BORG Guide Scope & 55FL F3.6 Back Focus

Since 7615 – 153mm long white tube has been discontinued, system packages have been renewed. Previous systems are no longer available. 7615 is replaced with 7604 and 7603 etc. Each system weight gets a little lighter. We will add “Recommended accessories” section later on.



BTW, following is the test shot by David Losano after changing system configuration. He had some issue in image result. After researching, I realized the cause was the camera’s flange back distance. According to the manufacturer’s official note, the camera has +/- 0.5 mm manufacturing variation as flange back distance. 55FL F3.6 is super sensitive to the optical distance and angle. So this camera’s variation number is not small to the scope at all. Actually variation is fine if we know each camera’s measured unique number. But unfortunately, not done by them. After some try-n-error process while using some difference spacers,  eventually we have achieved setting the sensor location at (or around) 60.00 mm which is optics’s optimum number. Here is the result. (I’m surprised these targets are still up in the west sky)

Median_ngc7000_Area DBE final

Now David is completely enjoying wide-field of view with KAF8300 sensor.

Again, BORG 55FL F3.6 is extremely sensitive to the distance.  If someone feel the result is not what you expected, check back if the optics’s back focus distance matches with camera’s flange back. I noticed an another brand has +/- 0.5 mm variation as well. To optimize 55FL F3.6, we should work by 0.1 mm unit at least. For full frame sensor, I should say by 0.01 mm.

BTW, to verify the issue is camera? or optics? I recommend using consumer cameras first. Those are much more precisely manufactured. So we can at least eliminate the camera flange back factor unless the camera is modified improperly.  In Hutech/Canon case, we set +/- 0.002 mm as specified at


and we keep the measurement record on each Hutech-modified camera’s 3-point flange back distance, so that we are still able to mechanically re-set the sensor if something happens.


~ by tedishikawa on March 28, 2016.

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