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6mm eyepiece on a 70/900 skywatcher is a good option?

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#1 gabrielvcosta


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Posted 15 July 2020 - 02:30 PM

Hello, I have a 70mm by 900mm focal length skywatcher refracting telescope. I use it for observing planets and I would like an eyepiece that gives me the maximum possible magnification of quality. I'm looking to buy a 6mm eyepiece. Do you think the image will look good or do I risk losing quality due to perhaps exceeding the magnifying capacity of the telescope?


#2 photoracer18



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Posted 15 July 2020 - 02:37 PM

That is 150x which is a little high for a 70mm telescope. 50x per inch is 125x and that is under really good seeing. 7mm would be closer but a lot depends on your sky conditions. Be nice if you could borrow both and see what works. Any clubs in your area?

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#3 Astrojensen


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Posted 15 July 2020 - 02:37 PM

That's an impossible question to answer, without knowledge of the specific telescope in question. There can be HUGE variation between samples. Most are fairly good, some are bad and some are truly excellent. It's impossible to answer, without actually using your scope. 


Generally, I'd say it should work fine with a 6mm eyepiece, if the seeing is good. 



Clear skies!
Thomas, Denmark

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#4 tony_spina



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Posted 15 July 2020 - 02:44 PM

Hello and welcome to Cloudy Nights! 


6mm is typically a bit high in magnification for a 70mm , but for bright stuff like planets, moon, and double star splits you are fine



Here is a cheap good 6mm eyepiece 




You will find the 66 degree clones on Ebay as well.  The 6mm and 9mm are very, especially in long focal ratio scopes like the one you have 

#5 gnowellsct



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Posted 15 July 2020 - 03:01 PM

Well he did say that he wanted the maximum magnification of quality. The maximum recommended magnification of most any telescope is 2x per millimeter of aperture which suggests he would top out at 140X. It looks to me like the scope is a little less than f/13. So half the focal length in eyepiece focal length gives you an ideal eyepiece of 6.5 mm.

If you use a six or seven millimeter eyepiece on this scope the view will be dimming out and the jittery qualities for which these kinds of scopes are famous, due to their mounts, will be greatly exaggerated. I would suggest eight or nine millimeters as a more practical upper boundary for this telescope that would give a good view often whereas a 6 mm would give a good view only occasionally from time to time. That would be due to the seeing and also the jittery mount.

Even though the focal ratio is a little long, as it should be on an achromat, putting in an eyepiece that has 0.5 mm exit pupil will bring out the chromatic aberration.

If however the objective is lunar observing then a 6 mm is a good choice because it will allow you to get the high magnification lust out of your system.

Another argument for sticking to 1.5x per millimeter of aperture or thereabouts is that on a very small aperture instrument like this one will quickly encounter the airy disc limit where stars become plainly visible as fat discs, an optical artifact That has nothing to do with the quality of the scope but everything to do with its aperture.

As I recall Edmund optical makes a nice little 1.25-in 8 mm eyepiece for about 50 or 60 bucks. I think it would do the trick nicely. An XW or a delos would probably overwhelm the telescopes capacity for balance.

Edited by gnowellsct, 15 July 2020 - 03:04 PM.

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