Question About Magnification Limitations (and 8mm recommendations)

[markleton]

Hello All -

I have an Apertura AD8 8" dob (1200 mm focal length) which I'm now using a 12mm Explore Scientific 92 series eyepiece with. I'm very happy with the eyepiece and it even barlows well with my 2.5x Celestron barlow.

I was looking into buying a smaller focal length eyepiece to have a couple options for my observation. I've read that magnification limits are 50x aperture, so I've been researching with the assumption that I can magnify up to 400x.

My question is... If I want to use my barlow with my new eyepiece, am I still limited by this 400x number? I assume I am, so I'm looking at the 8mm range for a new eyepiece.

So, the second part of my post... If anyone has recommendations for 8mm, I'd really appreciate it.

Right now, I've narrowed down to the Explore Scientific 8.5mm LEF 82 Series and the Pentax 8.5mm XF Series. I think I'm leaning towards the ES piece since I'm so happy with my current one, and I saw a couple reviews saying that the Pentax isn't that great. I also came across the Baader/Orion 8mm, but saw that it's not a good eyepiece for faster scopes.

Thanks again!

[Kyle Allen]

In answer to your first question, yes, your maximum useful magnification will still be 400x because it is based on the aperture of your telescope. I wouldn’t expect to be able to use 400x most nights because the seeing most likely won’t support it (you will just be magnifying a blurry image) but an 8mm eyepiece would still be a good addition to use on it’s own on the moon and planets when the seeing is not great or on deep sky objects such as globular clusters and planetary nebulae.

I use an 11mm Explore Scientific 82 degree eyepiece in my 8” Dobsonian and I’m really pleased with it. I’m sure the whole line of eyepieces gets good reviews so perhaps look at the 8.8mm?

[Trikeflyer]

I was writing this when Kyle replied but I think we are on the same page. The practical challenge of magnification is atmospheric conditions especially in the UK and exit pupil size limiting the image quality and contrast. The theory and the practice are in my experience, very much different. You can magnify an object as much as you like so in theory there is no limit but the problem is the quality of the image suffers in less than ideal seeing conditions. There are occasions - like the last few nights when the seeing is so good you can really magnify things - especially the moon - people with 12 inch scopes are talking about magnifications in the order of 400x or more but nights when you can achieve that without image degradation are few and far between in my experience.  Anyway the main point I want to make is that there is a practical limit and it varies according to many factors. An 8mm without Barlow will give 150x - a nice working mag and an exit pupil of just over 1mm - again reasonable so on its own the 8mm will be a good choice anyway.   With 2.5 barlow this will be your theoretical limit of 400x - you will probably count on one hand the amount of times you will actually use this combination. Regarding specific eyepieces, it’s tricky to recommend because there are many factors. You will most likely get many recommendations from people on here. 
 

Steve 

[vlaiv]

I also thought that x2 aperture in mm is some sort of magical number, but it is not.

There is actual number based on a bit more science and it will surprise you - for 8" scope it is about x94. Yep, you read it correctly - it is only x94 magnification.

Let me explain. Most people with 20/20 vision can resolve 1 arc minute feature (high contrast). Rayleigh criterion (again high contrast) for green light and 200mm aperture is 0.64" - arc seconds. How much do you need to magnify to make something that is 0.64" be 1' large? - that is rather simple 60" / 0.64" = x93.75 - there you go.

This basically means - if you use magnification below x94 - you won't see what you can see because you did not magnify enough - you are below resolving power of human eye.

Going above x94 - just makes things easier to see. At some point, image just becomes too magnified and blurry - we call that "falling apart" of the image, and it depends on observer, scope and object being observed.

I used x500 magnification on my 8" dob and it did not fall apart for me - it was dim and floaters were all over the place, but I could still observe. I prefer now to keep things up to x300 and often at x200.

I've got 6.7mm and 11mm 82 degrees ES and 5.5mm 62 degrees ES as higher power eyepieces. I also have x2.7 APM coma correcting barlow - but I noticed that I don't really like to use barlows if I can get wanted magnification with eyepiece alone.

 

Edited June 2, 2020 by vlaiv
arc seconds / arc minutes - more or less the same :D

[Ags]

I have the 6.7 mm ES too and it was a nice high power eyepiece in the fast 6 inch Newt I used to have. Regarding @vlaiv point about moderate magnifications showing everything there is to see: my own personal experience is that a magnification roughly equal to aperture in millimeters shows the best view of planets for me. At that magnification I am not struggling with floaters, colors are richer and detail is all there. The Moon and doubles can benefit from more magnification.

Edited June 2, 2020 by Ags

[markleton]

Thanks for the replies, guys. I figured an eyepiece in the 8mm range would give me a good high-magnification (non barlowed) option while also giving me the chance to barlow it under awesome conditions.

[Louis D]

1 hour ago, markleton said:

So, the second part of my post... If anyone has recommendations for 8mm, I'd really appreciate it.

At 8mm, I don't have much experience since I tend to skip over that magnification.  The BST Starguider 8mm is decent, the discontinued Speers Waler 5-8mm zoom is very nice at that power.  Nearby, I've been really impressed with the Morpheus 9mm.  The Meade HD-60 9mm isn't that far behind it, just narrower.  At 7mm, the Pentax XW is good, but I'm not that pleased with the edges.  There's an 8mm Delos which should be superb if it's as good as my Delos 10mm.

Comparison images of some of my eyepieces in those ranges: