An interesting observation - stacked OIII filters

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.

Thanks for sharing that Howard. I also have a Lumicon OIII from the 90's it's 1.25. Last year I started upgrading to 2" filters and got the DGM OIII. My first 2 comparisons were on the Helix and Veil. The Lumicon was a clear winner on both which left me kicking myself for not putting up the extra cash for the 2" Lumicon. But now at my next opportunity (who knows when that will be with the weather here) I will try the double filter with the 2" in the bottom of my Glatter parallizer and the Lumicon in the eyepiece.

Eric

Hi there Howard. You posted:

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!

The Lumicon OIII is narrower than the DGM OIII, so it should work a little better on improving the contrast of some of the filaments. Stacking the two should have only given you a little less light transmission, so I suspect that what you saw might have been either a change in local conditions or perhaps a personal effect due to your eyes getting a little more sensitive. However, a couple of weeks ago, in my 14 inch at 135x, I was able to see the filaments without a filter, which surprised the heck out of me. Overall, I still like a narrow-band filter for the Crab, but higher power often helps nearly as much on that nebula as a filter does (at least from a dark sky site). Clear skies to you.

You make a couple of good points David. I checked for changing conditions and becoming more sensitive during the observation by repeating it several times and having my observing partner, Chuck, check it out. The results were consistent.

But there's no way to be sure based on one night and on one object so I take this result with a big grain of salt for now. If it was the conditions changing, they changed for the worse during my observations, but my eyes getting better adapted throughout the observation isn't something to dismiss.

I'll try this observation again the next time I'm out, hopefully at the end of the month, and see what happens. However, it would be interesting to exactly measure the transmission profiles of my two OIII filters to see if they do combine to improve contrast.

Howard, you make us "smaller" scope people jealous with a big 28" WOW! Seeing any filamentary structure is pretty to me, I struggle to pull detail out on it on a good night here (Red Zone) with my 12.5".

Howard, your pals down in Eugene were discussing your "visual stacking" observation over lunch, and described it as likely the result of cutting down the "slop" (as Jerry O calls it), on either side of the primary targeted center of what both filters let pass - assuming what David can likely verify, that they are not symmetrical. That is, one lets through a bit more nm on one side of its center, and the other filter is "tilted" a bit on the other side of its own center. Thus, you might be getting a narrowed and more "efficient" pass of the most effective bandwidth.

All best,

Jon

It's virtually guaranteed that the bandpasses of the two filters differ. And so the combination of the two will result in a narrower resultant bandpass which the two share in common.

Interesting. Presumably all interference filters are different and stacking them is averaging; like a bunch of musicians playing the same tune sound cleaner than each one ever could. I had never thought of this though. If I ever get to observe again with this weather, I may stack my 2 and 1.25" Orions - surely they are not identical either.

Howard, any idea why the sky was brighter than usual?

It's virtually guaranteed that the bandpasses of the two filters differ. And so the combination of the two will result in a narrower resultant bandpass which the two share in common.

Filter tilt would change the passband location, so if one were tilted in its mount and coupled with another filter, that might result in a narrower passband. On the DGM OIII filter I received for my review, a spectroscope showed the passband to be nearly 1.3 times wider than that of the Lumicon OIII. The DGM OIII I got has a fairly symmetrical passband that is centered between the two OIII lines, so again, if stacked, what would result would be a passband width that would be pretty close to the same as that of the Lumicon OIII with nearly 90% transmission on the lines. If you really want a test of what narrowness does on the Crab nebula, go use the Baader OIII that has the 85 angstrom FWHM passband. I have seen my eyes change sensitivity slightly as the night progresses as well as seeing small but definite rapid changes in transparency in a given area of sky, so I feel those effects may need to be considered unless you have the stack in a filter wheel where the change between the stacked system and a non-stacked can be instantly done. Clear skies to you.

Yes, the Lumicon OIII is in my filter wheel and the DGM OIII was screwed to the bottom of my eyepiece making the comparison nearly instant by rotating the Lumicon in and out of the light path. This should have minimized tilt as well, and at the very least kept it constant.

But as they say, more research is warranted and I'll check out the Baader OIII as well.

Interesting. Presumably all interference filters are different and stacking them is averaging; like a bunch of musicians playing the same tune sound cleaner than each one ever could. I had never thought of this though. If I ever get to observe again with this weather, I may stack my 2 and 1.25" Orions - surely they are not identical either.

Howard, any idea why the sky was brighter than usual?

That's a memorable way to think about this Ivan! Also, the northern sky was glowing from a low grade aurora brightening the entire sky.

To be meaningful, the filter tilt would have to be at least several degrees at these passbands. Unless cross-threaded, filters will be more than sufficiently perpendicular. The de-tuning resulting from the outer rays of the light cone from a fast objective is the *much* more significant factor.

Lets not forget that the DGM O-III has a WIDE band pass and compares to the Orion Ultrablock very close in performance.

Remember that comparison we did Eric?, (Beatlejuice).

Lets not forget that the DGM O-III has a WIDE band pass and compares to the Orion Ultrablock very close in performance.

Remember that comparison we did Eric?, (Beatlejuice).

Actually, the DGM OIII that I received for review offers a performance that is similar to that of the DGM NPB narrow-band, although there is a slight boost in performance for the DGM OIII. The DGM OIII's bandwidth is on the order of 150 to 160 angstroms, so it is somewhat wider than that of the Lumicon OIII. The NPB and Ultrablock offer similar performance as well, although I do like the slightly higher throughput of the NPB, as well as the NPB's inclusion of a deep red secondary passband for viewing red coloration in M42 and M8. Indeed, the first good look I got at "the Jellyfish" nebula (supernova remnant IC 443 in Gemini) came in my 9.25 inch SCT equipped with the NPB filter. It was visible in the OIII but I liked the view of it better in the NPB filter. Clear skies to you.

Hi Howard,

interesting observation! You likely narrowed the passband by stacking the two filters, similar as when double stacking solar H-alpha filters.

This narrowing of the pass band is actually not really an "averaging", it is rather a multiplication of the transmission curves. It happens also when you stack absolutely identical filters.

The narrowing effect is less pronounced with identical filters having a the optimal box-type pass band and is more pronounced, for instance, with filters having a more Gaussian-type pass band. Below is shown what would happen if you double or triple stack identical filters having either a typical box-type or a gaussian pass band

Nice examples, and of course you are right. Relative to the maximum the product is similar to the geometrical average, I may say in my defense