ED80 Focal Reducer Trials
Review by
Paul Russell
While it cannot be
denied that the release of the ED80 range of refractors coupled with popular
DSLR cameras has been a boon to low cost astroimaging, with an f ratio of
f/7.5, the current crop of ED scopes are still somewhat slow for imaging
purposes. Along with the moderate f ratio, when using standard chip format
DSLRs there is a distortion of stars brought about by field curvature at the
outer edges of the field of view. This is particularly obvious in the corners
of images.
To improve the f
ratio it was suggested to me that possibly the Meade 6.3 focal reducer might be
an acceptable option for use with the ED80. This would certainly lower the f
ratio from 7.5 to 4.725 (assuming the correct separation of FR to imaging chip)
and reduce exposure time for a given object and ISO by 60%. This is a
significant decrease in imaging time. As well as the increase in speed, the FOV
was increased from 130.1arcmin x 86.7arcmin (2.1 deg x 1.45deg) to 206.5arcmin
x 137.7arcmin (3.44deg x 2.3deg). Unfortunately this increase in speed and
wider FOV brought with it a much more pronounced distortion at the outer edges.
Instead of looking like small ovals the stars now looked like broad triangles.
Through
IceInSpace
and via correspondence with similarly frustrated amateur astronomers I became
aware that possibly the Williams
Optics 0.8X Focal Reducer and Field Flattener might be a suitable
substitute for the Meade FR. The WO 0.8 was designed for the WO range of 66mm
scopes and with their smaller aperture I was a bit doubtful that full chip
illumination could be achieved and there would be a significant vignette to
deal with. As well, the focal reduction achieved would only be a modest
reduction from f/7.5 to f/6, bringing about a reduction of imaging time by 36%.
I purchased a WO 0.8X II FR (a version III has been released since aimed at the
80 – 100mm aperture scopes) and while the imaging reduction time is only modest
the field flattening abilities of this FR is excellent for a focal reducer that
has not been specifically matched to the ED80 optics.
Even though the
results were promising, I was also interested in looking for a FR that was
relatively inexpensive yet obtained reasonable results, especially for those of
us on a limited budget and thought a shoot out between the available contenders
would be of benefit to anyone in a similar boat to me, i.e. that wanted a good
quality, value for money, focal reducer that works well in combination with an
ED80 and a DSLR camera.
The Meade Focal
Reducer (and similar) is priced from around $200, while the Williams Optics FR
is priced from $250 in Australia. I contacted a number of suppliers of
astronomy equipment here in Australia to see what other FRs were available.
Apart from 1.25” reducers, only the above mentioned Meade and WO, and the
Celestron 6.3 from upwards of $260 (and depending on who you talked to,
different to the Meade one with better optics.) were readily available on the
Australian retail market. There is also a Lumicon thread in Focal Reducer on
the Australian market, but one was not available at the time of this shootout.
I had purchased FRs
for other cameras and scopes before from Steve Mogg so on the off chance he
might have something I decided to check his site out. Steve had a
0.66x FR for use with refractors with a 2” nose piece to fit into a 2”
focuser for about $90. After contacting Steve he was more than happy to allow
me to use one of his reducers in a trial of similar products.
Procedure.
I chose NGC104 for
the trial object, principally because it was in the right area of the sky at
the right time, it would be available from my limited shooting gallery for a
couple of months if I was unable to complete the trials over a few nights due
to weather, and obtaining clear, sharp images of the density of stars near the
center would be a challenge for a DLSR/ED80 imaging setup.
All imaging was
carried out through an Orion ED80 mounted on a Losmandy G11 using a Carton 60mm
f/7.5 scope with ToUcam 900 for autoguiding via K3CCDTools V3. The Orion has had the original Crayford
removed and replaced with a Williams Optics 10:1 focuser. All exposure timings
were automated through Rob Beck’s Star-Mate . All focusing was
carried out using DSLR Focus with the
aim to achieve both a vertical and horizontal FWHM of 2.0 or as close as
possible on a star near the center of the field of view but outside the
cluster.
For each trial
composite image I took 10 light frames @ ISO800 @ 60 sec exposure, along with
the same number of darks for calibration. Flats were not used as I did not
intend to fully process the images and the lack of flats would also assist in
evaluating vignetting effects. The resultant subsets of images were processed
initially in Images Plus using the
automated processing function, then aligned and rotated using the same stars
for all the sets of images to try to ensure alignment and combination
uniformity across the samples. A simple average was used to combine each image
set. All images were saved as TIFFS, then opened into Photoshop CS2 and had
identical curves and levels applied to them. The idea was not to process them
as such, but to simply make them more easily visible for evaluation and to
gauge roughly how much increase in image brightness was achieved simply by
focal reduction alone. There was no attempt of an accurate colour balance (see
NB. below). I have tried to keep individual image scales the same when creating
checkerboards, however at times differences in outcomes of the processing will
create an illusion that NGC104 appears with more stars defined and looks
“bigger” in some samples than in others. This is particularly noticeable in
image 6 – “Center of Image”.
The following is a
summary of each set collection:
1.
Control: Orion ED80, no focal reducer.
Focus achieved relatively easily
using DSLR focus.
FWHM
both V and H = 2.0.
10 lights at ISO800@60 sec
2. Meade
6.3. FR attached via Orion Prime Focus Adapter. The silver 2” sleeve was
unscrewed from the adapter then screwed into the female side of the FR. The
male side of the FR was then screwed into the adapter itself. It has been
suggested that the Meade 6.3 FR needs to be 105mm from the imaging surface to
obtain the stated 0.63x focal reduction. I was unable to use the normal SCT T
adapter to reach 105mm, due to insufficient in focus so it is unlikely that the
expected focal reduction will be achieved.
