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What new eyepiece?


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Hi all. I've a celestron 8 hd edge that came with a 40mm eyepiece. I bought a celestron x cell 9mm eyepiece and now want to purchase another x cell eyepiece. Would an 18mm one be a good choice? If not what would you recommend? 

Many thanks. 

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At f10, most eyepieces will work well with your telescope, so you pretty much have a free choice. You don't have to match Celestron eyepieces with a Celestron scope either, as astronomical eyepieces are standardised (usually 1.25" or 2" fit). The 18mm X-Cel LX would be a good choice and pretty much split the difference of your existing eyepieces if you are happy with the 60° field, but wider fields of view are available if you would prefer a wider field. For example you could instead choose the 18mm Explore Scientific 82° eyepiece, although this is a 2" eyepiece and would therefore also require you to upgrade the visual back and diagonal to a 2" fitting.

Personally, I always consider my eyepiece choices in two ways. The first is to step up and down in focal length in steps of approximately 1.41X (root 2), which for human vision equates to one noticeable step in image brightness. At high magnifications where the atmosphere is the limiting factor you may want closer steps or a zoom eyepiece to adjust for the conditions at the time. In the UK this will start to come into play int he 150X-200X magnification range.  If you were to follow these steps using your 9mm as a base you would be looking for 9 - 12.7 - 18 - 25.4 - 35.8. Your existing 40mm replaces the 35.8 and then the X-cell LX range matches the other numbers quite nicely. That isn't a coincidence because the eyepiece designer used the same method when deciding which focal lengths to produce.

The second is to consider the exit pupil that an eyepiece will give with a certain telescope and compare that to the type of object you wish to observe. Exit pupil is given by eyepiece focal length / telescope focal ratio, so in your case this is easy as it is just eyepiece focal length / 10. Your 9mm gives an exit pupil of 0.9mm and your 40mm an exit pupil of 4mm. As a starting point consider the following exit pupil ranges:

  • Splitting double stars: 0.5mm-max
  • Lunar and Planetary: 0.85-1mm
  • Star Clusters: 1mm-max
  • Unfiltered extended objects (galaxies, nebulae): 2mm-max
  • UHC Filtered nebulae: 2.5mm-max
  • OIII filtered nebulae: 3mm-max

Under urban skies your maximum exit pupil is likely to be around 5mm before the sky background becomes too light. If you have an observing location with very dark skies and your eyes are young enough to still dilate far enough you may be able to go to 7mm.

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This is for general observation, planets and galaxies and I really like the x cell 9mm which was recommended to me by Tring astronomy. Because I bought my edge 8 hd I get £35 off the next eyepiece from them. Love the view from the x cell so just wondering which next one I should get. Thanks 

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3 minutes ago, Ricochet said:

At f10, most eyepieces will work well with your telescope, so you pretty much have a free choice. You don't have to match Celestron eyepieces with a Celestron scope either, as astronomical eyepieces are standardised (usually 1.25" or 2" fit). The 18mm X-Cel LX would be a good choice and pretty much split the difference of your existing eyepieces if you are happy with the 60° field, but wider fields of view are available if you would prefer a wider field. For example you could instead choose the 18mm Explore Scientific 82° eyepiece, although this is a 2" eyepiece and would therefore also require you to upgrade the visual back and diagonal to a 2" fitting.

Personally, I always consider my eyepiece choices in two ways. The first is to step up and down in focal length in steps of approximately 1.41X (root 2), which for human vision equates to one noticeable step in image brightness. At high magnifications where the atmosphere is the limiting factor you may want closer steps or a zoom eyepiece to adjust for the conditions at the time. In the UK this will start to come into play int he 150X-200X magnification range.  If you were to follow these steps using your 9mm as a base you would be looking for 9 - 12.7 - 18 - 25.4 - 35.8. Your existing 40mm replaces the 35.8 and then the X-cell LX range matches the other numbers quite nicely. That isn't a coincidence because the eyepiece designer used the same method when deciding which focal lengths to produce.

The second is to consider the exit pupil that an eyepiece will give with a certain telescope and compare that to the type of object you wish to observe. Exit pupil is given by eyepiece focal length / telescope focal ratio, so in your case this is easy as it is just eyepiece focal length / 10. Your 9mm gives an exit pupil of 0.9mm and your 40mm an exit pupil of 4mm. As a starting point consider the following exit pupil ranges:

  • Splitting double stars: 0.5mm-max
  • Lunar and Planetary: 0.85-1mm
  • Star Clusters: 1mm-max
  • Unfiltered extended objects (galaxies, nebulae): 2mm-max
  • UHC Filtered nebulae: 2.5mm-max
  • OIII filtered nebulae: 3mm-max

Under urban skies your maximum exit pupil is likely to be around 5mm before the sky background becomes too light. If you have an observing location with very dark skies and your eyes are young enough to still dilate far enough you may be able to go to 7mm.

Thanks for reply. I will order the 18mm eyepiece then and next month get the 12mm eyepiece. 

I also want to get a zwo camera also as want to enjoy both sides of this great hobby but camera choices are vast so will have to research that topic more as I want to do planet imaging and also things like andromeda etc. 

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8 minutes ago, Maclean156 said:

It's a stupid question but what's star hopping?

Star hopping is a method of finding faint objects when you are using a manual telescope. You start by pointing your telescope at a bright star you can see, then "hop" from that star to a series of fainter stars you can see through the eyepiece or finder scope until you find the object you are looking for. Your 40mm could be used for this purpose. 

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Oh I see what you mean now. Thanks. I have starsense fitted to my scope so I don't think I need to do that but on the other hand it would be good to learn that in manual mode to add another skill to this hobby. 

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