Last updated: 31 August 2006
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Subject:	Orion 0.5x Focal Reducer
Sent:	Tuesday, August 29, 2006 06:32:46
From:	Ted Rafferty (raff650@erols.com)
Orion 0.5x Focal Reducer
Orion recently (summer of 2006) came out with a 0.5x Focal Reducer for
their StarShoot Imaging camera. Since some of the Messier objects are a
little too large for the combination DSI-I imager and LXD75 8-inch f/4
Schmidt-Newtonian I am using, I have been looking for a focal reducer.
Most focal reducers are made for the Schmidt-Cassegrain telescopes that
screw into the back of the telescope where the Orion focal reducer
screws into the end of the 1.25-inch nose piece of the imager. Though
Orion doesn't indicate that their focal reducer can be used with other
imagers, I decided to get one for my DSI-I anyway.

The focal reducer causes the focus for the imager to shift along the
line-of-sight towards the primary. The amount of shift is dependent on
how far the focal reducer is from the imager's chip (the closer the
reducer is to the chip, the further in the imager much to located to
focus), so one needs to have the necessary travel with their focuser to
reach the focus. In my case, I had replaced the original 1.25-inch nose
piece that came with my DSI-I (I didn't like the under-cut steps on the
draw tube), as well as the original IR-blocking filter (the quality
didn't look very good). The thread on the original nose piece is the
standard one used for t-adapters so I replaced it with one that had a
smooth draw tube. And I replaced the IR blocking filter with one that
screwed onto the end of the draw tube of the nose piece. When I screwed
the Orion focal reducer to the IR-blocking filter, I was unable to reach
the focus. Switching the position of the filter and focal reducer also
would not focus. Since the original nose piece for the DSI-I was a
little shorter than the one I replaced it with, I put it back on the
imager and was able to reach the focus if the focal reducer was screwed
into the noise piece first and the filter screwed onto the end of the
focal reducer. I only provide these details as an illustration that
reaching the focus with the focal reducer might be difficult.  

Besides reducing the effective focal length, the focal reducer also
decreases the f-ration of the optical system. The plus side to this is
that it reduces the exposure time necessary to reach the same magnitude
limit that would be required without the reducer. The negative side is
that the lower f-ration makes focusing more critical.

Here are two images taking of M27; one using the focal reducer and one

Combination of ten 1-minute exposures using the DSI-I with the 0.5x
Focal Reducer on a Meade LXD75 8-inch f/4 SN:


Combination of eight 2-minute exposures using the DSI-I (without the
0.5x Focal Reducer) on a Meade LXD75 8-inch f/4 SN:


I measure the actual scale reduction to be 0.64x instead of 0.5x, but
that is just due to the differences in the dimensions of the DSI-I and
StarShoot imagers. This changed the effective focal length from an f/4
to an f/2.6.

Some degrading of the image quality is likely due to the additional
optics of the focal reducer. I am not comfortable with recommending this
reducer due to the difficulties of reaching the focus. It would likely
be better to not use the focal reducer and take the necessary images to
create a mosaic.

Going the other way, there are a few Messier objects that are a little
too small for the DSI-I/8-inch f/4 and I have tried using a Barlow to
increase the effective focal length. Here are two images of M57; one
using a 2x Barlow with the DSI-I and one without the Barlow.

Combination of twelve 30-second exposures using the DSI-I with a 2x
Barlow on a Meade LXD75 8-inch f/4 SN:


Combination of thirty two 8-second exposures using the DSI-I (without
the Barlow) on a Meade LXD75 8-inch f/4 SN:


Measuring the scale change the 2x Barlow enlarged the field by a factor
of 2.55. This changed the effective focal length from an f/4 to an f/10.
Ted Rafferty
Gaithersburg, Maryland

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