Focus was
considerably more difficult to achieve than when using no FR. Considerable time
was spent ensuring accurate focus was achieved, (approximately 3 times longer
than without a FR).
HFWHM V
=2.0. VFWHM =3.0
10 lights at
ISO800@60 sec
3.
William Optics 0.8X FR and Flattener. Attached directly to the EOS T Adapter
(ring). The WO FR is setup to screw directly into a T ring. No other adapter is
needed and the other end slides comfortably into a 2” focuser. The WO snapped
to focus using DSLR Focus. It focus very easily achieved with very clear FWHM
both V and H = 2.0 and much quicker than using no focal reducer.
HFWHM V =2.0. VFWHM =2.0
10 lights at ISO800 @ 60 sec
NB. These first three
trials were carried out under an 10 day old moon with NGC104 still low to the
SE (about 20-25 deg) so there is considerable light pollution from both
moonshine and sky glow from the coast and Brisbane. I notice as well when
processing these images that there is a definite red cast to each image that
wasn’t noticed in later images when NGC104 was much higher in the sky(45 deg
plus). Also the images were not at bright as I anticipated. I am assuming that
both these problems have been brought about because of various extinction
effects of the atmosphere (atmospheric density and pollutants etc).
4. Mogg
0.66 FR. Attached directly to the EOS T Adapter (ring). No other adapters
required, the 2” barrel slides into 2” focuser easily. (Very similar in build
to the WO, though constructed of strong rigid plastic rather than metal. If you
are familiar with Steve’s adapters you will know what I mean.) Focusing was more of a challenge than without
the FR, but easier than using the Meade 6.3.
FWHM both V and H =
2.0
10 lights at ISO800 @ 60 sec.
5.
Celestron 6.3. FR attached via Orion Prime Focus Adapter. The silver 2” sleeve
was unscrewed from the adapter then screwed into the female side of the FR.
Male side of the FR was then screwed into the adapter. It is a similar
situation with the Celestron Reducer as there is with the Meade Reducer
insomuch that the required lens/imaging distances cannot be achieved, so it is
unlikely that the expected focal reduction will be achieved. Focus was much more difficult to achieve than
using no focal reducer. I was unable to achieve VFWHM of 2 after considerable
time.
VFWHM =3.0 HFWHM =
2.0.
10 lights at ISO800
@60sec
Center of Image
(Processed)
Crop of Lower Left of
Center – Full Size Image. (Processed
Evaluation of Center
of FOV
All
of the tested FR seemed to soften the image slightly as none were as sharp as
the native setup. However none were excessively soft under full view, with the
Mogg 0.66 being marginally the softest. The Mogg 0.66 did show some minor star
elongation, brought about by chromatic aberration but was only obvious when
viewing the full size image under zoom, otherwise it was unobtrusive.
Amounts of Focal Reduction
While
I haven’t attempted to measure the exact amount of focal reduction for each
reducer mathematically, I have compared each image with the native image in
Photoshop and attempted to determine the zoom factor required to achieve a
similar spacial relationship. The results are;
Meade
and Celestron – 0.75x
William
Optics – 0.83x
Mogg
– 0.68x
As
my approximated reduction for the WO and Mogg reducer are very close to the
advertised reductions (within 3.6% and 2.6% respectively) I will assume that the
results I determined for the Meade and Celestron are relatively accurate as
well. If this is the case then the achieved focal reduction in this situation,
is very different to the advertised 0.63x reduction for both the Meade and
Celestron reducers. As mentioned previously this result was expected due to the
inability to correctly space the reducer and imaging chip.
Conclusion
Without
doubt the William Optics 0.8x Focal Reducer and Field Flattener performed the
best under the conditions at the time, with good field flattening and a
moderate focal reduction with a flow on (though not measured) exposure
reduction times. It showed a clear reduction of edge of field distortions as
well as giving a reasonable increase in field of view. There was moderate vignetting
when compared to the native setup, but I would expect that the use of flat
fields would alleviate most of this issue.
The
Mogg 0.66x reducer has good field flattening abilities and excellent focal
reduction with flow on exposure reduction times. It certainly held its own with
the WO reducer with only a slight softening of the image and some chromatic
aberration. As with the WO there was noticeable vignetting, but again, flat
fielding would help to remove this issue. For someone looking for a well priced
focal reducer I felt the reducer I had to be a stand out performer when
considered next to its more expensive cousins. The WO pipping it in colour
correction for chromatic aberration and ease of focus and degree of sharpness
achieved. It may be possible to improve the ease of focusing by increasing the
length of focusing exposures or using a relatively bright star with a focusing
aid, such as a Hartmann mask, in conjunction with computer aided focusing.
The
Celestron and Meade reducer’s performances were disappointing in this
situation. While the center of the image was comparable to the WO reducer, the
edges of field performances were dismal to say the least. This is not to say
that these reducers aren’t great reducers, just not in the situation of ED80
refractor/DSLR combination. If all that is needed is the center of a field of
view then these reducers would get you out of trouble, but don’t expect to
achieve an image that is useful across the full field of view.
NB.
It seems to me that these two reducers are very similar in their construction.
For an exercise I layered a Meade reduced image over a Celestron one using
Photoshop and found that they showed almost identical reduction and
distortions. The Celestron image beating the quality of the Meade one possibly
because when the images were taken, NGC104 was about 75 deg above the horizon
for the Celestron and when NGC104 was only around 30 deg above the horizon for
the Meade reducer.
Thanks
to Steve Mogg.
