ACCESSORY REVIEW Last updated: 6 February 2006

Subject:	ETX site: review of Borg Helical Focuser
Sent:	Sunday, February 5, 2006 01:26:57
From:	Jonathan Putsman (jon@putsman.plus.com)
Thanks for an excellent site, it is a great source of information. I
recently bought a Borg helical focuser for my ETX and since you don't
have a review of this accessory, I thought I'd write one.


I have a Meade ETX-105 which suffers from some mirror shift when
focusing. I don't find the image shift objectionable with low-power
eyepieces, but when using high-power eyepieces or doing webcam planetary
imaging it is quite troublesome. With the Meade LPI and a 2x barlow the
image shifts by almost the full width of the sensor! Achieving sharp
focus on a shifting target is quite difficult.

Looking for a solution to the problem of image shift and hoping to get
improved fine focus control, I settled on a helical focuser made by
Borg. They have quite a range of helical focusers; I chose the
non-rotating 'S' focuser 7315 with the 7316 1.25" adaptor. The
"non-rotating" description means that when you adjust the focus, the
eyepiece does not rotate - a must for imaging. The 1.25" adaptor lets
you put the focuser into a standard 1.25" eyepiece port. This focuser
offers 10mm of travel with a graduated scale on the barrel with 50
micron marks - that's 20 divisions per millimetre! It has two locking
screws for securing an eyepiece and a focus-lock screw for photography.
The rear of the focuser has a T-thread. It weighs around 85g including
adaptor and measures between 50mm and 60mm in length, depending on focus
position. The length of the focuser means that small eyepieces are
easily used on the ETX without the stock finderscope getting in the way
- a nice bonus.

In use, I set the Borg focuser to it's midway point, then achieve
approximate focus using the ETX control. I then 'dial-in' the fine focus
using the Borg. The focuser has a very smooth action and is very
precise. Before I had the Borg focuser I would often hunt for best focus
by moving the ETX focus back and forth, or by defocussing the image then
gradually moving towards focus. In times of poor seeing I would do this
repeatedly, to try and improve the focus, unsure if best focus had been
achieved. I find that this trial and error approach is much reduced with
the Borg. The focus control is so fine that 'perfect' focus is much more
easily found. If you over-shoot the focus point you can reverse focus
with no image shift. I find the lack of image shift a joy, though of
course you do setup vibrations in the telescope by touching the focuser
- something that motorised focusers and focus extenders avoid. The Borg
focuser can be easily moved between telescopes so will survive upgrades,
unlike ETX-specific focus improvements.

In the UK, the 7315 focuser retails at 55 GBP, the 7316 adaptor at 15
GBP. For the cost of two or three budget eyepieces, I think this is a
very useful accessory for those who seek to eliminate image shift or
want finer focus control.


Best regards,
Jonathan

And:

I have used the Borg focuser with the following eyepieces and they all
reach focus:

Meade Series 4000 32mm Plossl
Meade Series 4000 26mm Plossl
No-name 8-24mm zoom
Televue 8mm Plossl
Takahashi LE 7.5mm

I have also used these eyepieces with a Meade #126 barlow inserted after
or before the focuser and they reach focus. Indeed, I still seem to have
plenty of travel remaining on the ETX focus.

At its smallest, the active part of the Borg measures ~50mm, which is
only 8mm longer than the "shorty" #126 barlow.

Hope that helps,
Jonathan

Return to the top of this page.

Go back to the ETX Home Page.