Don Pensack

Using a newtonian for projection of the solar disc works OK with simple eyepieces like Ramsdens or Huygens as long as they are glass and the housings metal. Good luck finding such an eyepiece. They were common in the '60s when I got started, but the ones out there now are usually plastic.

The 4 bright short diffraction spikes are from the spider holding the secondary mirror, and are normal. The other long spike is of more concern, however, and you gave a clue of its cause--the focuser. You mentioned the focuser was nearly all the way in when focus was achieved, and it seems logical to conclude it is sticking into the light path to the mirror and causing a massive spike. If the focuser is at approximately that angle relative to the spider vanes, that would cinch the identification as caused by the focuser. So how to get it out of the light path to remove the diffraction? Well, it would focus farther out if it were shortened, and I think I have a way: --get rid of the 1.25" adapter that you now use and instead use a tall 1.25" adapter like the Baader Click-Lock, or an Antares Twist-Lock, or something with at least 10mm of height above the focuser. This would move the eyepiece in quite a bit, and require the focuser be moved out to compensate. That might be enough to get the focuser out of the light path. You can check to see how much it intrudes into the tube when you're in focus. You want the focuser to not be inside the tube at all when the eyepiece is focused. Chances are there is not a lot of clearance between the mirror and the tube. If the inside diameter of your upper tube assembly is less than, say, 40mm larger than the mirror, the tight clearance could easily lead to having the focuser intrude into the light path. You also have the possibility of simply dropping the UTA toward the primary by 10-20mm to experiment with seeing whether the focuser is at fault. That would require moving the focuser out by the same amount. If the long extra diffraction spike disappears, then you know what causes it and what you have to do to eliminate it.

As for the sun, one poster described leaving his big dob on the porch when he went to bed. The morning sunlight reflected off the mirror on the eave of his house and set the house on fire. Fortunately, very little damage was done. But if you ask if sunlight can melt a Barlow or eyepiece? Yes, it can.

There is one "fly in the ointment" with the hypothesis that we can see colors under low light. Psychology lab tests show that when 2 identical grey squares are presented side-by-side to the subject, and both are illuminated so low that the only way to see the squares is with scotopic vision, and one square is illuminated just a bit more than the other, the mind fills in the color by seeing the slightly brighter one as greenish grey and the dimmer one as reddish-grey. So if you see red and green in a nebula, which, unfortunately, are the predominant colors in nebulae, it is impossible to tell if the colors are spurious if the object has a surface brightness of 18 mpsas or dimmer. This shouldn't apply to the Orion Nebula, which has a high enough brightness to damage night vision in a scope, but definitely applies to, say, M27, where the people who report seeing color see the "apple core" as greenish, and the long wings of the oval as reddish, when color images show the opposite to be true. A few posts back, I explained why the DGM filter is believed to work for red colors and the size of large bright nebulae. It works quite well even if color is not seen, but I believe this is likely related to its quite-narrow bandwidth in the blue-green.

