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Venus + 6" f/6.5 achromat + filters (Observational Notes)
On three recent evenings I experimented with visual observations of Venus using
a 6-inch f/6.5 achromat and various filters. Temperatures ranged from +12F to +26F. A magnification of 166x was used for all observations. Seeing conditions were, in all cases, a bit unsteady but not too bad. The primary filters used we Fringe-Killer, #8 Yellow, #12 Yellow, #15 Yellow, and #11 Yellow-Green. The #8 Yellow combined with the Fringe-Killer was not sufficient to completely mask the blue halo of chromatic aberration, but the CA was nevertheless greatly reduced. I concluded that the #8 Yellow was the least effective of the filters tried. The #12 Yellow + Fringe-Killer completely (or almost completely) eliminated all visible blue-purple CA. The #15 Yellow with or without the Fringe-Killer seemed to eliminate all the blue-purple CA. All yellow filter combinations tried resulted in images that had some colors other than yellow - such as small amounts of red/orange or magenta along parts of the planet's sunward limb, but in all cases these colors covered very little area – generally little more than very narrow, short arcs of color along the planet's limb. I felt that the #15 Yellow was denser than necessary to mask the CA . I therefore preferred the #12 Yellow over both the #8 and #15. I don't recall if I tried the #11 Yellow-Green without the Fringe-Killer, but in combination with the Fringe-Killer there was no noticeable blue-purple CA. On one of the evenings I noted my preferred filter arrangement (for Venus with this telescope) as being the #11 with Fringe-Killer. With this arrangement the only colors noted on Venus were variations of yellow and green. On another note: I have two #12 Yellow filters from different sources, having different clear apertures. One was of recent acquisition (came with a set of 'cheap' eyepieces purchased for public use/abuse), the other is several decades old. A comparison (indoors) reveals differences in color and density. Therefore, beware! Not all filters having the same Wratten numbers are equivalent! Besides Venus, quick comparisons were made with and without the Fringe-Killer on M45, the Orion Nebula, and the Tank-Track Nebula (at 38x). The blue stars of M45 seemed noticeably brighter without the filter. The Pleiades nebulosity covered smaller areas and had more distinct outer boundaries with the filter, but with all the CA it wasn't always clear what was nebula and what was CA. A pair of future, careful sketches might clear this up. I preferred the non-filtered view of the Orion Nebula. For the fainter Tank-Track Nebula it was more difficult to arrive at any clear preference. In general, the Fringe-Killer alone or with other filters will likely be my preferred arrangement for solar, lunar, and (bright) planetary observing with this fast achromat. For most DSO observing I'll probably prefer the unfiltered view. Globulars might be an exception. Another night or two or three of experimentation on globulars seems to be called for . . . Sketcher, To sketch is to see. |
#2
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Venus + 6" f/6.5 achromat + filters (Observational Notes)
On Friday, 20 January 2017 01:18:30 UTC-5, Sketcher wrote:
On three recent evenings I experimented with visual observations of Venus using a 6-inch f/6.5 achromat and various filters. Temperatures ranged from +12F to +26F. A magnification of 166x was used for all observations. Seeing conditions were, in all cases, a bit unsteady but not too bad. The primary filters used we Fringe-Killer, #8 Yellow, #12 Yellow, #15 Yellow, and #11 Yellow-Green. The #8 Yellow combined with the Fringe-Killer was not sufficient to completely mask the blue halo of chromatic aberration, but the CA was nevertheless greatly reduced. I concluded that the #8 Yellow was the least effective of the filters tried. The #12 Yellow + Fringe-Killer completely (or almost completely) eliminated all visible blue-purple CA. The #15 Yellow with or without the Fringe-Killer seemed to eliminate all the blue-purple CA. All yellow filter combinations tried resulted in images that had some colors other than yellow - such as small amounts of red/orange or magenta along parts of the planet's sunward limb, but in all cases these colors covered very little area – generally little more than very narrow, short arcs of color along the planet's limb. I felt that the #15 Yellow was denser than necessary to mask the CA . I therefore preferred the #12 Yellow over both the #8 and #15. I don't recall if I tried the #11 Yellow-Green without the Fringe-Killer, but in combination with the Fringe-Killer there was no