Magnification, AFOV & TFOV

[pete_81]

So, with a 1200mm focal length scope, high magnification is relatively easy but not recommended with the atmospheric conditions, lower exit pupil, etc etc...

When I look at calculating TFOV's from eyepieces, I can use a 52º_30mm, or 68º_24 and get the same TFOV.
Obviously there's a 20% difference in magnification (40x or 50x) but what are general reasons for the purchase of either EP?
The 52 is likely cheaper, but is the 68 more 'useful' with same TFOV but higher mag?
Is a 52 easier to use in that the observer doesn't need to move head around EP to see full image, so lower magnification is more preferential to see the full field in one go? Wider FOV is nice to get the 'immersive' experience but equally at the expense that isn't a necessity?
For instance, the Pleades occupy 1º51', which will be fully visible in the following EPs with the 1200mm FL
52º 40mm (30x),
68º 34mm (35x),
82º 30mm (40x)

The higher magnifications are better for splitting double stars, etc, but what factors/reasons do folks have for different EPs?
I do like the wide field but budget wise and coming up to winter observing (and Christmas), looking at a larger TFOV EP. Those observant may also note my scope is f/4.8. Explore Scientific are likely my choice (hence the list of EPs above too).
What's the ES70 like - they never seem to be recommended to the 68, and the 52 seem to have very good reviews throughout also...

[John]

I tend to prefer higher magnifications even when wanting a good sized true field because with the moderate light pollution I have, I get a darker background sky which helps deep sky objects to stand out a little better.

So I gravitate towards eyepieces with larger fields of view.

I also like wide or very wide fields of view when observing at high power as well because my scopes are on undriven, alt-azimuth mounts.

From what I've read the ES70 eyepieces are OK but nothing special but the ES68, 82 and 100 degree eyepieces (which I have owned and used) are really pretty good. The ES92's are superb but rather bulky and heavy. I've not used an ES52 degree eyepiece.

My scopes vary from a 12 inch F/5.3 dobsonian to a 100mm F/9 refractor (and a few in between !)

 

 

 

Edited October 30, 2020 by John

[Pixies]

Yep - as John implies, it all depends on the exit pupil. As magnification increases, the bright objects and the dark sky both get darker - but the sky appears to darken more initially.  

There is a trade-off between the brightness of the object and the brightness of the sky. The sweet spot varies. Some say a 2mm exit pupil offers the best contrast. There have been quite some arguments over this on Cloudy Nights! Personally, I find a 2mm exit pupil great. Exit pupil = Eyepiece focal-length / focal-ratio of scope. So with my F/6 dob, a 12mm eyepiece gives a 2mm exit pupil. 

So your 52º_30mm and 68º_24 EPs have the same TFOV, but the latter will have darker sky and better contrast between sky and stars. If your scope was F/6, the former eyepiece will have an exit pupil of 5mm, and the latter will have a 4mm one.

Also - and this could well not apply to you, smaller exit pupils are easier for your eye to handle if it suffers from any aberrations, like astigmatism. I know there are some SGL members who can use the more powerful EPs without their specs, but once the exit pupil hits a certain size, they need their glasses to observe.

[Don Pensack]

22 hours ago, pete_81 said:

So, with a 1200mm focal length scope, high magnification is relatively easy but not recommended with the atmospheric conditions, lower exit pupil, etc etc...

When I look at calculating TFOV's from eyepieces, I can use a 52º_30mm, or 68º_24 and get the same TFOV.
Obviously there's a 20% difference in magnification (40x or 50x) but what are general reasons for the purchase of either EP?
The 52 is likely cheaper, but is the 68 more 'useful' with same TFOV but higher mag?
Is a 52 easier to use in that the observer doesn't need to move head around EP to see full image, so lower magnification is more preferential to see the full field in one go? Wider FOV is nice to get the 'immersive' experience but equally at the expense that isn't a necessity?
For instance, the Pleades occupy 1º51', which will be fully visible in the following EPs with the 1200mm FL
52º 40mm (30x),
68º 34mm (35x),
82º 30mm (40x)

The higher magnifications are better for splitting double stars, etc, but what factors/reasons do folks have for different EPs?
I do like the wide field but budget wise and coming up to winter observing (and Christmas), looking at a larger TFOV EP. Those observant may also note my scope is f/4.8. Explore Scientific are likely my choice (hence the list of EPs above too).
What's the ES70 like - they never seem to be recommended to the 68, and the 52 seem to have very good reviews throughout also...

What others have said is very true, but another reason for not going lower in power than the 24mm is simply that 50x is a low enough low power to frame large objects well, and see improved resolution of objects compared to the

lower magnification of a 32mm.  I advocate for 50x steps in magnification, which would be focal lengths of 24mm, 12mm, 8mm, 6mm, 4.7mm to make a complete set.

Make the 24mm an 82° 2" if you want a wider field of view than a 1.25" eyepiece will yield.

The Pleiades really look best when the field of view exceeds 2° and even better at 3°.  This is the realm of the short focal length refractor (480-750mm focal lengths).  It's pretty close to the only object that large that you might care to look at, and it is fantastic at 15x in a short 80mm refractor.  I wouldn't plan my eyepieces around that object when 99.99999% of all objects you're likely to view are smaller than 1° and probably 90%+ are smaller than 1/2°.

If you really love large fields, look into a pair of large binoculars or an 80mm refractor.  Either makes a great complement to a 10" scope.

 

[Louis D]

1 hour ago, Don Pensack said:

The Pleiades really look best when the field of view exceeds 2° and even better at 3°.  This is the realm of the short focal length refractor (480-750mm focal lengths).  It's pretty close to the only object that large that you might care to look at, and it is fantastic at 15x in a short 80mm refractor.

Collinder 70 (Belt of Orion open cluster) is another large favorite of mine at low power.  15x70 binos show both it and the Pleiades rather well.