Introduction:
My interest in astronomy started when I
was 15 and saw Haley¡¯s comet through a pair of Bushnell binos.
I had always wanted to buy a telescope but growing up in light and
particulate polluted Bombay (now Mumbai), I gave up all notions of
pursuing the hobby. Then in grad school in a small town and away from
the madness, I picked up a 60mm Meade refractor that I enjoyed using
on brighter planets and moon. I purchased a Zhumell Z10 two years
back in an effort to revive my passion. I am an environmental
scientist by profession and have been living in Mason, Ohio for 10
years. I tried to use my scientific background while evaluating the
filters. Although I use a Zhumell telescope, I shall try to give a
fair review using elaborate scientific testing and avoid bias. Also
this is my first scientific article in astronomy, so excuse me if I
do not sound technically correct. I cannot do an apples to apples
comparison because I do not own Baader, Lumicon or any other filter
for that matter.
Equipment:
Zhumell Z10 dobsonian telescope flocked
with Jo-Ann¡¯s black velvet fabric
Zhumell 32 mm and 9mm Plossl eyepieces
Knight Owl 2¡± 20mm, 15mm 82¨¬AFOV
eyepieces
Knight Owl 6mm 66¨¬ AFOV eyepiece
Zhumell 2¡± ¡°see it all¡±
filter set including LPR, variable polarizer, UHC and OIII filters
Telrad and 8x50 RACI finder
Shimadzu UV1601 scanning UV-Vis-NIR
spectrophotometer
Hach DR4000 scanning UV-Vis-NIR
spectrophotometer
A few cups of coffee
Test conditions:
After a lot of deliberation, I
purchased a 2¡± Zhumell filter set on 09/18 from telescopes.com.
They have an introductory sale going on and at $150 a set, the price
was irresistible. You could spend as much for just one filter from
other manufacturers. It arrived neatly packaged on 09/23 by UPS
ground. Then came the standard curse in form of clouds and rain,
humidity and dew. I did get a chance to do some field test with the
filters. More about that later.
The filters were scanned at 1nm/sec at
wavelengths 200-1100 nm using Shimadzu UV1601 spectrophotometer. As
expected, glass blocks UV and there was no transmission in 200-320nm
range. The scan results on Shimadzu were cross checked using a Hach
DR4000 spectrophotometer and the scans agreed extremely well.
Eventually, the Hach spectrophotometer was used because of ease of
removal of cuvette holder and placement of filter in optical path for
testing. Five replicate scans were made 1 cm off the center and at
positions shown in the first figure. The resulting spectrographs are
result of 5 replicate scans each and because of excellent agreement
between replicates, the spectrographs have virtually overlapped and
cannot be discerned.
Fig 1:
Scan areas on the 2¡± filter
Results
Zhumell Moon and Skyglow (LPR) filter:
I was pleasantly surprised by the
results. As seen in the graph, the filter showed very little
transmission in 570-590nm range, the primary wavelength for low and
high pressure sodium vapor lamps (used in street lighting). In case
of mercury vapor lights though, except 579nm line, other emission
lines were not attenuated by the filter. Overall about 25% of the
light was filtered and hence moonlight (which has a spectrum similar
to sunlight) will only be partly attenuated. I wish it had blocked
the major Hg-vapor emission line at 546nm but I have been pleased
with the filter. If Zhumell improves the filter to block ¥ë546
and ¥ë435 lines, it will be truly an exceptional filter. I am
not sure if there is a filter that blocks both Na-vapor emission
lines at 590 as well as Hg-vapor emission lines but would like to
hear from other owners/users about their version of LPR.
In the field though, the skyglow filter
performed very well. On Jupiter, I could clearly make out extra
contrast and details with the filter that I normally need darker
skies to perceive. For example, the GRS was easier to see and the
orange colors looked a touch more pronounced and vibrant. On M13,
M27, M57 it did not improve the views much but, made the background a
tad darker making the view more pleasing. Even a not-so-exciting
object like Vega made me stay at the eyepiece longer. Actually, the
improvement was similar to what I obtained after I flocked my
telescope¡¦. if it gives a better idea of what to expect with
the filter on. I wish I could give a more detailed report but the
weather has played spoilsport.
Fig 2:
Spectrograph of Zhumell LPR filter on Hach DR4000 spectrophotometer.
To obtain baseline, air was scanned prior to placing filter in
optical path. The graph is result of 5 replicate scans overlapped
UHC filter:
UHC filter performed exactly as it
should. It blocked most of visible spectrum except a 25nm bandwidth
in 480-505nm and partial transmission in the violet-indigo region of
the spectrum. On ring nebula and dumbbell nebula, the views were
dramatically improved. I liked the UHC filter better than the OIII
filter on ring nebula for instance because the background was not
lost by the narrower bandwidth of the OIII
I saw NGC7000 (North American nebula)
for the first time using UHC. I have seen Deneb so many times but
never imagined the splendor nearby. With my light polluted burbs, I
could not make out colors in the nebula but did not expect it anyway
with a 10¡± instrument.
Fig
3: Spectrograph of Zhumell UHC filter.
OIII filter:
OIII filter looked okay on ring nebula
but I must admit I was disappointed. The lab test revealed why. At
desired ¥ë501 and ¥ë496 nm for OIII lines, transmission
was just 68% and the ¥ëmax (wavelength at which maximum
absorption, or in this case transmittance occurs) was at 510 nm where
transmission was 88%. The bandwidth was okay but I wish the entire
spectrum was skewed 10nm to the left. There is a caveat while
reporting transmission data. I used air for my baseline. Using 2mm
polished Zeiss uncoated optical quartz cell for baseline (to simulate
uncoated 100% transmission glass), I got better transmission of 78%
at the desired wavelengths. I am not sure if Lumicon/Baader/Televue
use air as blank for testing or use 2mm uncoated glass. Zhumell OIII
filter passes only 75% as much light as Lumicon OIII (data obtained
from Lumicon website). The Zhumell UHC filter passed more OIII
wavelengths than the OIII and that might explain why UHC performed
better than OIII on the nebulae tested. I repeated the experiment
with OIII filter several times and at slower slew rates (0.5 nm
instead of 2.0nm) but got identical, reproducible and similar
results.
I did not test the variable polarizer
but in field it looked good on moon and reduced glare and intensity
to make the view more pleasing.
Fig
4: Spectrograph of Zhumell OIII filter.
Conclusions:
The skyglow and UHC filters met or
exceeded my expectations. I wish the OIII filter was better in
isolating doubly ionized oxygen lines at ¥ë501 and ¥ë496.
It still transmitted 68% of the light and was halfway decent on the
aforementioned nebulae. The views were generally better than without
filter if it makes sense. I am not sure if I got a bad filter because
at $37 a filter ($150 for the entire set), I am not sure if QC
experts at Zhumell manufacturing plant check each filter prior to
shipping. It is not uncommon for ¥ëmax to get skewed to the
right at higher concentrations. In other words, The Zhumell filter
isolated ¥ë510 line instead of ¥ë501 because the filter
coating was heavier than intended. A surprising bonus however was 80%
transmission of singly ionized oxygen line OII at ¥ë373nm.
Recommending the set would have been a no-brainer had the OIII filter
performed as intended. If you are an amateur and in the market for a
decent starter filter set for making DSO seeing easier, the Zhumell
set is very attractively priced, is a heck of value, and will meet
most of your needs. If you are a dedicated DSO nut needing better
OIII filter, please look elsewhere.
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