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| F and Primary Suggestions http://webstertelescopes.com/forum/viewtopic.php?f=7&t=116 |
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| Author: | djfunkshowkid [ Mon Mar 29, 2010 6:04 pm ] |
| Post subject: | F and Primary Suggestions |
Im rather new to LARGE dob scopes, but throughly enjoy reading about designs and configurations of many units available. However, being rather new to this hobby, Im not sure of a couple of things that may help me decide which sizes I would need for order. First off, what does the "F" mean, when many scopes are referencing F5 or F8's. I don't expect anyone to hold my hand and teach me, however a link to a website where I could read in detail, all the information would be quite helpful. Second, of course I am deffinatly interested in solar planetary viewing with great clarity and crisp images, But my main interest is galatical viewing, nebula's, black holes, etc. I understand that I won't see anything simular to a hubble scope photo due to it being a combination of UV, and x-ray overlays...but seeing these wonders would be phenomenal to me. So, my question is...what size mirror's would I need to view these wonders? |
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| Author: | jimwebster [ Tue Mar 30, 2010 12:51 pm ] |
| Post subject: | Re: F and Primary Suggestions |
In astronomy, f/ratio is simply the Focal Length (the distance that the mirror comes into focus) divided by the primary mirror diameter. So, if we had a mirror that came to focus 100 inches away from its front surface, and that mirror was 25" in diameter, we would say that the mirror was a f/4 mirror (100 / 25 = 4 ). If the same 25" mirror was ground to come to focus 150" away from its surface it would be called a f/6 mirror (150 / 25 = 6 ) . If you just wanted to view the planets and our moon, a quality 10" telescope would be a good choice. But, for Galaxies, Nebula and other "deep sky" viewing you want as much aperture as you can afford. The deep sky objects are dim, so you need a big mirror to collect a lot of light. |
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| Author: | n0nuf [ Wed Mar 31, 2010 12:16 pm ] |
| Post subject: | Re: F and Primary Suggestions |
Hello. You might find this useful: In photography the focal ratio varies the focal-plane illuminance (or optical power per unit area in the image) and is used to control variables such as depth of field (the background blurred while the subject is in focus). When using an optical telescope in astronomy, there is no depth of field issue, and the brightness of stellar point sources in terms of total optical power (not divided by area) is a function of absolute aperture area only, independent of focal length. The focal length controls the field of view of the instrument and the scale of the image that is presented at the focal plane to an eyepiece, film plate, or CCD. (Wikipedia) What does all of this mean? The lower the f/ number, the wider the field of view. This is more visible sky in a given eyepiece. The lower the f/ number the less magnification an eyepiece will render. A lower f/ number makes a shorter telescope. Clear Skies, -Scott |
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