Louis D

Not a clue what you mean by a barlow being parabolic with same brand EPs. Do you mean parfocal? Mirrors can be parabolic, but I've never heard of a parabolic barlow or eyepiece. Parabolic describes a curve, not a relationship between two optical items. Yes, TFOV is halved with a 2x barlow. AFOV remains the same, assuming no vignetting as described above. The exit pupil is also halved with the doubling of the power. Yes, for extended objects and the sky background brightness (which is technically a huge extended object). As I recall, point sources will also start to dim as well once the magnification resolves the Airy disk.

I think he means by doubling the power, the brightness is quartered, that's all.

Be aware that barlowing low power eyepieces can lead to vignetting, sometimes difficult to hold exit pupils due to extending eye relief, and sometimes adds spherical aberration of the exit pupil. Televue's Panoptic Barlow Interface (PBI) was created to use with TV's Big Barlow to eliminate these issues. I can verify it also works well with the GSO 2" 2X ED barlow. Alternatively, telecentric magnifiers like the TV Powermate and ES Focal Extender work in the same way but as a single unit. Anecdotally, I notice much reduced coma when using a 2x barlow with or without the PBI. I have to use it without the coma corrector because I can't reach focus with both at the same time. It's not reduced astigmatism because there wasn't any to start with when using the coma corrector alone.

The coma correctors you listed are not general purpose visual correctors. The only three general purpose ones that I'm aware of are the TV Paracorr, the ES HR CC, and the GSO/Revelation CC. The GSO version is the cheapest and easiest to use because it has a 20mm wide sweet spot. Unfortunately, it isn't plug and play straight out of the box. It requires adding a 25mm extension between the optics nose piece and the eyepiece holder. It will flatten the field as well. It also adds only 10% magnification and requires 11.3mm of infocus.

It would depend on the field stop of the eyepiece to some extent. If it is less than 27mm, there shouldn't be any vignetting at all. If it is bigger, there would probably be some, but it might be very difficult to detect with the human eye. It would also depend on the distance between the reducer and the field stop. The farther away the better. If you put it on the front of the diagonal, you'd probably never notice the vignetting. I use a 2" diagonal on a 127 Mak with a 27mm diameter rear port. Since the port is pretty far from the field stop, vignetting is all but undetectable to my eye.

There is no Meade 5000 HD-60 5mm, only a 4.5mm and a 6.5mm. There is a BST Starguider 5mm which is seemingly just as good.

Periodic cleanings are "snow" cleanings using CO2 "fire extinguisher"-type blasts. Deep cleanings generally use really pure water, shampoo (no additives, just the foaming agent), and natural sponges if I recall correctly.

The ES-52 line was just introduced a few years ago, and they had a recall on the 3mm version, so there just haven't been a lot of reports about them so far. This was the only one google could find. I find the 4.5mm Meade 5000 HD-60 to be a decent performer. It's not quite as good as their 6.5mm, but it's very close. Here's a FOV image comparison I posted a couple of days back that includes the Meade, Paradigm, and a couple of Pentaxes.

If you have deep pockets, a 3-6mm Televue Nagler Zoom would be ideal so you could dial in the highest usable power for the conditions. Alternatively, @John recommends a 7.2mm-21.5mm zoom with the Baader Q-Turret barlow. I have a similar 7.2mm-21.5mm zoom, and it is indeed quite sharp even without a barlow f/6. Look for my zoom's images in my eyepiece FOV postings from last night in the 6.5mm to 8mm, 12mm to 12.5mm, and 18mm to 22mm sections. With a barlow in your f/5, it would be operating at f/10, so this might be a good budget option.

