Bolton Astronomical Society

Experimenting with an OIII filter
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Author:  jbrannan [ Thu Jul 28, 2011 12:07 pm ]
Post subject:  Experimenting with an OIII filter

Here are my humble observations from using the society's Skywatcher 1.25” OIII filter. I chose the M57 Ring Nebula and the M27 Dumbbell nebula as objects to try out the filter as these are reported to emit the wavelength of light these types of filters are designed to pass through.
Two separate observations were carried out. First was on the 14th July (1 night from a full moon) and the second was the27th July (3 nights from a new moon). Observation time was between 11pm and 11:30pm on both occasions.
From the first observation night the M27 could be easily seen without the filter in both 25mm and 10mm eyepieces. With averted vision, subtle detail of the ring could be seen. Fitting the filter enhanced the contrast of the object but without improving the detail. Moving to M27, with the filter fitted to the 25mm eyepiece, a fuzzy outline could clearly be seen. I could not really make out much detail of the shape of the object. The filter was then removed and I could not see M27 under these conditions.
The same two objects were targeted during the second observation night. The view of the Ring Nebula (M57) was very clear without the filter and with the filter the contrast increased but again without any enhancement to detail of the object. With the Dumbbell Nebula (M27), the object was much clearer than the previous observation with the filter in. I could start to see the hourglass shape outline of the object. Without the filter I could see the object but this was nowhere near as clear as when the filter was used.
So to conclude, I found the filter extremely useful when looking at M27. In fact on the 1st night of observation I do not think I would have been able to find the object without the filter. With the M57 I did not see any clear advantage as the object was clear enough without. A general observation was that with the filter in, the background is darker and the surrounding stars are much dimmer. This effectively increased the contrast of the planetary Nebula making it much easier to see.
Would I buy one? Yes! This device certainly will help with the location and viewing experience of these faint planetary Nebulae.
Telescope used was a Skywatcher 200p 8” f/5 Newtonian.
J Brannan

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