Puzzled need help

[blind pew]

When I use my star travel 80 with a skywatcher 8-24 zoom I get a clear picture up to about 45x.when I add the skywatcher 2x barlow I can only get to about 55x before the picture starts to get grainy.Is this because the barlow is poor quality or are the two incompatible.As the scope is supposed to magnify up to 160x I am puzzled as to the solution.Any help would be appreciated please.

[John]

An ST80 maxes out at around 80x - 100x because of the chromatic aberration (CA or false colour) that an F/5 achromat refractor produces. 160x would be for a slower scope (eg: F/10 or more) under excellent conditions. The ST refractors are designed for low / medium power wide angle viewing of deep sky objects really.

Add to the above a zoom eyepiece being used with a low cost barlow lens and I reckon that explains why your views are less than pin sharp.

[Capricorn]

Consider the ST80 and what it is built from, then consider the WO GT-81.

Both about the same size, ST80 costs £90, the WO GT-81 costs £680.

The ST80- is a short achromat made from a piece of Flint and a piece of Crown glass. Unspecified.

The WO GT-81 is a triplet made from FPL-53.

Yet we say both will take twice the mm diameter in magnification, 160x.

The ST 80 will have a large amount of CA, also a fair degree of SA. Both degrading the image you see.

Realistic max on the ST80 is 80x and possibly occasionally 100x. Take no notice of the claimed maximum.

[blind pew]

Puzzle solved.I screwed the barlow lens element to the base of the EP and now I get up to 80x.I suddenly remembered reading it in an earlier post on SL.Thanks for your input I appreciate it.