Which Barlow for Prime Focus Photography

[andy77]

So we want to take some close up images of the moon ect, which barlow lens would i need to be able to do this, i will be using it on our sw150p-ds with canon 600d/5d mk2.

Im thinking it will need to be a barlow with a 2" fitment so i can fit the 600d to it with the 2" t-adapter to it? Links to the barlow we need would be great

Thanks Andy & Abigail

[andy77]

Anyone??

[ronin]

Cannot suggest anything as it depends on how much more of a close up do you want?

A 1.6x is slightly closer, a 2x a bit more a 5x quite a bit more, sure there are 3x's and 4x's around.

So really how much bigger do you want and pick the barlow closest to that.

As the magnification is increased I would say you would need a better quality barlow and at 5x could be in powermate territory.

The other minor arguement being if you stick a barlow in then it is no longer prime focus.

[JamesF]

The Moon is half a degree across. To find out how much space that represents on your imaging sensor, you can first calculate the image scale in degrees per mm as 57.3 / focal length, so 57.3 / 750 for your scope, or about 0.08 degrees per mm. That means the half degree wide moon will take up 0.5/0.08 mm on the sensor, or about 6mm.

The 600D has an APS-C sensor I think, so it'll be about 15mm high which means you can't really do more than double the size of the image before it gets too big to fit on the sensor. That means you can't really go bigger than a 2x barlow if you want to fit the entire Moon on the sensor. In fact that would probably work quite nicely.

With the 5D (full frame?) you may be able to go to 3x though I'm not aware of a 3x 2" barlow off the top of my head, and obviously if you're not worried about getting the entire Moon in one frame you can use a larger barlow too.

I thjink I'd go with the 2" barlow. If you're filling the sensor with the image then a 1.25" barlow with, say, a T to 1.25" adaptor may result in some vignetting. I'm not sure it will, but it's close enough that I'd want to work it out to be sure.

James