APM 2.7 coma barlow ccd spacing query?

[ebdons]

Hi, not having tried this yet, the APM 2.7 coma barlow is stated to have a back focus of 112mm from the shoulder of the element? but in the paperwork I got with it is stated  104.1 mm from the shoulder of the element. have I got the wrong printed info?   all the other info states 112mm so I presume that is correct? or is this barlow able to adjust to smaller sensors and you would have to adjust the spacing? obviously you can adjust the spacing to suit sensors ccd etc. not important as I will try and test later if I can.

[vlaiv]

Here is some info on this barlow:

- focal length of this barlow is 62.9mm

- for magnification factor of x2.7 you need to place it 105mm "from the center of the last lens", or another way to put it - from "highest" point on the surface of the last lens (that being the center of lens of course).

You can actually calculate required distance by using barlow formula:

Magnification = 1 + D/F

Where D is distance from the lens and F is focal length of the lens. If you plug in the numbers - you can work out distance from the "center" of the lens assembly (principal point)- not to be confused with center of last lens - it is "middle" of the lens assembly rather than point on surface of the lens.

2.7 = 1+D/F => 1.7 = D/F => D = 1.7*F

1.7 * 62.9 = 106.93mm

This agrees well, so principal point to focal point is 106.93mm - that puts principal point about 1.93mm "inside" last lens.

Anyway - here you have it. It should be about 104.1mm from shoulder because it is about 1.1mm "in front of the last lens".

If you place your sensor further or closer - you will get different magnification - handy for adjusting sampling rate, however nothing to do with sensor size.

[ebdons]

Hi many thanks for your explanation, as the printed info differed from the website info I was a bit dubious with the setting but and have set it to 104mm (ish) and will try it and see, but this barlow does have a good rep. many thanks.