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It's All In M', Mr And M"...

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Posted by Andy Kidd on August 5, 2002 06:07:39 UTC

What you have to realize is that using that kind of magnification with a 4.5" primary mirror [which is probably spherical and not parabolic in this case] is really pushing it- the reason you were seeing just big fuzzy blobs.

There are three mag powers you want to keep in mind for your particular OTA [which are somewhat subjective, as mentioned bearing on seeing conditions as well as the seeing individual]. The most important of these is what is sometimes called your Mr, or magnification of resolution. This is the magnification that will make all the detail on the focal plane visible to your eye [this detail is always aperture-dependent as well as optical-quality-dependant and nightly as atmospheric-dependent and even subject to dependency on the individual individual].

Generally speaking you can get pretty close to the theoretical Mr of your OTA for the 'average person' by dividing the dia. of your aperture [in mm] by 2. In the case of your 4.5" newt, this would be in the neighborhood of 57x. Using less than this magnification wastes resolution, using more wastes light [either of which may or may not be critical, depending on what you're veiwing and is done in common practice but without regard, this fact does remain].

M' and M", the lowest and highest practical magnifications, respectively, for your aperture tend to range from more to very subjective, especially the latter of these two, which is very dependent on the quality of your optics in addition to the other factors already mentioned here and in the previous posts.

For your M", or highest magnification, i think you'd be better off with about 120x [around 25x per inch of aperture, which many would find very conservative, but i happen to think this about 'normal'. People getting good results pushing their OTAs to 50x per inch are #1: probably using larger apertures [or long f-ratio apos] and #2: better optics to begin with and even then rely on good seeing conditions, as they've already mentioned].

Unless you have a short-focus OTA [i.e. f4] it's unlikely you'll be able to get an eyepiece that matches your M' [in your case maybe about 15x, or the lowest usable magnification w/ acceptable field illumination (this is of course dependent on several factors and this particular estimate is also based on a 'rule of thumb', as such].

IMO, if you want to be happy with your 'little newt' [i've got one, my 'camping scope' that i've 'tweaked' (i should probably say 'completely rebuilt') into a usable little wide-field 'star-grazing' scope] you should get a good-quality eyepiece that will generate 55 or 60x w/ your particular OTA, a 2x barlow lens and the longest focal-length eyepiece that will fit into and work with your focuser [probably something on the order of a 32 or 40mm plossl?] and you'll have all three bases covered with just those 2 oculars and the barlow.

Just my $.02
:)


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