“I am the only person to ever ace a 1951 USAF resolution test. My 'to observe' list says 'done'. I do not use charts or atlases when I starhop; men do not use maps. One of my sketches won an SBIG deep sky imaging contest. I am the life of star parties I have never attended. I never say anything looks like a faint fuzzy - not even a faint fuzzy. Pilots aim green laser pointers at me. Don Pensack proofreads my CN forum posts.” - The Most Interesting Astronomer in the Universe
Steve Verba 76mm Tasco 10TE 180mm Astro-Physics StarFire 20in. Obsession 15 x 15 Rolloff (Backyard Observatories)
Zhumell Z10 Telrad w/DIY dew heater ES82 30mm, ES100 20mm,14mm,9mm,5.5mm
Quote:When there is a deep sky object in the field, there is more than mere framing going on.Personally, I find Jeff's 75% rule to be unaesthetic. I would say an object width of 50% is more pleasing for the view because it gives more context for the object.Context is the nature of the immediate neighborhood of the object. Whether or not there are other nearby objects that form nice pairings or groups with the object is not only nice to know--sometimes it is nice to see those other objects at the same time.But just because it is nice to see those other objects at the same time and in the context of closeness to the object in study doesn't mean we necessarily want to use such a low power that the details of the object disappear with the low power.It can be nice to see a couple of close objects in the same field at the same time but with sufficient magnification to actually see them well--at the correct power to observe details in those objects. To not only see the objects in context but to also see them with the right amount of magnification, like M13 and NGC6207 in the same field at 180X.Now if you want to view the objects in context and with enough power to observe them well, where does that lead? To large apparent fields of view.But this doesn't only apply to groups of objects. Say I want to view an edge-on galaxy like NGC891, which is an edge-on galaxy in Andromeda. This object's maximum length is about 13'. If I stay with my 50% rule (and with this object the field is magnificent), I will want a field about 26' wide. But, experience with this object tells me that a magnification over 200X is necessary to show details in the dark lane and show irregularities in the interior of the galaxy. I can easily achieve that with an 8mm eyepiece having a 100 degree apparent field. It would take a 16mm eyepiece with a 50 degree field to see the same size field, but then the magnification would be too low to see the details that can be seen at higher power.OK, NGC891 is a larger object. What about, say, M75, the globular in Sagittarius? It's only 6.8' wide. Well, like most globulars, the stars are faint. The brightest members hover around magnitude 14.6 and the horizontal branch is clustered around magnitude 17.5. How do I see as many stars in this cluster as possible? The answer: high power. At 300X, in good seeing, this cluster appears to resolve at least partially in my 12.5" and provide a view of stars all over the cluster. It begins to resemble a globular and not a faint nebula. How can I use sufficient power to see the faint stars and still see sufficient field to note the shape of the outlying stragglers and to see a nice context for the object?A wide apparent field.Jeff's article points out that most deep-sky objects aren't that large. That's true. But you only need a 30' field to see the whole Moon, while a 1 degree field gives a much more pleasing view of the full moon. In fact, it could be argued an even larger field is more aesthetically pleasing.Advertising hyperbole aside, there is a valid reason for wider apparent fields with excellent correction. They do a better job of framing the objects we view, and of showing the field around the object (its context) with sufficient magnification.It's not a matter of "drinking the Kool-Aid" or "falling for advertising hype", its a practical matter of wanting to use higher powers to study and view those, as Jeff points out, largely small objects and still see them in a field size sufficient to show context if they are grouped with other objects (and many, if not most, deep-sky objects are). Yes, a shorter focal length telescope (likely to have a smaller aperture) would suffice to yield the wider fields of view, but at the sacrifice of magnification necessary to see details in those small objects.And the wider the apparent fields of view, the larger the spread in the magnifications that can be tolerated because the field size doesn't seem to shrink so dramatically when shortening the focal length. Three eyepieces and a barlow can yield all the magnifications you need.Narrower fields have to have their magnifications closer together to be able to come up with a sufficient field size for the objects. Paradoxically, expensive hyper-wide field eyepieces may form a cheaper collection than a case filled with narrow field eyepieces, looked at from that standpoint.To be fair to Jeff, he is not advocating for the use of only narrow field eyepieces. And, whereas we differ in how much context we'd like to see when framing an object being observed, we agree on the idea of framing. I spend a lot of time with other observers at a popular observing site, and this is a lesson that is not well-known. I see most observers observing at too low a magnification to see details in the object being viewed--a 6' galaxy in a 1 degree field. Certainly there is a place for this (as when trying to see several galaxies in Markarian's Chain in Virgo), but not if one wants to see all that can be seen in the scope.So kudos, Jeff. Thanks for bringing up the idea of framing and explaining it. I hope it becomes a commonly-used technique of observing.
Vixen 140mm Neo-achro, 2" AP Maxbright diagonal, 40mm Orion Optilux, 35mm, 30mm, 18mm, and 15mm Ultrascopic/Ultima, 28mm & 20mm ES 68, 19mm TV Panoptic, 5.5mm Meade UWA, 2.4x 2" Dakin barlow (prototype barrel),1.6x Antares barlow.
