Eyepieces - the very least you need.

[Naughty Neal]

Yes a UHC and possibly an Olll filter will increase the chances of teasing  out the nebulae .

Most of us will be looking at the not only  #8 but also   #2 from your list and be thinking lucky sod.

Edited February 14 by Naughty Neal

[Commander Cody]

On 28/05/2009 at 22:52, The Warthog said:

I have considered the question of what a person needs in his eyepiece kit, as a bare minimum, for quite a while. Personally, I don't have a lot of disposable income, and I recognize that a lot of amateur astronomers are getting along on a shoestring budget. So, if you can afford to go out and buy a full set of Naglers, or even Radians, go ahead, this article isn't for you. It is for those of us who have to choose between a new eyepiece and a new spring jacket, and are already garnering disapproving looks from our partners for buying that natty little refractor at a higher price than they really, truly expected.

I will talk first about scopes on equatorial or tracking mounts, and later about Dobsonians.

I am assuming that, as we don't have a lot of money, we are not buying large catadioptics or refractors, and cannot afford a Newtonian of larger than 8". These general principles apply to most scopes, however.

SCOPES ON EQUATORIAL, GOTO, OR TRACKING MOUNTS

I am going to talk about Plossls, mostly, as they are the best value for money. If you get a branded Plossl, you will seldom get a piece of junk. You can expect reasonable sharpness across most of the field in all but the fastest scopes. Plossls also have a field of view of 50 - 52º, which is quite reasonable. I am also going to suggest a set of three or four eyepieces, and no Barlow,except in the case of a fast scope.

You should have a high power, a medium-high and/or medium-low power eyepiece, and a low power eyepiece. The eyepieces that came with your scope probably fill the medium-high and low power slot. If they are satisfactory, keep them for now. If they are marked 'H' or 'SR' don't even think about keeping them! If they are marked with a 'K', they are Kellners, which are generally acceptable eyepieces, but a little limited on field of view, being about 45º, usually.

Find out the focal ratio of your scope. It should be printed on a plate on the scope, usually near the focuser, and be represented by a number like f/5 or f/8. F/6 or lower is a fast scope, and f/7 or higher is an intermediate to slow scope. Scopes with focal ratios of f/8 or higher are generally more forgiving of lower-quality eyepieces, while fast scopes tend to reward lower-quality eyepieces with fuzzy stars anywhere from 1/3 to 1/2 way from the edge to the centre.

If you can't find the focal ratio, but you know the aperture and focal length, the focal ratio is (focal length/aperture).

Take your focal ratio, and multiply it by 3/4. So, if you have an f/8 scope, the result is 6. If you have an f/10 scope, the result is 7.5. This result is the length in millimetres of your high power eyepiece. It will give about 2/3 of the theoretical maximum power of your scope. This is the actual maximum if you do not always enjoy perfect seeing and transparency. If you have a 100mm scope, this eyepiece will give 133x.

IF YOU HAVE A FAST SCOPE, say, f/5, this formula will suggest a 3.75mm or 4mm eyepiece. Looking through a Plossl at this length is a miserable experience. If this is the case, I would suggest you buy an eyepiece with a length equal to 1½ times your focal ratio, and buy a 2x Barlow lens in the same price range as your eps. These purchases give you your high power and medium-high power magnifications, so skip the next paragraph.

Now multiply your focal ratio by 1¼. For our f/8 scope, the result is 10, and for an f/10 scope, the result is 12.5. This is the length of your medium-high power eyepiece. For our 100mm scope, it gives a magnification of 80. Eyepieces in these lengths are not hard to find, and you can go up or down a millimetre if your dealer doesn't stock them.

Multiply your focal ratio by 2, now. By now, you can do the math yourself! In our 100mm scope, this gives a magnification of 50. This is your medium-low power eyepiece, and your low power eyepiece is given by multiplying your focal ratio by 3, and you get a magnification of 33 in your 100mm scope. IF YOU HAVE A FAST SCOPE, you want an eyepiece of 3 to 4 times your focal ratio, or 15 to 20 mm for an f/5 scope as your medium-low power eyepiece, and about 5 times your focal ratio for your low power eyepiece.

An eyepiece of 5 times your focal ratio also gives you an 'exit pupil' of 5mm. This is the longest eyepiece you want to use if you are older, as this exit pupil is approximately equal to an older (45+) person's maximum pupillary dilation. You can't use more light than that. If you are younger, you could go up to 7 times your focal ratio, or an exit pupil of 7mm.

To summarize, for an f/8 scope, we suggest a kit consisting of 6, 10, 16 and 24mm. For an f/10 scope, 7.5, 12.5, 20 and 30mm. For an f/5 scope, 2x Barlow, 8, 18, and 25mm.

If your budget allows for only three eyepieces, drop one of the medium power eyepieces. If you are a lunar/planetary observer, then we would suggest dropping the medium-low eyepiece, and if you are a DSO observer, the medium-high eyepiece. In the latter case, we could suggest dropping the high power, but let's face it, there will always be times you want to get a good look at Saturn, or a good planetary nebula, so keep the high power.