Barry, There will be coma visible in this instrument as long as you don't use a coma corrector, so NO eyepiece will be perfect to the edge. I assume this is a SkyWatcher Flex-Tube dob with a 1500mm focal length, as such, f/5. Normally, a complete set of eyepieces for the scope might be 60x/120x/180x/240x/300x That would be focal lengths of 25mm, 12.5mm, 8mm, 6mm, and 5mm. Your current set lacks only a 6mm, so the jump from 8mm to 5mm is a big one. That is not set in stone. A more minimalist set might be 21mm, 11mm, 7mm, 5mm. A more maximalist set 30mm, 25mm, 20mm, 15mm, 12.5mm, 10mm, 7.5mm, 5mm, 3.5mm You have all the eyepieces you need, though there are eyepieces with wider fields and sharper image quality in the outer field (taking into account that coma will make stars in the outer field distorted so no eyepiece will be perfect). With that aperture, it is unlikely you would want or need more than one eyepiece to yield a magnification under 100x. Likewise, if you need eyepieces above 300x, it's just as desirable to get there by adding a Barlow lens to a lower power. Any eyepiece shorter than 7-8mm will be limited by the seeing conditions, so whether you get sharp stars won't likely be determined by the eyepiece. Any eyepiece longer than 15mm is also less likely to be used except for the largest of objects, and, even then, you'll likely go higher to investigate details more. "Planetary" eyepieces will be 7-8mm and shorter. Highest visual acuity and best overall views will be eyepieces from 10mm to 15mm (2-3mm exit pupils). Low powers will be 12.5mm and longer eyepieces. It looks like you don't need long eye relief for glasses, so just about everything will work. I have a 12.5"(31.8cm), but the focal length is 22% longer than yours, so my eyepieces are a little different. I have a "maximalist" set of 30, 22, 17.5, 14, 12.5, 11, 10, 8, 6, 4.7, 3.7mm focal lengths. I would note that the seeing conditions at my normal sites are very good, so 300x is always usable and sharp. Note also that some of the magnifications are a bit close together, so I tend to break the focal lengths down into "sets"--one for poor seeing, and one for good seeing. So it's really more like a set of 5 and a set of 6. And, truth be known, I sometimes spend the entire night using only 2 focal lengths--especially if all my targets are small and faint. So it's normal you might have a night when you use only the medium powers (120-240x on your scope), and another night mostly lower powers (50-120x). It will depend on the seeing, and the targets, as to which focal length is best. P.S. The Catseye nebula will be incredible in your scope with > 200x, or a shorter focal length than 7.5mm. The amount of detail will be leaps and bounds above what can be seen at 100x. If looking at the Morpheus, I recommend the 12.5mm for starters. I think you will use it a lot and it hits a point of high visual acuity (2.5mm exit pupil)

It's a difference in philosophy. I'll explain. Generally, most people have felt through the years that having any red transmission only seemed to create funny looking star images, so the preference was for filters that had no red transmission, like the TeleVue BandMate II, the current Lumicon filters, or the Orion Ultrablock (as an aside, Orion filters come directly from Korea and are not Synta products sold under the Orion label). But the DGM NPB filter, which is almost like a broadband in its red transmission, despite a narrow blue-green bandwidth, challenged that idea. Here is why: Any nebula's light is a combination of light from the sky PLUS the light from the object. Any DSO is brighter on the Earth's surface than it would be in space. But, so is the sky background brighter on Earth, which reduces contrast, and the human eye is a contrast sensor. Otherwise, using filters would have no value, because every DSO is brighter without a filter. So how do you increase contrast without dimming the object? Let through the wavelengths emitted by the nebula, but block all other wavelengths. It works, and we have had good nebula filters for 40 years. But what if the wavelengths to which the eye is relatively insensitive were allowed to pass unimpeded. The nebula would be bright at those wavelengths because its brightness would be that of the sky and object. And if those wavelengths happened to be strong emission lines, like S-II, N-II, and H-α, would that have an effect? So a fellow named Dan McShane (of DGM) experimented with filters when he worked for a company making filters for industry. And he found that passing a lot of red made the nebulae appear brighter and of larger extent than not passing the red, and at the sacrifice of only a small amount of contrast. To enhance the nebula in the center of our visual peak sensitivity, he narrowed the bandwidth more than in other narrowband filters. Most of the high-end UHC-type filters have a 26-27nm bandwidth these days. My DGM sample measures 21nm. That requires EXACT placement of the bandwidth, which is often the Achilles heel of filter making (and DGM's too), but the result is incredible--superb contrast in H-ß and O-III wavelengths, but a larger extent seen in nearly every large hydrogen-emission nebula. My lifetime-best view of M17 was with a DGM NPB, where the "Swan" shape only appeared to be about 10% of the visible nebula and nebula could be traced, visually, all the way to M16, the other bright point on the same large nebula. It comes at a cost, however--all the stars are red. This is obviously because the bandwidth in the red end of the spectrum is so much wider than the one in the blue-green. Some observers find that terrible. It's a fairly unique approach to making filters, and one that is not favored by Lumicon or TeleVue. The Astronomik basically has a sharp spike in transmission at the H-α wavelength for photographic purposes because the red transmission isn't sufficient to have the effects of the DGM NPB (narrow pass band) filter. And there are a lot of nebulae where having the red transmission doesn't help, pointing to the design philosophy of TeleVue and Lumicon as being better in application due to increased contrast for visual use. Still, I've found it effective to have both designs on hand, and to compare nebulae with them. I've found nebulae that responded better to the DGM philosophy and others that responded better to the Lumicon and TeleVue philosophy. Filters that take more out of the spectrum cost more, due to more coating layers (called cavities) being required, so the advantage to DGM's approach is that the filters ended up less expensive to make. So if you experiment, I'd try one of the DGM philosophy and one of the Lumicon/TeleVue/Orion philosophy and make your own conclusions. For me, I'll keep both.