Quote:+1 Agree 100%. Multiple objects are more enjoyable with wider TFOV - the more you can see in the same FOV the more the merrier. Wider TFOV also allow more days planet conjunctions and or comet/DSO/planet conjunctions are in same TFOV. Those who don't have tracking and find objects manually, wider TFOV and AFOV make it more likely you will actually find the objects, and not miss it while sweeping/hunting the sky. Lastly, manually tracking the ISS is far easier with wider TFOV; once you lose it it may be gone.
Jeff Morgan - Wile E. Coyote School of Telescope Making
Quote:I've seen people express different approaches on when the want the 82/100 deg AFOV. Some people want the arrest AFOV at low mag and are ok with smaller AFOV at high mag. I prefer the opposite approach. I like the ultra wides at high mags but am good with the 48 deg AFOV of my 48mm Brandon at low mags. Dave
Quote:Jeff, thanks for the great article! I really enjoyed your approach.I happen to have taken issue with nearly everything you said, though. My two day-to-day instruments are 400mm and 750mm, rich-field and multiple-object framing are important to me, and I'm with Jon Isaacs that I like very wide (say, 50%) framing. And you say we always increase the magnification if the object fits and the seeing isn't atrocious, and it is like reading a letter from my penpal on Venus...the thought bubble opened over my head, "when is seeing *not* atrocious?!"And of course probably the biggest nitpick is that even though most objects are quite small, my favorites are often large, some are even binocular objects.But I'm a cheap eyepiece aficionado, I accept a lower magnification to get a wider view (and I don't mind squinting through a short plossl when the seeing does line up), and I think you're spot on that finding fun ways to look at it is more important than finding new equipment to look through. Thanks for something fun to think about.
John H Visual astronomy on the Blue Ridge since 2004
Quote:Of the 2405 objects, three groups (OCs, BNs, DNs) total about 1050 of those objects, or about 44% of the total. Each of the three groups average 24 arcmin in size. When I look at the top chart, it does not look like it contains 1000+ objects of 24 arcmin average size.And when I look at your tables, the "frame field" for that average would be 32 arcmin. Closest to that is the low power 2800mm fl scope with the 21 Ethos, giving a frame field of 33.3. The chart says that includes 2,215 objects. That leaves max 190 objects above that 24 arcsec to weight things to get to the average.I'm no mathematician, and I'm sure it's possible; but it seems worth asking about. Could you please clear this up for me?Lee
Quote:Try magnitude 8. The resulting size distribution curves will start shifting where you want them.
Quote:Some specific things I notice in the tables:Low power1700mm + 35Pan ff = 34.6 and 2800mm + 35 Pan ff = 34.6?High power1000mm + 3.7E of = 2021 and 2800mm + 3.7E of = 2021?There are some more that seem worth double-checking.
Quote:Not that it changes anything, but I think the most common f.l. are:400mm-750mm (many 4" and under refractors, a few 4.5" f/4 Newtonians)750mm (6" f/5)1000mm (4" f/10, 4.5" f/9, 120mm f/8)1200mm (6" f/8, 8" f/6, 10" f/4.8)1500mm (12" f/5)1800mm-2000mm (8" f/10, 15" f/5, 16" f/4.5)2800mm (11" f/10)I think that a low percentage of telescopes in use except SCTs have focal lengths of 2000mm or more.Some consideration of exit pupil should be included in this. I am not sure that a 3.7mm focal length ep in an f/10 system is useful very often (0.37mm exit pupil). (This improves the strength of your argument for this case).If you exclude the LDN catalog, then 90%+ of the objects available to view in moderate or large apertures are smaller than half a degree (30 arc min). Even then, about half the objects over 30 arcmin are Barnard objects. Over 50% of objects are smaller than 5 arcmin. Over 75% are smaller than 10 arcmin. About 10% of those 2400 objects are 1 arcmin or less (according to SkyTools). So a case could be made, and I think has been made, that your main instrument should have a useable field of view of 30 arcsec. It makes sense to me to buy one eyepiece that gives you the widest field that your telescope can provide and that matches well with your eye; the latter part largely determines how "wide field" that eyepiece needs to be. For an f/10 SCT, that 55mm Plossl may be ideal. That same eyepiece is not ideal for an f/5 Newtonian; a 26mm or 31mm Nagler or 35 Pan may be, depending on your eye.I want an eyepiece that gives me the maximum magnification under the best skies I anticipate, and one that gives me the maximum magnification under my typical conditions. These need to be around the 1mm exit pupil range, probably one somewhat below and one above. Wide fields and good eye relief are very nice, here.Other than those 3, it seems more a matter of taste and funds; that's 3 eyepieces, and either one or two more seems like enough to make a complete set.The attraction for a lot of people for premium ($$$) eyepieces may be the very wide AFOV. For me, the fundamental appeal is that it seems that things like excellent performance at low focal ratios and availability of consistent adequate or long eye relief, independent of the focal length of the eyepiece, are only available in these lines of eyepieces, and are, for me, more important factors. Lee
Quote:2) Unless you have supra-human physiology, you are only going to see one of those three galaxies better at one time. Sure, you can "see" the others separated by 80+ degrees of apparent field (remember, the Nagler didn't work!).
Tom Karpf (tkarpf) Vice President, Astronomical Society of Greater Hartford www.asgh.org