-
Posts
5,337 -
Joined
-
Last visited
Content Type
Profiles
Forums
Gallery
Events
Blogs
Posts posted by The Warthog
-
-
Glad you are back in the saddle warthog. I cannot remember how many people I have recommended your guide to.
That's very generous of you, Damian. We got whacked by snow and cold in late January and February, and went 46 days without going above zero, with temperatures sometimes as low as -25, so what with the constant cloud I've hardly been outside except to take the dog for a pee. However, we are being promised the start of above zero temps, and once the 40cm of snow in my back yard melts, I'll think about putting my scopes on the lawn. After all this time, I have some refurbishing to do. The sky in my location allows only for planets, and things like double stars and brighter clusters, but I will try to get away to some better locations, which we have aplenty in Canada.
-
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.
-
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.
- 5
-
Great post.
I got a second hand WO 102 doublet f7 came with a prism and one eyepiece a 15mm plossl. I'm on a budget and have been searching for info. on what would be the best eyepieces for me too get.
Your post has made things clearer at what ep's I should be looking.
Thank you
Alan
Sent from my LT30p using Tapatalk
Off the top of my head (which is only 70" up) I would say a 5mm, a8 or 9mm, and a 25mm, and keep the 15 as it fills the gap neatly.Get the best quality you can afford, and remember you don't have to buy them all the same day.
-
Thanks for all the kind comments. I have not been really well the last few months, and seldom have the energy to get out after nightfall these days. I've probably had my scope out no more than three times this year, when I wanted to show my great-niece something special. We even missed our annual perseid safari this year.
-
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.
-
Hi guys and i have spent alot of time reading this post, but i cannot work out the formula above?
I got to the 250mm (size of mirror or aperture) / 5mm (or what exit pupil i require) = 50x (which means an EP which gives this mag will give a exit pupil of 5mm) it is the next bit that throws me, why does it mean a 25mm eyepiece, where is the formula?
I am sure i am missing something, doh!
To work out what an eyepiece can do for my own scope i take the focal length of the scope and divide it with the focal length of the EP = Magnitude, so (1250/25=50x) and from this i can then work out what the exit pupil will be, still assuming we have the 250mm (10") F5 scope, i then use the aperture or mirror size of the scope and divide it with the mag of the EP (250/50=5mm)
In any scope, the exit pupil will be whatever multiple of the f/number is represented by the focal length of the eyepiece. So for an f/5 scope, a 5mm eyepiece will give you an exit pupil of 1mm, and a 25mm eyepiece will give you an exit pupil of (focal ratio/ focal length of ep =) 5mm. This is independent of the magnification calculation.
-
Perhaps this is the kind of guide you are talking about:
http://stargazerslounge.com/primers-tutorials/63184-primer-understanding-choosing-eyepieces.html
It also goes into the difference between 1.25" and 2" eyepieces.
HTH
Andrew
The difference is in the width of the field of view. For a plossl lens, you get the full FOV at up to a 24mm eyepiece, but above that you need a 2" eyepiece. At 32mm, your FOV is restricted to about 48º, no matter how great the FOV of the eyepiece may be. This makes a great difference in the Widefield eyepieces with FOVs of 60 to 100º.
-
Am I missing anything by having such a large gap in the range of powers?
Alan
Only if you feel you are missing something. Are there times when 93x isn't quite enough, but 200x is too much? If there are, then you might want to stick something in the 12mm range in there.
I have a gap in my kit at 16mm, but I don't want it, and I get 12mm very occasionally by Barlowing my 24mm, but this is on very rare occasions. I sometimes use my Barlow with the 7.5mm to get 200x on the Moon, but usually I go straight from my 24mm at 31x to the 7.5mm (100x) or the 4.8 (156x).
I do mostly Lunar observing. If my skies were better, I might be interested in filling that 16mm gap.
-
When you put a Barlow into the mix, you want to consider what focal lenghts it gives you. The 10 mm with your 3x Barlow will give you 3.3mm, which is close enough to the 3.75mm you get using my formula. Try it and see if it works; this article is meant to give you a starting point, not to be the final word. If the quality of the view suffers with the barlow, consider getting a 3 or 4mm ep.
-
Thanks very much - as a beginner and hopefully soon to buy, it's very informative and hopefully it will help me avoid spending money on things I don't need.
Once I'd worked out that the "100mm scope" you used as an example throughout was actually 1000mm focal length, I could do the maths.
Not exactly. It is a 100mm aperture, and if you follow the formula I give, your ep will give 133x no matter what the focal length is. If you have an f/8 scope and are using a 6mm ep, then 800/6=133. For the f/10 scope, 1000/7.5=133, and for an f/6 scope, 600/4.5=133. This result surprised me a little when I was putting this article together.
Thanks to everyone for your kind comments. Family matters, health and work have kept me away from the ep and from the forums this year, but I am planning a resurgence.
-
The SW EXP 200p come with 10mm and 25mm eps are they Plossl? and Would the Eyepiece Sets - Celestron Eyeopener Eyepiece and Filter Kit be any good for it. planetary detail and dso's etc....
And again with the SW Skyliner 250xp??
