I will take delivery first week of October of a factory new achromatic 60mm f15 refractor that comes with a simple equatorial mount and aluminum tripod.
So before it arrives I'd like to get some advice on what to expect as far as visual viewing of lunar features as compared to the slightly larger but much faster refractor (70mm f5) we're currently using.
For now here are some things that I could use some feedback on:
1) Our ETX 70 tracks the moon stably enough that we've used a maximum of 175x magnification with a 3x Barlowed 6mm eyepiece for crater viewing. Can we expect to stably track the moon with a simple equatorial mount with magnifications of 150x, and to a maximum of possibly 225x?
2) The manufacturer is supplying accessories to reach 338x magnification, but this would give an exit pupil of only 0.177 mm - is it really possible to view lunar features with such a tiny exit pupil?
3) The Conrady criteria for the f15 scope is well over 5 so my understanding is that chromatic aberrations should be essentially zero. Is this lack of CA true even at magnifications of 225x? The fast f5 achromat we now have is less than 2 on Conrady's scale - our expectation is that details on the moon should be more visible with the slower refractor. On the other hand the objective lens is 10mm smaller with the slower scope. Is it reasonable to expect to see more and sharper details with the 60mm f15?
4) The kit includes a 90º mirror diagonal which may degrade the image quality so if possible I'll try to evaluate this by using the scope without the diagonal. On the other hand I've read that a 90º prism may actually be a better alternative than a 90º mirror for refractors slower than f8. If we end up keeping the kit, would a prism be a good accessory to have?
5) The ETX 70 included a 45º erecting prism that we can hopefully attach to the new scope, does anyone know if this will in theory help or not? Of course, I realize that depends on the quality of the erecting prism we now have but it would be nice to understand what the physics says about this.
My hope is that this 60mm f15 kit will perform better for lunar observing than the 70mm fast f5 refractor that we now use from under the light dome of several large cities that surround us that give us Bortle 8 skies. (We do have access to a quite large park without city or housing lights that is not accessible by car where we first saw comet Neowise two weeks after peak viewing days - cloudy skies were the culprit - through 7x35 binoculars. So we are looking for portability also, for us – over 60 years young - that means being able to hand carry for a 3km walk, half of which is dirt paths, to a site that is locally dark but still under a huge metropolis light dome that never shows the Milky Way.)
Though, in closing, I understand that the manufacturer is probably playing a numbers game (sort of 'the dilemma of the marketer' when having to compete with unscrupulous producers that exaggerate the capabilities of their products) by advertising stable views at 338x with this small, slow refractor with a lightweight German equatorial mount and aluminum tripod.
To be honest, I do not expect this kit to perform as the marketing specs promise - though in all fairness I do expect it to manually track well at least at 150x along with giving nice sharp views of the moon's details, or is this asking too much? Or have I been royally duped into buying the infamous department store ripoff?
In any case I'll have 30 days to test the complete package with a full refund and no additional cost for either shipping on delivery or return if we decide to not keep it.
Comes what may, with your advice/suggestions I will gladly send it back if it does not meet some reasonable minimums for performance in real life conditions - hopefully a return would in some measure contribute to holding telescope kit suppliers to some minimal standards for marketing claims that exaggerate performance specifications.