How about some discussion of the results? I'm surprised by just how good today's step-up 60 degree eyepieces are relative to premium eyepieces from 12mm on down. The HD-60 and Paradigm eyepieces are very good across their fields. I was a bit disappointed with the field curvature of the 12mm Pentax XF by comparison. I'll have to see if it has superior polish under the stars allowing for better contrast and dimmer details to be picked out to atone for its edge issues. I just can't get over how much SAEP some eyepieces have. I always knew I had issues with some of them even with fully dilated pupils. They're definitely easier to use under those conditions, but they're still fussy. The AstroTech AF70 eyepieces are quite good for 70 degree eyepieces if you're on a budget and can pick them up used for a good price. The older Konig and Erfle wide angles are very sharp in the center, but fall off rapidly toward the edges. They really make you appreciate modern wide angle eyepieces. Those generic Kellners, the RKE, Plossls, and the zooms are really pretty good, all things considered. Even the $10 Aspheric is decent. The same can't be said of the reversed Kellner. Thus, there are some good budget choices out there if you know what to look for. Televue eyepieces are sharp across the field if you can get past the SAEP of some of them. Only the 22mm Nagler is in my A-Team case, though. The rest have been ousted by newer, better designs. ES-92 eyepieces are phenomenal eyepieces. Given their size and price, they'd better be. They ousted my 12mm and 17mm NT4s from my A-Team case. The Speers-Waler zoom is very sharp across the field. Again, if you can live with the SAEP (which I find mild in use). The Morpheus eyepieces are very sharp across the field. The 7mm Pentax XW is rather disappointing by comparison. I may get the 6.5mm Morpheus the next time it goes on sale. The Meade MA Astrometric is no replacement for the Celestron Ortho Astrometric. The former is not sharp across the field. What good is a wider field if it isn't sharp in a measurement eyepiece? The 15mm and up HD-60s and Paradigms are definitely not the in premium eyepiece category, but by comparison to Konigs and Erfles, they're not all that bad. Again, they're good budget buys, just not great budget buys like the 12mm and below versions. The 24mm APM UFF is a bit of a disappointment given its size, price, and hype. The 30mm APM UFF really does live up to the hype by comparison. It ousted my venerable 27mm Panoptic from its spot in my A-Team case. The 30mm Kasai Super Wide View, which was their $400+ copy of the 30mm Leitz Super Wide, is just awful by any measure. It's not even as good or wide as the $50 Rini MPL. The Agena UWA looks very usable by comparison. Under the stars, I find the Agena UWA very sharp in the center 50% and very easy to hold the view with eyeglasses. It just suffers from massive field curvature and some edge astigmatism. The ES-82 is slightly less sharp in the center (compare the barcodes between the two). Of course, it is very nearly sharp to the edge, just suffering from chromatism out there. The Scopos is a real keeper. It is very sharp in the center and holds most of that sharpness to the edge. It is a huge and heavy eyepiece, so it had better do something well. The Aero ED is a nice compromise on size and weight by comparison. The military eyepiece show how far premium eyepieces have come over the last 50 years. The Meade 5000 Plossl while sharp in the center is a bit of a disappointment further out. The 5000 SWA is a keeper showing a nice sharp field nearly from edge to edge.

I guess I forgot to mention the telescope and diagonal. I was using my AstroTech 72ED with a 2" GSO dielectric diagonal with a TSFLAT2 field flattener attached to the front of the diagonal with 20mm of spacing between it and the diagonal. The net result is very close to a perfectly flat field from edge to edge.

I spent 3 or 4 days of solid work on this. My back has been paying the price from hunching over all these eyepieces with a cellphone. Editing at the computer sitting in a nice office chair was a delight by comparison. 😁 All of the "regular" camera images were taken at f/1.7 and 4mm focal length. All of the "ultrawide" camera images were taken at f/2.4 and 2mm focal length. It's pretty obvious that the latter camera is much more sensitive to SAEP than the former. The weird part about using these eyepieces with SAEP is that I had never noticed it in the Speers-Waler because I can't get close enough in normally due to its somewhat short eye relief (despite Waler meaning Wide angle long eye relief). The 12mm NT4 is much less bothersome to me than the 17mm NT4 despite them showing about equal amounts. The 22mm NT4 shows the least by far of the three; and indeed, it is the easiest to use. The 14mm Meade UWA also is not normally very bothersome. Of the 5 with dramatic SAEP, I'd say the 17mm NT4 is my least favorite because even with a fully dilated pupil, it is still incredibly difficult to hold the exit pupil once the field stop comes into view. Thanks for your appreciation of my efforts. I just want to help others get a better idea of what the view looks like through various eyepieces if they don't have easy access to them at retail shops.

