Useful Magnification & Exit Pupils

Every time I think I'm starting to understand the basics of this hobby I learn something new. Last night one of the fellows on the CN forums pointed me over two an article explaining the concept of "Exit Pupils" and from there I also read up on the idea of "Useful Magnification". These are two different methods of choosing eyepieces. I wish I'd fully understood these before I'd bought my first eyepieces. I'm not sure if I'd have made different choices, but I would have had a better feel for "why" I chose specific eyepieces. I am considering returning the expensive Ethos which would allow me to purchase a different eyepiece plus a new focuser, but I'm not sure yet.

Highest Useful Magnification
Basically, the more magnifying power you apply, the bigger something looks, but that object also decreases in brightness at the same time. So, bright objects like planets can take great magnification, but dim objects like nebula and galaxies will become invisible if you apply too much magnification and are better viewed under low power. (This part I knew already). It turns out that there is a recommended magnification for each type of viewing. It goes like this:

Highest magnification per inch of telescope aperture:

• 5-8x power for galaxies and large nebula
• 12-15 power for globular clusters and small nebula
• 25-30 power for binary stars and planets (actually, a long focal ration scope could take up to 50x magnification for this, but my scope, being a short tube, probably can't go much above 30x reliably.)

Since my Orion 120ST has 120mm of aperture (4.73 inches) this means that for galaxies and large nebula I should use an eyepiece that will get me between 24x (or less) and 38x magnifying power. For viewing globular clusters and small nebula I should go for 57x and 71x and for binary stars and planetary viewing I should go for 118x to 142x power. In all cases less magnification will work, and will result in smaller, but brighter images.

To determine the specific focal length of the eyepiece needed for the above requirements, you divide eyepiece focal length into the telescopes focal length. So, because the Orion 120ST has a focal length of 600mm, these sizes of eyepieces become apparent:

• Galaxies and Lg Nebula: 25mm to 16mm focal length
• Globular Clusters & Small Nebula: 11mm to 8mm focal length
• Binary Stars and Planets: 4mm to 5mm focal length

Before I bought my new scope I had a cheap 25mm Kelner that I scavenged off an old kid's scope. It isn't great quality, but it works well enough for my untrained eyes. And, it's the right size for the first category above.

And, the next eyepiece I bought was a 4.3mm W70 Antares, which is right in the range for the 3rd category.

Finally, the 13mm Ethos. This one isn't quite in either the 1st or the 2nd category but straddles them both. I'm not sure what to make of this. These are the two categories I'm most interested in, and why I picked this model telescope in the first place (along with its portability). If I interpret this correctly, I think it means that the Ethos is well positioned to provide a good view of the 2nd category, globular clusters and small nebula, but is probably too high a magnification to pick up the galaxies and large nebula. Since I've been learning how to use the Ethos better, I may be happy keeping it, in spite of its high price. Then, unless I decide I'm happy keeping the 25mm Kelner, I will likely need at least one more higher quality eyepiece in the 16mm to 25mm (or bigger) range.

Exit Pupils
Using the idea of exit pupil size is a second way to choose eyepieces. Basically, every eyepiece has a specific diameter of light emitted from it based on the relationship between the size of the eyepiece and the focal ratio of your telescope. It works like this:

Exit Pupil (mm) = eyepiece focal length / focal ratio of telescope

So, for example, my cool new 13mm Ethos eyepiece combined with my f/5 telescope has an exit pupil of 2.6mm. As I understand it, the human eye never has a pupil size of more than about 7-8mm so anything larger than that is wasted. And, the pupil of your eye shrinks as you age down to about 5mm in your 50s or 60s.

And, just as with the "highest useful magnification" method, there is a recommended exit pupil size for each type of astronomical viewing you might want to do. Here's a quote from the astronomics guide I linked to up at the top of this post:

"The brightness of extended objects (galaxies and nebulas) is proportional to the square of the exit pupil. Therefore, a low power 4mm exit pupil (42 = 16) is four times as bright on galaxies and nebulas as a medium power 2mm exit pupil (22 = 4). To put it another way, twice the power results in one-fourth the brightness on the faint fuzzies outside our solar system."

Basically, for different kinds of deep space objects (DSOs) you want eyepieces with different exit pupil diameters. This link here provides a handy table which I used to generate one of the tables below.

Results: (Note: only valid for a 120mm f/5 telescope)

Using the Highest Magnification method of choosing eyepieces, I generated this table:

And using the Exit Pupil method I generated this table:
The reason for this exercise is to a) determine what sorts of eyepieces I'll need for my chosen purposes and b) figure out if my 13mm Ethos is worth keeping.

My goals with the 120ST are to primarily view deep space objects like star clusters, nebula and other galaxies. Given that, it appears that I would want 2-3 different eyepieces. The first table suggests one eyepiece with a focal length of 16-25+mm for galaxies and large nebula and a second eyepiece with a focal length of 8-11mm for globular clusters and small nebula.

For similar goals the second table suggests 3 eyepieces: 20-35mm for widefield, large deep space objects; 10-20mm for general deepspace viewing and 5-10mm for compact deepsky objects.

The two methods give results that are very similar, though not identical. But, they do both suggest that my 13mm Ethos, with a 2.6mm exit pupil and 46x magnification, is good for general deepsky viewing of most DSOs, but it may be too high a magnification and too small an exit pupil for many galaxies and large nebula. For that purpose I may want to consider a larger eyepiece with a 4-7mm exit pupil and a focal length around 25mm or so. I do still have the 1.25" 25mm Kelner I scavenged, and I can use it for that, but I'll likely want to consider something of higher quality. I've been eyeing a 27mm Televue Panoptic eyepiece for that purpose.

Since I do want to look at the planets or split binary stars occasionally, the results also suggest that my 4.7mm Antares eyepiece is a good one for that as well.

Basically I think I can be happy with my 2 main eyepieces at high and mid power, but that I may want a 3rd eyepiece for widefield DSO use. At some point I may think about something around 8mm. We'll see.

I still think I went a bit overboard buying the 13mm Ethos. If I were starting over again I might not have bought that one, but now that I have I'm finding it difficult to give it up. Its widefield views really are impressive.

So, I might want one more eyepiece, maybe two, and a new focuser. I hate how sticky the stock focuser is.