Defining eyepiece power.

[Foo Fita]

In 'Turn Left At Orion', the author recommends whether to use Low/Medium/High power to view each object. What ranges would you give to each one?

Thanks,

FF

[GazOC]

Very roughly and depending on seeing, scope, conditions I'd venture 0-75, 75-175 and 175+ as being not too far off.

[Andrew*]

i was thinking just the same question th'other day and was considering starting a poll to get opinions!

Anyway, I think it's to an extent a personal thing, and I would say (slight alteration from Gaz...):

low = 20-70

(low-medium = 50-70)

medium = 70-150

(medium-high = 100-150)

high - 150+

For me, below 20 is not even "low power" - it's in binocular or finderscope territory.

Andrew

[Ant]

It depends, for me, on the scope...

I only have a little ED80 so x175 is actually not possible...

I have two EP's (don't do a lot of visual)....

25mm and 10mm so thats 22x and 55x respectivly. I think of them as low and high - it should probanly be more low and medium and I need to add a 4mm (or thereabouts) fcor the high power...

Ant

[acey]

The "right" magnification does depend on the scope, the sky, and what you're looking at. My workhorses are Televue plossls: 32mm, 20mm, 8mm, which I use with a 200mm F6 dobsonian (giving magnifications of 37.5, 60 and 150). I have a 2x barlow which I sometimes use on planets, but it's tiresome (to me at any rate) keeping an object in view at 300x with a dob. Anyway, I mostly look at deep sky objects, and in that case the main thing is darkening sky background and making the object appear large enough (i.e. using a high enough power) while not making it spread out and disappear altogether (i.e. not going too high). The 20mm and 8mm EPs turn out to be just right for this - I don't actually use the 32mm very much for observation, rather it serves when I want to use the main scope as a "finder". So on that basis I'd suggest:

"low": up to 50

"medium": 50-100

"high": 100+

Andrew

[philsail1]

Yes, I like your estimates Acey.

I tend to view my 32mm and 25mm as giving a "low" powers, at 31x and 40x on my 200mm Newt.

Medium are 80x to 100x (acheived by "Barlowing" the 25mm) and in making use of the 10mm Vixen NPL.

High power for me is 200x by using the 10mm Vixen with a 2x Barlow.

(I have a 6.3mm Orthoscopic which used with the 2x barlow will give me 317x on the Newt, but the image goes "soft," and the eye relief is tight too).

The best images I've really enjoyed so far is in using the Antares "Binoviewers" I recently bought. Used with two 25mm plossls and a 2x Barlow (giving a magnification of 80x) the views of Saturn has been truly mesmerising for me anyway. Seeing things in "stereo" seems to make them appear bigger, and more "3D."

Sorry to drift off the point there.

Magnification is largely (as the others have said) an individual preference. What's comfortable to you is right whether "low," "medium," or "high."

Regards,

philsail1

[Ajohn]

Look at in terms of exit pupils. Take the diameter of the scope divide that by the exit pupil that's wanted and that gives the magnification needed. eg 200mms scope 5mm exit pupil = 200/5 = 40X. Say it was an F5 scope. The eyepiece needed would be focal length = 200*5 = 1000 so the eyepiece needed would be focal length/ magnification=1000/40= 25mms.

Why do it this way? Well the resolving power of a scope increases with diameter so bigger scopes can take more magnification and still give decent images.

Useful exit pupils are in the range of 0.5mms to 6mms. At the high end things are limited by how large the observers pupil can get up to. 6mms might be a bit extreme for older people and obviously the observing site would have too be very dark. Some achieve that by using a sort of light proof bag fitted to the scope and over the observers head.

The other end, 0.5mms is limited by optical effects and the resolving power of the scope. 0.5 mms is unlikely to give good planetary views. The usual range for that would be 0.8 to 1.2 mms. Scopes of dubious quality need to err on the larger size. 0.5mms can be regarded as the right sort of area for telescope testing and final collimation of the scope. A decent scope will give good diffraction rings on bright stars at this sort of level of magnification. That would be 400x on a 200mms F5 scope.

The only catch in this area is the amount of magnification needed as the size of the scope goes up. The atmosphere starts to have an effect. A 200mms scope can expect to use a 0.8mms exit pupil on a good night. A 250mms scope has less chance and so on.

There is another often overlooked aspect of exit pupils. This indicates that smaller scopes can show more stars in the field of view than larger ones. There is some info on this link but I think that the author has upped the size somewhat in relation to what is practical and and also neglects refractor effects etc. He also shows another way of working out exit pupils. I prefer mine - it gives a clear idea of what is actually going on. In reality a 6 or 8ins reflector will show the most providing it F ratio isn't too small a number.

http://cdcc.sc.usp.br/cda/telescopios/hp-mbartels/scopes/rft.html

John

[GazOC]

TBH when using exit pupil rather than magnification to choose eyepieces you just tend to end up with the same spread of focal lengths in any case.

[Ajohn]

That might be the case - eventually - Gaz but exit pupils do give people an idea of where to start and what to expect. Many people just don't know. Some think that the image can be just blown up and up. The truth is that a 0.5mms exit pupil is extremely unlikely to give a good image of a planet. A lower mag with an exit pupil of some where round a mm will give a much better more detailed view even though it's smaller. At the other end of the scale a wide field eyepiece giving an exit pupil of about 5mms is a fairly safe bet on any scope. The only complication is high mags on bigger scopes. That depends one where you observe. The other complication is a poor quality scope.

John