bands may not be suitable for your scope due to aperture restrictions.

[alocky (Andrew lockwood)]

Actually - I use the same eyepieces in both, as I'm too tight to buy a set for my refractors! You need enough of a range to give low to high power, and I've found eyepieces like my 17mm and 12mm nagler are both in the sweet spot for my 25"f5, 10"f4.5, 4"f15 and 4"f5 (OK it's 102mm, not 80!). Obviously the 4mm radian doesn't see a lot of use in the 25", but 4 to 5 eyepieces from 4mm to 22mm will definitely keep you out of trouble regardless of aperture and fl - although the magic 50x per inch limit for scopes above 4" is a fairly reasonable thing to keep in mind.
cheers,
Andrew.

Originally Posted by ausastronomer

Hi,

I agree with this comment. Without knowing what telescope(s) you have it's impossible to recommend anything. You run a completely different set of eyepieces in a 25" Obsession to what you run in a 80mm refractor.

Tell us your Scopes and we can help.

Cheers,
John B

[ausastronomer (John Bambury)]

Originally Posted by technofetishism

John, the telescope doesnt matter in this case at all.

It matters a whole lot.

For instance in a 14"/F10 SCT (FL 3.56 metres) you would choose either a 40mm ~ 70 deg Apparent Field of View (AFOV) eyepiece or a 55mm/56mm plossl with a ~50 deg AFOV eyepiece as your lowest power widefield eyepiece. These eyepieces give the maximum True Field of View in that scope. Similarly, those eyepieces would also be the best choice for low power widefield views in say a 100mm F8 refractor (FL 800mm) In those respective scopes, an eyepiece say for example a 40mm Pentax XW (70 deg AFOV) gives 90X magnification and a TFOV of 47'. The same eyepiece in the 100mm/F8 refractor will give 20X with a TFOV of 3.5 degrees. Worlds apart.

Then insert that same eyepiece in a 32"/F4 reflector (FL 3.25 metres) and its almost unusable because of the 10mm exit pupil it creates. With that 32"/F4 reflector your longest "practical" eyepiece focal length would be ~30mm. Lets take a 30mm Pentax XW (70 deg AFOV). In the 32"/F4 reflector that eyepiece gives 108X with a TFOV of 39'.

Let's go a step further and consider Jimmy Lowrey's 48"/F4 reflector (FL 4.9 metres). The longest focal length eyepiece he uses in that scope is a 31mm Nagler T5. In his scope that eyepiece gives 157X with a TFOV of a fraction over .5 degrees. Not your classical low power widefield view.

A combination that gives 20x will still give 20x in a 25" SDM or a 80mm ED.

They might both be giving 20X but there's a whole world of difference due to the significantly greater resolution of the larger aperture telescope. A 25" telescope at 60X is a whole world of difference to a 6" telescope at 60X.

Further there is no eyepiece available which will allow you to get anywhere near as low as 20X in a 25" telescope. Assuming a 25"/F5 you would need an eyepiece with a focal length of 160mm which would need a 4" plus diameter barrel, so that it did not vignette; and it would create a 32mm exit pupil which an owl would find difficult to use.

I suppose if the question is phrased in the manner of something like, what would be the ideal magnifications for OC, GC, Nebula, Planetary etc targets it might be more clear.

This depends enormously on the aperture of the telescope.

In a large telescope most of the bigger brighter globular clusters are easily resolved at low power. For instance in my 18" Obsession most of the Messier and Caldwell globulars are easily resolved at under 100X. However, you need a lot more magnification to resolve some of those same globulars in a 6" or 8" telescope and in many cases a smaller refractor will not resolve them, regardless of magnification. I like to observe globulars at about 120X in my 18" Obsession, using a 20mm Pentax XW with a paracorr, or my 22mm Nagler with a paracorr for 110X. In my 14" SDM I usually observe globulars using my 14mm Pentax XW with a paracorr for 130X or my 12mm Nagler for 153X. The effect can sometimes be similar with open clusters. The bigger the telescope the less magnification you need to make them look good because of the brighter image and greater resolution at any given magnification.

Cheers
John B

[sopticals (Stephen)]

Well said John. Even a blind man could follow your desciption. And yes aperture rules.

[technofetishism (Nick)]

Very good point John B regarding TFOV. For some reason that had slipped my mind while thinking about this matter.

