Introduction
Just to share my thoughts and set the records straight with some of the observed differences I read on the internet with this Meade (now no longer in production) 4 inch achromat doublet refractor with 800mm focal length, or f7.8. Second hand units comes up now and then.
https://telescopes.net/store/media/e...escope-3e1.jpg
or
https://www.youtube.com/watch?v=VhZTuBIFNDI
Note: the current Meade Starnvavigator NG Infinity series is a 102mm with a 660mm f6.5 short-ish tube scope.
My discussions below is restricts to aspects of the optical tube assembly (OTA) only, in the hope that those who have this scope can modify and improve their unit, so they can actually see better with this scope.
I will very briefly comment that the pressed aluminium tripod mount legs with plastic connecting joints is too short in height when fully extended and too shaky, enough said.
The modified achromat MA 26mm eyepiece has a plastic field lens and not a glass field lens element when I took it apart and lend edge blackened it...
No comments about the Goto feature as I don’t use it, I use what's between my ears for Goto instead.
Back to the OTA for the rest of this review:
Objective lens
I took the objective lens out and measured it, it is 104mm across. The objective lens cell retaining ring measured 102mm across, so once mounted, this scope is a 4 inch, ie, it is a full 102mm, not 100mm as some others suggested. Flashlight test also showed it is 102mm, so it is f7.8 and not f8. The sticker label on the bottom underside of the focuser assembly is therefore correct, it is 102mm with 800mm focal length.
The objective lens is actually a cemented doublet achromat, with the thinner crown facing outwards and the thicker flint facing the telescope focuser (swapping the orientation actually resulted in inability to achieve clear focus with this OTA). The lens edge not blacken, I blackened it with permanent marker and sharpie.
My unit suffered quite significant astigmatism when I first got it, partly inherent in the lens itself and partly as the plastic rack and pinion focuser draw tube was out of collimation, which can be corrected by the 4 hex screws on the rack and pinion focuser assembly.
One option maybe to de-cement the lens and rotate the crown and flint lens elements to cure the astigmatism at the expense of light loss, ie, four surfaces instead of two. However, an unknown is the quality of the two lens surface polished on the cemented end.
Telescope tube
The telescope tube (rolled aluminium – very light in weight) is slightly over 90mm and so the 102mm objective is housed in the dark brown glossy plastic lens housing covered in part by the black plastic dew shield, that by the time it is connected to the scope tube, the light path is not truncated by the smaller diameter telescope tube.
My 102mm unit suffered from very significant loss of contrast when I first got it. The first culprit which I identified and fixed was the dew shield, the inside surface plastic was very glossy black. Once I flocked with matt black paper, the contrast improved dramatically.
Baffles
The telescope can be described as having two baffles. One close to the focus tube. The other is actually part of the plastic lens cell that narrows the aperture opening behind the objective lens, but does not vignette the light path. The problem here is that it is a glossy dark brown plastic lens cell on the inside surface, so ample light bouncing and scattered all over the objective lens cell.
I flocked the inside of the objective lens cell with matt black paper to improve contrast also.
How does it compare?
Comparison with a 70mm air spaced doublet achromat with 500mm focal length, ie, f7.1 resulted in the smaller unit doing much better on both star test diffraction ring pattern and violet fringing than this 102mm f7.8 unit.
Note: Not comparing light gathering power or resolution here of course as we are comparing apples with oranges with such size difference.
I suspect that the optics quality on this cemented 102mm doublet varies unit by unit. I also suspect the actual glass material used to make this 102mm objective lens is not that good, this is inherent and can’t be fixed.
Maybe nice if the optics is air spaced instead of cemented, so astigmatism inherent in the lens elements may be cured by rotating the lens but then again the quality of the glass and the extent of polishing on the two cemented surfaces may not allow this option.
Conclusion
After all the above fine-tuning...
The pros:
It is a cheap and cheerful 4 incher. Very light for the OTA, quick to set up and use in no time, which is real nice for a 4 incher of f7.8.
The Cons:
Optics OK, but not great (on my unit) once collimated, clear diffraction rings during star test is hinted or evidenced at times only on parts of the lens surface, and not clearly evident at all times.
Achieving crisp and sharply focused image during star test is not easy with my unit despite the above modification and collimation, which should be expected for a refractor, therefore I suspect the glass quality on this 102mm objective is average, at least with my unit. Not expecting an Astrophysics here but I have seen better achromat doublets than this.
