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Telescope Questions - Revised

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bbrock

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Thought I would Update and Start a new Thread. <br /><br />Question 1, Does a Barlow extend the telescope focal length or does it divide the EP focal length? Exampel, a 2X Barlow will double the power of an eye piece 2X. If a 1200 mm telescope focal length is fitted with a 10mm EP, the power is 120. If a 2X Barlow is used, does this double the telescope to 2400mm/10mm=240 power or half the EP ( 1200/5 = 240 )? IF the EP focal length is cut in half, is the Exit Pupil also cut in half?<br /><br />Question 2. What is the best way to smooth out the Azimuth motion of a Dob. I have an Orion XT Intelliscope that moves a little rough in Azimuth. Orion suggested using Pledge Furniture Polish. Would a teflon grease work?
 
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heyscottie

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I can answer question 1:<br /><br />It is the focal length of your telescope that is multiplied. Exit pupils of any eyepieces should remain unchanged. I hook up an SLR camera to a Barlow attached to my telescope, giving me a "camera lens" with a F.L. of 1820 mm.
 
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nevers

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White Lithium Grease works but only temporarily. Eventually it gets "gummy" and has to be cleaned and replaced. Over time, without being cleaned it will make the action worse.
 
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bbrock

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Thanks Guys. I will try the milk jug fix for the Azmimuth motion of the Dob. <br /><br />I am most intriqued by the answer to my first question, and I've been wrestling with that answer for a couple of days. This is really quite a profound answer. <br /><br />I guess the reason is that "Exit Pupil" is actually a measure of the efficiency which our eye and the EP work together. Since the average adult dialated retina is approximately 6.5 to 7mm diameter, anything much over this diminishes available light to our cones and rods. Anything much under this admits all the light, but doesn't take advantage of all the cones and rods available, diminishing contrast, resolution etc.. <br /><br />My understanding of telescope and eye piece "Exit Pupil" is very fundamental. However. it is defined ExPu=EP focal length / Telescope focal ratio. Thus the "Exit Pupil" is not just a function of the telescope exclusively or the eye piece exclusively, but both combined. Which I think is quite profound. <br /><br />The reason I am intriqued by the answer to question one, if that answer is true, this definition of "Exit Pupil" appears to be more complex then my understanding is. <br /><br />For Example. If my telescope has a focal length of 1200 mm, focal ratio of f/f.4 ( apature = 254mm ) and an eye piece focal length of 32 mm, I arrive at a very efficient and bright image "exit pupil" of: ExPu=(32/(1200/254)) = 6.77 mm. Almost exactly the right exit pupil for maximum efficiency of viewing all of the available light. If I add a barlow 2x and double the magnification, it would be great if I could maintain the maximum efficiency. However, if a barlow doubles the focal length of the telescope, then according to this calculation:<br />ExPu = (32/(2400/254)) = 3.386 . Which is what I would expect, but not really want. So if the "Exit Pupil" does in fact remain constant, then something is missing from this relationship. and my understanding of it ??? Which is why intrigued by it
 
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nevers

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Hi Bill - <br /><br />That is exactly the reason why I try to stay in the shed where it's nice and dark. I understand "milk jugs" a lot better! <img src="/images/icons/wink.gif" /><br /><br />When intrigued I have often found myself resorting to being content by looking at the pretty colors. I wish I could be of more help; I'm just not a "numbers" person at all. Good luck in your quest...
 
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bbrock

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ok, lets try a more down to earth question. I have found that "dew" is my personal public enemy #1. Dew had stopped my observations in their tracks. I refuse to take a towl and start wipeing water off of my lenses (EP's, spotting scope and E-Z Finder). I take everything inside and let it evaporate. <br /><br /> Question --- is ther anything like a cordless hair drier, or portable - battery operated warm air blower. I can't find anything like this anywhere. <br /><br />Bill
 
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nevers

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Hi Bill - <br /><br />Try this: 12-Volt Dew Remover Gun <br /><br />I've seen Dew Covers for Telrads but not for the EZ-Finder. Maybe you could rig something. I don't see where it would hurt to make a little "Dew Shield" for the finder. I love the desert: tonight - 13% humidity. Sorry...
 
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bbrock

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Brad<br /><br />Thanks for the lead on the Orion 12v Dew Gun. I have looked at Orion Accessories and completely overlooked it. This is probably going to be as close to what I am looking for as I will ever find. As far as a heat signature with astrophotography, I can't answer that. All I can say is try it and see. Perhaps you might discover a new nebula using this method !<br /><br />Regards<br />Bill
 
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bbrock

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I had a thought.<br /><br />I wonder if a small hand air pump would work. The compression would warm the air and drive the air dew point (wet bulb temperature ) lower. You really don't need warmer air, just air with a lower relative humidity that would evaporate surface moisture. As long as you don't discharge the pumped air through too small an opening ( throttling would re-cool the air ) then it might retain the lower humidity. <br /><br />Bill
 
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heyscottie

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BBrock:<br /><br />You are right; I am wrong.<br /><br />The exit pupil will necessarily change with a Barlow, as you stated.<br /><br />It IS, however, the focal length of the telescope, not the eyepiece, that it is modified.
 
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bbrock

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Shucks<br /><br />I was really hoping you were correct. One of the big reasons a low power EP is so bright is because the "Exit Pupil" more closely matches the human eye iris ~~ 7mm. <br /><br />Im not overly experience with amature astronomy and telescopes. But my thought was that using a low power EP with a Barlow would be one way to go up in power and keep the image relatively bright and sharp, rather then using higher power EP oculars. <br /><br />I wonder if EP manufacturers could develope an ocular that had a variable "Exit Pupil" on the back end. In my example, going to the 2X Barlow cut the exit pupil by 1/2. Now if you took the Exit Pupil and lensed in back to ~ 7mm, you would be using all of the available rods and cones in your eye to look at the dim image - which might (at least) give the image more contrast. <br /><br />Sounds like pie in the sky to me. Such an EP would probably cost $2000. <br /><br />Thanks<br />Bill
 
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nevers

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I don't recall anybody asking about Dew vs. Astrophotography. Dew vs. Observing yes. Yours is a cleaver name suggestion: I shall condsider it.
 
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nevers

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Hi Alex - <br /><br />The more I thought about it the more I think your right. I should change my name to "DOOM" but use a different meaning: Don't Order Obsolete Meade.<br /><br />I broke my 'scope over the weekend or it broke itself, I don't know. It's just not working. I should have waited for the newer version instead of getting the model that's going out of style. Now, I have to send it back to the factory. Luckily, I have another 'scope I can fall back on while it's away. I'm just kidding about the Meade jab. It's very good equipment considering the price when compared to the same set up as something like Tele-Vue: $800 vs. $6000. I'm just ticked off about the whole thing.
 
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bbrock

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I haven't tried my hand at astrophotography, but would be interested. I love the 10" Dob I use for observations. What would be a good, reasonably priced and portable telescope with good tracking that could be used for photography? My first love is observing and this would be a secondary telescope. It would need adaquate aperture and probably a slower focal ratio then my Dob (f/4.7). How much magnification is reasonable for photography? This would determine the least amount of aperture. But I don't have a clue! <br /><br />Bill
 
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bbrock

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It appears to me that good tracking is the most critical part of astrophotography. It would not take much magnification for most deep sky objects, but good optics and a good camera are essential. I don't understand the principals involved, but a slow focal ratio is also better then a fast ratio for photography. Why is this? I would think that the greater the aperture per telescope focal length the better?
 
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bbrock

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Another Question: Why are mirrors coated with aluminum rather then chrome?
 
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