Placement of a barlow for prime focus imaging.

[sophiecentaur]

I use two extension tubes for connecting my DSLR and wanted to use my 2X barlow  for a moon image. A field flattener position relative to the sensor plane is critical and I was wondering what difference it would make to fit the barlow right on the focuser or to fit it between the two extension tubes.  It's a 2" barlow so balance could be an issue in some circumstances.

[vlaiv]

Depends on magnification that you want to achieve.

Barlow magnification factor depends on distance to sensor (or eyepiece), so x2 barlow will in some cases be x1.9 and in some cases x2.5, depending on extensions used. You can calculate what magnification factor you will get if you know focal length of your barlow lens (can sometimes be found in item specs). Use following calculator if you know barlow focal length:

http://astunit.com/astunit_tutorial.php?topic=barlow

(look under Barlow math section).

Position affects focus change requirement as well (because of changed magnification) so you might not be able to focus in certain configurations.

Best advice is to try what works best for you.

P.S. Telecentric amplifiers don't have such change of magnification with distance, they tend to stay at the almost same magnification, but that depends on particular model.

[sophiecentaur]

@vlaiv

Thanks. That makes sense and I will try both positions. Aamof, with the existing layout, focus is only just possible by pulling out the tube past the detent position - not the best thing. It could also account for why the image I get is a bit  larger than the image that Stellarium predicts.