Magnification

[Cam (Cam)]

Had trouble focusing an eyepiece.Hope I have done the calculations right, maybe someone could correct me if Im wrong.
Im new to astronomy. I have an 8" f6 Dob. Ive been told up to 50x magnification per inch of aperture. I was out the other night observing the skies and tried a 4mm planetary II eyepiece that by my calculations gives me 304.8 x mag(38x per inch)and I just couldnt focus it.I also have an Explore Scientific 82 degree FOV 14mm (definately my favourite so far) which gives 87x only 10.8 x per inch.. I can barlow up the 14 mm x2 and even x3 giving me 261 x mag (32.6x per inch) and the views are great. really enjoyed checking out Jupiter, eta carina nebula and tarantula nebula and 47 Tuc.. Now what Im wondering.. If my calculations are correct a 5mm eyepiece would give me 243.8 x mag coming in even under the 3x barlowed 14mm. Can 1mm difference in focal length of an eyepiece really make this much difference??? Did the "seeing" suck??? Or is this it??? Is around 30x mag per inch of aperture more realistic??? Ive also been told by someone recently that over 250 x mag is rarely supported by the atmosphere..Is that correct??? Think I already need more aperture lol Im hooked!!!

[leon]

Mate you are pushing it to the outer limits, 50x per inch is just to much, 25 to 30 will be most beneficial and your views would be much better.

Leon

[Cam (Cam)]

Thanks Leon. So the "seeing" would have been ok then? Maybe I should have bought a 5mm TMB instead of the 4mm.. Oh well live and learn haha. Thanks for ya reply mate.

[Robh (Rob)]

I don't find those maximum magnification rules of much use. I think they were designed for smaller apertures, say to 4 inches, as their light dropped significantly with power up to 200x.

For larger apertures (e.g. 8 inch), the seeing will always be the limiting factor. Only on odd occasions will you get a sharp view above 200x.
The higher magnification will in fact exaggerate any jitter due to poor seeing.
And the lower the object is to the horizon, the worse the effect.

Regards, Rob

[Cam (Cam)]

Thanks for the input Rob

[Barrykgerdes]

Those calculations are theoretical only. The maximum magnification you can use will depend mostly on the seeing conditions. An 8" Dobsonian of the type you are using will go to 200+ magnification when the viewing is perfect as found in some dark locations sometimes.

In the city about 100 x is the best you will do before focus appears blurry.

Barry

[koputai (Jason)]

Cam, yes your 4mm is just way too much magnification, but don't swap it for the 5mm, that will be almost as bad.

In my 8" f/6 Dob, I find 8-9mm about the normal limit for giving decent views. There aren't many days per year where you can use less than a 7mm.

Cheers,
Jason.

[Cam (Cam)]

Thanks everyone... Im lucky. I live in rural vic so my backyard is a dark sky site

[koputai (Jason)]

Better seeing allows more magnification, not darker skies.