realflow100 wrote: ↑Tue Aug 17, 2021 3:57 pm
Whats the typical maximum resolution in arcseconds of a 70mm achromat/doublet refractor telescope?
I have a cheap one and ive been messing around with it
and the limit seems to be reached between 60x and 120x magnification
it looks perfect crisp sharpness at 60x but becomes noticably softer at 120x
Shouldn't it still be crisp at 140x if I follow the rule of 2x magnification per mm of aperture?
Also the contrast seems decent at 60x but the whole image looks a bit washed out/greyish at 120x
The answer to your question really just depends as it will vary for the scope depending on many things, like the objects you are looking at, the sky conditions (seeing, transparency, altitude of observing site), and of course the particular optics. With Apochromats I find that the view stays nicely crisp for planetary with exit pupils as low as 0.65mm (1.5x/mm or 40x/in) when the optical figure is descent and the atmosphere is cooperating. So with your
aperture that would be about 108x if its color correction was fairly perfect as with an apochromat, less since you are working with an achromat. Remember, as the exit pupil gets smaller the view gets dimmer and the contrast appears lessened. So you are noticing this effect when you say it looks high contrast at 60x (1.16mm exit pupil) but washed out at 120x (0.58mm exit pupil). Given that the scope in question is an achromat and not an apochromat, the resulting color blur will mean that you will not be seeing as crisp of a rendition at the same magnifications an apochromat will, so you need to reduce expectations some. But you can experiment and use a green or red filter and am sure the views will appear crisper as a result since you are limiting the color blur by removing some of the spectrum. If viewing the Moon instead of a planet, with its very bright and high contrast features you will find you can reach a lot more magnification and still see a crisp appearing view. With my apochromats while a planet (especially Jupiter with its more subtle features) I like to go no lower than a 0.65mm exit pupil, the Moon I find can take even a 0.25mm exit pupil (4x/mm or 100x/in) and still show very well.
Since the seeing and transparency and altitude and site darkness can have quite an impact on the outcomes, you might want to try instead seeing how much magnification you can put the scope thru with daytime observing in the back yard. So pick a familiar target like a flower no more than 25 meters or yards away, closer preferred, and see how much magnification the view can take BEFORE it looks "off" (i.e., not as sharp or contrasty as naked eye up close). This I find is a much better way to evaluate what a particular scope can do as it can often take weeks or months to find an evening when the atmosphere is really good enough to let an optic strut its stuff.
As far as the resolution of the 70mm scope, that would be about 1.65 arcsec (as already mentioned by others). However, I would still take that with a grain of salt as that is assuming no color blur. So given you are working with an achromat I'd say maybe expect a resolution not quite that and perhaps closer to 2 arcsec unless you have a filter on the scope to reduce the spectrum and thereby the amount of color blur coming through as well. So if the figure of the scope is diffraction limited and you have a green filter like a #56 or #58 then likely you will get much cleaner looking splits of close doubles nearer the theoretical 1.65 arcsec for your 70mm
aperture scope.