Yes, but that depends on the light intensity. We see a red flashlight because it is so bright. Bright stars, planets, Moon, even some brighter DSOs, turn vision "mesopic", where cones are partially activated. It's why we see colors in stars and planets. I've noticed M42 can actually reduce my night vision, so it's no wonder I often see colors in M42.

Orion in filters means Orion in the US, a California company. The red transmission in the Astronomik filter can be seen as a red character to the star images, but it has almost no effect on nebulae visually. it's primarily there for photography.

My sample of the Baader UHC-s has a 62nm bandwidth in the blue-green. bandwidth FWHM H-B line O-III (1) O-III (2) Ha low wavelength High wavelength 62 93.3 94.7 95.0 87.1 464 526 Comparing with my ES CLS: FWHM H-ß O-III O-III H-α low High 75 92.1 95.2 94.0 89.9 451 526 Both are decent broadband filters. The Baader UHC-S has the edge for visual contrast on nebulae simply due to a narrower bandwidth. The CLS and Baader differ in their long wavelength cutoffs, though. The Baader cuts off at 675nm, with almost no output at all up to 1000nm., while the ES CLS cuts off at 756nm, with a broad bump centered on 950nm. I'd say that the ES CLS would be better for imaging because of that.

Uncommon on filters made since 2005.

A couple points: 1) If buying used, avoid the TeleVue filters called BandMate. You want the BandMate II, the later ones made by Astronomik. Much better bandwidths and performance. 2) if you use a Paracorr coma corrector in a dob, the lowest possible setting is still not optimum for the Nikon HW 17. They left a long length of filter threads on that one, and it simply cannot get close enough to the lens for optimum coma correction. It's pretty close, though, and certainly no worse than the 31mm Nagler in the original Paracorr, which also could not achieve an optimized position. 3) APM is back in stock on the 20mm XWA HDC eyepieces.

Original Nebustar was made by a different company than the Nebustar-II. The original was a lot wider. The results on the ES filters shows the effect of making things very cheaply without any QC. The #1 problem with the inexpensive filters isn't the too-wide bandwidth, it's inconsistency from filter to filter. Unless you test it, you don't know what you have. This thread should be mandatory reading for those who want to know about filters: https://www.cloudynights.com/topic/527199-spectroscopic-analysis-comparison-of-nebula-filters/?hl=%2Bspectroscopic

I don't know if they changed it, and I regard it as one of the best, if not THE best broadband filter. I think the tune up of contrast is there, and it is noticeable, but I always have to add the caveat that it is subtle because so many are led to believe nebula filters provide a huge improvement, and they will be disappointed if using a filter this wide. Additionally, in high light pollution, often the case with city dwellers, the broader the bandwidth, the more the internal scattered light in the filter. Using an Astronomik CLS filter (99nm bandwidth in the blue-green and wider in the red) here in Los Angeles made the view WORSE than the view without the filter in place. Light scatter was horrendous. So I think that a broadband filter has to have certain caveats attached to it. I think they work their best at sites where the sky is already quite dark and many/most objects need no filter at all, and the presence of the broadband just turns up the contrast a bit, making things a bit more visible than without the filter, but not dimming the stars quite so much as in the narrowband filters. I also cannot know how sensitive an observer is to small changes in contrast. A narrowband drops the brightness of the background sky by around 2.3-2.5 magnitudes and an O-III filter around 3 magnitudes. That is not subtle. Here is a simulation of various nebula filters: https://www.cloudynights.com/topic/385867-filter-simulator/?p=4939179 I wouldn't call the broadband filter improvement subtle, but I think most would compared to the narrowband. [this nebulae doesn't respond well to H-ß, which explains that simulation.] There are objects where the broadband shines, though, like NGC7023, IC405, or Sh2-155. And, quite certainly, if used in appropriate circumstances, it will be much better than no filter at all.

