Am I using the wrong barlow?

[jrkirkham]

For solar system photography I have been using my C11 with a ZWO ASI120MC camera and a 2.5X Powermate. I have watched a couple of videos lately that suggest I am over shooting my magnification. The videos mentioned that I should keep the system below f/20 with that camera, which means anything over a 2X barlow is going to be out of focus. Do you all agree or disagree?

[JayTee]

My gut says I disagree, but my system is the same as yours, C11, ASI 120MC, but I use a 2X shorty Barlow, so I don't have empirical data greater than F20 myself. I was under the assumption to get your focal length as long as you can make it because the atmosphere is the limfac on whether you're in focus or not.

[Lady Fraktor]

Just throwing it out there, but perhaps the Powermate is not playing nice with the spherical SC mirror?
Maybe try a regular barlow and see what happens.

[sdbodin]

Disagree too, could be just bad seeing. I have shot as high as f30, but my seeing is really crap except for September, jet stream maybe.

I shoot now at f18, this is with a 2x barlow, you never really know what you get with a moving mirror system until you measure it. So, maybe you are at f30?? or maybe at f20?? Shoot a star field and do the math, usually an open cluster will work. I use the math from here, http ://www.wilmslowastro.com/software/formulae.htm#FR

Steve

[jrkirkham]

Thank you, I am probably crummy at focusing an SCT. I used to shoot through my 80 ED triplet, but switched to the SCT for solar system work. I'm learning, but it is fun journey. My planets aren't terrible, considering my experience, but my lunar shots are not as crisp as I would like.

[JayTee]

I most likely have different equipment for focusing my SCT than you might. I use both the feather touch focuser and my screw on a crayford dual speed focuser to fine tune the focusing when I'm doing planetary Imaging. I even do minor refocusing on the fly with the crayford focuser.