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Some alternatives to Ethos and Panoptic widefield eyepieces

Stu

By Stu
in Discussions - Eyepieces

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iPeace

iPeace

Posted
Posted
9 hours ago, Stu said:

Just posted this on the Eyepiece Cases thread, but here so you don't miss it :) 

IMG_8861.JPG

I really like the look of these, Stu, and you describe them as solid performers insofar. I hope to read that they are making the most of clear nights for you soon.

I have enjoyed their green-lettered counterparts for a while, now, and while they are all superb, I have regularly experienced that no two of them fit into a single session with a single scope, so to speak. Recently, I have even pondered the actual need for more than one of these in a single EP case (believe it or not) , carrying out "what if" experiments by limiting myself to only one of them on a given night.

This brings me to the question I would like to put to you. Given that you, having missed the widefield views, have acquired all three in short succession, were there any specific considerations behind the acquisition of each one, naturally given your current array of excellent scopes, etc.?

In other words, I've started to wonder why I have all three, nice as they are. I hope to gain insight from why you have all three.

Are they meant for use in different scopes, or is each one suitable for specific targets, or something else?

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Stu

Stu

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So.... to come up with an excuse for having all three eyepieces.... tricky.... ;) 

Seriously though, it is simply about having flexibility to cover different observing situations, and with different scopes. Let's say I only had the Tak FC-100D, 100mm f7.4, 740mm focal length. At home, my skies are not very good, say mag 19 at best, often more like 18.5 and that is at the zenith, significantly worst below 45 degrees altitude. That means large exit pupils just result in washed out views unless filtered heavily.

The 40mm gives x18.5, 3.68 degrees fov and an Exit Pupil of 5.4

The 30mm gives x24.7, 3.32 degrees afov and an Exit Pupil of 4.0

The 20mm gives x37, 2.70 degrees afov and an Exit Pupil of 2.7

I’m unlikely to use the 40mm at home, whereas the 20mm was really quite effective the other night observing with my club just a little way down the road where it is slightly darker. Next time I will try the 30mm and see whether the sky background is acceptable; with a UHC filter it is likely to be really quite effective.

Ultimately it is a balance of actual field of view vs exit pupil, fitting the object within the fov whilst maintaining a perceived contrast which is acceptable (or better). I remember quite a while ago, using a 41mm Panoptic to try to view a conjunction between a two planets (I think but I can’t quite remember). Basically I ended up fitting them in the field of view, but it was so washed out and they were so close to the opposite edges that it wasn’t a particularly rewarding experience. One of those, “I did it, but….” things where I could chalk it up as something seen but not that memorable. Learning here was that field of view per se is not important, nice, contrasty field of view is.

When I travel to a dark site in the UK, it is often during the late summer, when Cygnus is well positioned. My two favourite targets are the Veil and the North America Nebula. Under dark skies (best I normally see are around mag 20.5 or so, so not really pristine but much better than normal). The Veil needs over 3.5 degrees, nearer 3.7 to comfortably fit the whole object in, so here the 40mm will be really useful, with an OIII filter it will look fabulous. The Tak is a little slower than I would ideally like for this sort of thing, in the past I have had a 106mm f6.5 triplet  which gave the same field of view with a 31mm Nagler. Potentially better contrast due to the smaller exit pupil, and same fov as the Tak with the 40mm. That said, under a very dark sky, the larger exit pupil may well work nicely.  The 20mm under a dark sky should be great for DSO observing of smaller galaxies etc.

Taking different aperture and focal length/ratio scopes into consideration, the justification becomes even easierJ. For instance, I have a C925, 2350mm focal length, the 40mm gives a 4mm exit pupil, 1.16 degree fov so is perfectly useable from home and gives a nice maximum fov for this scope. Put the same eyepiece in my MN190 which is 190mm aperture, f5.3 so 1000mm focal length and you get an unuseable 7.55mm exit pupil.  Even the 30mm gives 5.66mm, great from a dark site, but  probably too much from home, so the 20mm becomes very handy at 3.77mm exit pupil and a 2 degree fov.

So, a justification of sorts :). If you have scopes of similar focal ratio, say a number of fracs around f6 to f7 or just a big fast dob then you can get away with fewer eyepieces. With a range from f5.3 to f14, it's necessary to have more bases covered. My nag zoom which is excellent in the Tak, becomes way too powerful (in magnification terms) in the OMC140; 3mm gives x667 vs x247 in the Tak.

I find these sites useful for calculating object surface brightness and visibility:

http://www.unihedron.com/projects/darksky/NELM2BCalc.html

http://www.users.on.net/~dbenn/ECMAScript/surface_brightness.html

http://www.bbastrodesigns.com/VisualDetectionCalculator.htm

I’ve also just re-discovered observer.pro on iOS as a great way of choosing targets based upon their visibility. It lists surface brightness in the object details which is handy for understanding whether they are likely to be visible or not.

Finally, this is a pivot table of my 2" eyepieces in each scope, excluding combinations which are not useful.

 

IMG_9190.PNG

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iPeace

iPeace

Posted
Posted

Wow.

Well, that's rather enlightening - especially the bit about exit pupil. Never was aware that a larger true field could get washed out due to this.

Well worth the wait. I appreciate you taking the time to formulate this so thoroughly.

I'll have to take all of this in before I can draw any conclusions with regard to my own EP collection.

:happy11:

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