I keep the distance from the telescope to the ruler(s) constant across tests. I also use the same camera for all tests (except where noted for ultrawides). You have to move the camera up and down relative to the eyepiece until the field stop just pops into view. This guarantees that the magnification is the same across eyepieces. You also have to adjust for centering and tip to exactly center the image on the sensor. I also take advantage of the wider field of view across the diagonal of the sensor for wide angle eyepieces. I lock down my alt-az mount once I have the image where I want it. I then hold my cell phone camera above the eyepiece using my index finger and thumb to brace between the two. I then practice shallow, careful breathing to avoid inducing vibrations as I finalize the alignment and take a series of images by lightly tapping the screen each time I think I've got the best image possible. I then select for the best image at the computer after downloading them. SAEP becomes a problem because some eyepieces blackout horribly once the entire field of view is seen. Today's cell phone cameras very closely mimic the reaction human eyes have to various eyepieces. In particular, camera lenses with smaller maximum apertures (larger f-numbers) are more sensitive to SAEP, just like a human eye with a constricted iris as in daytime viewing. The regular camera above is more immune to SAEP because it has a maximum aperture of f/1.7 while the ultrawide camera has a maximum aperture of f/2.4. If you have to tip your head to the side to see the edge because the field stop is the end of the barrel (as is the case for the 29mm Rini MPL above), the camera will show a fuzzy edge before the true edge of field. This is because it shows what can be seen from the center of the eye lens. You can tip the camera to capture the edges as I did for the ultrawides above to show the true edge because the regular camera didn't have a wide enough field of view to capture it or because it is hidden from the center. You can calculate the true FOV (TFOV) but not the apparent FOV (AFOV) with this technique (though someone might prove me wrong on that last point). You can appreciate the relative differences in AFOV, but directly calculating it is not straight forward at all, so I'll leave the derivation to others. TFOV and thus the effective field stop diameter can be easily calculated using similar triangles. The distance shown on the ruler is directly related to the diameter of the field stop by this relationship. The only trick is calibrating the scaling coefficient. I did it by noting the relationship between my Televue eyepieces which have published effective field stop numbers and the distances I measured. This measurement is immune to magnification distortion across the field of view. If you don't have any Televue eyepieces for calibration, I'd start with a 1.25" 32mm Plossl which generally has a 27mm field stop diameter. I could never work out what distances are relevant to use the similar triangles method directly, so I back into it using known good values as described. Using this method, I'm generally within 0.1mm of the values measure by others using calipers on accessible field stops (those below the eyepiece's field lens). To calculate AFOV, I use the flashlight (torch) projection method. It is immune magnification distortion across the field of view. First, use a miter (mitre) box to keep the eyepiece and flashlight aligned and stable. Second, I mount the eyepiece in a 2" extension tube (with 1.25" adapter as needed) to back off the flashlight slightly and to contain stray light not entering the eyepiece. I then move the flashlight forward and back until the projected image circle has as sharp a field stop image as possible. Sometimes, you just can't get a sharp field stop image due to the design particulars of certain eyepieces. This makes the measurement a bit "fuzzy" because you have to make a judgement call as to where the field actually ends. I then measure the distance from the top of the eyepiece to the projection screen (a white box works well for this purpose). Next, I measure the diameter of the projected image circle. Lastly, I move the screen and eyepiece toward each other until the image circle reaches its smallest point. I then measure the separation between the top of the eyepiece and the screen as this is the usable eye relief. Again, this can be a fuzzy measurement when there are exit pupil aberrations causing the minimum point to cover a range of distances as is the case with SAEP and chromatic aberration of the exit pupil (CAEP). I generally accept the center of these distances as the eye relief. Finally, get out the scientific calculator and perform the following calculation: AFOV=2*arctan[((circle diameter)/2)/(screen distance - eye relief)].

32mm to 42mm eyepieces.

29mm to 30mm eyepieces.

23mm to 28mm eyepieces.

18mm to 22mm eyepieces.

16.7mm to 17mm eyepieces.

13mm to 15mm eyepieces.

12mm to 12.5mm eyepieces.

9mm to 10mm eyepieces.

Next up, the 6.5mm to 8mm eyepieces.

First up, the 3.5mm to 5.2mm eyepieces.

I decided to create eyepiece field of view (FOV) images for all of my eyepieces with my newer cell phone camera and an ultrawide angle camera on another junked cell phone camera for the ultrawides. I'll post them here one group at a time to break them up into manageable sized posts. The ultrawide angle camera images (labelled "full view" below) were up-sampled to match the center magnification of the regular camera images since distortion should be at a minimum there. This allows for easier comparison. However, the regular camera has higher resolution to start with and a much larger percentage of the field is being utilized, so the ultrawide angle images are rather low resolution and are presented mainly so the reader can more easily grasp the relative width of the ultrawides. I also ran into severe spherical aberration of the exit pupil (SAEP) with several eyepieces, so you'll see varying levels of blackouts in some images. The eyepiece exterior images are in the same order left to right as the FOV images from top to bottom.

Makes me glad I have a concrete patio to setup on. We've had the opposite problem here in our part of Texas. We've only had 0.25" of rain over the last three months and are in a severe drought. I'd probably try the concrete pavers made for parking lots to allow water to run through to the ground to allow trees to get enough water. Grass would probably grow up and through them over time.