It depends on what you can afford. I started with this kit, and eventually acquired a better set of eyepieces, and then sold all my old eps to pay for the motors on my mount. The case is great; I still use it to hold my present ep collection, and the filters are usefull for planets and the moon. The whole kit costs less than buying the pieces separately. The Barlow makes some ot the eps in the kit redundant, but it's a fairly good Barlow. I never had any complaints about it.
You might want to look further afield for a set of wide angle and/or long eye relief eps. There are some on the market, and they usually come in cases too. As I said, it depends on your budget.
Alternatively, you could buy eps one at a time to fill in the gaps between your present eps and the set you would like to have. The Rigel WA eps are on offer for about £30 each.
If you don't have a Barlow, that would be a good first purchase.
The Rigel WA eps have an AFOV of 65º, which is about the limit of a 25mm 1¼" ep, if you want to build your set around those or similar eps. The 25mm ep will give you an exit pupil of 5mm, which may be all you need. Your scope will not accommodate a 2" ep, and a 32mm ep will be restricted to about 48º AFOV, giving no real advantage over a 25mm one.
I believe the eps that Skywatcher supplies are Plossls, but I am not 100% sure.
- 2
-
I did mention eye relief as a reason to NOT buy a 4mm or 6mm Plossl. Although I wear glasses, I observe without them, and any eyeglass wearer who does not suffer from significant astigmatism should be able to observe without glasses. For people who must wear specs, there are long eye relief eps which start at about twice the price of Plossls. Still, that's not bad.
-
Brilliant post, thanks!
I have recently purchased SW explorer 150PL so using your guide I should go for 6, 10, 16 and 24mm EP's. The EP's supplied with this scope are 10mm, 25mm and 2x barlow, giving effective range of 5, 10, 12.5 and 25mm which seems like a reasonable match.
Any recommendations on makes/models of reasonably priced quality EPs would be appreciated.
Thanks
My guide is a guide not a precise mathematical rule, so being off by a millimetre or two isn't going to make a whole lot of difference. With your f/8 scope, your 10mm and 25mm eps may produce very good results indeed. I believe SW ships standard quality Plossls with their scopes, which are OK. I agree with John. When you get to the point that you can afford some really good eps, consider getting a bigger scope.
-
Hi
Been reading up a lot on EP. Seems like I am going to get an 8 inch f/5 reflector. It will come with a 20mm plossl so thats 50x.
I've read that fast telescopes need better EP and more expensive barlow lenses, I was planning on getting the cheapest ones which were the Skywatcher plossls/barlows, do you think they will work well, and if so, what size, if I only buy one I would buy the 7.5mm one and maybe a barlow lens with it
That would be a good choice with that scope, but if you are going to buy only one ep, spend a little more money and get a good one. Apparently the Meade 4000 Superwides are a good choice. You can use the 20mm as a low power ep and add eps as you save for them. It doens't take long to accumulate a good set of eps.
-
A very imformative post WH must have took some time to type out.
I agree with everything you say and eventually my plan is also to slim my collection down to 4 good eyepieces.
I do not like barlows so my ideal collection to accompany my dob will be.
26T5
17E
13E
8E
Better start playing the lottery
Yeah, my dream kit would look like that, too, but I find my Antares Speers-WALERs good value. This took me about two weeks to write and edit, and there are still a couple of minor things I could change.
Thanks!
- 3
-
- Popular Post
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!
- 173
- 17
-
I quit. Now I can turn my computer off at night.
-
When I first got my first decent scope, a 4.5" Meade, I went out and looked at Saturn first, as both Jupiter and Saturn were in the sky. It was so cool I made all my kids come out and look at it in the freezing night air. It is always a wonderful sight! Congratulations.
-
Mine's been running for over two weeks, and my average is down to about 7, from just over 40 some time ago. Maybe I should take my computer and wind up the springs tighter.
-
OK, then. It is an astropulse work unit. I allow 70% of my computer time to the projects, and haven't changed anything. Most of the time, Seti has my computer all to itself. I'd better get a huge improvement in my average after this!
-
I seem to have been handed a very big task. It has been running for four days, and according to the task page, has 312 hours to go, or about two weeks. I've never had a task that took more than a couple of days. I haven't changed any of the parameters of the system, so I don't think I'm being unusually slow. Maybe a bit thick, but not slow...
Has anyone seen a big task like this before?
-
I have withdrawn from the SETI thingy for now, and my ancient computer works much better without it. I may start up again when I have a 21st century box on my table.
-
Send me a copy of that PM while you're at it, bud!
Eyepieces - the very least you need.
in Getting Started General Help and Advice
Posted
HI. I just dropped in to this topic as I may need my own advice after nearly 10 years away from the hobby. My health and energy levels have not been stellar, especially after the MI in 20112 that left me with a quad bypass and combined with 54 years of T1 diabetes, arthritis an circulatory problems, as well as being old. I'm 71 now and temporarily unable to drive. Bummer! Anyway, I've hunted up my collection of eyepieces and find the ones I have still fit the bill. I tried to look up Jupiter and Saturn a couple of nights ago, but was successful only with my 32mm ep, and with a focal length of 1000mm didn't give me much of a view. My higher mag eps wouldn't focus, but in the daylight I corrcted a sim[le problem and am ready to try again. Saturn and Jupiter have not been this close together for about 20 years, so it's a great opportunity, and in summer too, so I don't have to freeze my, er, toes off seeing them. Clear skies, everyone!