noticeable blue-purple CA. On one of the evenings I noted my preferred filter arrangement (for Venus with this telescope) as being the #11 with Fringe-Killer. With this arrangement the only colors noted on Venus were variations of yellow and green. On another note: I have two #12 Yellow filters from different sources, having different clear apertures. One was of recent acquisition (came with a set of 'cheap' eyepieces purchased for public use/abuse), the other is several decades old. A comparison (indoors) reveals differences in color and density. Therefore, beware! Not all filters having the same Wratten numbers are equivalent! Besides Venus, quick comparisons were made with and without the Fringe-Killer on M45, the Orion Nebula, and the Tank-Track Nebula (at 38x). The blue stars of M45 seemed noticeably brighter without the filter. The Pleiades nebulosity covered smaller areas and had more distinct outer boundaries with the filter, but with all the CA it wasn't always clear what was nebula and what was CA. A pair of future, careful sketches might clear this up. I preferred the non-filtered view of the Orion Nebula. For the fainter Tank-Track Nebula it was more difficult to arrive at any clear preference. In general, the Fringe-Killer alone or with other filters will likely be my preferred arrangement for solar, lunar, and (bright) planetary observing with this fast achromat. For most DSO observing I'll probably prefer the unfiltered view. Globulars might be an exception. Another night or two or three of experimentation on globulars seems to be called for . . . Sketcher, To sketch is to see. Nebula filters work on emission nebula. Fringe killers might work on global and older open clusters owing to their (sometimes) orange stars. |
#3
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Venus + 6" f/6.5 achromat + filters (Observational Notes)
On Friday, January 20, 2017 at 1:18:30 AM UTC-5, Sketcher wrote:
Besides Venus, quick comparisons were made with and without the Fringe-Killer on M45, the Orion Nebula, and the Tank-Track Nebula (at 38x). JOOC, is the Tank-Track Neb like the Cali like fornia nebula? |
#4
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Venus + 6" f/6.5 achromat + filters (Observational Notes)
I've heard that some people leave their Fringe-Killers or other minus-violet filters in their optical trains regardless of what they're observing - with achromats. I had the Fringe-Killer in place and decided to check on a few DSOs. M42 was targetted because it was a bright emission nebula. The Pleiades were targetted because of its blue reflection nebulae (The Fringe-Killer works by blocking some of the blue - I was curious). The Tank-Track Neb. was targetted because it was near M42 - and significantly fainter - a spur of the moment addition.
The Fringe-Killer doesn't block much light. As such, I wondered if it would be beneficial for me to leave it in the achromat's optical train - regardless of the objects being observed. My conclusion, based on those few observations was that leaving it in would be detrimental to at least some DSOs - at least for me, under my sky conditions, with the 6-inch f/6.5 achromat.. I have other filters (OIII, Ultra-Block, and H-Beta) designed specifically for various nebulae. They of course help with some objects, but even so I prefer (the vast majority of time) to make my DSO observations without them.. I have a fairly dark (NELM better than 6th magnitude) sky. I've observed B33 more often without an H-Beta filter than with one. The various nebula filters alter my visual experience in ways that I don't generally prefer over my unfiltered views. It's a simple matter of personal preference - not a blanket statement condemning the functioning of those filters. Sketcher, To sketch is to see. On Saturday, January 21, 2017 at 12:07:25 AM UTC-7, RichA wrote: Nebula filters work on emission nebula. Fringe killers might work on global and older open clusters owing to their (sometimes) orange stars. |
#5
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Venus + 6" f/6.5 achromat + filters (Observational Notes)
NGC 2024, aka the Flame Nebula, aka the Tank-Track Nebula is about four degrees north-northeast of M42 right beside Alnitak - the eastern-most star in Orion's belt. One night, after viewing the Horsehead Nebula (B33) with an unfiltered 5-inch apochromat I swung the scope over to NGC 2024 and saw a wealth of detail at 200x with my extremely dark-adapted telescope eye. The visual appearance resembled a tank's track through mud far more than it resembled a flame. Ever since, I've preferred "Tank-Track Nebula" over "Flame Nebula". I don't know who first called it the Tank-Track Nebula, but that's the name I'm sticking with!
