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The Warthog

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    Niagara Region, Canada

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    roger.n.tulk

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  1. 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!
  2. Hi Warthog sorry to hear you been ill, hope you feel better now 

  3. 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.
  4. 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.
  5. 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.
  6. 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!
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