SW Coma Corrector.... Is it reducing too ?

[badgers]

Hi All,

On my Newt I use the Skywatcher coma corrector. It's advertised as being a 1.0x coma corrector.

One thing I've noticed though is that I get consistently more sky coverage when I image with it in place according to astrometry.

Here's an example from the Crescent (click to enlarge):

As you can see according to Stellarium and astrometry I'm getting a wider-field than predicted (top right is what I should get)

My refractor images come out exactly as they should when I examine them the same way, but the newt images all look like this with the corrector.

The images obtained look like they've come from a 690mm focal length instead of 750 (bottom right image).

So, the SW corrector seems to be acting as approximately a 0.92 reducer given the images I'm getting.

Has anyone else seen this kind of effect ?

I wouldn't mind a raw image or two from someone with the Coma corrector to see if I'm going crazy.

[A320Flyer]

I have the SW CC. I read somewhere that it gave a 10% reduction so,in the plate solving parameters in Maxim, for my 750mm newt, I altered the focal length to 675mm and it seems to work ok.

Bill

[badgers]

Ahh, good to know it's not just me.

Shame they don't tell you about this when you buy the corrector... I don't mind a little reduction, helps hoover up more photons, but still it would be nice to know.

Anton