Posts Made By: dan hilts

Enrique,

http://www.heavens-above.com/

enjoy!
dan

From what you have shown us, I wouldn't be surprised to see that the eyepiece is an old symmetrical design, two identical achromats facing each other with only a small gap between. In your case, two 30mm dia x 45mm FL spaced to give about 1" focal length eyepiece.

Use the Surplus Shed lens finder to get an idea of what is available then give Fred a call and explain that you need the one that is the closest match to your thicknesses. Or send him an email with a photo of the one good lens and the dimensions along with his stock numbers for what the lens finder shows you. I'd be very surprised if he didn't help you get a decent physical match. If worse comes to worse, replace both lenses with a matched pair. Most of Shed's achromats are $7.00 ea and $5 total for shipping. Like John said, you're getting close to the point of "not practical" and from an optical viewpoint, a replacement lens is just a "hope that it works".

good luck
dan

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The documentary actually addresses this question and provides an answer. The climatologists state that the effects of global warming have been significantly reduced by the simultaneous global dimming. Without the dimming, the greenhouse effects we are now recognizing would have been much more obvious and the results much more dramatic. The air pollution problem is twofold: one side has to do with particulate matter causing global dimming, the other side has to do with greenhouse gasses causing global warming.

clear skies?
dan

Use your other eye and see if the view changes. I get all kinds of spurious flares and things when I view the planets with my right eye and a setup that produces a small exit pupil.

dan

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Somewhere around F22 and longer, a 4" achromat will put all the visible light inside the Airy disk so secondary spectrum is as good as it gets. F22 is a loooong scope.

dan

Hi Ansuman,

I've been more than once. A great trip any time of the year!

If you are near the north end and get a chance, the caverns are worth the trip. Luray is probably the biggest and the best. If you've never been in a big cave, it is really remarkable.

have a good trip,
dan

I don't think you will be happy with the GSO, Rini's or the Surplus Shed Erfle when working at F4.5. However, I tried my 19mm Ultra from Harry Siebert in a 10" Orion and it was really very nice right out to the edge. It is not as wide a view as some eyepieces but it is a very good preformer and available in 1.25" or 2". I think the actual FOV is about 67 degrees.

clear skies,
dan

The Shed sold out of that lens fairly quick. While I was deciding to order one or not, the decision was made for me. I hope your scope shows a good improvement for you.

clear skies,
dan

Hi Robert,

It's not a simple thing.

Focal length controls the size of the image at the focal plane. For instance, the sun and moon are about 1/2 degree in angular size. The size of the image at the focus of a telescope is about .009 times the focal length. In a 1000mm scope the size of teh moon's image is about 9mm. You can view the entire moon in any eyepiece with a field stop 9mm or larger. In a 3000mm scope, the size of the moon's image is about 27mm and it won't fit in our little eyepiece with only a 9mm field stop. It follows that for any given eyepiece, a long focal length scope provides more magnification than a shorter focal length scope. However, you can always change eyepieces to get higher magnification to match what you get with a longer focal length scope. You cannot always change eyepieces to match the low magniifcation and wide field images that you get with a shorter focal length scope.

It is important to know the size of an image if you plan on taking pictures because you can calculate the focal length needed to fit the target onto a ccd chip (or film if you are so inclinded). For that, you need to know focal length.

Aperature of a telescope determines the faintest star that can be detected and the resolution that can be obtained. Given two scopes with the same focal length, the scope with the greater aperature will show fainter stars and will also separate closer doubles. An 8" Newt will show lots more than a 4" refractor of the same focal length. (Hence aperature rules.)

The F-ratio attempts to say something about a scope that relates the focal length and aperature together. It is determined by teh focal length divided by the aperature. Given two scopes of the same F-ratio, extended objects such as nebulae, galaxies, sun, and moon, will appear to have equal surface brightness if viewed at the same magnification. However, the longer focal length scope will be showing a much larger image and will show more detail in that image. The image in the shorter focal length scope will have the same surface brightness but the image will be smaller and contain less detail. So, if you are looking at a faint fuzzy object in two scopes of the same F-ratio and you choose eyepieces so the image looks about the same size in each scope, the larger scope will show a brighter image with more detail. (Images were the same brightness until you magnified the one in the smaller scope to appear to be the same size as in the larger scope.) If you are imaging, the exposure time for targets is dependant on the surface brightness of the image produced and this is directly related to F-ratio, not aperature. However, image size is dependant on focal length.

hope this helps,
dan

Nice photos. I spent a few hours looking at the comet under poor seeing conditions on the 25th. I think what I could see was very close to what your photos show.

clear skies,
dan