So yes, in that way it's like the California Nebula (and the North America Nebula, and a few others). Sketcher, To sketch is to see. On Sunday, January 22, 2017 at 3:38:09 AM UTC-7, wrote: JOOC, is the Tank-Track Neb like the Cali like fornia nebula? |
#6
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Venus + 6" f/6.5 achromat + filters (Observational Notes)
On Sunday, January 22, 2017 at 12:14:45 PM UTC-8, Sketcher wrote:
NGC 2024, aka the Flame Nebula, aka the Tank-Track Nebula is about four degrees north-northeast of M42 right beside Alnitak - the eastern-most star in Orion's belt. One night, after viewing the Horsehead Nebula (B33) with an unfiltered 5-inch apochromat I swung the scope over to NGC 2024 and saw a wealth of detail at 200x with my extremely dark-adapted telescope eye. The visual appearance resembled a tank's track through mud far more than it resembled a flame. Ever since, I've preferred "Tank-Track Nebula" over "Flame Nebula". I don't know who first called it the Tank-Track Nebula, but that's the name I'm sticking with! So yes, in that way it's like the California Nebula (and the North America Nebula, and a few others). Sketcher, To sketch is to see. On Sunday, January 22, 2017 at 3:38:09 AM UTC-7, wrote: JOOC, is the Tank-Track Neb like the Cali like fornia nebula? The main visual difference between the California Nebula, NGC 1499, and other bright nebulae is that the California Nebula responds well to the h-beta filter whereas most others don't. Also, the CN is huge, about 2.5° in length, and has a very low surface brightness. |
#7
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Venus + 6" f/6.5 achromat + filters (Observational Notes)
On Sun, 22 Jan 2017 12:14:42 -0800 (PST), Sketcher
wrote: NGC 2024, aka the Flame Nebula, aka the Tank-Track Nebula is about four degrees north-northeast of M42 right beside Alnitak - the eastern-most star in Orion's belt. One night, after viewing the Horsehead Nebula (B33) with an unfiltered 5-inch apochromat I swung the scope over to NGC 2024 and saw a wealth of detail at 200x with my extremely dark-adapted telescope eye. The visual appearance resembled a tank's track through mud far more than it resembled a flame. Ever since, I've preferred "Tank-Track Nebula" over "Flame Nebula". I don't know who first called it the Tank-Track Nebula, but that's the name I'm sticking with! Another interesting question is who first called it the Flame Nebula. It can be remarkably difficult to track down the origin of common astronomical names. I've just spent a couple of hours going through a bunch of old books on my shelf, as well as a large number of Internet references. The earliest reference to the name I've found is 1987. Do you have a recollection of when you first heard the name? Because the Flame Nebula so strikingly resembles a flame photographically, I wonder if the name is of relatively recent vintage and based on its appearance on film, or even CCD, rather than through an eyepiece. |
#8
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Venus + 6" f/6.5 achromat + filters (Observational Notes)
On Sunday, January 22, 2017 at 3:14:45 PM UTC-5, Sketcher wrote:
NGC 2024, aka the Flame Nebula, aka the Tank-Track Nebula is about four degrees north-northeast of M42 right beside Alnitak - the eastern-most star in Orion's belt. One night, after viewing the Horsehead Nebula (B33) with an unfiltered 5-inch apochromat I swung the scope over to NGC 2024 and saw a wealth of detail at 200x with my extremely dark-adapted telescope eye. The visual appearance resembled a tank's track through mud far more than it resembled a flame. Ever since, I've preferred "Tank-Track Nebula" over "Flame Nebula". I don't know who first called it the Tank-Track Nebula, but that's the name I'm sticking with! So yes, in that way it's like the California Nebula (and the North America Nebula, and a few others). Sketcher, To sketch is to see. On Sunday, January 22, 2017 at 3:38:09 AM UTC-7, wrote: JOOC, is the Tank-Track Neb like the Cali like fornia nebula? The California Nebula looks more like tank tracks than the Flame Nebula does. That why I was, like, asking. |
#9
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Venus + 6" f/6.5 achromat + filters (Observational Notes)
On Sunday, January 22, 2017 at 8:29:51 PM UTC-5, Chris L Peterson wrote:
Another interesting question is who first called it the Flame Nebula. It can be remarkably difficult to track down the origin of common astronomical names. I've just spent a couple of hours going through a bunch of old books on my shelf, as well as a large number of Internet references. Clearly, you have WAY too much time on your hands. |
#10
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Venus + 6" f/6.5 achromat + filters (Observational Notes)
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