Observation Notes: Thursday, November 16, 2000

I had a definite plan in mind for this evening's observations. (And fortunately God didn't laugh.) Basically, the idea was to determine whether or not there was any real value in using "filters" to enhance the view of deep-sky objects (now that the moon isn't hogging up so much of the sky) and the planets.

Deep Sky:

I started pretty early (around 6:30pm while the sky was still darkening) and made a quick assessment of the seeing conditions. Turning to Epsilon Lyrae, I found that it could not be cleanly split at 120X but that it was most definitely a double binary. By experience I know this means that atmospheric stability is about average (5/10). Under such conditions I would expect to have a difficult (but not impossible) time making out Cassini's division in Saturn's rings and I would be limited to 210X when viewing the gas giants later in the evening. (When I would shift over to filter tests on Jupiter.) But first "deep sky" and the associated OIII filter test program...

After assessing the seeing, I made a quick check to roughly estimate how much limiting magnitude is lost when viewing the night sky through an OIII filter. I know from consulting star charts that the Epsilon Lyrae "double-double" actually includes 9.5, 12.0 and 12.5 comes within the same basic region of space. (There are others but these are good test subjects.) I found that as the sky darkened I could make out the 9.5 and 12.0 magnitude comes -- but not the 12.5 magnitude star. On installing the OIII filter, I could still make out the 9.5 magnitude star. (This looked slightly brighter than the 12.0 magnitude attendant had.) So under current conditions (with a naked visual limiting magnitude of 4.5), I could just detect 10th magnitude stars with the OIII filter in place and 12th magnitude stars without. (Generally speaking my 150mm 34% obstructed Orion Argonaut/Intes MK-67 gives me a plus 7.5 magnitude boost over naked eye visual limiting magnitude.)

With the OIII filter still in place I quickly located M57. Even at 120X (and during relatively poor sky transparency -- 4/10) I had no trouble locating it about 1/3 the way between Beta and Gamma Lyrae. I found it farirly straightforward to distinguish the darker interior of the smoke ring. I also verified that the roughly east-west ansae were darker than its north-south flanks (but lighter than the center). I could also detect the fact that the southern flank was marginally brighter and slightly wider than the north. Removing the filter made no difference to my eye in making out these same generalities as to shape and brightness. So far, the use of an OIII filter was not much of an advantage in a telescope of this aperture, focal ratio, and large central obstruction ratio.

My next test was to see if, with an OIII filter in place, I could still locate a moderately difficult object that was not expected to be enhanced in any way by selective narrow-banding. The obvious choice for this was M56, a relatively dim globular cluster of the 8th magnitude about 2/5ths the distance between Albireo and Gamma Lyrae. (M57 is listed as magnitude 9.7 but its smaller size suggests a surface brightness comparable to the larger M56.)

After some searching, I was able to locate M56 using my 35mm (50x) ep. Under current observing conditions (less than average transparency) it was just barely possible to make out M56 (as a fuzzy star) in the 7*35 finderscope. (And this only after I had found it by sweeping the star fields in the immediate area using the main telescope.) Since the light from globular clusters is broadband, there is no advantage (in fact appreciable disadvantage) to viewing it through the filter. So having found it, I switched over to the 15mm (120X) ep and gave it the kind of scrutiny I had just done with M57...

M56 is obviously larger than M57 (perhaps by a factor of 3 under the conditions I was viewing it in). It is also far more spherical. Unlike M57 it has no defined edges (although the north quadrant appears to more sharply drop off into the darkness of space than the "rounder" south quadrant). Globularity seems to extend a little further along the east-west axis (than the north-south).

Like most low surface brightness objects, the best views of M56 occur when you first acquire it with the eye, or when you flit the eye about in the eyepiece, or when you view it through averted vision. Unlike the brighter globulars (M13 & 22 for instance) no amount of patient observation will ever reward the owner of a 6" telescope with the much sought after 3D effect of resolution into hundreds of stars. At best you will see the scintillation effect of a dozen or so member (and brighter line of sight field) stars followed by an apparent "granularity" when the eye settles down to viewing it directly. Perhaps under excellent deep sky conditions two dozen or so stars may remain persistently apparent in a 150mm but "seeing" precedes believing for this particular object.

Now on to my final test of OIII enhanced perception: The Veil Nebula(!?). There is so much "nebulosity" in the area south of Epsilon Cygni that every time I attempt to find the famed "Veil Nebula" I am completely afflicted by doubt about identifying it! (Will the real Veil Nebula please stand out?) Meanwhile, tonight's "Veil Nebula" was a lovely arc of stars interspersed by obvious reflection nebulosity somewhere in the 52/41 Cygni area. To help others in finding what I am referring to here let me provide a brief description...

Once you find the southern tip of the Cygnus cross, locate the first 4th magnitude star that drops down from Epsilon Cygni (parallel to the backbone of the cross). Center this star in a low power ep and look east until you identify a parabolic arc of stars some half-degree in length. You can verify this particular "Veil Nebula's" identity IF you also notice a 7th magnitude star taking up a position that corresponds to the end of an arrow's shaft, if that arrow was placed against a string connecting the two ends of the parabolic arc of stars comprising the bow.

