Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_banner_30_second_exp_2_winners.thumb.jpg.b5430b40547c40d344fd4493776ab99f.jpg

The Warthog

Eyepieces - the very least you need.

Recommended Posts

All good advice and I think where I was heading to anyway. The is such a bewildering amount of information and opinion out there that it can become confusing for the novice.

That's where you guys come in with no allegiances to the profit margin and sound advice.

Share this post


Link to post
Share on other sites

Outstanding information, thank you very much.

I'm not completely new to astronomy, and have degrees in physics, so I understand basic optics.

I'd like some opinions. I've decided to buy a Celestron 130 SLT for a starter telescope. I like the fast F5, the ability to use 2" EPs, the Newtonian design, and the goto and tracking functions. It will be used occasionally in my dark-sky hilltop home in central Texas, USA. Most of the viewings will be with family and friends, mostly age 50+ (old eyes). I'm looking for fun and learning, not astrophotography or serious deep space. If I find I enjoy some particular aspect of the hobby, I can always buy more telescope.

With that in mind, I purchased the Celestron 2" eyepiece kit, 94305, which adds 26mm, 32mm, and 40 mm 2" E-Lux plus a 2x Barlow to the 25mm and 9mm 1.25" EPs that come with the telescope. I figure these will give me plenty of options for comfortable viewing of extended objects in the 4000-object catalog in the telescope's computer.

I am thinking of buying a Celestron X-Cel LX 5mm 1.25" EP for planetary viewing. I'm hoping the longer eye relief will make it more pleasant viewing than a Plossl. Amazon sells them for $70. While that's a significant fraction of the cost of the telescope, I realize that EP optics are often the limiting factor. I also rationalize that I may use this set of EPs on a larger instrument if I find I really do enjoy and reach the limits of the 130mm aperture.

I'd appreciate any comments, criticisms, or suggestions for alternative choices for a high magnification eyepiece for this telescope. Thanks in advance.

Edited by ahutch

Share this post


Link to post
Share on other sites
great post! im thinking of buying a f/5 scope,would the 3 ep you suggested for f/5 all be plossls?

Not necessarily. That is decided by what you can afford. The advice on the focal length of the ep is the same, whatever the type of ep.

Share this post


Link to post
Share on other sites

Sticky eyepieces? That really is the very least you need.

Anyone done that gag yet?

Share this post


Link to post
Share on other sites

Being new to astronomy, ive found there is so much to learn before you even start looking towards the heavens, posts like this are a great help. cheers

Share this post


Link to post
Share on other sites
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!

Being dyslexic I get a little lost in this sorry, that why I am still asking about it.

Share this post


Link to post
Share on other sites

Really informative post. Thank you.

Will be a very useful guide when It's time to upgrade from the standard 10mm & 25mm ep's that came with my scope

Share this post


Link to post
Share on other sites

Just want to say thanks for this post. So much information.

I'm picking up a 150P DS soon hopefully and seeing as it only comes with a 25mm, I'm picking up a 8mm and 2x Barlow with it to start with.

I hope I've made the right decision based on everything you said! :)

Thanks again.

Share this post


Link to post
Share on other sites

Very useful info, thanks! As a beginner, I'm finding my glasses a right pain, especially since I'm sufficiently astigmatic to be unable to go without. I'll look into some of these LV types you mention.

Share this post


Link to post
Share on other sites

Great thread, I currently have an 8" Skywatcher type (Helios as they were) and the two Plossls of 10 and 20mm that were supplied plus a 5mm Vixen Lanthanum and a cheap 32mm widefield Plossl which I use mainly for star hopping, I get on alright with this set, but reading through this thread and recent observations convinced me to get a really good eyepiece, so I have sent for a Baader Hyperion 13mm and a TS Apochromatic 2.5 Barlow. Later once I figure out how it works I can get the tuning rings and spacers for the eyepiece which should increase its range considerably. I am definatley looking forward to (hopefully) an increased contrast to enable those faint fuzzies to betray there presence a little better.

Thanks for all the informative posts.

Share this post


Link to post
Share on other sites

Thanks for the post, some really good info! :hello2:

For my Nexstar 127 Mak I get the following results:

8.85mm : Maximum EP

14.75mm : Medium-high EP

23.6mm : Medium-low EP

35.4mm : Lowest EP

I've already purchased a 10mm Hyperion Baader with the 28mm FTR, which should give me my maximum powered EP. I'll keep the stock 25mm for the medium-low EP (for now at least) and I'm looking at getting a 32mm GSO plossl to fit the lowest powered EP category.

The question is, what Hyperion would be best suited for the medium high? The 13mm or the 17mm?

I'm thinking I should have gone 13mm instead of the 10mm to begin with :)

Share this post


Link to post
Share on other sites
Thanks for the post, some really good info! :hello2:

For my Nexstar 127 Mak I get the following results:

8.85mm : Maximum EP

14.75mm : Medium-high EP

23.6mm : Medium-low EP

35.4mm : Lowest EP

I've already purchased a 10mm Hyperion Baader with the 28mm FTR, which should give me my maximum powered EP. I'll keep the stock 25mm for the medium-low EP (for now at least) and I'm looking at getting a 32mm GSO plossl to fit the lowest powered EP category.

The question is, what Hyperion would be best suited for the medium high? The 13mm or the 17mm?

I'm thinking I should have gone 13mm instead of the 10mm to begin with :)

If you get the 17mm Hyperion then you can also by the 14mm Fine Tuning Ring which will transform it in to a 13mm hyperion.

Link for the fine tuning rings below

http://www.firstlightoptics.com/baader-planetarium/baader-hyperion-finetuning-ring.html

Share this post


Link to post
Share on other sites

Thanks, Warthog. That was a fantastic informative read.

I had never thought about the speed of the scope restricting the choice of EP's until just now.

It almost makes me think that I should have bought a top-line super-expensive eyepiece first and then decide on which scope to attach to it! :grin:

Stevie

Edited by Bluemaris

Share this post


Link to post
Share on other sites

sorry if I'm being a bit thick

but can anyone clarify for me

Focal length is the distance from the mirror at the bottom of my reflector to the mirror mounted in the centre of the tube below the eyepiece ??? or do I include the distance from the little mirror to the eyepiece ?

Aperture is the diameter of the tube ?

My eyepiece says H20mm , so I need to bin that ?

Edited by bobglen

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.