How to use a Barlow for AP?

[Star Dad]

Hi all - I have a 2x barlow - looks like this one: https://agenaastro.com/gso-2-2x-ed-barlow-lens.html . The write up states it needs 19mm more in-focus travel. The ASI294MC requires 55mm backfocus - so could I remove 19mm from the ASI and just screw the actual 2x lens onto the camera?

[yobbo89]

The distance between the Barlow and ccd/cmos is subjective, different spacing can alter the magnification slightly , it's up to you how you want to use it. There might be a position for eyepiece viewing (probs the 19mm) for optimal aberration correction but for imaging I don't think it is too important, correct me if I'm wrong.

The 55mm back focus specification really only applies to optical add ons that specifies that distance and isn't reliant on the camera. There are coma correctors that need a back focus of 80mm-90mm or more, general most corrector, are about 55mm. Barlow's should need no apply,they work alright at a variable distance.

A barlow should move the focal point away from the main optics, and with a reducer you usually need to rack the focuser in closer to the main optics.

[Star Dad]

OK, thanks. That is what I thought. Sounds like I just didn't achieve focus with the barlow - not sure exactly where focus is. Next time out I'll try and spend more time fiddling with it.

[yobbo89]

I have hunch you'll be moving the focus point out, grab some extention tube while you're out in the field. If you Can find the moon you can just hover the Barlow and camera over the draw tube and move in and out to give a quick indication of focal point while checking the live view.

Edit:when I mention extension tube, it is for the focuser, if you have such, don't be adding anything between the Barlow and the camera.

[sdbodin]

Check our this Article on the forum;

viewtopic.php?f=94&t=6459

Should give all the general facts.
Steve

[Star Dad]

I have hunch you'll be moving the focus point out, grab some extention tube while you're out in the field. If you Can find the moon you can just hover the Barlow and camera over the draw tube and move in and out to give a quick indication of focal point while checking the live view.

Edit:when I mention extension tube, it is for the focuser, if you have such, don't be adding anything between the Barlow and the camera.

Great idea! I probably did not move the focus far enough out - don't ask why. Senility, maybe. Of course, now we're moving into the new moon. Murphy's Law

Check our this Article on the forum;

viewtopic.php?f=94&t=6459

Should give all the general facts.
Steve

Ah, yes I remember that discussion - which is why I'm going down this path. Thanks!