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  #1  
Old 15-05-2018, 04:42 PM
Icearcher (Chris)
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Eyepiece advice for lunar viewing

Hi All

I have a new SW Evostar 72ed and want to do some lunar viewing, what eyepieces would you suggest I look at?

Is there a general rule-of-thumb/formula for calculating aperture vs the diameter of the moon?

Should I look for a zoom eyepiece? Are these any good?

Also, as this is really not the main purpose of the scope, i would like to keep the costs at a reasonable level.

Regards
Chris
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  #2  
Old 15-05-2018, 06:36 PM
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ZeroID (Brent)
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What eyepieces have you got ? Any eyepiece is good for the moon, it just alters the magnification. What may help is a green moon filter to reduce the glare although many view it unfiltered, your eye gets used to it but careful walking around in the dark afterwards as you will have some serious dark blindness till it recovers.
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  #3  
Old 15-05-2018, 06:52 PM
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dannat (Daniel)
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in the 72ed youll want short focal lengths, or barlow a zoom. i like plossl or the pseudo masuyamas for luna
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  #4  
Old 15-05-2018, 07:21 PM
Tropo-Bob (Bob)
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A Meade HD EP may be suitable, and secondhand ones are not normally expensive.
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  #5  
Old 15-05-2018, 07:31 PM
Icearcher (Chris)
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Thanks all.

I don't have any eyepieces at present, I went into this game on the astrophotography side so I'm starting from scratch.

The 72ed has a focal length of 420mm, is there a formula to figure out what view an eyepiece will give?
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  #6  
Old 15-05-2018, 08:10 PM
Tropo-Bob (Bob)
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Quote:
Originally Posted by Icearcher View Post
Thanks all.

I don't have any eyepieces at present, I went into this game on the astrophotography side so I'm starting from scratch.

The 72ed has a focal length of 420mm, is there a formula to figure out what view an eyepiece will give?
A telescope's focal length divided by the eyepieces focal length gives the magnification.

So for your scope, a 10mm EP would give 42x. whereas a 6mm EP would give 70x.
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  #7  
Old 15-05-2018, 08:35 PM
Wavytone
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Quote:
Originally Posted by Icearcher View Post
...want to do some lunar viewing, what eyepieces would you suggest I look at
Hi Chris, yes there is a rule of thumb:

1. For a good quality refractor (an ED doublet or triplet APO) the highest useful magnification is usually M = X2 per mm of aperture, in your case 140X.

Focal length / magnification = eyepiece focal length,
in this case 420/140 = 3mm.

2. There is a tradeoff between sharpness vs more magnification which to some extent is dependent on your vision. For most observers this is around X1 per mm.

Using the same as above, this gives 6mm.

3. These are not exact, so basically any eyepieces in the range 3...6mm should do nicely.

For what its worth I have a quadruplet 70mm APO, f=475mm and this can show Uranus as a neat little disk with a 4mm eyepiece, at 118X. Moon is excellent with that magnification.
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  #8  
Old 15-05-2018, 09:31 PM
Icearcher (Chris)
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Thanks everyone

That's the sort of info I was looking for.

Wavytone, is the full disc of the moon visible with a 4mm?
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  #9  
Old 15-05-2018, 10:15 PM
Wavytone
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That depends on the eyepiece apparent field of view (AFoV), its distortion, and the net magnification.

Bear in mind most eyepieces have pincushion distortion so through the eyepiece, objects are wider than actual angular size X magification.

For example an eyepiece with a 50 degree AFoV will not quite accommodate the moon at 100X. At say 140X the moon will not quite fit in a 72 but should fit in a 83-degree eyepiece.
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  #10  
Old 15-05-2018, 10:56 PM
Icearcher (Chris)
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Wow this is opening up a whole new world to me and Im still only just dipping my toe into astrophotgraphy, good thing I like a challenge.

Are there any eyepieces that you have that give you a nice full disc view?

If I was looking at getting some Plossls with around a 52 degree FOV, what sizes do you think would be good and do you have a preferred place of purchase, maybe around the $50 each mark?
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  #11  
Old 16-05-2018, 07:50 AM
Tropo-Bob (Bob)
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Chris,

Since U brought an OTA without eyepieces, you may also need a star diagonal to be able to use this scope for observing.

