APT - Bahtinov aid - what focal length should I specify?

[BrendanC]

Hi,

I have a Bahtinov mask that I originally got for my scope (a Skywatcher 130PDS) before I got a coma corrector.

The scope has a focal length of 650mm. The coma corrector reduces this to 590mm.

So, in the APT Bahtinov aid, should I specify the focal length for which the Bahtinov mask was intended (650mm), or the focal length now that I use the coma corrector (590mm) - or should I get a new Bahtinov mask?! Or doesn't it matter?

Thanks
Brendan

[tooth_dr]

I have a Bahtinov mask that I use for both 1200mm F4.7 and 500mm F2.8, and it works as intended on both.  I dont use the APT, but I would input the actual working FL of the scope.  

Edited July 14, 2020 by tooth_dr

[BrendanC]

Thanks - I think that, if it weren't for APT actually asking me to specify the diameter of the aperture and the focal length, I would assume it's fine too. But the fact that it's asking, so that the Bahtinov aid can do its calculations correctly, makes me think I probably should absolutely get this one right! Otherwise everything might be very slightly out of focus, which I'd rather not realise several months down the line...!

[dunc]

Hi BrendanC,

I would think it has to be the effective focal length of the entire optical train including flattener, coma corrector, reducer and whatnot and anything else such as filters so that the displacement of the line is calculated correctly but think a bit more it shouldn't matter for exact focus, only when APT is trying to tell you how close you are to exact focus, as focus i.e. intersection of the three lines, is always at 0 displacement regardless of the optics, which is the point of the Bahtinov method. There, I sound like I may know what I'm talking about! I've used APT for years and the Original Bahtinov Grabber for longer - which I still prefer as I find its display clearer than the APT version.

Duncan

[BrendanC]

Thanks! I just got an 'official' reply from Ivo at APT and he confirms what you say - so 590mm it is then.

I think I'll also try the FWHM focusing tool too, just for laughs.