Corrector before or after barlow (Baader FFC)?

[multiweb (Marc)]

I have a corrector for my refractor that works well at 119mm from sensor. I have a Baader FFC that I'd like to use in the imaging train. The FFC magnification is proportional to the spacing between the glass and the CCD. Do I:

1_ place the FFC at the native focal point of the refactor then have the corrector then the CCD at 119mm?

or

2_ place the corrector up the tube 119mm ahead of the FFC glass and then the CCD can be spaced as usual to get the required magnification.

In #1 I can't get magnification smaller than x4. With #2 I'd still be able to go down to x3.

TIA.

[Hans Tucker (Hans)]

Originally Posted by multiweb

I have a corrector for my refractor that works well at 119mm from sensor. I have a Baader FFC that I'd like to use in the imaging train. The FFC magnification is proportional to the spacing between the glass and the CCD. Do I:

1_ place the FFC at the native focal point of the refactor then have the corrector then the CCD at 119mm?

or

2_ place the corrector up the tube 119mm ahead of the FFC glass and then the CCD can be spaced as usual to get the required magnification.

In #1 I can't get magnification smaller than x4. With #2 I'd still be able to go down to x3.

TIA.

I would think that option 2 would be the preferred setup. I have a Baader FFC and plan to use it on an f/12 refractor once I have the correct extension tubes which are hard to source. Is your corrector just that, a corrector or does it have a reducer/extender function as well? Still, I think any correction device should be placed ahead of the FFC....trial and error will reveal the correct option.

[multiweb (Marc)]

Originally Posted by Hans Tucker

I would think that option 2 would be the preferred setup. I have a Baader FFC and plan to use it on an f/12 refractor once I have the correct extension tubes which are hard to source. Is your corrector just that, a corrector or does it have a reducer/extender function as well? Still, I think any correction device should be placed ahead of the FFC....trial and error will reveal the correct option.

Thanks Hans. I'm trying #1 tonight and will post results. #2 would be a lot more convenient and practical obviously. Here's a pic of the setup barlowed x4 so 2400mm. Distance from FFC to sensor is a bit over 150mm. At that FL I had to get the OAG working.

The native focal point is just a bit behind the red line with a 9 next to it. The FFC is inside the second (small) GSO extension tube. The corrector is inside the FFC extension tube 119mm to the CCD. Managed to fit the OAG in there as well about 50mm before the sensor so the lodestar comes in focus too. Worked well. Hopefully tonight I can find a guide star. I'd like to try M83 as a Lum.

PS: Paddle pop sticks are temporarily needed for spacing because the OAG thread goes too deep inside the QHY9 opening and would jam the shutter. I need to get a small washer made out of aluminium.

Originally Posted by Hans Tucker

Is your corrector just that, a corrector or does it have a reducer/extender function as well?

Not sure. I don't think it changes the F ratio. That's the one here.

[Barrykgerdes]

If you are going to upset the focal length of a telescope using a "barlow" lens you might as well forget the corrector as it will no longer match the optical path. Besides what sort of quality is the barlow. Is it multi-element?

Barry

[multiweb (Marc)]

Originally Posted by Barrykgerdes

If you are going to upset the focal length of a telescope using a "barlow" lens you might as well forget the corrector as it will no longer match the optical path. Besides what sort of quality is the barlow. Is it multi-element?

Barry

Yeah it's pretty good. Specs here. Probably better glass than any of the scopes I have.