I was able to get out and observe this past Saturday and Sunday evenings with Chuck and Judy Dethloff at their place in the Coast Range west of Portland. The skies were clear but brighter than average with SQM readings around 21.1 on Saturday and 20.95 on Sunday night. It's usually a few tenths darker at their place so we were a little disappointed, and the seeing was pretty soft too. All that aside, the transparency was decent and it felt great to get in a few hours of observing this time of year.
I've been looking forward to trying my new DGM OIII filter on the Crab Nebula. Back in early November I had an astonishing view of the Crab through Jimi Lowrey's 48 inch scope in Fort Davis Texas using a DGM OIII and had to get one for myself - there were filaments everywhere! I've had a Lumicon OIII since the early 90's and even though I've never felt it performed poorly, I just had to try the DGM version to see how it worked with my 28 inch. Plus, it was on sale at the time.
So Saturday night was my big chance to compare these two OIII filters. I started with the view through my old Lumicon filter and noted that the sky conditions allowed me to see only the main two filaments with the rest of the nebula a soft glow.
Then the DGM filter - drum roll please - and the filaments were barely seen at all - dang! So my twenty year old Lumicon OIII is the clear winner on the Crab, but then I had the sudden and wild notion to stack the filters to see how they worked together...
To my surprise, the view through the stacked filters was superior to the Lumicon OIII by itself. The Crab showed the main two filaments well along with a faint but distinctly lacy background overlain on a soft nebulous background glow. The view was not perceptively darker than with a single filter, which at first surprised me, but with a moments thought made sense - the transmission lines of both filters must line up pretty well, but with just enough difference to enhance the Crab's filaments. Pretty cool!
I didn't try this Sunday night nor did I look at other objects with either filter so I won't try to generalize this observation beyond what I saw on the Crab - that will have to wait for other nights. But it was a happy, unanticipated result I want to share with you all even though this effect may be specific to how my two OIII filters interact.
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