As 8 or 9 eyepieces may seem too many for some to hve in ones inventory, one could also pick one from each magnification group still appropriately spaced.
So for a f/5 scope.. it could be
Exit pupil --- 0.6 - 1.2 - 2.4 -- 4.8 - 6.8.
Focal Length --3 - --6 - -12 - -24 - -34.
3mm can archived by using a 2X barlow the others in between by using a 1.5X one.
As 8 or 9 eyepieces may seem too many for some to hve in ones inventory, one could also pick one from each magnification group still appropriately spaced.
So for a f/5 scope.. it could be
Exit pupil --- 0.6 - 1.2 - 2.4 -- 4.8 - 6.8.
Focal Length --3 - --6 - -12 - -24 - -34.
3mm can archived by using a 2X barlow the others in between by using a 1.5X one.
Clear Skies!
Interesting table, thanks. I like the layout.
But I liked your first range suggestion better than the second one above.
To my mind, in an f/5 dob, I'd go for
0.7 to 1mm for high power (i.e. not too hard to track an object by pushing the dob)
2mm for finding galaxies and other faint stuff.
4mm or 5mm for low power, depending on how bright one's "dark" sky is.
Which is in accord with your first range.
A 2.8 to 3mm exit pupil I find okay for the smaller open clusters in a dob, but I don't use it often unless it happens to be the lowest power I can get with a 1.25" eyepiece in a MAK. But plenty of people around here think the 3mm exit pupil is the best one to go for when viewing most DSOs.
Though definitely not for everyone, I also use an 8.8mm exit pupil with a broadband filter - mainly because I can tilt my eye withing the exit pupil beam and get rid of my eye's astigmatism without wearing glasses.
Personally, I've never found a good use for a 1.4mm or 1.5mm exit pupil, which many of my eyepieces delivered in various telescopes I've owned.
Do you use it much?
Regards,
Renato
The eyepieces I have as of now which I use in f/5 or f/6 scopes are 6mm, 10mm, 15mm and 20mm in the high to mid power range. 15 being the most used on dso's(hunting and general observing) followed by the 10mm(observing details) and then the 6. Sometimes I do 2X barlow the 15mm for small plaqnetary nebulae as it has a wider apparent field of view and a longer eyerelief than the 6.
The eyepieces I have as of now which I use in f/5 or f/6 scopes are 6mm, 10mm, 15mm and 20mm in the high to mid power range. 15 being the most used on dso's(hunting and general observing) followed by the 10mm(observing details) and then the 6. Sometimes I do 2X barlow the 15mm for small plaqnetary nebulae as it has a wider apparent field of view and a longer eyerelief than the 6.
Clear Skies!
Thanks. I was unaware of that interesting observation about the Pentax eyepieces.
Interesting that you too are using 3mm and 2.5mm exit pupils for DSOs, like lots of other here.
Maybe it's because I'm a relic of the pre-GOTO days, but I did try 1.5, 1.8, 2.6, 2.7 and 3mm exit pupils for hunting galaxies and all those little clusters in the Magellanic clouds. But I always gravitated towards my 2mm exit pupil eyepieces in my f/10 C8 and f/4.5 Dob because it made things easier for me than with those other exit pupils.
Though in my 80mm refractor, I must admit that I do tend toward a 2.6 to 3mm exit pupil most of the time.
Regards,
Renato
That's a reasonably sound way of choosing eyepieces, IMHO, though there are other things to consider. Forgive me for being lazy, but as this has come up before, and not too long ago, I'll link instead of type: eyepiece f/l selection
Wow! Nice to see someone thinking on the same lines to choose eyepieces. Liked the way you presented it there. Wonder why not many ppl do it so as not to get too close or to far a gap in between EP focal lengths, brightness or image size. You also speak of other things to consider. Would be very interesting to hear your thoughts on that.
Well, pretty much already covered: select minimum and maximum useful magnifications for your scope and viewing habits (e.g. 50x and 300x) and the corresponding exit pupils, and minimum and maximum useful exit pupils for your eyes and conditions (e.g. 0.5mm and 6mm). Combine the two to get your upper and lower limits. Fill in between the limits using f/stops.
A couple of example calculations (all figures approximate):
10" SCT, F/L 2500mm, f/10: 300x >> 8mm eyepiece with exit pupil = 0.8mm. Result OK.
10" SCT, F/L 2500mm, f/10: 0.5mm exit pupil >> 5mm eyepiece with magnification 500x. Result unusable.
Depending on scope parameters, a good exit pupil size may give an unusable magnification, and vice versa.
For scopes with a central obstruction, there is a lower limit of magnification else the shadow of the obstruction becomes visible - I've seen ~35-40x quoted, but I generally stick to 50x.
How often do you get to use 500x? I never have. The best that I've ever managed, in many years of viewing, and even then only once, is 420x. To me, 500x is unusable and it is pointless to own an eyepiece to deliver that magnification.
As always, YMMV.
(But if you seriously suggest to some newbie that they should spend money on an eyepiece that gives 500x then ...... )
It doesn't matter how often one gets to use it, if the seeing is good 500x is eminently usable with a 10" scope. I'm frequently using 360x with my 6" and sometimes wish I had eyepieces to go higher (the shopping list has been adjusted accordingly).
Well, there's difference of opinion. I don't buy (nor do I recommend) eyepieces that will never get used.
But seriously, you want to pick an argument in this thread based on example calculations I gave just to show how I do it? Don't you have something better to do?
I'm not picking an argument, I rather thought I must be missing something in your explanation that ended with a declaration of "500x = unusable". Well, it turns out that's really what you meant, and if you want to stick with it then that's fine with me.
But "don't you have something better to do"? Seriously?
How often do you get to use 500x? I never have. The best that I've ever managed, in many years of viewing, and even then only once, is 420x. To me, 500x is unusable and it is pointless to own an eyepiece to deliver that magnification.
As always, YMMV.
If that's the case one of several things are going on:-
1) You must observe in areas that have poor seeing conditions ALL THE TIME,
2) You haven't spent enough time observing to experience good enough conditions
3) The telescopes you have are not of large enough aperture to support 500X, or if they are large enough, the optics may be are not good enough, are not collimated properly or are not cooled enough.
Whilst 500X isn't a regular thing for me, it is certainly achievable on a handful of nights each year, in my 10", 14" and 18" Newtonians.
I have on a handful of occasions over the past 10 years gone to 1050X in my 18" Obsession with a 5mm Pentax XW combined with a 2.5X Televue Powermate for .45mm exit pupil. In the same scope over the same period I have also used 750X with the 7mm Pentax XW in the 2.5X Powermate on more than 6 occasions.
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