DOBSONIANS

Dobsonians tend to be large, fast scopes. If your Dob is 6" or less, you can safely follow the guidelines for the scopes listed above, as the highest magnification this will give you is 200.

At about 200x, it gets hard to follow things with a Dob. Some people can do it, and your ability to follow objects will improve with time, but 200x is a good start. You will want to have an eyepiece kit between 200x, and a 5mm (or 7mm if you are a youngster) exit pupil. Suppose you have a 10", f/5 Dob. You will have a focal length of 1250mm, and will get 200x with a 6.25mm eyepiece. In practical terms, a 6.5 to 7.5mm eyepiece will be what you will find available. To get a 5mm exit pupil out of a 250mm mirror, you will need an eyepiece that gives you 50x. This means a 25mm eyepiece. To get a 7mm exit pupil out of the same mirror means a magnification of 36, and a 35mm eyepiece.

Having decided on your low and high power, it is fairly easy to pick two more eyepiece focal lengths that will fill in the gap. If your spread is 6mm to 25mm, try 10mm and 16mm as your intermediate lengths. If the spread is 6mm to 35mm, then use 12mm and 20mm as your intermediate eyepieces.

So, for an 8" f/5 Dob, you would be getting something like a 5mm, 10, 16 and 25mm.

These guidelines will give you a useful set of eyepieces without breaking the bank. You can buy one eyepiece a month until you have your set, and use the eyeieces you have until your set is complete.If you can afford slightly better eyepieces, then buy those, with the length guidelines still in mind. If you have a fast scope, ask specifically if the eyepiece you are considering is appropriate for a fast scope. Some less expensive wide-angle eyepieces perform well only in a f/8 or slower scope, and you don't want to buy a set of these with a fast scope.

Best wishes, and enjoy your new hobby!

WARTHOG- I was so blown away by your post that I have come back multiple times to just let the info sink in,  just a brilliant menu for us beginners to absorb again and again. Thank you so much, please continue with your bits of knowledge they are much appreciated'

[bosun21]

11 hours ago, Commander Cody said:

WARTHOG- I was so blown away by your post that I have come back multiple times to just let the info sink in,  just a brilliant menu for us beginners to absorb again and again. Thank you so much, please continue with your bits of knowledge they are much appreciated'

@WARTHOG hasn't visited SGL for 4 years.

[Commander Cody]

Like a sponge I am trying to soak up as much info as possible, thanks to all who participate in this and glad this info will always be available in this thread.

[Commander Cody]

In thinking about EPS I have a z130 and SW8" waiting on the side, not using yet but also eventually using both when thinking of EPs. So I must think of eventually switching to the 8" and any EP bought must do justice to both. Am planning to spend the best deal on any premium EP if I catch a deal. I am currently viewing Baader Hyperions and Baader Planetariums, there is a huge difference in price for what reason I cannot figure out reading the specs. So I have decided to go with the Baader Planetarium as am not planning on photography, if that was the point. but what do you think of the choice? I do have all the plossls, GSOs in the beginners kit, but am thinking ahead as the SW 8' will be my baby. Thank you for reading my blabber but trying to move on and up. Joe

[Bivanus]

@Commander Cody , Baader PLANETARIUM is a company that makes some pretty nice astronomy kit including a couple series of eyepiece lines like :

CLASSIC ORTHO: Apparent Field of View 52° /  Focal length 6-10-18mm / with correspondent Eye Relief 5-8-15mm 
CLASSIC PLOSSL: Apparent Field of View 50°  / Focal length 32mm with Eye Relief 21mm 
HYPERION: Apparent Field of View 68° / Focal length 5-8-10-13-17-21-24mm with Eye Relief 20mm
ASPHERIC: Apparent Field of View 72° / Focal length 31-36 mm /Eye Relief 17-19mm  
MORPHEUS: Apparent Field of View 72° / Focal length 4,5-6,5-9-12,5-14-17,5mm with correspondent Eye Relief 17.5-18.5-21-20-18.5-19mm 

To get a feeling of Eye Relief, anything under 10mm eye relief means you kinda have to clean eyelashes and skin oils from the EP after every use aaaand  😁 if used vigurously you'll have swollen eyelids in no time. Personally I try to stay at 15 or above. 

To get a feeling of the AFOV quite a lot of people me included use astronomy.tools, here's a chart for the 10mm Ortho and Hyperion , 12.5mm Morpheus and 31mm Aspheric for a Sky-Watcher - Quattro 8" 200P f/4

[bosun21]

5 hours ago, Bivanus said:

MORPHEUS: Apparent Field of View 72° / Focal length 4,5-6,5-9-12,5-14-17,5mm with

The Morpheus range are actually 76°.

[Bivanus]

Yup,  they are 76° but I hit Copy/paste from aspherics and forgot to change  😂