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Yogesh

Explanation on different eyepiece types

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Posted (edited)
23 hours ago, Stub Mandrel said:

The OP's 200PDS is a 1000mm focal length.

This eyepiece calculator:

http://telescope.eyepiece-calculator.co.uk/

Says the revelation 50mm will give a 10mm exit pupil. That's absolutely huge so you won't be able to see most of the view and will waste a lot of photons, making the view much dimmer than it beeds to be.

The 42mm gives an 8.2mm exit pupil, and will give brighter views if you are youngster (typically with 7mm eye pupils) without losing the 'immersive' effect.

The 30mm will give a 6mm exit pupil (ideal for older astronomers!)

Hi Neil, myself and the OP were talking about extension tubes, not ep's.  However, your suppositions, along with Louis' are absolutly true as regards exit pupil. 

Edited by rwilkey
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41 minutes ago, rwilkey said:

myself and the OP were talking about extension tubes

DOH!

Sorry folks....

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On 12/06/2018 at 09:26, Yogesh said:

Hi Robin,

Thanks a lot for giving that link. I am thinking of getting the 50mm one. One question though, Is there some maths I can use to get the required length for the eyepiece to attain focus ?

Thanks again!

 

Now I understand the question.  I would point the telescope at the moon and center it as best you can.  Try looking down the front of the tube and move the reflection off the wall of the tube to the center.  Now, move a piece of white vellum or simply printer paper toward and away from the focuser tube fully retracted with nothing in the focuser until you get the sharpest projected image of the moon possible on the paper.  Measure that distance as best you can, it need not be super accurate.

Figure that most eyepieces will focus within 10mm above or below the eyepiece shoulder, so you'll want about equal amounts of travel above and below best focus.  Now rack your focuser all the way out and measure the total travel distance available and divide it by two.  Now subtract that number from the moon's focus position number and that should give you a good idea of the ideal extension tube length.  For example, let's say your scope's best focus at infinity is 3 inches above the most inward position of the focuser, and the focuser can be racked out 2 inches.  In that case, you'd want a 3-(2/2)=2 inch or 50mm extension tube.  When in the focuser, you'd have focus travel from 2 inches to 4 inches above the focuser's normal most racked in position, giving you one inch on either side of best focus at 3 inches above the focuser's most inward position.

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13 hours ago, Louis D said:

Now I understand the question.  I would point the telescope at the moon and center it as best you can.  Try looking down the front of the tube and move the reflection off the wall of the tube to the center.  Now, move a piece of white vellum or simply printer paper toward and away from the focuser tube fully retracted with nothing in the focuser until you get the sharpest projected image of the moon possible on the paper.  Measure that distance as best you can, it need not be super accurate.

Figure that most eyepieces will focus within 10mm above or below the eyepiece shoulder, so you'll want about equal amounts of travel above and below best focus.  Now rack your focuser all the way out and measure the total travel distance available and divide it by two.  Now subtract that number from the moon's focus position number and that should give you a good idea of the ideal extension tube length.  For example, let's say your scope's best focus at infinity is 3 inches above the most inward position of the focuser, and the focuser can be racked out 2 inches.  In that case, you'd want a 3-(2/2)=2 inch or 50mm extension tube.  When in the focuser, you'd have focus travel from 2 inches to 4 inches above the focuser's normal most racked in position, giving you one inch on either side of best focus at 3 inches above the focuser's most inward position.

Thanks Louis for your reply! That is quite a good way to get the focusing distance. I will try that.

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Just an update. Many thanks to all the wonderful people who responded.  I did some digging around based on the replies that I got and also referring the very useful articles written by Robin(rwilkey) at http://www.swindonstargazers.com/beginners/eyepieces.htm . Thanks a lot John for giving me that link, that website has a wealth of information. I have now bought myself the following:

For the light pollution filter I was very much confused if I should get a Explore scientific 2" UHC or the Skywatcher LPR. Mainly I would be using it for imaging. Cost being one factor, usefulness being another. 

There are still some things which I want to buy. This would be for planetary imaging but don't know much yet.

  • Barlow 2x,3x or 5x ? Which one ?
  • For planetary images webcam or DSLR or any other option. 

Thanks again for the help. I cant say in word how valuable all the information in here has been.

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Just a thought but you might need a 2" to 1.25" adapter as well, for your 5mm ep.

 

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4 hours ago, rockystar said:

Just a thought but you might need a 2" to 1.25" adapter as well, for your 5mm ep.

 

Thanks for your reply. I already had that with me. Thanks anyways !

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On 08/06/2018 at 14:50, Yogesh said:

Thanks for your reply. Appreciate you taking out time to post links for the eyepiece. I will certainly have a look at those. My telescope which is a 200PDS has 1000mm focal length. The eyepieces will still hold good right?

 

 

All you need do is divide the focal length of your scope by that of the eye piece.

So for example a 25mm eye piece will give magnification of 40x (1000 ÷ 40 = 25).

True field of view, the bit of sky you see through your eye piece in your focuser, is Apparent field of view divided by the magnification. So for a 50 degree Apparent field of view,  using a 25mm eye piece giving 40x magnification, your True field of view is 1 1/4 degrees of sky, 50 ÷ 40 = 1 1/4.

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