Basically, the main thing your saying is for a given aperture, leaving the AFOV an eyepiece constant, what can be regarded as medium power may be said to be say 10-20x per inch of scope? And for planets that may be up to 40-50x per inch?

[technofetishism (Nick)]

Would you happen to know anything worth reading to really get a better understanding of how and why everything interacts with everything else?

[alocky (Andrew lockwood)]

Originally Posted by technofetishism

Would you happen to know anything worth reading to really get a better understanding of how and why everything interacts with everything else?

Telescope Optics by Rutten and Van Rooj. Available from Wilman-Bell online. At a higher level you could get Born and Wulff's treatise on Optics.
The first is extremely accessible and perfect for an amateur astronomer.

[Varangian (John)]

Great post, thanks.

Originally Posted by ausastronomer

It matters a whole lot.

For instance in a 14"/F10 SCT (FL 3.56 metres) you would choose either a 40mm ~ 70 deg Apparent Field of View (AFOV) eyepiece or a 55mm/56mm plossl with a ~50 deg AFOV eyepiece as your lowest power widefield eyepiece. These eyepieces give the maximum True Field of View in that scope. Similarly, those eyepieces would also be the best choice for low power widefield views in say a 100mm F8 refractor (FL 800mm) In those respective scopes, an eyepiece say for example a 40mm Pentax XW (70 deg AFOV) gives 90X magnification and a TFOV of 47'. The same eyepiece in the 100mm/F8 refractor will give 20X with a TFOV of 3.5 degrees. Worlds apart.

Then insert that same eyepiece in a 32"/F4 reflector (FL 3.25 metres) and its almost unusable because of the 10mm exit pupil it creates. With that 32"/F4 reflector your longest "practical" eyepiece focal length would be ~30mm. Lets take a 30mm Pentax XW (70 deg AFOV). In the 32"/F4 reflector that eyepiece gives 108X with a TFOV of 39'.

Let's go a step further and consider Jimmy Lowrey's 48"/F4 reflector (FL 4.9 metres). The longest focal length eyepiece he uses in that scope is a 31mm Nagler T5. In his scope that eyepiece gives 157X with a TFOV of a fraction over .5 degrees. Not your classical low power widefield view.

They might both be giving 20X but there's a whole world of difference due to the significantly greater resolution of the larger aperture telescope. A 25" telescope at 60X is a whole world of difference to a 6" telescope at 60X.

Further there is no eyepiece available which will allow you to get anywhere near as low as 20X in a 25" telescope. Assuming a 25"/F5 you would need an eyepiece with a focal length of 160mm which would need a 4" plus diameter barrel, so that it did not vignette; and it would create a 32mm exit pupil which an owl would find difficult to use.



This depends enormously on the aperture of the telescope.

In a large telescope most of the bigger brighter globular clusters are easily resolved at low power. For instance in my 18" Obsession most of the Messier and Caldwell globulars are easily resolved at under 100X. However, you need a lot more magnification to resolve some of those same globulars in a 6" or 8" telescope and in many cases a smaller refractor will not resolve them, regardless of magnification. I like to observe globulars at about 120X in my 18" Obsession, using a 20mm Pentax XW with a paracorr, or my 22mm Nagler with a paracorr for 110X. In my 14" SDM I usually observe globulars using my 14mm Pentax XW with a paracorr for 130X or my 12mm Nagler for 153X. The effect can sometimes be similar with open clusters. The bigger the telescope the less magnification you need to make them look good because of the brighter image and greater resolution at any given magnification.

Cheers
John B

[ausastronomer (John Bambury)]

Originally Posted by technofetishism

Basically, the main thing your saying is for a given aperture, leaving the AFOV an eyepiece constant, what can be regarded as medium power may be said to be say 10-20x per inch of scope? And for planets that may be up to 40-50x per inch?

IMO this is a much better way to look at this. However I would break it up a little differently and in respect of Telescope aperture. I wont refer to telescopes < 4" as they are IMO suitable only as "finderscopes", or for imaging

4" to 10" telescope (low power 10X to 50X, medium power 50X to 150X, high power over 150X)

10" to 20" telescope (low power 30X to 60X, medium power 60X to 180X, high power over 180X)

20" and larger telescope (low power 50X to 100X, medium power 100X to 200X, high power over 200X.

