Powermate/Barlow?

[martin_h]

What's the difference? If any.

[vlaiv]

There is difference, both in optical layout and how they perform.

Barlow is negative lens, here is diagram:

Powermate is telecentric lens

Both work as focal extenders, but there are differences:

Barlow:

- magnification changes with barlow element to eyepiece distance (or sensor distance - further away greater magnification)

- will impact eye relief of eyepieces (increase it)

- some eyepieces will vignette with barlow

- in some applications it is less suitable than telecentric - like Ha filters for solar observation

- usually fewer glass elements (most often 2 elements)

Telecentric:

- magnification stays relatively the same regardless of the distance (there can be very small shift depending on telecentric design)

- will not change eye relief of eyepiece

- usually no problem with eyepiece vignetting (where barlow produces problems)

- it is more suitable for some applications like Ha filters for solar. This is due to fact that emerging light from telecentric is parallel - look at diagram, so total angles remain the same - important with Ha etalon where exact wavelength depends on angle of incidence of light ray, barlows on the other hand have principal ray at the angle when further of axis, so beam that is spread over principal ray will have one side less angled but other even more angled - and that is bad for Ha etalons

- more glass elements (4 I believe is most often)

 

[martin_h]

Many thanks for that.