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

I need to work out what's the best xmas present option. . . bst 25mm, a planetary 7mm, a 32mm of some description or maybe a better barlow

Share this post


Link to post
Share on other sites

Given that choice, knowing what I have, and knowing that we have different scopes, I don't think you would be unhappy with a 32mm  Panaview.  However your telescope suggests an EP of between 28mm to 14mm as your optimum EP! the minimum magnification of about 19x power equates to a 47mm EP and your highest magnification suggests 260x power with a 3.5mm EP.

Again this is almost the limits of your telescope, I would suggest to try and stay inside the limits for the better results. You have near as dammit the 7mm by way of the BST 8mm, and with your present 2x Barlow , you  could Barlow your 15mm BST giving you 7.5mm. so I wouldn't necessarily buy a dedicated 7mm just yet? Optimum could suggest anything around the 25mm mark, so at some time, yes a replacement BST 25mm will give you a wider field of view over the SW25 and 47mm is just a waste, so maybe be happy with the BST 25mm again, or a 28 or 32mm Panaview. As these two Panaviews are the same price, I would opt for the larger focal length.

Edited by Charic

Share this post


Link to post
Share on other sites

No Barlow at all would be the best option as there is less glass in the focal train? Barlowing  helps build your EP selection by effectively letting you use one eyepiece covering two focal lengths.

Try barlowing the 8mm or the 15mm on the Moon, see for yourself. Say you use the 15mm, the image will be closer when Barlowed ie 7.5mm but unless you see a discolouration I would not bother about changing the Barlow.

Edited by Charic

Share this post


Link to post
Share on other sites

Yes only in 2". This gives you the very good 70° afov. However I now believe your telescope will only accommodate a 1.25" EP, so back to the BST 25mm as a reliable 60° afov EP. That's the longest focal length  for a BST Starguider. £49.

  • Like 1

Share this post


Link to post
Share on other sites

Also. . does the 'no Barlow is best' statement not lead me back to thinking maybe a 5/6/7mm ep may be worthwhile for Saturn viewing? Money better spent on this than a barlow?

Share this post


Link to post
Share on other sites

Useful page , and see what your saying now. . just can't help thinking a 5mm bst would be better value than a Barlow for rare occasions of planet viewing.

Page is bookmarked for future use! :)

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!

Really great post! Thanxs for sharing this!

Share this post


Link to post
Share on other sites

excellent thread i have just got my skywatcher 130/900 with motor and am already looking at possible EP upgrades,only managed to get out in the back garden on tuesday when it came and has been wind and rain since then,not to mention being on nights thurs,fri,tonight and tomorrow night. :sad:

Share this post


Link to post
Share on other sites

I've been away for a very long time, and to be honest haven't had my scopes out very often since my second heart attack (and quadruple bypass) three years ago. I hope to do better this spring, as my health has gotten a lot better although I still can't go out on the -10 nights we've been having. I noticed a very nice crescent moon last night, and would have liked to get my refractor out, but it's just too damn cold.

I really appreciate all the kind comments I've seen in response to my post. It seems to have done what I wanted to do, which is to give a reasonable and simple way to create a usable and versatile set of eyepieces that can grow as you save your pounds.

I'll try to hang about here more often. This was my favorite of all the astronomy forums I visited, and the longest lasting by now.

  • Like 5

Share this post


Link to post
Share on other sites

Happy New Year to you Warthog - it's good to hear from you again ! :smiley:

Sounds like you have had a rough time of it but I'm pleased that you are picking up now.

Your piece on eyepieces is a "classic" in my humble opinion and has deservedly been one of the most visited sections of the forum :icon_salut:

Hope to see you on the forum again from time to time !

  • Like 1

Share this post


Link to post
Share on other sites

Wondered where you had got to.

Sorry to hear about the heart problems, guess you won't be going over the falls in a barrel any time soon. :rolleyes: :rolleyes:

What are the temperatures like in your bit of Canada?

Think another Canadian has sort of been buried under snow. I know that your area can get very heavy falls owing to the effect of Lake Ontario.

Share this post


Link to post
Share on other sites

Well, no going over the falls in a barrel, but I am thinking about taking up horseback riding, and maybe hitching a ride in a hot air balloon. They are both on my bucket list.