While 200X in a 20" telescope is still only 10X per inch of aperture and might really seem to be "low power" in many peoples eyes for such a large aperture, you are severely limited by seeing conditions and thermal equilibrium and in this regard larger aperture telescopes are more severely affected than smaller aperture telescopes. I have had countless nights where you cannot run past 200X or so in the 18" telescope because of local seeing or thermal stabilisation of the mirror, yet I can easily run 240X to 280X in my 10" scope under the same conditions. Of course when it all comes together with seeing and thermals the bigger scope can pull 700X plus, which you cannot reach with smaller aperture scopes.

You also need to remember that you can see a lot more detail and get a much brighter image with say an 18" telescope at 200X than what you can in say a 10" telescope. Similarly and as a result of the same physics, double stars separate at much lower power in larger telescopes than they do in smaller telescopes. Consequently on most targets you don't need as much magnification in a large telescope as you do in a small telescope to get the same view and detail.

Cheers,
John B

[ausastronomer (John Bambury)]

Originally Posted by technofetishism

Would you happen to know anything worth reading to really get a better understanding of how and why everything interacts with everything else?

The best place to start is this website.

http://www.telescope-optics.net/

It is free and has lots of good information, although in many cases (as with most complex optical theory) the mathematics can get quite complicated for people not that way inclined.

An excellent book is Telescope Optics by Rutten and Van Venrooij.

http://www.bintel.com.au/Accessories...oductview.aspx

This book is aimed at the amateur astronomer as opposed to the professional optician and provides a good explanation of most of the optical concepts the amateur astronomer might want to be aware of. Again however the amthematics might be daunting for some but you can just skip the complex maths and grasp the concepts.

Cheers,
John B

[Colin_Fraser]

Excellent description. I certainly learned a lot from this snippet, thanks John.

Originally Posted by ausastronomer

It matters a whole lot.

For instance in a 14"/F10 SCT (FL 3.56 metres) you would choose either a 40mm ~ 70 deg Apparent Field of View (AFOV) eyepiece or a 55mm/56mm plossl with a ~50 deg AFOV eyepiece as your lowest power widefield eyepiece. These eyepieces give the maximum True Field of View in that scope. Similarly, those eyepieces would also be the best choice for low power widefield views in say a 100mm F8 refractor (FL 800mm) In those respective scopes, an eyepiece say for example a 40mm Pentax XW (70 deg AFOV) gives 90X magnification and a TFOV of 47'. The same eyepiece in the 100mm/F8 refractor will give 20X with a TFOV of 3.5 degrees. Worlds apart.

Then insert that same eyepiece in a 32"/F4 reflector (FL 3.25 metres) and its almost unusable because of the 10mm exit pupil it creates. With that 32"/F4 reflector your longest "practical" eyepiece focal length would be ~30mm. Lets take a 30mm Pentax XW (70 deg AFOV). In the 32"/F4 reflector that eyepiece gives 108X with a TFOV of 39'.

Let's go a step further and consider Jimmy Lowrey's 48"/F4 reflector (FL 4.9 metres). The longest focal length eyepiece he uses in that scope is a 31mm Nagler T5. In his scope that eyepiece gives 157X with a TFOV of a fraction over .5 degrees. Not your classical low power widefield view.

They might both be giving 20X but there's a whole world of difference due to the significantly greater resolution of the larger aperture telescope. A 25" telescope at 60X is a whole world of difference to a 6" telescope at 60X.

Further there is no eyepiece available which will allow you to get anywhere near as low as 20X in a 25" telescope. Assuming a 25"/F5 you would need an eyepiece with a focal length of 160mm which would need a 4" plus diameter barrel, so that it did not vignette; and it would create a 32mm exit pupil which an owl would find difficult to use.



This depends enormously on the aperture of the telescope.

In a large telescope most of the bigger brighter globular clusters are easily resolved at low power. For instance in my 18" Obsession most of the Messier and Caldwell globulars are easily resolved at under 100X. However, you need a lot more magnification to resolve some of those same globulars in a 6" or 8" telescope and in many cases a smaller refractor will not resolve them, regardless of magnification. I like to observe globulars at about 120X in my 18" Obsession, using a 20mm Pentax XW with a paracorr, or my 22mm Nagler with a paracorr for 110X. In my 14" SDM I usually observe globulars using my 14mm Pentax XW with a paracorr for 130X or my 12mm Nagler for 153X. The effect can sometimes be similar with open clusters. The bigger the telescope the less magnification you need to make them look good because of the brighter image and greater resolution at any given magnification.

Cheers
John B