Posts Made By: Markus Ludes

Russ,

a 175 mm apo offers ultrahigh contrast , excellent choice for douple stars and Planets. Due its smaller aperture and no central obstructionit is also way less effected by seeing turbulences, which means the real good nights with 175 mm aperture will be many more than with larger size.

The SC scopes I know will have zero chance on Planets and douple stars against the Apo. On Deep Sky you may see in the SC a brigher Image but much less contrasty, so the Apo will show you some details in Deep Sky stuff which you will not see in the SC

The classic Newtonian can be optized for everything, but you need to accept to deal with tweaking the collim,ation, always , mirrors which need to be cleaned from time to time and a open tube which also will be effected very much by the seeing many times

Leaves the Mak. A bigger Mak 10" or 12" will be also way more often effected by seeing, but if you buy a very high quality Mak and you get seeing like we have had last year in Florida it will beats the Apo , excapt on widefield observing where the apo offers wider field due its shorter focallenght.

Solution: buy a Apo 175 for everyday use and a 16" Mak for the small faint stuff and best seeing for planets

best wishes
Markus

John

if you see astigmatism in a lens, no matter if achromat or apo or whatever, than mostly it is not in the glas.

there are several possibilities alone or together :

1, most comon problem in low priced achromats: cells are not matched in inside diameter with glas diameter, so lenses can shift sidewards
2, spaces are not using matched thickness.

how to correct such problem without return of the optics:
- you can drill for each lens at the edge of the cell with 120° spacings threads and use plastic screws, they can allow you to push the lens onto perfect axis

good luck
Markus

If someone want such a scope with no delay, I mean from stock, we selling them all over the year here in europe and have happy owners

a review of our modell was posted here
http://www.cloudynights.com/reviews4/apm-achro.htm

I am self promoting this refractors as low power rich field refractors and cannot undelrine all findings of the reviewer

best wishes
Markus Ludes

Alan say :No lens has perfect color correction, and there is always some variation in focus across the spectrum. For simplicity, assume we have a lens corrected for C and F. The green light focuses slightly closer to the lens than either the blue or red.

Markus : sorry but shall a real apo not show 3 wavelenghtes at excact same focuse ???????????

Alan say :.....Based on this, it is hard to see why there would be symmetry in white light. Different things are going on inside focus than outside focus. This will always be true, unless all colors come to exactly the same focus (reflector, anyone?).

Markus:if the optics is superb polished you will see a perfect symmetrie even in achromats with same contrast, only the colors you see on each side of focuse are diffrent.
The diffrence in a perfect corrected lenses between inside of focuse and outside of focuse is nothing than the diffrent colors, but the fressnell rings and here contrast must be 100% equal if the optics is perfect polished

Alan : If you want to see what the correction for spherical aberration is, you really need to isolate the green light.

Markus: Alan here above you are correct , but with a real superb Fraunhofer achromat design or other semiapo or apo design, all optimized for green , usual the spherical aberration on outer wavelenghte is very similar. Only photographic corrected systems show diffrent wavelenght optimisations.

Look again the the white lioght startest images of perfect corrected, achros, semiapos and apos and see its true, simply because its a physical law, that it have to be this way

Markus

next 25 mm TMB eyepiece image

Markus

Sun filters can be always used because screwed on front of the objective, or just fittet over.

On our modell we use eyepieces with special filterthread, so we can offer custommade screw in filters

best wishes
Markus

more images

image MN 56