Just had a long conversation with Al Nagler of TeleVue who confirmed some thoughts of mine: There are 3 different ways chromatic aberration expresses itself in an eyepiece near the edge of the field: 1) Chromatic aberration of the exit pupil, as found in the 31 and 26mm Nagler and 30mm Explore Scientific (and others). It is a ring of color extending well into the field. This is what is termed "the Ring of Fire". It will be different in different f/ratios of scope since the exit pupil will differ. He suggested experimenting with aperture stops to see the difference. 2) lateral chromatic aberration that yields a thin ring of color right at the field stop. It is produced by the upper section of the eyepiece and it is caused by chromatic aberration of the system, which is why it is seen more in wider eyepieces than narrow ones. I asked if it would be covered completely if the field stop were made narrower, and he said the ring would grow thinner, but unless the field stop were substantially smaller, it would not be gone. He suggested the experiment of putting a small tab of tape on the field stop of an all-positive eyepiece like a Panoptic (which is below the bottom lens) to see where the color fringe gets thinner and disappears. 3) Chromatic smear, or prismatic smear of a star image as the star gets closer to the edge of the field. This is the most common form of chromatic aberration in eyepieces and it can be reduced or eliminated with additional lenses in the system. That may or may not be practical if the eyepiece is already large and heavy. This one is from all the elements in the eyepiece, whether positive or negative.

You should not refer to the thin color ring at the field stop, blue or otherwise, as the "Ring of Fire". This was a mistake made by some observers who had never seen it and assumed it was the same thing. It is obviously not. Your link does show the issue with a 31mm Nagler, which was the eyepiece that prompted the coining of the term a couple decades ago. As Ernest pointed out, it is "chromatic aberration of the exit pupil". The thin ring of color at the edge of the field shows up in some 50° all-positive eyepieces, as well as many 60-70° eyepieces without a negative field lens. So it is not necessarily just caused by the use of a negative field lens in an eyepiece. It is different than the lateral chromatic aberration in most eyepieces.

Excellent pix! Especially the 30mm ES. There is a very slight SAEP there, too. The ring at the edge extends farther into the field on the 31mm Nagler. Over the years, the very thin colored line at the field stop that I've seen in eyepieces have also varied in width. The line at the edge in the new TeleVue Apollo 11 is about the thinnest I've seen. That's really never much of a problem at night, of course, but chromatic aberration in the form of lateral prismatic effects is an issue in some wide/ultrawide/hyperwide field eyepieces. Photographing the issues can be hard when it is in the form of Edge of Field Brightening (EOFB), though Bill Paolini has captured some images that reveal it in at least one eyepiece. It goes to show there is no perfect eyepiece.

Lumicon has had 4 owners and there have been at least 5 different makers of the filters. The first owner was 1979-2001 The second owner was 2001-2012 The filters were VERY high quality, but expensive. The 3rd owner was 2012-2016 She got bitten by an unscrupulous filter provider The 4th owner is 2016 till now. At first, he was selling off old stock, but moved into another provider in 2018. They were relabeled "Gen.3" to differentiate. I can see that. Calling them Gen.9 would have been really confusing to customers. LOL. The Baader UHC-S, by the way, is a misnomer. It's a broadband filter of about a 62nm bandwidth. it's one of the very best broadbands, but it's not a narrowband UHC-type filter. Ditto the Astronomik UHC-E, 49nm bandwidth, which, though narrower than the UHC-S is more of a broadband than a narrowband UHC-type. I'd call it a "medium bandwidth" filter. This site allows you to superimpose any filters you want that have been tested: https://searchlight.semrock.com/?sid=a08a1af9-84ee-49d2-959d-153d7e7c0eb8# Note that all the filter graphs displayed are as they were tested in the lab, NOT what the manufacturer claims. You can see he has tested a LOT of filters.

All. But I note that Harry Siebert of Siebert optics makes a 1x OCA, which is a marvelous thing for yielding lower powers. If the binoviewer is primarily used for Moon and planets, though, a higher power OCA might be preferable.