If you find this configuration, you will see that many of the 7.5+ magnitude stars making up the "bow" display a wispy nebulosity around them. Under poor transparency conditions (like I experienced this evening), little of the nebulosity actually interconnects the two dozen or so stars together in the parabola. UNLESS you install an OIII filter! With the filter in place, most of the stars are interconnected by pale luminosity. $100 narrow band OIII filter vindicated!

Jupiter:

I wrote the above while waiting for the Saturn and Jupiter to ascend. Around 9:30 I headed out into the "Northern California Cold" to resume my filter checks -- this time on Jupiter.

Took a quick look at Saturn -- strictly for stability-checking purposes, of course. Cassini and the SEB were clearly visible. Things had improved over earlier. I mentally ticked up the stability rating by a notch (to 6/10). Some of the haze had dissipated so I indexed the transparency rating up to 5/10. Nice solid, slightly than better average for the Jupiter series.

Broke out my 3x Ultrascopic barlow along with the 25 (210X) and 15mm (360X) Ultrascopic eps. I find that two eyepieces are essential (not for filter tests) -- due to dew. (One ep can be de-fogged by ensconcing its cold barrel in your not so warm hand while the other remains in the OTA for viewing.)

First views of Jupiter were sans filtration. The SEB and NEB were easily visible. Irregularities in the edge of both equatorial belts (but especially the SEB) were noticeable. Hints of intra belt turbulences were suggested but not explicit. Both the SEB and the NEB were seen as clearly twinned. Four belts were accessible with no hints of the two temperate belts. Polar darkening also visible. The four Galileans were queued up linearly in groups of one and three along the equatorial plane. Tiny disks of various sizes besmirched with "skyglow" were perceptible.

Based on this Jupiter view I mentally broke my observational goals up along the following lines:

1. Which filter(s) (if any) would assist in showing the two temperate belts?

2. Which (if any) would tend to better display irregularities in the outlines of the two equatorial belts?

3. Which (if any) would improve the sense of texture and detail within the more distinctive regions of the equatorial belts?

4. Which would tend to better separate the two divisions of the twinned equatorial belts?

5. Which would display the texture of the polar regions to best advantage?

6. Which would give the most pleasing aesthetic view of the planet as a whole?

Under the 6/10 stability conditions of the evening, it was obvious that the 25mm/barlow configuration (210X) gave the best views of the planet. 360X was not showing very good limb delineation or contrast on this particular occasion. (There would be no 540X observation this evening.)

#15 Deep Yellow

I started with the yellow filter (#15 Deep Yellow) 210X. Aesthetically the view was poor. (Too much yellow already in Jupiter -- all I could think of was lemons.) In addition to the aesthetic, the yellow seemed to deprecate the detail visible within the EBs. Overall, yellow was a dis-improvement. Another observing session will be needed to determine if it has anything worthwhile to offer as a Jupiter image enhancement tool...

#25 Red

Before switching over to #25 Red, I reviewed the appearance of the planet without filtration. By this time I began noticing a hint of the NTB in the ep. (This was not suggested during use of the yellow filter just previously.) Use of the red filter immediately reduced the surface brightness of the planet. Because of this the equatorial belts stood out fairly dramatically. However, there was little additional detail present to the eye. (Although my scribbled notes indicate a possible improvement in making out the outline of the EBs with another possible improvement in perception of intra-belt texture.) The red filter also seemed to improve belt contrast even more at 360X under the current observing conditions. The red filter did not reveal the NTB so it did not appear to bring out elusive detail.

#58 Green

Overall seeing of the planet began to peak just as I began the #58 Green filter tests. The NTB was more readily apparent (without a filter) and hints of the STB were also seen occasionally. 

The use of green cleaned up the edge of the planet. (It was not possible to fully relieve the eyepieces and barlow lens of condensation, so internal light diffusion was a constant problem.) Although green reduced surface brightness considerably (like red) it did a better job of revealing general detail. Green was the only filter that enabled views of the NTB. It also enhanced the sense of surface "mottling" associated with the planet's polar regions. So Kermit, Be Proud!

#80A Blue

By the time I moved on to #80A Blue, the filterless image of the planet began to deteriorate due to condensation on the OTA meniscus. (Even with a dew cap.) Previous observations had already proven that blue gives, by far, the most pleasant aesthetic look. This it does this without undermining the visibility of any particular feature type.

My Conclusions:

Owners of 150mm telescopes may consider adding some filters to their observing kits. Those with smaller telescopes may want to consider the fact that filters do not add anything to what is already present in the native image. (Theoretically, we all recognize that filters block some frequencies of light so that other frequencies can better impact the eye.) If native luminosity is already low, there is no advantage to using a filter as an image enhancer. The value of filters does seem to increase as light gathering capacity improves. Larger apertured scopes will no doubt profit more from filters than smaller ones. However, filters may still be of advantage while using imagers or long period photographic exposures in smaller scopes.

For those of us using 150mm reflectors (or 125mm or larger refractors) certainly the filter types described above may offer some visual advantages. Personally, I'm pleased with the effect of an OIII filter when viewing reflecting nebula. The aesthetic effect of the medium blue filter on Jupiter can be satisfying, while green is probably best overall (even if less pleasant aesthetically pleasant) at enhancing low contrast detail and "sharpening up" images.

Hopefully, if anything, this write up will encourage others to rtecord their own experience with filters on this and other observing sites. I personally look forward to reading these discussions -- especially on cloudy nights...

Clear and Steady Seeing to You all,

Jeff Barbour

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