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Problems Reaching Focus

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I had the first opportunity today since my Feathertouch adapter arrived to test out my white light imaging setup. I thought back focus would be an issue as I was adding a 2” focuser to the existing 4” focuser on the scope - and it was. The only way to reach focus was to screw the camera directly into the Baader wedge (pictured) which gave a few mm to play with.

So success, and I took a few test captures. However, I don’t want to image at the native focal length of the scope as I am under sampling. Simple, I thought, I would add the Baader VIP Barlow element or the 2.6x glass path corrector used with my Baader binoviewers to increase image scale and push the focal plane out which would be required to accommodate the physical size of the bayonet, Barlow and spacer. 

However, no matter what spacing I used with the Barlow I could not reach focus. I tried spacers between the Barlow and camera that resulted in a magnification increase from about 1.2x to over 2x but each time I couldn’t reach focus. Racking focusers out made things worse so it seems like I need more in focus. I am clearly missing something or the same rules for accommodating binoviewers don’t apply here. Please can someone shed some light on what I am doing wrong?





Edited by astro_al
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From the document calculating_different_magnifications_with_the_vip_barlow.pdf

"If the sensor is 64 mm away from the upper end of the Barlow element, you're working with double focal length.

If you are using a DSLR with a standard T2-adapter, the distance is always 55mm. Thus, if you screw the Barlow- element directly into the T-ring, the distance to the sensor is 50 mm, and you are working with 1,8x magnification."

To me this suggests you need around 50 to 60mm between the barlow element and the sensor.

But as you have found, this will result in insufficient in-focus.

Unless you mount the Barlow between the focuser and the wedge ??


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Thanks Michael. I guess I was hoping that introducing the Barlow of GPC would push the focal point out far enough to offset the physical distance required to accommodate it. Sounds like that’s not going to happen unless I introduce the Barlow further down the light path. Thinking a bit harder about it, when I introduce the GPC to my binoviewers it is not taking any additional in-focus and has the length of the body of the binoviewers to get to the 2.6x.

My concern with adding the GPC before the diagonal is that I am not sure if its housing it will clip the light cone and get very hot. I don’t really like the idea of anything going before the wedge so may just end up looking for a camera with smaller pixels in the longer term.

Thanks again.


Edited by astro_al
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