One thing to note about these filters. They work by lowering the background brightness several magnitudes while dimming the nebula maybe only 0.1 magnitude. The contrast enhancement is how they work. As the magnification increases, the background in the scope dims, as does the nebula. Above a certain point, the darkening of the background by the filter is small because you can't get darker than black, but the nebula has dimmed. This lowers the improvement in contrast and the filters stop being particularly effective. With narrowband or O-III or H-ß filters, this occurs at about 10x/inch of aperture in the telescope. That means they will be used at low power and if even your only 2" eyepiece is your low power, then get a 2" filter. You can always thread it onto a 2" adapter to use with 1.25" eyepieces. Now, broader filters, like a broadband, can be used at a bit higher magnifications without dimming the objects quite as much--maybe up to 12x-13x/inch of aperture. But their effects are, at best, somewhat subtle, and only improve contrast a tiny bit.

excellent. it is nearly identical to the Noblex/Docter 12.5mm 84°. It is not an eyepiece you would use for scanning through the sky, though as the design has some angular magnification distortion, seen as "globe" or "rolling ball" distortion. other than that, which is different than most astronomical eyepieces, it's a fine eyepiece.

Nope, that is not the "Ring of Fire". That is the chromatic issue at the field stop seen in nearly all widefield or wider eyepieces. It's even visible in eyepieces without a Smyth lens, i.e. positive systems. I've seen red, amber, yellow, green, orange, blue and even violet in different eyepieces. The so-called "Ring of Fire", which is present in only a very few eyepieces, like the 31mm TeleVue Nagler, is not just at the field stop but extends inward 5-10 degrees into the outer field. The term "Ring of Fire" was coined because the 31mm Nagler's outer field, when used on the Moon or in the daytime, has a pronounced orange-red color, like a flame. The 30mm Explore Scientific 82° also has this, though it is more yellow than the Nagler. The 26mm nagler as well. There are others I can't remember off the top of my head Al Nagler called the 31mm Nagler issue "Chromatic Aberration of the Exit Pupil" (abbreviated CAEP) and it's cause is different than the explanation given by Ruud of the reason for the color ring right at the field stop. Ruud's explanation also doesn't explain why a positive-only assembly of lenses would have the same CA right at the field stop, but that is an aside.

But do you get a filter to look at stars, or nebulae? I do hear you, though. There are times when I prefer a wider filter to look at an object that is better seen in a narrower filter. One example is NGC2359, Thor's Helmet, in Canis Major. It's best with a 11-13nm O-III filter, but because it's in the Milky Way, you lose the context. So if I'm at a site that already has very dark skies, I'll use a filter like a Baader UHC-S (62nm bandwidth) because, though the nebula becomes a lot harder to see, the stars are still there and the nebula is magnificent with a billion stars in the background. That doesn't mean I won't go back to the narrower filter to look at striae in the central bubble, however. Here is a picture of what I mean: http://www.jburnell.com/NGC2359.html

The red/green split in your UHC is because there is a lot of red transmission in the Astronomik filter. The TeleVue filter, made by Astronomik, has no red transmission at all. Though this has a funky effect on the star images, the red does yield a bit more size to the nebula, so it's a trade-off. The DGM NPB isn't on my list, though I love it, because of its variable manufacturing QC. If you get a good one, it's fantastic. The blue-green bandwidth is the narrowest of all the UHC-type filters, and the huge amount of red yields larger sizes to nebulae, sacrificing only a small amount of contrast to do it. All the stars appear red in the NPB. The DGM O-II is nothing special, and the VHT has too wide a bandwidth. It resembles the Astronomik UHC-E.

Over the last 3 years, I've tested and reviewed about 52 different nebula filters. These were the best in the field and in the lab: Narrowband: TeleVue BandMate II Nebustar (2018 on) Astronomik UHC (2017 on) Lumicon UHC Gen.3 (2018 on)* O-III: TeleVue BandMate II O-III (2018 on)* Astronomik O-III Visual (2017 on) Lumicon O-III Gen. 3 (2019 on) H-ß: TeleVue BandMate II H-ß (2018 on)* Astronomik H-ß (2017 on) Orion (US) H-ß (2018 on) Broadband: Baader UHC-S (2017 on)* Lumicon Deep Sky (2016 on) DGM GCE (2017 on) Best ones in the lab have a star by them. A lot of other filters failed to live up to their missions, by using too wide a bandwidth, or having a misplaced bandwidth that clipped a line, having a large variation from filter to filter, or failing to cover the necessary wavelengths at all. My review was of filters for visual use, not photographic. Photographically, the list would be different. Brands I tested: Explore Scientific, TeleVue, Astronomik, DGM, Baader, Lumicon, Yulong (StarGuy, Optolong), Sirius Optics, Orion (US), Thousand Oaks.

Moved to relevant thread.