Besides an EP for the Moon, U will also need a lower power EP. They see a larger region of the sky and make it easier to find what U are looking for. When found, then a higher-power EP may be then used to have a closer look.

Also, do U have a mount for this scope or what is your plan re this?
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  #12  
Old 16-05-2018, 09:09 AM
Icearcher (Chris)
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Morning Bob

Iv got a diagonal on my list, currently deciding between a 2inch or a 1.25 with and adapter as they are a bit more affordable.

My main focus will still be astrophotography as I'm loving that at present.

The main reason i started this thread is that the ABC stargazing world record attempt is in a few weeks and its visual only, no cameras so i need to get set up for that.

Mount wise, I have a SW Star adventurer so as long as I can get ok polar alignment then Ill have the moon in view most of the night.
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  #13  
Old 16-05-2018, 09:18 AM
Tropo-Bob (Bob)
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Chris,

I would probably recommend the 1.25 inch diagonal; it keeps the telescope nice and light. However, sometimes a heavier diagonal may be more appropriate if the telescope does not balance with the lighter load.

For EPs, check out Myastroshop and have a look at the Vixen Plossls. They are suitable EPs in your price range.

Best of luck,

Bob
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  #14  
Old 16-05-2018, 09:50 AM
astro744
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Lots of good info here:

http://televue.com/engine/TV3b_page....212&plain=TRUE

http://televue.com/engine/TV3b_page....n=Advice&id=79

http://televue.com/engine/TV3b_page....=Advice&id=230

http://televue.com/engine/TV3b_page....n=Advice&id=80

The Moon is 0.5 degrees in diameter (+/- 0.05 approx.). Select an eyepiece that gives approx. 0.65-0.85 degrees true field of view (TFOV) for nice framing of the subject. The Moon will fit inside 0.55 deg. but looks better when there is a bit of space around.

TFOV = Field Stop Diameter of Eyepiece x 57.3 / focal length of telescope.

The following can also be use to calculate TFOV and is not 100% accurate due to distortion but close enough.

TFOV = AFOV / Magnification

Magnification = Focal length of Telescope / Focal Length of Eyepiece

See also http://televue.com/engine/TV3b_page.asp?id=214 and look in Field Stop column and apply to formula above or simply click on first link above.

Tele Vue publish their FSD data whereas not all others do.

The AFOV determines the 'width of the window' that you look through. You can see the same amount of Moon/Sky through separate eyepieces at different magnifications. The one with the larger AFOV makes the object appear larger/closer and you have to look around to take it all in. Eye relief too comes into play here and determines viewing comfort to some degree.

The other term you will hear is Exit Pupil which relates to the brightness of the image.

Exit Pupil = Eyepiece Focal Length / Telescope Focal Ratio

also

Exit Pupil = Telescope Aperture / Magnification

Select eyepiece for an Exit Pupil range of 0.5mm to 7mm typically although the lower and upper limits can change on telescope type and observer. Don't worry about exceeding the upper limit on refractors and too on Reflectors (with obstruction) as the impact of the obstruction is often overstated but be wary of it for the latter. At the lower limit eye floaters come into play and on the Moon in particular they are a nuisance. The image too starts getting dim but the type of object you are observing also comes into play. If a Planet image brightness and contrast will be low but for double stars this is not so much a problem.

Whatever you choose, enjoy!
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  #15  
Old 16-05-2018, 10:04 AM
Icearcher (Chris)
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You lot are all bloody legends, that why I love this community, the amount of knowledge here is just next level.

Thanks a million everyone, Iv learnt more in this one thread then what hours of google gave me.

Like the old saying goes, its not what you know but who you know.

Last edited by Icearcher; 16-05-2018 at 12:18 PM.
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  #16  
Old 16-05-2018, 05:17 PM
astro744
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You can calculate the true field of view when Astro imaging using the same formula as for eyepiece TFOV. i.e.

TFOV = Sensor size in mm x 57.3 / Telescope or lens focal length

E.g. For APS-C sensor of 24 x 16mm and 420mm lens

TFOV = 3.3 X 2.1 degrees

For full size 36 x 24mm sensor and 420mm lens

TFOV = 4.9 x 3.3 degrees
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  #17  
Old 23-05-2018, 07:02 PM
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Hi please drop me a pm I have something you might like to have .



regards
graham
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