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Shooting star

Exit Pupil vs Max Magnification

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Hello. True beginner given an Orion Skyquest XT8 (1200mm focal length; f5.9). I have had to collimate (all sorts of fun that was) as the scope had been moved quite a bit over time. Believe I have it very near perfect but will star test. 

Scope came with an Orion 25mm Plossl eyepiece so I am exploring what range of additional eyepieces I would like. From what I’ve read this scope is capable of a theoretical 400x magnification. Again in theory that would take me to 3mm as limit of eyepiece.

But then I read about exit pupil limitations and scratching my newbie head. The majority of what I see suggests .7mm exit pupil minimum...? But it appears I would need to buy a much larger eyepiece focal length to avoid too small exit pupil. 

I wear eye glasses so would be buying longer eye relief pieces in case that is relevant.

Advice truly welcome.

Neill

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Welcome to the forum Neill

Maximum mags are a variable and much debated feast really. Much depends on your targets, higher powers are useful for planetary, lunar and double star observing. Generally you might use the highest powers for lunar and splitting tight doubles. Next might be planetary and mag for that depends a lot on the stability of the atmosphere at the time of observing and the altitude of the object above the horizon. Currently the planets are generally low from the UK so magnifications of x150 to x200 are the norm. The moon will take more on good nights.

So, Exit pupil. I think this depends on the scope to a degree. With smaller, high quality apo refractors Exit pupils down to 0.5mm or even less are useable, although floaters in your eyes can become very distracting at this level.

Your 0.7mm suggestion is probably about right as a max mag. 1mm is potentially more useable on more occasions, giving you x200.

Eyepieces such as the Vixen SLVs are high quality, very sharp and with good eye relief. What sort of budget do you have?

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In terms of magnification per aperture, I always feel x1 per mm gives the best combination of detail resolution and contrast. Any less and you lose detail, any more and you lose contrast.

I will stretch that a little on nights of good seeing, but normally it's only double stars I use high magnifications on.

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Welcome to sgl.

I would recommend erring on the side of less rather than more magnification at least at first, exit pupils of 0.7mm and 0.5mm are not going work well so often. An 8mm eyepece would give 150x magnification and a 1.3mm exit pupil, thats a good start and would be winning on many targets in most conditions. A 6mm eyepiece would be 200x and an exit pupil of 1mm which would still be sensible but you begin to trade things off... more magnification when it is possible but it but it will be possible less often.

As well as the issue of amplifying noise and losing contradt at high magnification other things also get tougher... collimation accurracy matters more, focussing accuracy matters more, finding a target gets harder, and following a target gets harder. For those reasons I would recommend starting off lower than the theoretical maximum magnification and go higher when youve got your bearings on what works for you.

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There is an age effect as well. For us "oldies", floaters can get in the way of viewing with 0.5mm or less, although I find personally no problem with 0.67mm and even 0.5mm; the eye relief is more of a problem for me. I bought a cheapo 8-24mm zoom a year ago, and find this invaluable to start off viewing and find the best EP focal length - if necessary, I switch to a fixed focal length EP for a slightly better view. Best to check though, with your relatively fast scope, I am not sure how well a cheap zoom will work?

Chris

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With my 8" f/6 newt (as with any scope), highest enjoyable magnification depends on conditions and observing target. For me, shortest eyepiece for Jupiter is 7mm or 5mm (as conditions allow), while 3.5mm works great for Lunar detail.

As for specific eyepiece types, there are several with long eye relief at differing price points. I will only comment on those I have experience with.

I use Pentax XW with the newt, top quality with great eye relief and nice wide field of view.

TeleVue DeLite are also excellent with equal eye relief, a bit narrower field of view and weighing less than the Pentax.

For eye relief, I also enjoy using the Vixen LVW, which are discontinued but appear occasionally. Very, very nice.

I also own a Baader Morpheus; good eye relief, good quality and great value. Eye relief does vary slightly across the range of focal lengths so check the specs.

TeleVue Delos is the final long eye relief option I've tried. It's a premium item with excellent build quality.

Good luck and enjoy!

:happy11:

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12 hours ago, Stu said:

Welcome to the forum Neill

Maximum mags are a variable and much debated feast really. Much depends on your targets, higher powers are useful for planetary, lunar and double star observing. Generally you might use the highest powers for lunar and splitting tight doubles. Next might be planetary and mag for that depends a lot on the stability of the atmosphere at the time of observing and the altitude of the object above the horizon. Currently the planets are generally low from the UK so magnifications of x150 to x200 are the norm. The moon will take more on good nights.

So, Exit pupil. I think this depends on the scope to a degree. With smaller, high quality apo refractors Exit pupils down to 0.5mm or even less are useable, although floaters in your eyes can become very distracting at this level.

Your 0.7mm suggestion is probably about right as a max mag. 1mm is potentially more useable on more occasions, giving you x200.

Eyepieces such as the Vixen SLVs are high quality, very sharp and with good eye relief. What sort of budget do you have?

Thanks Stu. Sounds like 1mm is a reasonable way to go. I’ll check out the Vixen SLVs. BBC has written a glowing review as well.

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Apologies. New to forums and replying to all the helpful responses was not intuitive  only iPhone. Great advice, so glad I joined in. Really appreciate your time in guiding me. 

 

1mm eye pupil seems to be the broad limit consensus for most viewing experience and I will give it a go, reserving anything higher for lunar and the few occasions where conditions allow. I'll review the eyepiece recommendations from you all and go with 6mm. Oldie floater issue noted! x2 Barlow might be the way to allow that lunar option as we'll as double any higher focal length pieces between 25 and 6mm…?

 

Looking forward to cooler, darker, moonless autumn nights! Thanks all.

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No one has mentioned them, but the relatively close and large globular clusters like M13 and M22 really benefit from higher powers.  Below a certain power, they just look like fuzzy balls.  Get above a certain power, maybe 160x to 200x in an 8" Dob, and they suddenly resolve into tiny grains of diamonds on velvet.  Of course, it takes steady skies, a well figured and collimated objective, and a well designed and polished eyepiece all working together to achieve this.

I've found that doubling my aperture allows me to double my magnification under Texas skies, at least.  While I top out around 200x of usable magnification in my 8" Dob, I can easily push to 350x in my 15" Dob.  Folks with 20"+ Dobs around here regularly go above 500x with ease.  This is especially true in the summer when a ridge of high pressure settles over us leading to dry and steady skies for weeks on end.  I've seen multiple nights in a row where there is no perceptible twinkle to the stars.  For our skies, it really is all about exit pupil.  Keep it above 1mm and you're golden, even with very large mirrors.

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Lots of envious folks this side of the pond Louis. With weather fronts piling across the Atlantic pIcking up plenty of moisture we don’t benefit from the stable seeing you get in the US. High powers are useable occasionally but are far from the norm.

I mentioned this in my earlier post, but wanted to clarify. I tend to agree with the 1mm optimum for newts above around 8” but know from much experience that sticking at 1mm for well figured fracs leaves a lot on the table. Any of the scopes I’ve had between 60mm and 120mm have been capable of mags resulting in 0.5mm exit pupils and better. The limiting factor is normally my eyes or the skies, a 120ED will cope up to x300 I think on doubles and lunar when the skies co-operate but floaters do become an issue for me particularly for planetary work so I tend to Binoview exclusively now.

There is a bit of a grey area for newts between say 6 and 8”, particularly over here where often x200 is the limiting factor imposed by the atmosphere. A 6” Newt should be fine with an Exit pupil of 0.75 ie x200, whereas an 8” would atart to hit the seeing conditions at a similar Exit pupil giving x267.

I hope some of that makes sense!

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