We have had only two significant snowfalls this winter, and at present all the snow has meltted. We are getting temperatures as low at -13, but no significant snow in the forecast.

Share this post


Link to post
Share on other sites

The balloon ride sounds good, pick one that knows how to land.

Friends in Canada (Alberta and BC) advised me against riding a horse (thinking of a trekking holiday) they said it was not as easy as it looked. I said I thought all you had to do was get the horse to understand where he came in the food chain. :eek: :eek:

Have you nowhere to set up a small refractor for the occasional view but out of the weather a bit?

I suppose standing at a window with one might make neighbours think the wrong thing. :grin: :grin: :grin:

Share this post


Link to post
Share on other sites

Well I have finally finished slimming down my eyepiece collection and I have reached I hope equilibrium. I based my thinking around some of what Warthog says in his OP.

I used to have 12, mainly high quality ep's and decided, given the amount of viewing I am doing that I could no longer tie up so much money in them. I had a 5,6,7,8,9,10,12,14,17.3,22,25 and a 28mm eyepiece.

So I sold on 10 of these eyepieces to hopefully good homes and now have replaced with 3 good mid price quality eyepieces (12,15,18)  and a decent 2x barlow. My existing Celestron barlow is actually 2.25 so I actually have more or less the same range with these 2 barlows and I am quite happy with the setup.

Forgot to mention I also bought a cheaper 30mm Moonfish eyepiece to cover low power viewing which compliments the retained SW 22mm.

This released as mentioned a fair wedge of cash and takes me from two to one eyepiece box.

I think going forward I will leave it here for some time now but if they ever bring out a MWA eyepiece (100 +) in certain lengths that was useable for a glasses wearing people as myself I may look at things again.

Share this post


Link to post
Share on other sites
I've been away for a very long time, and to be honest haven't had my scopes out very often since my second heart attack (and quadruple bypass) three years ago. I hope to do better this spring, as my health has gotten a lot better although I still can't go out on the -10 nights we've been having. I noticed a very nice crescent moon last night, and would have liked to get my refractor out, but it's just too damn cold.

I really appreciate all the kind comments I've seen in response to my post. It seems to have done what I wanted to do, which is to give a reasonable and simple way to create a usable and versatile set of eyepieces that can grow as you save your pounds.

I'll try to hang about here more often. This was my favorite of all the astronomy forums I visited, and the longest lasting by now.

Great to have you back. :)

Share this post


Link to post
Share on other sites

I'll try to hang about here more often. This was my favorite of all the astronomy forums I visited, and the longest lasting by now.

That is very good news  :smile:

You have been missed. 

Steve

Share this post


Link to post
Share on other sites

Now that is good news :)

Hope you are well ! and look forward to seeing you around more.

Ant

Share this post


Link to post
Share on other sites

Hi there, a good informative post. But as I'm new to this it looks like a lot of numbers and equations at the moment...and maths is not my forte :-) I've finally purchased a decent beginners scope which is a dobsonian f/5, now want to make sure I make the most of it by getting the right eyepieces. So if I've read it correct I would be best with a 2x Barlow, 8mm, 18mm and 25mm? I want to observe deep sky and planets. I already have a 20mm plossl through which I could see Jupiter (mostly as a bright disc) and the four moons, which last night were all in a line, it was amazing. So I don't want to ruin it by buying the wrong eyepieces. I was looking at 6mm..until I read Warthogs post. Any help is appreciated :-)

Share this post


Link to post
Share on other sites

Use the telescope parameters, although these can be exceeded.

f/5 telescope, then take for granted a 5mm eyepiece as your guidline highest magnification in good conditions, double this for the sweetspot, and treble for the lowest power. But dont go too low, ie beyond your pupil size. I chose a 32 mm low power eyepiece and satisfied with the result. I can reach 3.2mm on the Moon, but between 6mm and 12mm is great on my scope. The size of the image on my scope is small, and Barlowing increases the image size, but does'nt always improve the detail in the larger image, whereas a larger aperture in my case would improve the detail / resolution.

Edited by Charic

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.